7.
ORGANIZING COMMITTEE
38TH ANNUAL MEETING
INSTITUTE ON LAKE SUPERIOR GEOLOGY
General
General Chairman: Albert B. Dickas, University
University of Wisconsin-Superior
Wisconsin-Superior
Program Chairman and Proceedings
Proceedings Editor: Bruce
Brace A. Brown,
Brown,
Wisconsin Geological and Natural History Survey
Survey
Volume 38 consists of
Part 1:
1: Program
Program and
and Abstracts
Abstracts
Part 2: Field Trip Guidebook
Guidebook
Published and distributed
distributed by
Institute on Lake Superior
Superior Geology
M.G. Mudrey, Jr., Secretary-Treasurer
Secretary-Treasurer
do
c/oWisconsin
WisconsinGeological
Geologicaland
and Natural
Natural History
History Survey
Survey
3817
38 17 Mineral Point Road
Madison, Wisconsin 53705-5100
53705-5100
ISSN 1042-9964
1042-9964
INSTITUTE
INSTITUTE ON
ON
•LAK E•
SUPERIOR
SUPERIOR
GEOLOGY
PROCEEDINGS
PROCEEDINGS
38m ANNUAL MEETING
MAY 6-9,1992
HURLEY, WIscoNsJl4
ORGANIZED BY
ALBERT B. DIcicAs,
UNIvERsn'Y O WISCONSIN-SUPERIOR
BRUCE A. BROWN,
WISCONSIN GEOLOGICAL
ANI) NATURAL HISTORY SURVEY
VOLUME 38
PART 2
MAY 1992
FIELD TRIP GUIDEBOOK
CONTENTS
CONTENTS
Trip
Trip1:1:Archean
Archeanand
andEarly
EarlyProterozoic
ProterozoicGeology
Geologyofofthe
theGogebic
GogebicDistrict,
District,
Northern Michigan
Michiganand
andWisconsin
Wisconsin
Northern
Leaders
Leaders
GeneL.
L. LaBerge,
LaBerge,University
UniversityofofWisconsin—Oshkosh
Wisconsin-Oshkosh
Gene
Richard W.
W. Ojakangas,
Ojakangas,University
UniversityofofMinnesota—Duluth
Minnesota-Duluth
Richard
J. Licht,
Licht, St.
St. Norbert
Norbert College
College
Kathy J.
Kathy
Trip2:
2: Evolution
Evolutionof
of the
theKeweenawan
Keweenawan Sedimentary
SedimentarySequence
Sequence
Trip
Leaders
Leaders
Albert B.
B.Dickas,
Dickas,University
UniversityofofWisconsin—Superior
Wisconsin-Superior
Albert
M.G.
M.G. Mudrey,
Mudrcy,Jr.,
Jr., Wisconsin
WisconsinGeological
Geologicaland
andNatural
NaturalHistory
HistorySurvey
Survey
Trip
Supergroup Rocks near
Trip3:
3: Geology
Geology of Keweenawan Supergroup
near the
the Porcupine
PorcupineMountains,
Mountains,
Ontonagonand
andGogebic
GogebicCounties,
Counties,Michigan
Michigan
Ontonagon
Leaders
Leaders
William
WilliamF.
F. Cannon,
Cannon, Suzanne
SuzanneW.
W. Nicholson,
Nicholson, Cheryl
Cheryl A.
A. Hedgman,
Hedgman,
Schulz,
Laurel G.
G. Woodruff,
Woodruff,and
andKlaus
Klaus J.J. Schulz,
Laurel
U.S. Geological
GeologicalSurvey,
Survey,Reston,
Reston, Virginia
Virginia
U.S.
Trip4:
4: Geology
Geology of the Great
Great Lakes
Lakes Tectonic Zone in the Marquette
Trip
Marquette Area,
Michigan-A Late
Late Archean
Archean Paleosuture
Paleosuture
Michigan—A
Leaders
Leaders
P.K. Sims
Simsand
andZ.E.
Z.E. Peterman,
Peterrnan,U.S.
U.S.Geological
GeologicalSurvey,
Survey,Denver,
Denver,Colorado
Colorado
P.K.
41
75
103
ARC HEAN AND
EARLY PROTEROZOIC
GEOLOGY OF THE
GOGEBIC DISTRICT,
NORTHERN MICHIGAN
AND WISCONSIN
Gene L. LaBerge
University
Universityof
ofWisconsin—Oshkosh
Wisconsin-Oshkosh
Richard W. Ojakangas
Richard
University
Universityof
ofMinnesota—Duluth
Minnesota-Duluth
WITH A CONTRIBUTION
CONTRIBUTION FROM
Kathy J. Licht
St. Norbert College
This
This field
field trip
trip is
is the
the outgrowth
outgrowth of
of numerous
numerous student
student field
field
trips
trips led
led by
by the
the authors
authors over
over the
the past
past quarter
quartercentury.
century. Most
Most of
of
the
the stop
stop localities
localities have
have been
been suggested
suggested by
by geologists
geologists who
who have
have
worked in
in the
the district.
district. However,
However, some
some localities
localities are
are the
the result
result
worked
One
of
us
(LaBerge)
spent
of
our
own
studies
in
the
area.
of our own studies in the area. One of us (LaBerge) spent
several
1991, supported
supported by
by the
the
several weeks
weeks mapping
mapping in
in the
the area
area in
in July,
July, 1991,
U.S.
U.S. Geological
Geological Survey,
Survey, and
and Ojakangas
Ojakangas worked
worked in
in this
this region
region
intermittently
intermittently with
with U.S.
U.S. Geological
Geological support
support from
from 1978
1978 until
until 1990.
1990.
2
EARLY
EARLYPROTEROZOIC
PROTEROZOICGEOLOGY
GEOLOGYOF
OFTHE
THEGOGEBIC
GOGEBICDISTRICT
DISTRICT
NORTHERN
AND WISCONSIN
WISCONSIN
NORTHERNMICHIGAN
MICHIGANAND
INTRODUCTION
The Gogebic
Gogebic range
range extends
extendsfrom
from Lake
Lake Gogebic,
Gogebic! Michigan,
Michiganl
The
westward
128 km
kmto
tonear
nearLake
LakeNamekagon,
NamekagonlWisconsin
Wisconsin
westward approximately
approximately128
The major
major structure
structure in
in the
the area
area is
is aa north-facing
north-facing
1). The
(Figure1).
(Figure
and
monocline
monocline that
that exposes
exposes Archean
Archean rocks
rocks to
to the
the south
south and
and Middle
Middle and
-(~eweenawan
Supergroup) rocks
rocks to
to the
the north.
north.
Late Proterozoic
Proterozoic (Keweenawan
Late
Supergroup)
7
SUPERIOR
LAKE
/
—
(C
( \000EBIC
WAKEFIELD
HURLE>
LI. EN
—
-
KiARENI3CO
t
•
I.
,
+
I
+
4-
4-
0
10
5
4-
4-
4
+
-f
4-
4-
-
--
vJ-
. 4+
s
LAX
-
cl SUE E I
Ci
-4-
20
KM
EXPLANATION
EXPLANATION
MIDDLE PROTEROZOIC
PROTEROZOIC
MIDDLE
-
EARLY
EARLY PROTEROZOIC
PROTEROZOIC
Volcanics and
and
volcanics
sediments
sediments
I
I
Graywacke—slate
Iron—Formation
Iron-Formation
ARCH EAN
1-4-4-
4!
I
m
y
).
vt.I
Granitoids
Volcanics
Volcan ics
1. Generalized
Generalized map
map of
of the
the Gogebic
Gogebic range,
rangel showing
showing the
the Early
Early
Figure 1.
Figure
Proterozoic
Proterozoic rocks
rocks sandwiched
sandwiched between
between Archean
Archean rocks
rocks on
on
the south
south and
and Middle
Middle Proterozoic
Proterozoic Keweenawan
Keweenawan Supergroup
Supergroup
the
rocks on
on the
the north.
north. (Adapted
(Adapted from
from Morey
Morey and
and others,
others,
rocks
1982)
1982)
33
Early
Early Proterozoic
Proterozoic rocks
rocks lie
lie discordantly
discordantly between
between these
these major
major
sequences.
sequences. As
As aa result
result of
of the
the steep
steep northerly
northerly dip
dip of
of the
the
monocline
tiltingl1 the
the geological
geological
monocline (a
(a consequence
consequence of
of Keweenawan
Keweenawan tilting),
the
district
is
basically
a
stratigraphic
cross-section.
map
of
map of the district is basically a stratigraphic cross-section.
The
known
The general
general geology
geology of the
the Gogebic range has been known
since
(1892)1Van
Van Hise
Hise
since the
the pioneering
pioneering work
work of
of Irving
Irving and
and Van
Van Hise
Hise (1892),
and
and Leith
Leith (1911),
(1911)1Hotchkiss
Hotchkiss (1919),
(1919),and
and Aldrich
Aldrich (1929).
(1929). Although
Although
subsequent
the
subsequent studies
studies have
have done
done much
much to
to "refine"
I1refineIt
the geological
geological
picture
picture -- - and
and much
much still
still remains
remains to
to be
be done
done -- - most
most of the
the early
early
interpretations
interpretations have
have withstood
withstood the
the test
test of
of time.
time. The
The broad-scale
broad-scale
geology
geology of the
the eastern
eastern Gogebic district is
is presented on
on the Iron
Iron
River
Quadrangle (Cannon,
lo x 2°
2
' Quadrangle
(Cannonl1986).
1986)
River 1°
.
STRATIGRAPHY
The
The Gogebic
Gogebic range
range contains
contains excellent
excellent exposures
exposures of
of rocks
rocks that
that
range
range in
in age
age from
from Archean
Archean (about
(about 2,700
21700Ma)
Ma) and
and Early
Early Proterozoic
Proterozoic
ProterozoictKeweenawantI
(2,300
11900Ma),
Ma), to
to Middle and Late Protero~oic-~~Keweenawan~~(2#300-- 1,900
(1,200
(11200-- -600
-600 Ma).
Ma). Stratigraphic
Stratigraphic and
and structural
structural relationships
relationships of
of
several
several of
of the
the rock
rock sequences
sequences can
can be
be observed
observed in
in outcrops.
outcrops.
Thus,it
'Thuslitis
is possible
possible to
to demonstrate
demonstrate the
the relationship
relationship of
of the
the rock
rock
sequences
sequences to
to each
each other.
other. The
The general
general sequence
sequence of
of rock
rock units
units in
in
the
the Gogebic
Gogebic district
district is
is shown
shown in
in Figure
Figure 2,
Z1 aa diagrammatic
diagrammatic
longitudinal
longitudinal section
section along
along the
the Gogebic
Gogebic range.
range.
w
Keweenawan
Keweenawan
tr ofl
E
---
-
Prit
+
+
g
Puritan
*
-
-
Figure
Figure 2.
2. Idealized
Idealized sketch
sketch showing
showing generalized
generalized stratigraphic
stratigraphic
section
section on
on the
the eastern
eastern Gogebic
Gogebic range.
range.
4
ARCHEA&N
ROCKS
ARCHEAN ROCKS
Archean
Archean rocks
rocks in
in the
the Gogebic
Gogebic range
range are
are variably
variably deformed
deformed and
and
metamorphosed
greenstones
and
granitoid
rocks
typical
metamorphosed greenstones and granitoid rocks typical of
of Archean
Archean
greenstone-granite
greenstone-graniteterranes
terranes (or
(orbelts),
belts) and,
and, indeed,
indeedlare
are an
an
The district
district is
is
extension
extension of
of the
the Wawa
Wawa Greenstone
Greenstone belt
belt of
of Canada.
Canada. The
near
near the
the southern
southern margin
margin of
of the
the Superior
Superior Province
Province of
of the
the Canadian
Canadian
Because this
this fied
fieldtrip
tripis
isconcerned
concerned primarily
primarily with
with the
the
Shield. Because
Shield.
Early
only aa brief
brief description
description of
of the
the Archean
Archean
Early Proterozoic
Proterozoic rocks,
rockslonly
rocks is
is presented
presented here.
here.
rocks
Archean volcanic
volcanic rocks
rocks are
are exposed
exposed in
'in aa belt
belt
Ramsay Formation.
Formation. Archean
Ramsay
about
about 55 km
km wide
wide and
and 16
16 km
km long
long in
in the
the eastern
eastern Gogebic
Gogebic range
range
(Prinz and
and others,
othersl 1975).
1975). Schmidt
Schmidt and
and Hubbard
Hubbard (1972)
(1972) named
named
(Prinz
volcanic
volcanic and
and sedimentary
sedimentary rocks
rocks in
in the
the Ramsay-Wakefield
Ramsay-Wakefieldarea
area as
as
the Ramsay
Ramsay Formation.
Formation. The
The Ramsay
Ramsay Formation
Formation consists
consists mainly
mainly of
of
the
pillowed,
pillowedl fragmental,
fragmentalland
and massive
massive basaltic
basaltic rocks
rocks in
in the
the eastern
eastern
part
part of
of the
the area;
area; however,
howeverlfelsic
felsic volcanic
volcanic rocks
rocks and
and metametagraywackes
graywackes are
are dominant
dominant to
to the
the west,
westl near
near the
the Wisconsin
Wisconsin border.
border.
Most
Most of
of the
the rocks
rocks have
have been
been metamorphosed
metamorphosed to
to greenschist
greenschistfacies,
faciesI
Locallyl
with
with well-preserved
well-preservedpillows
pillows and
and other
other primary
primary features.
features. Locally,
especially
especially adjacent
adjacent to
to the
the Puritan
Puritan Quartz
Quartz Monzonite,
Monzonitelthe
the rocks
rocks
have
have been
been metamorphosed
metamorphosed to
to amphibolite
amphibolite facies,
faciesland
and few,
fewlor
or no,
nol
The Ramsay
Ramsay Formation
F o m t i o n may
may be
be the
the
primary features
features are
are preserved.
preserved. The
primary
oldest rock
rock unit
unit in
in the
the Gogebic
Gogebic range
range proper,
properl however,
howeverl older
older
oldest
gneisses
3#560Ma)
Ma) are
are exposed
exposed in
in the
the Watersmeet
Watersmeet area,
areal some
some
gneisses (to
(to3,560
30 km
km southeast
southeast (Sims
(Simsand
and others,
othersl 1984).
1984)
30
.
kritan Quartz
Ouartz Monzonite.
Monzonite. The
The volcanic-sedimentary
volcanic-sedimentarysequence
sequence of
of
Puritan
the Ramsay
Ramsay Formation
Formation has
has been
been intruded
intruded by
by large
large granitoid
granitoid masses
masses
the
named the
the Puritan
Puritan batholith
batholith (Schmidt
(Schmidtand
and Hubbard,
Hubbardl1972).
1972). The
The
named
Puritan
Puritan Quartz
Quartz Monzonite
Monzonite exposed
exposed in
in the
the central
central and
and western
western
Gogebic range
range is
is aa weakly-deformed
weakly-deformedpost-tectonic
post-tectonicpluton
pluton that
that
Gogebic
ranges
ranges in
in composition
composition from
from granite
granite to
to tonalite
tonalite and
and has
has been
been dated
dated
2#735±& 16
to 2,735
16 Ma
Ma (Sims
(Simsand
and others,
othersl1984).
1984).
to
Granitoid rocks
rocks are
are also
also extensively
extensively exposed
exposed in
in the
the
Granitoid
Marenisco area
area on
on the
the eastern
eastern end
end of
of the
the Gogebic
Gogebic range.
range. Parts
Parts of
of
Marenisco
the granitoid
granitoid has
has prominent
prominent gneissic
gneissic banding
banding and
and associated
associated
the
otherl more
more extensive
extensive phase,
phasel is
is aa weakly
weakly
amphibolite. The
The other,
amphibolite.
deformed medium-grained
medium-grainedgranite
granitewith
withwidely
widelyscattered
scatteredpeglnatite
pegmatite
deformed
(Frittsl1969;
1969;Trent,
Trentl1973)
1973)
bodies (Fritts,
bodies
.
The Archean
Archean rocks
rocks were
were eroded
eroded and
and are
are unconformably
unconformably overlain
overlain
The
by Early
Early Proterozoic
Proterozoic rocks
rocks of
of the
the Marquette
Marquette Range
Range Supergroup.
Supergroup.
by
Because the
the Gogebic
Gogebic range
range is
is aa north-dipping
north-dipping monocline,
monoclineI Early
Early
Because
Proterozoic rocks
rocks rest
rest on
on Archean
Archean rocks
rocks to
to the
the south
south along
along the
the
Proterozoic
entire length
length of
of the
the range.
range.
entire
EARLY PROTEROZOIC
PROTEROZOIC ROCKS
ROCKS
EARLY
At the
the ends
ends of
of the
the Gogebic
Gogebic range
range the
the Archean
Archean rocks
rocks are
are
At
unconformably overlain
overlain by
by rocks
rocks of
of the
the Chocolay
Chocolay Group
Group of
of the
the
unconformably
5
Marquette
Marquette Range
Range Supergroup,
SupergrouplSunday
Sunday Quartzite
Quartzite and
and Bad
Bad River
River
the eastern
eastern end
end and
and only
only Bad
Bad River
River Dolomite
Dolomiteat
at the
the
Dolomite
Dolomite at
at the
Chocolay
Group
rocks
are
absent
from
the
central
western
westernend.
end. Chocolay Group rocks are absent from the central
part
and evidently
evidently were
were eroded
eroded prior
prior to
to deposition
deposition
part of
of the
the range,
rangeland
of
overlying
units.
of overlying units.
The Sunday
Sunday Quartzite
Quartzite was
was deposited
deposited
Sunday Ouartzite.
Ouartzite. The
Sunday
unconformably
uncon~ormablyon
on Archean
Archean rocks
rocks (mostly
(mostlythe
the Ramsay
Ramsay Formation),
Formation)
The basal
basal
no regolith
regolith on
on the
the Archean
Archean has
has been
been observed.
observed. The
however,
howeverl no
unit is
is aa prominently
prominently cross-bedded
cross-beddedreddish
reddish quartzite.
quartzite.
unit
Conglomerate
cm
Conglomerate layers
layers with
with quartz
quartz and
and granite
granite cobbles
cobbles to
to about
about 88 cm
Most of
of the
the
are
are present
present within
within the
the lower
lower part
part of
of the
the formation.
formation. Most
formation
formation is
is aa gray
gray vitreous
vitreous quartzite
quartzite in
in which
which cross-bedding
cross-beddingis
is
common
common and
and current
current ripple
ripple marks
marks are
are well
well developed
developed in
in places.
places.
According
According to
to Schmidt
Schmidt (1973)
(1973)the
the Sunday
Sunday Quartzite
Quartzite is
is present
present only
only
on
on the
the eastern
eastern end
end of
of the
the range,
rangelwhere
where it
it has
has aa maximum
maximum thickness
thickness
of about
about 46
46m.
m. One
One of
of the
the type
type localities
localities of
of the
the Sunday
Sunday
of
Quartzite
Quartzite has
has been
been described
described in
in the
the center
center of
of the
the district
district near
near
the
the Newport
Newport Mine
Mine (Van
(VanHise
Hise and
and Leith,
LeithI1911)
1911) and
and the
the problem
problem of
of
this
locality"
has
this "lost
lllost
10cality~~
has been
been discussed
discussed by
by Schmidt
Schmidt (1973)
(1973).
The Sunday
Sunday Quartzite
Quartzite grades
grades upward
upward into
into the
the
Bad River
River Dolomite.
Dolomite. The
Bad
Bad River
River Dolomite.
Dolomite. The
The transition
transition is
is marked
marked by
by interbedded
interbedded
Bad
dolomite
dolomite and
and quartzite;
quartzite; the
the dolomite
dolomite beds
beds are
are thicker
thicker and
and more
more
abundant
abundant and
and quartzite
quartzite beds
beds are
are thinner
thinner and
and less
less abundant
abundant upward.
upward.
dolomite weathers
weathers to
to aa distinctive
distinctive tan/brown
tanlbrown color
color and
and
The dolomite
The
contains
contains abundant
abundant layers
layers and
and irregular
irregular patches
patches of
of gray
gray to
to black
black
chert. Stromatolitic
Stromatolitic layers
layers with
with mounds
mounds that
that range
range in
in size
size from
from
chert.
about
about 5-50
5-50 cm
cm in
in diameter
diameter become
become more
more abundant
abundant upward
upward in
in the
the
dolomite. Stromatolitic
Stromatolitic units
units tend
tend to
to be
be prominently
prominently silicified.
silicified.
dolomite.
Maximum
Maximum thickness
thickness of
of the
the dolomite
dolomite is
is about
about 120
120 m,
ml but
but in
in most
most
areas it
it is
is considerably
considerablyless.
less.
areas
AA period
period of
of erosion
erosion followed
followed deposition
deposition of
of the
the quartzite
quartzite and
and
dolomite
dolomite of
of the
the Chocolay
Chocolay Group,
Groupl removing
removing the
the Sunday
Sunday Quartzite
Quartzite and
and
Bad River
River Dolomite
Dolomite from
from all
all but
but the
the eastern
eastern and
and western
western ends
ends of
of
Bad
the Gogebic
Gogebic range.
range.
the
Palms Formation
Formation is
is the
the basal
basal unit
unit of
of the
the
Palms Formation.
Formation. The
The Palms
Palms
Menominee Group
Group of
of the
the Marquette
Marquette Range
Range Supergroup.
Supergroup. It
It rests
rests
Menomiaee
unconformably on
on the
the Bad
Bad River
River Dolomite
Dolomite and
and Sunday
Sunday Quartzite
Quartzite on
on
unconformably
rangel and
and on
on the
the Puritan
Puritan Quartz
Quartz
the east
east and
and west
west ends
ends of
of the
the range,
the
Monzonite and
and Ramsay
Ramsay Formation
Formation in
in the
the central
central part
part of
of the
the range.
range.
Monzonite
Little or
or no
no regolith
regolith is
is developed
developed on
on the
the erosion
erosion surface.
surface.
Little
The Palms
Palms Formation
Formation is
is about
about 146
146 meters
meters thick
thick and
and contains
contains aa
The
basal mud-rich
mud-rich unit,
unitl aa central
central interbedded
interbedded mud-silt-sand
mud-silt-sandunit,
unitl
basal
and an
an upper
upper sand-rich
sand-richunit
unit (Ojakangas,
(Ojakangas 1983).
1983) AA thin
thin (usually
(usually
and
less than
than 33 mm thick)
thick) conglomerate
conglomerate occurs
occurs locally
locally at
at the
the base
base of
of
less
(Aldrich! 1929).
1929). The
The Palms
Palms grades
grades abruptly
abruptly upward
upward
the formation
formation (Aldrich,
the
into the
the Ironwood
Ironwood Iron-formation.
Iron-formation. Thin
Thin (1-5
(1-5cm)
cm) beds
beds of
of granular
granular
into
iron-formationare
are interbedded
interbedded with
with quartzite
quartzite (Ojakangas,
(Ojakangasl1983)
1983)
iron-formation
across aa thickness
thickness of
of 10
10 m.
m. The
The abundance
abundance and
and thickness
thickness of
of beds
beds
across
of iron-formation
iron-formationincreases
increases upward with
with a
a corresponding
corresponding decrease
decrease
of
in detrital
detrital beds.
beds. Recent
Recent studies
studies by
by Ojakangas
Ojakangas (1983)
(1983) indicate
indicate
in
.
66
that
that the
the Palms
Palms Formation
Fomtion may
may have
have been
been deposited
deposited by
by tidal
tidal
currents
sea
currents in
in aa transgressing
transgressingsea(Figure
(Figure3).
3j
.
Figure
Sedimentational model
model showing
showing lateral
lateral relationships
relationships of
of
Figure 3.
3. Sedimentational
the
the siliciclastic
siliciclastic tidal
tidal facies,
faciesI the
the iron-formation
iron-formation
facies
shelfI and
and the
the deeper-water
deeper-waterturbidite-mud
turbidite-mud
facies on
on the
the shelf,
facies. Thicknesses
Thicknesses of
of units
units not
not drawn
drawn to
to scale.
scale.
facies.
(From Ojakangas,
OjakangasI1983)
1983) .
(From
Ironwood Iron-Formation.
Iron-Formation. The
The transition
transition from
from the
the Palms
Palms Formation
Formation
Ironwood
into
into the
the Ironwood
Ironwood Iron-Formation
Iron-Formationrecords
records an
an abrupt
abrupt change
change from
from
detrital to
to chemical
chemical sedimentation.
sedimentation. Like
Like other
other iron-formations,
iron-formationsI
detrital
the
the Ironwood
Ironwood contains
contains little
little detrital
detrital material
material even
even though
though it
it is
is
150-275mm thick.
thick.
150-275
Although iron-formation
iron-formationhas
has aa simple
simple composition
composition ---- chert
chert
Although
is extremely
extremely varied
varied in
in appearance.
appearance. Both
Both
and iron
iron minerals
minerals -- -- it is
and
the
the chert
chert and
and the
the iron
iron minerals
minerals are
are varied
varied in
incolor.
color. Although
Although it
it
is
is widely
widely cited
cited as
as indicating
indicating the
the chemistry
chemistry of
of the
the depositional
depositional
1954)1 the
the mineralogy
mineralogy of
of iron-formations
iron-formationsis
is
environment (James,
(JamesI1954),
environment
the product
product of
of the
the depositional,
depositionalI diagenetic,
diageneticI metamorphic,
metamorphicI and
and to
to
the
some extent,
extentI the
the weathering
weathering environment
environment that
that the
the rocks
rocks have
have
some
undergone. Still
Still moderate
moderate subsequent
subsequent changes
changes to
to the
the primary
primary
undergone.
mineralogy do
do not
not usually
usually reduce
reduce our
our ability
ability to
to infer
infer primary
primary
mineralogy
mineralogy.
mineralogy.
it
Iron-formationshave
have two
two basic
basic textural
textural types.
types. One
One is
is
Iron-formations
laminated with
with layers
layers of
of chert
chert about
about 5-10
5-10 nun
laminated
mm thick alternating
alternating
with layers
layers of
of iron
iron minerals
minerals of
of similar
similar thickness.
thickness. This
This
with
laminated type
type of
of deposit
deposit is.
is comonly
referred to
to as
as "slaty"
1lslatyll
laminated
commonly referred
4). It
It has
has aa grain
grain size
size and
and bedding
bedding
iron-formation(Figure
(Figure4).
iron-formation
characteristic of
of siltstones
siltstones (LaBerge,
(LaBergeI1964,
19641 Dimroth,
Dimroth! 1968).
1968). The
The
characteristic
l1chertyI1
other textural
textural type
type of
of iron-formation,
iron-formationIreferred
referred to
to as
as "cherty"
other
iron-formationIconsists
consists of 1-10
1-10 cm
cm thick
thick layers
layers containing
containing fine
fine
iron-formation,
(c2
mm)
llgrainsn
of
chert
with
variable
amounts
of
iron
sand-size
sand-size (<2 mm) "grains"
chert with variable amounts
iron
minerals
set
in
a
chert
matrix.
These
layers
of
llgranularll
chert
minerals set in a chert matrix. These layers of "granular" chert
are extremely
extremely variable
variable in
in thickness
thickness over
over short
short distances,
distancesI
are
commonly
forming
lenses
(Figure
5).
The
cherty
lenses are
are
The cherty lenses
commonly forming lenses (Figure 5).
7
_
Figure
Figure 4.
4. Photo
Photo of
of thin-bedded
thin-bedded (laminated)
(laminated)iron
iron
formation
formation with
with some
some thick-bedded
thick-bedded(granular)
(granular)
layers.
layers. Mt.
Mt. Whittlesey,
Whittlesey, nearMellen,
near-Mellen,WI.
WI.
.
,: :
*
A. '
;—
—
,.
:::::
:
.,:
..— .vr
:
•
.
•
'&.
.•
..
•
.
..
.
¶
,
.-
g_:L...'
I .Figure
Figure 5.
5. Photo
Photo of
of thick,
thick, irregularly
irregularly bedded,
bedded, granular
granular
iron-formation.
iron-formation. Mt.
Mt. Whittlesey,
Whittlesey, near
nearMellen,
Mellen,
WI.
WI
88
.
separated by 0.5-3
typically separated
0.5-3 cm layers
layers of laminated
laminated iron
iron
"Cherty"
iron-formations have the grain size
minerals.
minerals.
llChertyll
iron-formations
size and
and
bedding characteristics
characteristics of
of sandstones
sandstones (Mengel,
(Mengel, 1963;
1963; Dimroth
Dimroth and
and
mineralogy of the
minerals is
Chauvel,
Chauvel, 1973).
1973). The mineralogy
the iron
iron minerals
is almost
almost
completely
completely independent
independent of
of the
the textural
textural varieties
varieties of
of ironironironformation.
(llchertyll)
ironformation. Most geologists
geologists agree
agree that
that granular
granular ("cherty")
formation
formation represents
represents shallow-water
shallow-water deposition
deposition and
and laminated
laminated ironironformation
formation represents
represents somewhat
somewhat deeper
deeper (quieter)
(quieter)water
water deposition.
deposition.
Several
Superior region
Several iron-formations
iron-formations in
in the
the Lake Superior
region have
been
subdivided
into."members"
on
the
basis
of
their
been subdivided into."membersNon the
their bedding
Hotchkiss
(1919)
divided
(textural) characteristics.
characteristics. Hotchkiss (1919) divided the
(textural)
the
Ironwood
Ironwood Iron-formation
Iron-formationinto
into five
five members.
members. The -60
-60 m thick
thick
Plymouth
Plymouth Member is
is an
an irregularly
irregularly bedded
bedded granular
granular cherty
cherty unit
unit
with a basal zone
containing some detrital quartz
quartz and
and patches
patches of
zone containing
overlying 46
46 m thick
thick Yale
Yale Member
Member is
is
stromatolitic
stromatolitic jasper.
jasper. The overlying
mainly
an
even-bedded
(slaty)
siderite-chert
unit
with
a
basal
mainly an even-bedded (slaty) siderite-chert
carbonaceous
zone and a layer that is probably
probably
carbonaceous pyritic slate zone
The
succeeding
Norrie
Member
is
a
60
m
thick
unit
m
thick
unit of
of
The
succeeding
Norrie
Member
is
a
60
tuffaceous.
tuffaceous.
The
Pence
Member
is
The
Pence
Member
is
irregularly
bedded
cherty
iron-formation.
irregularly bedded cherty iron-formation.
about 45 m of siderite-chert
siderite-chert iron-formation,
iron-formation, and the uppermost
uppermost
Anvil
Anvil Member
Member is
is aa 15-60
15-60mm thick
thick mixture
mixture of
of slaty
slaty and
and cherty
cherty ironironThese
members
are
continuous
and
uniform
throughout
formation.
formation. These
continuous
uniform throughout
However, they are
are difficult
difficult to
the
length of
of the
the range.
range. However,
to
the 128
128 km length
distinguish
east
of
Wakefield,
Michigan,
where
a
thick
distinguish east of Wakefield, Michigan, where a thick sequence
sequence
of volcanic rocks,
1973), is
is
rocks, the
the Emperor
Emperor Volcanic Complex
Complex (Trent,
(Trent, 1973),
interbedded with
interbedded
with the
the iron-formation.
iron-formation.
Gogebic range
Iron ores.
Iron
ores. The iron
iron ores
ores on
on the
the Gogebic
range were produced by
chemical removal
the chemical
removal of
of silica
silica from
from the
the iron-formation.
iron-formation.
Dissolution of the
Dissolution
the silica
silica required
required the
the removal
removal of
of nearly
nearly 50
50
volume of the
percent of the
the original
original volume
the rock
rock and
and produced
produced very
very
The ore
ore consisted
consisted mainly of earthy
earthy goethite
goethite
porous orebodies.
porous
orebodies. The
hematite; however,
and hematite;
however, in
in open
open spaces
spaces produced
produced by removal
removal of
of
silica,
botryoidal masses of
silica, large
large botryoidal
of crystalline
crystalline hematite
hematite and
and
goethite were found,
goethite
found, as
as well as
as local
local concentrations
concentrations of
of
psilomelane
psilomelane (romancheite),
(romancheite), manganite,
manganite, rhodochrosite,
rhodochrosite, calcite,
calcite,
presence of
manganocalcite,
manganocalcite, barite,
barite, gypsum,
gypsum, and
and marcasite.
marcasite. The presence
of
iron
iron ore
ore cobbles
cobbles in
in Keweenawan
Keweenawan rocks
rocks in
in the
the district
district suggests
suggests
that at least
least some
some of the
the ore
ore was
was formed
formed prior
prior to
to deposition
deposition of
of
rocks of
rocks
of the
the Keweenawan
Keweenawan Supergroup.
Supergroup. Nearly all
all the
the mines
mines on
on the
the
Gogebic
Gogebic range
range were
were underground
underground mines
mines (Figures
(Figures66 and
and 7).
7). Mining
Gogebic range
on the
the Gogebic
range began
began in
in 1884
1884 and
and ended
ended in
in 1967,
1967, when
when the
the
underground mines could
no longer
open pit
could no
longer compete
compete with the large
large open
taconite mines
taconite
mines elsewhere
elsewhere in
in the
the world.
world. Ore was taken
taken by rail
rail to
to
Ashland,
Wisconsin, and
Ashland, Wisconsin,
and Escanaba,
Escanaba, Michigan, from
from where it was
shipped to
shipped
to steel
steel mills
mills in
in Indiana
Indiana and
and Ohio.
Ohio.
Emperor Volcanic
Volcanic Complex.
Emperor
Complex. The
The Emperor
Emperor Volcanic Complex
Complex
constitutes a
constitutes
a thick
thick pile
pile of
of volcanic
volcanic rocks
rocks and
and sills
sills in
in the
the
sedimentary sequence
sequence on the eastern Gogebic range where they
sedimentary
reach a thickness
thickness of
of at
at least
least 2,000
2,000 meters.
meters. Although
Although the
the
volcanic rocks
been known
volcanic
rocks in
in the
the eastern
eastern Gogebic
Gogebic range
range have
have been
known for
for
100
1892), few
studies have been
few studies
100 years (Irving
(Irving and Van Hise, 18921,
undertaken
undertaken on
on these
these rocks.
rocks. Irving
Irving and Van Hise (1892)
(1892) stated
stated
9
___
GOGEBIC RANGE
GOGEBIC
RANGE
GENERALIZED
GEN E R A L I Z E D CROSS
CROSS SECTION
SECTION
. (P.
I b
... .0..
°o0
0.•.
0
:. .o;,; 0 °
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fl
0
:.
-
PLTMQJTII
yap
-
)
--•------•
,O
•-
£pds,t
-
-
\ \
\, / --
\
./
.
c \
—.
\
/\
-<\
-.
\/ /
\
-,
\,
_y
-':':
—
\
FORSATION
—.
— .'
a
\\
IlOH*Uc)O
\
/,/\\
\\,S
,'—_-c'
\ \ \ \ //,;/
"
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-:
Poll I IOIUI.l l(P.
__1__ _____:_.__ ..
//
1.'
\
'a.
',
•-'
/ /,'•
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WK ST
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IAT ION
.
Figure 6. Schematic diagram showing
showing the
the occurrence
occurrence of
of iron
iron
(From
Goldich
orebodies on the
orebodies
the Gogebic
Gogebic range.
range. (From Goldich and
Marsden,
1956)
Marsden, 1956)
Lake mine in
in 1960.
1960. Note the
the stockpile
stockpile of
of iron
iron
Figure 7. Sunday Lake
ore in
in the
the upper
upper left.
left. Photo taken from
from the
the top
top of
of
In Wakefield,
Wakefield, Michigan.
Michigan.
Radio Tower Hill
Hill In
10
that
that the
the volcanic
volcanic rocks
rocks are
are interbedded
interbedded with
with the
the Ironwood
Ironwood IronIronformation,
formation, which
which has
has been
been demonstrated
demonstrated convincingly
convincingly by
by subsequent
subsequent
Mapping by
by Trent
Trent
exploration
or iron
iron ore
ore in
exploration drilling
drilling ffor
in the
the area.
area. Mapping
(1973)
documented the distribution of igneous
(1973) documented
igneous rocks
rocks and
and showed
showed
some
some of
of the
the lithologies
lithologies present.
present. Dann
Dann (1978)
(1978) showed
showed that
that the
the
volcanics
dacite, and
and that
that
volcanics range
range in
in composition
composition from
from basalt to
to dacite,
they have
have been
been metamorphosed
metamorphosed only
only to
to prehnite-pumpellyite,
or to
to
they
prehnite-pumpellyite, or
He also
also elaborated
elaborated on
on the
the possible
possible
lowest
lowest greenschist
greenschist facies.
facies. He
volcanic
volcanic setting
setting in
in which
which the
the rocks
rocks may
may have
have formed.
formed. Trace
Trace
element
studies
by
Schulz
(Sims
and
others,
1990)
element studies
Schulz (Sims
others, 1990) indicate
indicate that
that
the
the volcanic
volcanic rocks
rocks are
are rift-related
rift-relatedcontinental
continental tholeiites.
tholeiites.
Reconnaissance
Reconnaissance mapping in
in 1991
1991 by LaBerge
LaBerge and J. S. Klasner
Klasner
indicates
that
the
Emperor
Volcanic
Complex
indicates that the Emperor Volcanic Complex consists
consists of
of aa wide
wide
Both
variety
variety of
of mainly
mainly subaqueous
subaqueous volcanic
volcanic rocks
rocks and
and sills.
sills. Both
mafic
mafic and
and felsic
felsic volcanics
volcanics were
were recognized.
recognized. Mafic
Mafic rocks
rocks include
include
sills
(such
as
the
Wolf
Mountain
sill
of
Trent,
1973)
sills (such as the
Mountain sill
Trent, 1973) pillowed
pillowed
and
and massive
massive flows,
flows, some
some with
with columnar
columnar jointing,
jointing, and
and extensive
extensive
Felsic
hyaloclastites
and
pillow
breccias
(Figure
8).
rocks
hyaloclastites and pillow breccias (Figure 8). Felsic rocks
include
include hyaloclastites,
hyaloclastites, massive
massive felsite
felsite breccias,
breccias, and
and units
units
comprised
comprised of
of felsite
felsite breccia
breccia with
with 2-10
2-10 cm
cm clasts
clasts in
in a
a finer
finer
hyaloclastite
9). The
The felsite
felsite breccia
breccia -hyaloclastite matrix
matrix (Figure
(Figure9).
hyaloclastites may
may be
be debris
debris flows
flows from
from aa subaqueous
subaqueous felsic
felsic dome.
dome.
hyaloclastites
Although
in the
the Emperor
Emperor Volcanic
Volcanic Complex
Complex with
with
Although there
there are
are zones
zones in
strong
strong foliation,
foliation, large
large areas
areas are
are almost
almost undeformed.
undeformed. Because
Because of
of
the
metamorphic grade
the low
low metamorphic
grade and
and minimal deformation,
deformation, primary
primary
features are
are well-preserved.
well-preserved.
features
The
The relationship
relationship between
between volcanism
volcanism and
and iron-formation
iron-formationhas
has
long
(e.g., Van
Van Hise
Hise and
and Leith,
Leith,
long been
been debated,
debated, with
with some
some advocates
advocates (e.g.,
1911)
volcanic contributions
1911) arguing
arguing in
in favor
favor of volcanic
contributions of
of iron,
iron, and
and
others
others (e.g.,
(e.g., James,
James, 1954)
1954) contending
contending that
that there
there is
is no
no genetic
genetic
relationship
relationship between
between volcanism
volcanism and
and iron-formation
iron-formationdeposition.
deposition.
The
The presence
presence of
of aa thick
thick volcanic
volcanic sequence
sequence interbedded
interbedded with
with the
the
iron-formation
iron-formationin
in the
the eastern
eastern Gogebic
Gogebic range
range shows
shows that
that there
there
certainly
certainly was
was volcanism
volcanism contemporaneous
contemporaneous with
with iron-formation
iron-formation
deposition
deposition in
in the
the Lake
Lake Superior
Superior region.
region. Therefore,
Therefore, the
the Gogebic
Gogebic
range
range is
is similar
similar to
to the
the Hamersley
Hamersley range
range in
in Western
Western Australia
Australia and
and
the
the Proterozoic
Proterozoic iron-formations
iron-formationsin
in South
South Africa
Africa where
where numerous
numerous
volcanic
ash layers
layers are
are interbedded
interbedded with
with the
the iron-formations
iron-formations
volcanic ash
(LaBerge, 1966a,
1966a, 196Gb).
1966b)
(LaBerge,
.
Tyler and
and Copps
CORDS Formations.
Formations. The
The Ironwood
Ironwood Iron-Formation,
Iron-Formation, and
Tyler
locally the
the Emperor
Emperor Volcanic Complex,
Complex, are overlain
locally
overlain by a 2,000
2,000 m
graywacke-slatesequence.
sequence. The
The unit
unit is
is thickest
thickest in
in the
the
thick graywacke-slate
thick
western and
and eastern
eastern ends
ends of the
the range,
range, and has been completely
completely
western
removed by erosion
erosion prior to
to deposition
deposition of the Keweenawan
removed
Keweenawan
Supergroup in
in an
an area
area near
near Wakefield,
Wakefield, Michigan.
Michigan. Throughout
Throughout most
most
Supergroup
the range
range the
the unit is
is called
called the
the Tyler
Formation, however
the
of the
Tyler Formation,
however the
correlative(?)
portion
at
the
eastern
end
of
the
range
is
called
correlative(?) portion
the eastern
range is
the Copps
Copps Formation.
Formation. The
The Tyler
Tyler Formation
Formation is
is correlated
correlated with
with the
the
the
Michigamme
Formation
to
the
east,
and
with
the
Thomson,
Rabbit
Michigamme Formation to the east, and with the Thomson, Rabbit
Lake, Virginia and
and Rove
Rove Formations
Formations to
to the
the west and north in
in
Lake,
Minnesota and
and Ontario.
Ontario. All these
these units
deposited
Minnesota
units were probably deposited
in the
the same
same basin.
basin.
in
11
Figure 8. Pillow
Pillow breccia
breccia in
in Emperor
Emperor Volcanic
Volcanic Complex.
Complex. Pillows
Pillows
and
in aa hyaloclastite
hyaloclastite matrix.
matrix.
and pillow
pillow fragments
fragments in
Jackknife
Jackknife is
is on
on aa pillow.
pillow.
Figure 9.
9. Felsic
Felsic phase
phase of
of Emperor
Emperor Volcanic
Volcanic Complex.
Complex. Felsite
Felsite
Figure
clasts
clasts in
in aa finer
finer hyaloclastite
hyaloclastite matrix.
matrix. Jackknife
Jackknife is
is 88
.
cm.
cm
12
The
The relationship
relationship of
of the
the Tyler
Tyler Formation
Formation to
to the
the underlying
underlying
iron-formation is
iron-formation
is unclear.
unclear. According to
to Hotchkiss
Hotchkiss (1919)
(1919) the
the
basal unit of the
the Tyler
Tyler Formation
Formation is
is the
the Pabst
Pabst Member,
Member, aa
distinctive
distinctive unit
unit containing
containing mainly
mainly jasper
jasper and
and quartz
quartz pebbles.
pebbles.
10, from drill hole data (Atwater,
Figure 10,
(Atwater, 1938)
1938) shows
shows the
the
relationship
ironrelationship of the
the conglomerate
conglomerate to
to the
the underlying
underlying ironHotchkiss (1919)
relationships between
formation.
formation. Hotchkiss
(1919) interprets the relationships
Atwater
the Tyler
Tyler and the
iron-formation
to
be
unconformable.
the iron-formation to be unconformable. Atwater
(1938)
did not believe*
believe' that
that aa major
major unconformity
unconformity separated
separated the
(1938) did
the
Schmidt and Hubbard (1972)
Ironwood
Ironwood and
and Tyler.
Tyler. Schmidt
(1972) interpreted
interpreted the
the
formations
sedimentation to have been
formations to be gradational and sedimentation
continuous
Gocrebic. However,
However, Trent
Trent (1973)
(1973) stated
stated
continuous in
in the
the central
central Gogebic.
that
that field
field relationships
relationships indicate
indicate an
an unconformity
unconformity between
between the
the
Schmidt (1980)
Ironwood and Tyler
Ironwood
Tyler on
on the
the eastern
eastern Gogebic.
Gogebic. Schmidt
(1980)
summarizes
summarizes the
the field
field evidence
evidence pertaining
pertaining to
to the
the Ironwood-Tyler
Ironwood-Tyler
contact
contact as
as well as
as the
the several
several alternative
alternative interpretations.
interpretations.
1 ATLANTIC MINE NO 3 SHAfl D 0 HOI I
2 PLUMMER SHAF1 51H LEVEL CROSSCUI
3 PENCE NO 2 SHAFT AND 0 0 HOLE
4 MONTREAL NO 4 SHAFT 20TH I EVE1
'
' 1
5 MONTREAL NO 4 SHAFI BTH LEVEL I I 1 l HOLE
6 OTTAWA 10TH LEVEL SHAFT CROSSCIII
7 GARY 19IH LEVEL NO 16 CROSSCUI 1) U HOLE
8 NORRIE 14TH AND 17TH LEVELS
9 AURORA E SHAFT 13TH LEVEL
10 PABS1 G SHAD
I I BONNIL
R2E
Figure 10.
Figure
10.
2 MRFS
0
&-\TJ
I
0
H
~
I:
?
I ~ N I A I XAI
3 ~IKIMETERS
c
..
vCRTICAL SCALE
1 >(L;Iu!
n:ii
Longitudinal section
Longitudinal
section through
through part of
of the
the Gogebic
Gogebic
district, showing
district,
showing one
one interpretation
interpretation of
of
"Pabst Member"
Membern of
of the
the
relationship between
the "Pabst
relationship
between the
Tyler
Formation and
and the
the Anvil and
and Pence
Pence Members
Members of
of
Tyler Formation
the
the Ironwood
Ironwood Iron-Formation.
Iron-Formation. The upper
slate is
is aa
upper slate
layer
layer of thin-bedded
thin-bedded carbonate
carbonate iron-formation,
iron-formation,at
at
least
least partly
partly within
within the
the Anvil
Anvil Member.
Member. Compiled
from measured
in mines and
and drill
drill holes.
holes.
measured sections
sections in
Location
sections given in map
Ironwood
Location of sections
map of Ironwood
Iron-Formation in
in lower
lower part
part of
of diagram.
diagram. Modified
Iron-Formation
(1938,p.
p. 163,
163, fig.
fig. 2)
2) (From
from Atwater
from
Atwater (1938,
(From Schmidt,
Schmidt,
1980)
1980)
13
The
The Tyler
Tyler Formation
Formation consists
consists dominantly
dominantly of
of intercalated
intercalated
argillite
argillite with
with lesser
lesser amounts
amounts of
of metasiltstone
metasiltstone and
and graywacke.
graywacke.
Graded
sequences indicative
Graded metagraywacke
metagraywacke beds
beds containing
containingBouxna
B o w sequences
indicative
of deposition by turbidity
turbidity currents
currents are common,
common, but structureless
structureless
beds
beds are
are even
even more
more abundant
abundant and
and aa grain-flow
grain-flow mechanism
mechanism may
may also
also
have
have been
been operative.
operative. Sole
Sole marks
marks and
and small-scale
small-scale cross-bedding
cross-bedding
indicate
westindicate a
a paleocurrent
paleocurrent trend
trend from
from east-southeast
east-southeast to
to westnorthwest
northwest (Alwin,
(Alwin,1976).
1976)
.
The
The "average"
"averagengraywacke
graywacke of
of the
the Tyler
Tyler consists
consists of
of 28
28 percent
percent
micaceous
micaceous matrix
matrix and
and 72
72 percent
percent framework
framework grains
grains (Alwin,
(Alwin,1976).
1976).
Of
73 percent
percent are
are monomono- and
and polycrystalline
polycrystalline
Of the
the framework
framework grains,
grains, 73
quartz
quartz and
and chert,
chert, 10
10 percent
percent are
are plagioclase
plagioclase (mostly
(mostly altered
altered
plagioclase),
plagioclase), and
and 17
17 percent
percent are
are rock
rock fragments,
fragments, mainly
mainly of
of
granitic
granitic to
to quartz
quartz dioritic
dioritic and
and volcanic
volcanic origin,
origin, but
but some
some grains
grains
of sedimentary
sedimentary and
and metamorphic
metamorphic origin
origin also
also are
are present.
present. Thus,
Thus,
the
the "average"
"averagev graywacke
graywacke of
of the
the Tyler
Tyler is
is compositionally
compositionally submature
submature
quartzose
quartzose lithic
lithic graywacke
graywacke that
that is
is texturally immature
immature as defined
by
by angular
angular framework
framework grains
grains and
and poor
poor sorting.
sorting.
The
The paleocurrent
paleocurrent directions
directions and
and the
the composition
composition of
of the
the
graywacke
graywacke suggest
suggest that
that the
the Tyler
Tyler Formation
Formation was
was derived
derived from
from aa
dominantly
"graniticvsource
source to
to the
the southeast.
southeast.
dominantly "granitic"
MIDDLE
MIDDLE PROTEROZOIC
PROTEROZOIC ROCKS
ROCKS
Following
the Penokean
orogeny, about
about 1,860
Following the
Penokean orogeny,
1,860 -- 1,830
1,830 Ma,
Ma, the
the
Gogebic
and the
the rest
Gogebic range
range and
restof
ofthe
theLake
LakeSuperior
Superior region
region was
was
subjected
subjected to
to erosion
erosion for
fornearly
nearly700
700 million
million years.
years. Erosion
Erosion
removed
removed an
an unknown
unknown thickness
thickness of
of rocks
rocks in
in the
the range,
range, but
but more
more rock
rock
was
example, at
at
was removed
removed from
from some
some areas
areas than
than in
in others.
others. For
For example,
least
preserved in
least 2,000
2,000 meters of the
the Tyler
Tyler Formation
Formation is
is preserved
in the
the
western
western Gogebic,
Gogebic, and
and aa comparable
comparable thickness
thickness of
of the
the Copps
Copps
Formation
Formation is
is preserved
preserved at
at the
the eastern
eastern end
end of
of the
the range.
range. However,
However,
all
Ironall of
of the
the Tyler,
Tyler, and
and some
some of
of the
the underlying
underlying Ironwood
Ironwood IronFormation
Formation as
as well,
well, was
was removed
removed in
in the
the eastern
eastern Gogebic
Gogebic where
where the
the
range
range experienced
experienced more
more folding.
folding. The
The erosion
erosion interval
interval presumably
presumably
ended
ended with
with the
the onset
onset of
of the
the Middle
Middle Proterozoic
Proterozoic Keweenawan
Keweenawan rifting
rifting
event.
event.
Bessemer
Bessemer Ouartzite.
Ouartzite. In
Gogebic range
In most of
of the
the Gogebic
range the
the Bessemer
Bessemer
Quartzite,
Quartzite, the
the basal
basal unit
unit of
of the
the Keweenawan
Keweenawan Supergroup,
Supergroup,
unconformably
unconformably overlies
overlies the
the Tyler
Tyler Formation.
Formation. However,
However, locally
locally on
on
eastern part of
of the
the range
range the
the basal
basal Keweenawan
Keweenawan rests
rests on
on the
the
the eastern
Ironwood Iron-Formation
Iron-Formation or
or the
the Emperor
Emperor Volcanic
Volcanic Complex.
Complex. The
Ironwood
The
quartzite is
is approximately
approximately 100
100 mm thick,
thick, has
has channel
channel cross-bedding
cross-bedding
quartzite
and
and is
is aa relatively
relatively mature
mature quartz
quartz sandstone
sandstone (Ojakangas
(Ojakangasand
and
Matsch,
Matsch, 1982).
1982). The
The Bessemer
Bessemer Quartzite
Quartzite evidently
evidently originated
originated in
in a
fluvial environment
environment in
in aa slight
slight sag
sag that
that presaged
presaged the
the Keweenawan
Keweenawan
fluvial
rifting
rifting that
that developed
developed into
into the
the Midcontinent Rift System
System
(Ojakangas
(Ojakangas and
and Matsch,
Matsch, 1982).
1982).
14
Overlying the
the Bessemer
Bessemer Quartzite
Quartzite is
is aa thick
thick
Powder Mill
Mill Group.
Grouu. Overlying
Powder
sequence
sequence of
of mainly
mainly basaltic
basaltic lava
lava flows
flows that
that formed
formed during
during the
the
Volcanic activity
activity
main
main phase
phase of
of Keweenawan
Keweenawan rifting
rifting in
in this
this area.
area. Volcanic
(and
Ma ago
ago and
and produced
produced
approximately 1,200
1,200 Ma
(and rifting?)
rifting?) began approximately
hundreds
hundreds of
of lava
lava flows
flows that
that have
have an
an aggregate
aggregate thickness
thicknessof
ofmore
more
than 3,000
3,000m.
m. The
The lowermost
lowermost basaltic
basaltic lava
lava flows
flows that
that overlie
overlie the
the
than
Bessemer
Bessemer Quartzite
Quartzite in
in the
the Bessemer,
Bessemer,Michigan,
Michigan,area
areaare
arepillowed,
pillowed,
suggesting
either aa lake
lake or
or aa
suggesting that
that they
they were
were extruded
extruded in
in water,
water, either
fluvial
system.
fluvial system.
Keweenawan Supergroup
Supergroup volcanic
volcanic rocks
rocks are
are overlain
overlain
Oronto Group.
Group. Keweenawan
Oronto
by
a
thick
sequence
of
mainly
red
clastic
rocks
in
the
Lake
by a thick sequence of mainly red clastic rocks in the Lake
Various representatives
representatives of
of this
this sequence
sequence are
are
Superior region.
region. Various
Superior
The basal
basal unit
unit of
of the
the
well exposed
exposed north
north of
of the
theGogebic
Gogebicrange.
range. The
well
Oronto
the Copper
Copper Harbor
Harbor Conglomerate,
Conglomerate, which
which is
is as
as thick
thick
Oronto Group
Group is
is the
The
major
lithology
is
red
arkosic
sandstone
and
The
major
lithology
is
red
arkosic
sandstone
and
as 2,000
2,000m.
m.
as
siltstone with
with numerous
numerous conglomeratic
conglomeratic horizons.
horizons. The
The Copper
Copper
siltstone
Harbor
Harbor Conglomerate
Conglomerate probably
probably represents
represents alluvial
alluvial fan
fan deposits
deposits
formed
formed near
near aa rugged
rugged source
source area
area along
along the
the south
south side
side of
of Lake
Lake
Superior.
Superior.
The
The Copper
Copper Harbor
Harbor Conglomerate
Conglomerate is
is overlain
overlain by
by the
the Nonesuch
Nonesuch
Formation,
Formation, aa 100
100 meter
meter thick
thick clastic
clastic unit,
unit, that
that is
is dominantly
dominantly
The Nonesuch
Nonesuch was
was probably
probably
gray to
to black
black carbonaceous
carbonaceoussiltstone.
siltstone. The
gray
deposited
deposited in
in aa lake
lake developed
developed on
on the
the subsiding
subsiding pile
pile of
of volcanic
volcanic
(e.g., Suszek,
Suszek,1991).
1991). In
In
rocks and
and Copper
Copper Harbor
Harbor sediments
sediments (e.g.,
rocks
addition
addition to
to the
the world
world class
class copper
copper deposits
deposits of
of the
the White
White Pine
Pine
mine,
mine, the
the Nonesuch
Nonesuch Formation
Formation also
also contains
contains one
one of
of the
the oldest
oldest
known occurrences
occurrences of
of petroleum.
petroleum.
known
The Freda
Freda Sandstone
Sandstone of
of the
the Oronto
Oronto Group
Group consists
consists of
of
The
clastics, and
and conformably
conformably overlies
overlies
approximately 3,000
3,000 mm of
of red
red clastics,
approximately
the Nonesuch
Nonesuch Formation.
Formation. The
The Freda
Freda strata
strata is
is believed
believed to
to consist
consist
the
of fluvial
fluvial deposits
deposits formed
formed during
during continued
continued subsidence
subsidence of
of the
the
of
Middle Proterozoic
Proterozoic rift.
rift.
Middle
KEWEENAWANTILTING
TILTING
KEWEENAWAN
Following the
the onset
onset of
of deposition
deposition of
of the
the volcanic
volcanic and
and
Following
sedimentary rocks
rocks along
along the
the Keweenawan
Keweenawan Midcontinent
Midcontinent Rift
Rift System,
System,
sedimentary
the south
south side
side of
of the
the rift,
rift, including
including the
the Gogebic
Gogebic district,
district, was
was
the
(65-90') to
to the
the north.
north.
tilted steeply
steeply (659O0)
tilted
TECTONIC SEFING
SETTING
TECTONIC
Early Proterozoic
Proterozoic rocks
rocks in
in the
the Gogebic
Gogebic range
range unconformably
unconformably
Early
overlie
Late
Archean
greenstone
and
granite
of
the
Superior
overlie Late Archean greenstone and granite
the Superior
L
a
m
e
(1981,
1983)
suggested
Province
of
the
Canadian
Shield.
Province of the Canadian Shield. Larue (1981, 1983) suggested
that
the
sedimentary
rocks
of
the
Lake
Superior
region
(the
that the sedimentary rocks
the Lake Superior region (the
Marquette Range
Range Supergroup)
Supergroup) were
were deposited
deposited in
in aa number
number of
of grabengrabenMarquette
like basins
basins in
in aa platformal
platformal environment.
environment. Sims
Sims and
and others
others (1990)
(1990)
like
15
proposed that
that rocks
rocks of
of the
the Marquette
Marquette Range
Range Supergroup
Supergroup formed
formed on
on aa
proposed
rifted continental
continental margin.
margin. Break-up
Break-upof
of the
the Archean
Archean craton
craton
rifted
resulted in
in the
the development
development of
of oceanic
oceanic crust
crust off
off the
the southern
southern
resulted
margin of
of the
the Superior
SuperiorProvince.
Province. Therefore,
Therefore, the
the sedimentary
sedimentary
margin
rocks were
were deposited
deposited during
during aa rifting
rifting stage
stage that
that developed
developed into
into aa
rocks
(1988)
passive
margin.
Nd
isotope
studies
by
Gerlach
and
others
passive margin.
Nd isotope studies by Gerlach and others (1988)
indicate that
that iron-formations
iron-formationsin
in the
the Lake
Lake Superior
Superior region
region were
were
indicate
deposited
about
2,100
Ma.
deposited about 2,100 Ma.
concept of
of aa rif
rifted
margin in
in the
the Lake
Lake
The concept
The
ted continental
continental margin
Superior
region
is
probably
best
exemplified
by
the
eastern
Superior region is probably best exemplified by the eastern
Gogebic range.
range. As
As discussed
discussed earlier,
earlier,the
the stratigraphic
stratigraphic
Gogebic
Wakefield,
succession
increases
considerably
in
thickness
east of
of Wakefield,
succession increases considerably in thickness east
Michigan,
and
igneous
rocks
(volcanic
rocks
and
sills)
become aa
Michigan, and igneous rocks (volcanic rocks and sills) become
major
component
of
the
succession.
The
increase
in
thickness
of
major component of the succession. The increase in thickness of
the
sedimentary
sequence
and
the
influx
of
volcanic
rocks
is
the sedimentary sequence and the influx of volcanic rocks is
suggestive of
of aa half
half-graben
near Wakefield
Wakefield
suggestive
-graben structure
structure with
with a
a hinge
hinge near
and
a
boundary
fault
(or
faults)
near
Lake
Gogebic.
Emplacement
and a boundary fault (or faults) near Lake Gogebic. Emplacement
of basaltic
basaltic sills
sills (e.g.,
(e.g., the
the Wolf
Wolf Mountain
Mountain sill
sill of
of Trent,
Trent,1973)
1973)
of
and
eruption
of
continental
tholeiites
(Sims,
and
others,
1990)
and eruption of continental tholeiites (Sims, and others, 1990)
The
would be
be characteristic
characteristicof
ofaarif
rifted
continental margin.
margin. The
would
ted continental
presence
of
the
sills
and
volcanic
rocks
in
the
Ironwood
Ironpresence of the sills and volcanic rocks in the Ironwood IronFormation
as
well
as
dramatic
eastward
thickening
of
the
ironFormation as well as dramatic eastward thickening of the ironthat major
major graben
graben formation
formation and
and volcanism
volcanism
formation indicates
indicates that
formation
occurred
during
deposition
of
the
Ironwood
Iron-Formation
(Figure
occurred during deposition of the Ironwood Iron-Formation (Figure
11).
11)
Following the
the rifting
rifting and
and passive
passive margin
margin stages,
stages, aa phase
phase of
of
Following
convergent
tectonics
produced
a
volcanic
island
arc
(the
convergent tectonics produced a volcanic island arc (the
Wisconsin magmatic
magmatic terranes)
terranes) that
that collided
collided with
with the
the Superior
Superior
Wisconsin
Isotopic ages
ages in
inrocks
rocksof
ofthe
thernagmatic
magmatic terrane
terrane range
range
craton. Isotopic
craton.
(Sims
and
others),
and
docking
occurred
1.890
Ma
to
1,840
Ma
from
from 1,890 Ma to 1,840 Ma (Sims and others), and docking occurred
1.860
Ma
(Sims
and
others,
1990).
Barovich
and
others
about
about 1,860 Ma (Sims and others, 1990). Barovich and others
(1989) showed
showed that
that Nd
Nd isotopes
isotopes indicate
indicate that
that much
much of
of the
the
(1989)
graywackes
in
the
Michigamme
Formation
were
derived
from
an Early
Early
graywackes in the Michigainme Formation were derived from an
Proterozoic source.
source. This
This suggests
suggests that
that the
the graywackes
graywackes were
were
Proterozoic
deposited in
in aa foreland
foreland basin
basin during
during docking
docking of
of the
the island
island arc
arc on
on
deposited
the margin
margin of
of the
the Superior
Superiorcraton.
craton.
the
16
16
Deformation and
and metamorphism
metamorphism of
of rocks
rocks of
of the
the Marquette
Marquette Range
Range
Deformation
Supergroup
on
the
Gogebic
range
resulted
from
the
collision
of
Supergroup on the Gogebic range resulted from the collision of
the
island
arc
with
the
margin
of
the
Superior
craton.
This
the island arc with the margin of the Superior craton. This
collision fits
fits very
very well
well with
with aa foreland
foreland basin
basin model
model as
as developed
developed
collision
Hoffman
(1987)
for
the
Early
Proterozoic
rocks
that
surround
by
by Hoffman (1987) for the Early Proterozoic rocks that surround
Archean Superior
Superior Province.
Province. This
the Archean
This model
model was
was utilized
utilized by
by
the
Southwick
and
Morey
(1991)
to
explain
lithologic
and
structural
Southwick and Morey (1991) to explain lithologic and structural
relationships in
in east-central
east-centralMinnesota.
Minnesota. AA similar
similar foredeep
foredeep
relationships
model,
with
associated
southward
subduction
and
collision
of aa
model, with associated southward subduction and collision of
volcanic
arc
against
the
craton
to
the
north
was
proposed
by
volcanic arc against the craton to the north was proposed by
Ojakangas (in
(inpress)
press)to
toexplain
explainthe
theorigin
originofofthe
theMichigainme
Michigamme
Ojakangas
Formation
of
the
Upper
Peninsula
of
Michigan,
as
well
as the
the
Formation of the Upper Peninsula of Michigan, as well as
origin
of
the
Copps
and
Tyler
formations
in
the
vicinity
of the
the
origin of the Copps and Tyler formations in the vicinity of
Gogebic
range.
In
this
model,
the
iron-formation
was
deposited
Gogebic range.
In this model, the iron-formation was deposited
northwardon the
the peripheral
peripheral bulge
bulge on
on the
the north
north side
side of
of the
the northwardon
migrating
foreland
basin,
whereas
the
aforementioned
turbidite
migrating foreland basin, whereas the aforementioned turbidite
sequences
sequences were
were deposited
deposited in
in the
the deeper
deeper axial
axial parts
parts of
of the
the basin.
basin.
This
This would
would require
require that
that the
the Ironwood
Ironwood Iron-Formation
Iron-Formationand
and the
the
overlying
overlying Tyler
Tyler Formation
Formation have
have aa gradational
gradational and
and conformable
conformable
relationship,
relationship, and
and that
that they
they were
were deposited,
deposited,at
at least
leastin
inpart,
part,
tholeiitic volcanism
volcanism
contemporaneously. In
In this
this model,
model, axial
axial tholeiitic
contemporaneously.
(Hoffman,
1988) and hydrothermal activity
activity in
(Hoffman, 1988)
in the
the foredeep
foredeep would
would
have
provided
the
iron
and
silica
that
were
precipitated
have provided the iron and silica that were precipitated as
as ironironformation
formation on
on the
the shelf,
shelf, as
as upwelling
upwelling brought
brought the
the solutions
solutions to
to aa
favorable site
site of
of deposition.
deposition.
favorable
Note
Note that
that the
the foreland
foreland basin
basin model
model provides
provides an
an alternative
alternative
interpretation
interpretation for
for the
the Ironwood
Ironwood and
and the
the intertonguing
intertonguing Emperor
Emperor
Volcanic
ted continental
Volcanic Complex
Complexcompared
comparedwith
withaarif
rifted
continental margin
margin model.
model.
Obviously
Obviously more
more work
work is
is needed
needed to
to resolve
resolve this
this problem.
problem.
Deformation
Deformation during
during the
the Penokean
Penokean orogeny
orogeny may
may have
have resulted
resulted in
in
reactivation
reactivation of
of the
the normal
normal faults
faults in
in the
the graben
graben structures.
structures. We
We
suggest
suggest that
that the
the structure
structure in
in the
the eastern
eastern Gogebic
Gogebic may
may represent
represent aa
half-graben
half-graben that
that has
has been
been subjected
subjected to
to thrusting
thrusting during
during the
the
Penokean orogeny
orogeny (Figure
(Figure12).
12). Perhaps
Perhaps significantly,
significantly, erosion
erosion
Penokean
prior
prior to
to deposition
deposition of
of the
the Keweenawan
Keweenawan Supergroup
Supergroup cut
cut deepest
deepest into
into
the
the Early
Early Proterozoic
Proterozoic sedimentary
sedimentary sequence
sequence in
in the
the central
central
Gogebic, where
where the
the proposed
proposed thrust
thrust faults
faults are
are present.
present. Whereas
Whereas
Gogebic,
over 2,000
2,000 mm of
of the
the Tyler
Tyler Formation
Formation remain
remain on
on the
the western
western Gogebic
Gogebic
over
in Wisconsin,
Wisconsin, and
and aa similar
similar thickness
thickness of
of Copps
Copps Formation
Formationis
is
in
present at
at the
the eastern
eastern end
end of
of the
the range,
range, erosion
erosion removed
removed all
all of
of
present
the Tyler
Tyler and
and part
part of
of the
the iron-formation
iron-formationeast
east of
of Wakefield.
Wakefield.
the
Assuming that
that erosion
erosion cut
cut down
down to
to aa relatively
relativelyuniform
uniform level,
level,
Assuming
then the
the central
central Gogebic
Gogebic may
may have
have been
been uplifted
uplifted at
at least
then
least 2,000
2,000 mm
more than
than areas
areas to
to the
the east
east and
and west.
west. An
An Archean
Archean cored
cored gneiss
gneiss
more
dome is
is present
present near
near Watersmeet
Watersmeet about
about 50
dome
50 km
km to
to the
the east.
east. That
That
area may
may represent
represent an
an even
even more
more pronounced
pronounced uplift.
uplift. Therefore,
area
Therefore,
rocks on
on the
the eastern
eastern Gogebic
Gogebic range
range may
may record
record depositional
depositional and
and
rocks
deformationalevents
events of
of both
both the
therif
rifted
and convergent
convergent
deformational
ted margin
margin and
tectonics during
during the
the Early
Early Proterozoic
Proterozoic in
in the
the Lake
Lake Superior
Superior
tectonics
region.
region.
17
w
--
--
'---'---'——-— '-
-
E
-
tRON WOOD
4
—.-.—'.--— ___A._
Water Surface-7
-
-
EROR
4u ,
--- - -::,/_.-- —
/
M
A
vA
I
RAM SAY
/
/
b
A
y
I—
1-i
•
V
V
18
Figure 11.
Figure
11.
pre-Penokean
Diagram showing hypothetical pre-Penokean
relationships
on
the
eastern
Gogebic
relationships on the eastern Gogebic range.
range.
Figure
Figure 12.
12.
Simplified geologic map
Gogebic
Gogebic range.
range. (Not
(Not to
of the eastern
eastern part of
of the
the
scale)
scale)
STOP
STOP
DESCRIPTIONS
DESCRIPTIONS
Field Trip
Gogebic Field
19
Stop
stow 1.
1.
Tyler
Wler Formation.
Formation.
Location:
Location: Roadcuts
Roadcuts at
at junction
junction of
of U.S.
U.S. Hwy.
Hwy. 22 and
and Hwy.
Hwy.
51
51 north
north of
of Hurley,
HurleyI WI
WI
Rocks
Rocks exposed
exposed at
at this
this locality
locality are
are representative
representative
of
Tyler Formation,
Formationlwhich
which is
is
of the
the upper
upper part
part of
of the
the Tyler
about
21100meters
meters thick
thick here.
here. It
It thickens
thickens westward
westward to
to
about 2,100
about
about 2,900
2#900meters
meters in
in Wisconsin
Wisconsin and
and thins
thins eastward
eastward to
to
zero
Ramsay! where
where it
it was
was entirely
entirely removed
removed by
by
zero east
east of
of Ramsay,
erosion
erosion prior
prior to
to deposition
deposition of
of Keweenawan
Keweenawan strata.
strata. The
The
Tyler
Tyler is
is unconformably
unconformably overlain
overlain by
by the
the Bessemer
Bessemer
Quartzite
Quartzite and
and aa substantial
substantial thickness
thickness of
of Keweenawan
Keweenawan
volcanic
volcanic rocks.
rocks.
The
relationship of
of the
the Tyler
Tyler to
to the
the underlying
underlying
The relationship
Ironwood
Ironwood Iron-Formation
Iron-Formationis
is uncertain
uncertain because
because of
of poor
poor
outcrop.
Atwater,
outcrop. Most
Most geologists
geologists (e.g.,
(e.ge1
Atwater,1938)
1938)
consider
consider the
the two
two formations
formations to
to be
be conformable
conformable (Schmidt
(Schmidt
and
and Hubbard,
Hubbardl 1972);
1972); however,
howeverl Aldrich
Aldrich (1929)
(1929)presents
presents
evidence
evidence for
for erosion
erosion of
of the
the iron-formation
iron-formationand
and aa basal
basal
conglomerate
conglomerate at
at the
the base
base of
of the
the Tyler.
Tyler. The
The presence
presence of
of
aa graywacke-slate
graywacke-slatesequence
sequence overlying
overlying the
the iron-formation
iron-formation
is
is characteristic
characteristic throughout
throughout the
the Lake
Lake Superior
Superior region
region
elsewhere on
on the
the western
western Gogebic
Gogebic range,
rangeI the
the
As elsewhere
As
beds here
here strike
strikeeast-northeast,
east-northeastl
dip60°-75°
60'-75' northwest,
northwestl
beds
dip
block that
that
and top
top to
to the
the northwest,
northwest! part
part of
of aa large
large block
and
was tilted
tilted northward
northward during
during Keweenawan
Keweenawan rifting
rifting to
to form
form
was
aa large
large north-facing
north-facingmonocline.
monocline. Schmidt
Schmidt and
and Hubbard
Hubbard
(1972) and
and Kiasner
Klasner and
and others
others (1991)
(1991) pointed
pointed out
out that
that
(1972)
cleavagelbedding relationships
relationships in
in these
these (and
(andother)
other)
the cleavage/bedding
the
outcrops
outcrops are
are unusual
unusual and
and may
may be
be of
of tectonic
tectonic
significance.
significance. As
As shown
shown in
in the
the accompanying
accompanying diagrams
diagrams
13) the
(Figure 13),
(Figure
the graywacke-slate
graywacke-slatewas
was dipping
dipping gently
gently
southward prior
prior to
to Keweenawan
Keweenawan time.
time. Because
Because the
the
southward
cleavage
cleavage dips
dips less
less steeply
steewlv than
than the
the bedding,
beddingl rotation
rotation
of the
the rocks
rocks southward
southward to
to their
their pre-Keweenawan
pre-Keweenawansubsubof
horizontal attitude
attitude results
results in
in aa south-dipping
south-dipping
horizontal
cleavage.
cleavage.
Cannon and
and others
others (1990)
(1990)showed
showed that
that originally
originally
Cannon
vertical Early
Early Proterozoic
Proterozoic diabase
diabase dikes
dikes in
in the
the Archean
Archean
vertical
terrane to
to the
the south
south of
of the
the Gogebic
Gogebic range
range now
now have
have aa
terrane
southward
explained by
by aa northward
northward
southwarddip.
dip. This
This dip
dip is
is explained
rotationalong
alongthe
theeast
east-northeast-trending
Marenisco
rotation
-northeast - trending Marenisco
faultlaa major
major listric
listric thermal
thermal fault
fault with
with perhaps
perhaps 10
10 km
km
fault,
of
of southward
southward thrust
thrust motion
motion on
onit.
it. This
This motion
motion explains
explains
the present
present steep
steep northward
northward dip
dip of
of the
the major
major monoclinal
monoclinal
the
structure
structure of
of the
the Gogebic
Gogebic range.
range.
Alwin (1976)
(1976)showed
showed that
that the
the Tyler
Tylerconsists
consists
Aiwin
dominantly of
of intercalcated
intercalcated argillite
argillite and
and slate
slate with
with
dominantly
lesser amounts
amounts of
of siltstone
siltstone and
and graywacke.
graywacke. Graded
lesser
Graded
20
NORTH
NORTH
SOUTH
SOUTH
I
Rocks o f
Earlv Proterozo~c
METERS
6OO
METERS
NORTH
SOUTH
B
Figure
Figure 13.
13.
diagram and cross
Generalized block diagram
cross section
section
illustrating structural
illustrating
structural relationships
relationships in
in Ironwood,
Ironwoodl
Mich.-Hurley,
Heavy dashed
Mich.-Hurley, Wis.,
Wise1area.
area. Heavy
dashed line,
linel
A, Block diagram
fault.
Al
diagram showing
showing relationship
relationship of
of
fault.
pre-Keweenawan strata
Midcontinent
pre-Keweenawan
strata to
to rocks
rocks of
of Midcontinent
foliation in
in q
Tyler
Formation
rift.
rift.
Note that
that S1
S' foliation
l e r Formation
dips
dips less
less steeply
steeply than
than bedding.
bedding. Modified from
from
B,
Reconstructed
Schmidt
and
Hubbard
(1972).
Schmidt and Hubbard (1972) Bl ~econstructed
post-Penokean,
post-Penokeanl pre-Keweenawan
pre-Keweenawan position
position of
of the
the
Note
gently
south
dipping
strata shown
strata
shown in
in A.
A. Note gently south dipping S1
Sl
foliation.
foliation. Modified from
from Schmidt
Schmidt and Hubbard
(From Klasner
Klasner and
and others,
others! 1991)
1991)
(1972). (From
(1972).
.
21
graywacke
sequences indicate
indicate
graywacke beds
beds containing
containing Bouina
Bouma sequences
marks and
deposition
deposition by turbidity
turbidity currents.
currents. Sole
Sole marks
and
small-scale
paleocurrent trend
small-scale cross-beds
cross-beds indicate
indicate a
a paleocurrent
trend
toward
toward the
the west-northwest.
west-northwest. Framework
Framework grains
grains in
in the
the
quartz and feldspar
graywackes are quartz
feldspar and
and rock
rock fragments
fragments
granitic, volcanic,and
of graniticl
volcaniclandmetamorphic
metamorphic rocks.
rocks.
Therefore,
Therefore, the
the source
source area
area for
for the
the Tyler
Tyler sediments
sediments is
is
inf
erred to
to be
be an
an Archean
Archean granitic-metarnorphic
inferred
granitic-metamorphic
The Tyler
(cratonic)
source area
(cratonic) source
area to
to the
the southeast.
southeast. The
Tyler was
was
only slightly
slightly metamorphosed to
to sub-greenschist
sub-greenschistfacies
facies
during
during the
the Penokean
Penokean orogeny,
orogenyl and
and the
the metamorphic
metamorphic grade
grade
here
was
not
changed
during
the
Keweenawan
here was not changed during the Keweenawan rifting.
rifting.
At this
this exposure,
exposurel note
note graded
graded graywacke
graywacke beds
beds
topping
NW, mud-chips
mud-chips in
topping NWl
in some
some beds,
bedsl rare
rare sole
sole marks,
marksl
some soles,
loading on some
solesl flame
flame structures,
structuresl and
and smallsmallassemblage of sedimentary
scale cross-bedding.
scale
cross-bedding. The assemblage
sedimentary
structures
structures indicates
indicates a turbidity
turbidity current
current mechanism,
mechanisml and
and
deposition on a submarine
likely but
but not
not
deposition
submarine fan
fan is
is likely
In the fan
of
essential.
In
fan model the "packets"
npacketsll
of graywacke
graywacke
essential.
beds
beds would
would be
be channel
channel deposits
deposits whereas
whereas the
the mudstones
mudstones
would
would be
be overbank
overbank deposits.
deposits.
Continue east on
Continue
on U.S. Hwy 2
2 to
to Golf
Golf Club
Club Road
Road just
just east
east of
of
Ironwood.
Thrn
Ironwood.
Turn right
right (south)
(south) on
on Golf Club Road and proceed about
miles south
south to
1.1 miles
to large
large brick buildings
buildings of
of the
the Newport
Newport Mine.
Mine.
Walk
Walk to
to the
the northeast
northeast across
across the
the small
small field
field to
to old
old railroad
railroad
cuts,
cutsl that
that are
are just
just north
north of
of the
the main
main building
building behind
behind the
the trees.
trees.
Caution:
Caution: Area
Area is
is now
now part
part of
of aa gravel
gravel washing
washing operation.
operation.
Stop
2.
Sto~
2.
Newport mine (Archean/Early
Proterozoic
Old Nemort
(ArcheanIEarlv Proterozoic
Unconformity).
Unconformitv)
.
The Palms
Palms Formation
Formation of the Menominee Group
the
Group of the
Marquette
Marquette Range
Range Supergroup
Supergroup rests
rests upon
upon the
the 2,700
21700m.y.
m.y.
old Puritan
Puritan Quartz
Quartz Monzonite
Monzonite (Schmidt,
(Schmidtl1976).
1976). The
The
Marquette Range
Marquette
Range Supergroup
Supergroup in
in the
the Gogebic
Gogebic range
range was
was
deposited on
on aa moderately
moderately flat
flat erosion
deposited
erosion surface
surface
developed
developed on
on Archean
Archean "granite"
I1granitenand
and greenstone.
greenstone.
steeply dipping
dipping (to
(to the
the
At this
this locality,
localityl the
the steeply
north) unconformable
unconformable contact
contact between
between the
the Palms
Palms
north)
Formation
is exposed
exposed in
in the
the old
old
Formation and Archean granitoid
granitoid is
railroad
cut. Small depressions
in the
the surface,
surfacel as
as can
can
railroad cut.
depressions in
be seen
seen here,
herel contain
contain laminated
laminated cherty
cherty'material,
some
material, some
granular and
and somewhat
somewhat resembles
resembles ironironwhich is
is granular
of which
formation. The
mm in
in diameter
diameter
formation.
granules are
are 0.5 to
to 1.0 mm
The granules
and consist
consist of
of mixtures
mixtures of
of chert,
chertI chlorite,
chlorite, and
and
and
calcite. There
calcite.
few phosphatic
argillaceous
There are also a few
phosphatic argillaceous
chips just
just above
above the
the contact.
contact. Note that
that there
there is
is no
no
appreciable
appreciable evidence
evidence of
of weathering
weathering of
of the
the granite
granite
Palms.
beneath
the Palms.
beneath the
22
Red
Red argillaceous
argillaceous and
and silty
siltybeds
beds of
of the
thelower
lower
member
member of
of the
the Palms
PalmsFormation
Formationare
areexposed
exposedhere.
here. They
They
contain
contain wavy
wavy bedding
bedding and
and small
small mudcracks,
mudcracksfand
and have
have been
been
interpreted
interpreted as
as upper
upper tidal
tidal flat
flat deposits
depositsof
of aa
transgressing
transgressing sea
sea(Ojakangas,
(Ojakangas 1983).
19831
.
cross it,
it, and
and continue
continue north
north approximately
approximately
Return
Return to
to U.S.
U.S. Hwy.
H w y . 2, cross
on right,
rightlbeyond
beyond the
the Kangas
Kangas farm.
farm.
mile to
11 mile
to rock
rock knob
knob on
S t o3.
~3.
Stop
Contact
Contact between
between Bessemer
Bessemer Ouartzite
Ouartzite and
and Powder
PowderMill
Mill
Volcanics
Vol
canics
This
This exposure
exposure is
is approximately
approximatelytwo
twomiles
milesnorth
northof
of
and the
the interval
interval between
between these
these two
two stops
stops
Stop
Stop 2, and
demonstrates
demonstrates the
the thickness
thickness of
of the
the Early
Early Proterozoic
Proterozoic
The exposure
exposure shows
shows the
the contact
contact
sequencein
inthis
thisarea.
area. The
sequence
between
between the
the basal
basal Keweenawan
Keweenawan Bessemer
Bessemer Quartzite
Quartzite and
and the
the
Thus the
the
overlying
overlying basalt
basalt of
of the
thePowder
PowderMill
MillGroup.
Group. Thus
exposure
exposure illustrates
illustrates the
the abrupt
abrupt change
change from
from
sedimentation
sedimentationof
of quartzose
quartzose sandstone
sandstoneto
to flood
floodbasalt
basalt
volcanism
volcanism during
during the
the Middle
Middle Proterozoic
Proterozoic in
in this
this part
part of
of
the Lake
Lake Superior
Superiorregion.
region. Only
Only the
the lowermost
lowermost basalt
basalt
the
flows
softl silty
silty
flows are
are pillowed,
pillowedl and
and locally,
locallyI the
the soft,
sediment
sediment was
was squeezed
squeezed up
up between
between rubbly
rubbly blocks
blocks and
and
pillows. Some
Some units
units of
of Bessemer-type
Bessemer-typequartzite
quartzite do
do
pillows.
occur
occur higher
higher in
inthe
the flow
flowsequence.
sequence. These
These features
features
indicate
indicate that
that volcanism
volcanism here
here was
was initiated
initiated in
in aa
sedimentary
basinl overwhelmed
overwhelmed sedimentation,
sedimentationland
and
sedimentary basin,
resulted
resulted in
in aa thick
thick sequence
sequenceof
of subaerial
subaerialflows.
flows. Flows
Flows
just to
to the
the north
north are
aremassive.
massive.
just
S t -o 4.
~4.
Stop
Ironwood
Ironwood Iron-Formation
Iron-Formationand
and Ironwood-Tyler
Ironwood-Tvler
relationshi~s. (Modified
(Modified from
from Schmidt,
Schmidtl1972)
1972)
relationships.
Location: Exposures
Exposures on
on the
the east
east and
and west
west banks
banks of
of the
the
Location:
Black River
River upstream
upstream from
from Hwy.
H w y . 22 on
on the
the western
western
Black
(NE ',
XI SW,
SWAl Sec.
Sec. 12,
12,
outskirts of
of Ramsay,
RamsaylMichigan.
Michigan. (NE
outskirts
T.47N. R.46W.)
R.46W.
T.47N.,
The Ironwood
Ironwood Iron-Formation
Iron-Formationis
is aa major
major sedimentary
sedimentary
The
Gogebic range,
rangel and
and is
is the
the source
source rock
rock for
for all
all of
of the
the
Gogebic
Unfortunatelylthe
the ironironiron ores
ores mined
mined in
in the
the area.
area. Unfortunately,
iron
formation is
is very
very poorly
poorly exposed
exposed on
on the
the eastern
eastern
formation
Gogebic. Excellent
Excellent exposures
exposures of
of the
the Ironwood
Ironwood IronIronGogebic.
Formation are
are present
present on
on Mt.
Mt. Whittlesey,
Whittleseylsoutheast
southeast of
of
Formation
Mellenl Wisconsin,
Wisconsinl however,
howeverl that
that locality
locality is
is more
more than
than
Mellen,
30 miles
miles away,
awayl and
and will
will not
not be
be visited
visited on
on this
this trip.
trip.
30
Schmidt and
and Hubbard
Hubbard (1972)
(1972) described
described several
several
Schmidt
exposures along
along the
the Black
Black River
River at
at this
this locality
locality that
that
exposures
illustrate different
different phases
phases of
of the
the iron-formation
iron-formationas
as
illustrate
well as
as possible
possible stratigraphic
stratigraphic relationships
relationships between
between
well
qler
the Ironwood
Ironwood Iron-Formation
Iron-Formation and
and the
the overlying
overlying Tyler
the
Formation. They
They describe
describe thin-bedded
thin-bedded chert-carbonate
chert-carbonate
Formation.
23
23
iron-formation
Member of
iron-formation at the
the top
top of the
the Anvil Member
of the
the
Ironwood
Ironwood Iron-Formation
Iron-Formation on
on the
the east
east bank
bank of
of the
the river
river
600
upstream from
bridge on
600 feet
feet upstream
from the
the bridge
on the
the secondary
secondary road
road
south of
south
of Hwy.
Hwy. 2.
2. Granular
Granular jasper
jasper is
is described from
from
test pits
Sec. 121
12, T.47Na1
T.47N., R.46W.
test
pits in
in the
the SE(,
SE%# SW1X,
SWA1 Sec.
R.46W.
Schmidt
Schmidt and
and Hubbard
Hubbard (1972)
(1972) conclude
conclude that
that the
the rocks
rocks are
are
basically unmetamorphosed
basically
umetamorphosed examples
examples of
of the
the ironironformation. Time
formation.
Time limitations
limitations will
will not
not permit
permit us
us to
to
visit all of the
localities
described
by
Schmidt
and
the localities
Hubbard (1972),
and
those
interested
in
examining
(1972)1
interested
unaltered iron-formation
unaltered
iron-formationshould
should consult
consult their
their 1972
1972
guidebook.
guidebook.
Schmidt
Schmidt and
and Hubbard
Hubbard (1972)
(1972) also
also discussed
discussed
alternative
alternative interpretations
interpretations of the relationship between
Tyler and
the Tyler
and Ironwood
Ironwood Formations.
Formations. The
The contact
contact is
is
(600
the Black
Black River
River 180
180 mm (600
exposed on the east bank of the
ft.)
roadl where
where Schmidt
Schmidt
ft.) upstream from
from the
the secondary
secondary road,
and
Hubbard (1972)
(1972) interpreted
interpreted the
the contact
contact to
to be
be
and Hubbard
gradational.
gradational.
We will examine
examine only the exposure on the west bank
of the Black River just
just south
south of Hwy. 22 where
where aa lens
lens of
of
carbonate iron-formation
carbonate
iron-formation and a
a sulfide
sulfide unit
unit is
is present
present
within the
within
the lower
lower part
part of
of the
the Tyler
Tyler Formation.
Formation. Although
.ong the
the stream
stream
exposure
poor, there
outcrops a1
along
there are outcrops
exposure is
is poorl
slatel massive
bank consisting
consisting of highly carbonaceous
carbonaceous slate,
sideritic
pyrite/marcasite, and sideritic
and concretionary(?)
concretionary ( ? ) pyrite/marcasite,
iron- formation.
iron-formation.
Stop 5.
Radio Tower
Radio
Tower Hill
Hill in
in Wakefield.
Wakefield.
Palms
Marmette Range
RanqeSupercrroup
Su~erqrouq
Palms Formation
Formation of
of Marguette
Location:
Location: The
The purposes of this
this stop
stop are
are to
to (1)
(1) examine
examine
an excellent
(2)
an
excellent exposure
exposure of
of the
the Palms
Palms Formation
Formation and
and (2)
obtain
obtain a
a scenic
scenic view
view of
of this
this part
part of
of the
the Gogebic
Gogebic range,
rangel
and of the
of the
the Archean terrane
terrane to
to the
the south,
southl and
the
Keweenawan terrane
Keweenawan
terrane to
to the
the north.
north.
From
hill, several
From the
the top
top of the
the hilll
several abandoned
abandoned iron
iron
mines
mines can
can be
be seen
seen in
in the
the lowland
lowland immediately
immediately to
to the
the
thi
north (refer
(refer to
to Figure
Figure 7);
7); the
the ridges
ridges in
in the
north
intermediate distance
basalt
of early
early
intermediate
distance are
are underlain
underlain by
basalt
of
Keweenawan
(Middle Proterozoic)
Proterozoic) age.
age. The
The lowland
lowland in
in
Keweenawan (Middle
the far
far distance
is underlain
the late
late Keweenawan
Keweenawan
distance is
underlain by the
Jacobsville
Jacobsville Sandstone.
Sandstone.
The
to the
the south
south overlooks
overlooks Archean
Archean terrane
terrane
The view to
consisting of the
the Ramsay
Ramsay Formation
Formation (mafic
(mafic and
and felsic
felsic
consisting
metavolcanic rocks)
rocks) and the
the Puritan
Puritan Quartz Monzonite
metavolcanic
and related
related rocks
rocks (—2,700
(-21700r
n.y.1 which
which constitute
constitute aa
m.y.),
major batholith
batholith more
more than
than 150
150 km
km long.
long.
major
area, the Palms
Palms Formation
unconformably
Formation unconfombly
In this areal
overlies
overlies the Archean Ramsay Formation
Formation and is overlain
overlain
24
which
conformably by
by the
the Ironwood
IronwoodIron-Formation,
Iron-Formation,which
conformably
underlies
underlies the
the low
low area
area north
north of
of the
the hill
hill and
and also
also
southwest of
of the
thehill.
hill. Low
Low on
on the
the next
next hill
hill to
to the
the
southwest
Further
east, rare
rare Sunday
Sunday Quartzite
Quartziteis
ispresent.
present. Furthereast,
east,
east,
the Palms
Palms unconformably
unconformably overlies
overlies the
the Bad
Bad River
River
the
Dolomite,and
and has
has aa thin
thin phosphatic
phosphatic conglomerate
conglomerateat
at its
its
Dolomite,
with
The
Palms
dips
steeply
northward,
in
accord
base.
The
Palms
dips
steeply
northward,
in
accord
with
base.
the regional
regional dip
dip of
of Proterozoic
Proterozoic rocks
rocks in
in the
the Gogebic
Gogebic
the
This
regional
dip
resulted
mainly
from
range.
This
regional
dip
resulted
mainly
from
range.
toward the
the
northward
northward tilting
tilting during
during Keweenawan
Keweenawan time,
time, toward
axis of
of the
the (Keweenawan)
(Keweenawan)Lake
Lake Superior
Superior syncline.
syncline. The
The
axis
Early
deformed
Early Proterozoic
Proterozoic rocks
rocks were
were not
not appreciably
appreciably deformed
(Penokean
or metamorphosed
metamorphosed during
during the
the tectonic
tectonic event
event (Penokean
or
that closed
closed Early
Early Proterozoic
Proterozoic deposition,
deposition,but
but
orogeny) that
orogeny)
the
east-northeast
at
20-25°
20-25'
a
few
folds
plunging
to
the
east-northeast
at
a few folds plunging to
have been
been recognized.
recognized.
have
Exposures
Exposures of
of the
the Palms
Palms Formation
Formation at
at this
this locality
locality
are representative
representative of
of the
the formation
formation which
which extends
extends for
for
are
Representative
rock
types
80
km
to
the
west-southwest.
Representative
rock
types
80 km to the west-southwest.
well-exposed
well-exposedat
at the
the top
top of
of the
the hill
hill include
include the
the upper
upper
part of
of the
the middle
middle member
member which
which consists
consists of
of interbedded
interbedded
part
argillite,
argillite,siltstone,
siltstone, and
and sandstone,
sandstone, and
and the
the transition
transition
to the
the massive
massive quartzite
quartzite of
of the
the upper
upper quartzite
quartzitemember.
member.
to
The middle
middle member
member is
is thin
thin bedded,
bedded, with
with beds
beds generally
generally
The
2-10cm
cm thick.
thick. In
In general,
general, beds
beds of
of buff
buff to
to pink
pink
2-10
siltstone and
and white
white sandstone
siltstone
sandstone alternate
alternate with
with buff,
buff, red
red
or green
green beds
beds of
of more
more argillaceous
argillaceous material;
material;coarsecoarseor
grained,
grained, dark-red,
dark-red,hematitic
hematitic sandstone
sandstone beds
beds are
are also
also
present. Most
Most beds
beds are
are quite
quite continuous
continuousacross
across the
the
present.
outcrop but
but "pinch
"pinchand
and swell"
swellnirregularities
irregularities impart
impart aa
outcrop
waviness to
to the
the generally
generally uniform
uniform bedding.
bedding. Lenticular
Lenticular
waviness
beds of
of sandstone,
sandstone,commonly
commonlycross-bedded,
cross-bedded,
arecommon.
common.
beds
are
Flaser bedding
bedding is
is present
present locally
locally where
where the
the mud
mud supply
supply
Flaser
was subordinate
subordinateto
to the
thesand
sandsupply.
supply. Minor
Minor cut-and-fill
cut-and-fill
was
and soft-sediment
soft-sedimentdeformation
deformation structures
structuresalso
also are
are
and
present. Symmetrical
Symmetrical but
but irregular
irregular ripple
ripple marks
marks and
and
present.
mud cracks,
cracks,although
although not
not visible
visible at
at this
this locality,
locality,are
are
mud
(Ojakangas,1983).
1983).
present on
on the
the next
next hill
hill to
to the
the east
east (Ojakangas,
present
Measurement
Measurement of
of 42
42 cross-beds
cross-bedson
on this
this hill
hill shows
showsaa
strong paleocurrent
paleocurrent trend
trend to
to the
the west
west with
with aa weaker
weaker
strong
trend to
to the
theeast.
east. Nearly
Nearly 200
200 measurements
measurements in
inthe
the
trend
formation as
as aa whole
whole accentuate
accentuatethis
thisbimodality.
bimodality.
formation
Correction for
for plunge,
plunge, if
if indeed
indeed the
the entire
entire formation
formation
Correction
has aa plunge
plunge as
as well
well as
as aa tilt,
tilt, would
has
would rotate
rotate the
the major
major
and minor
minor modes
modes clockwise,
clockwise, thus
thus relocating
relocatingthe
themodes
modes to
to
and
the west-northwest
west-northwestand
and east-southeast,
east-southeast,respectively.
respectively.
the
General characteristics
characteristics of
of the
the formation,
formation, and
and
General
especially the
the bimodal
bimodal cross-bedding
cross-beddingpattern,
pattern, the
the
especially
bedding styles,
styles, and
and the
the overall
overall lithologies
lithologies are
are highly
highly
bedding
suggestive of
of deposition
deposition in
in aa tidal
tidal environment.
environment.
suggestive
Recall that
that the
the lower
lower member
member interpreted
interpretedas
asupper
uppertidal
tidal
Recall
25
flat, was
was seen
seen at
at Stop
Stop 2.
2. Here
Here on
on Radio
Radio Tower
Tower Hill,
Hill,
flat,
the
middle
member
is
interpreted
as
middle
tidal
flat,
the middle member is interpreted as middle tidal flat,
and
the
cross-bedded
quartz
sand
upper
member
is
and the cross-bedded quartz sand upper member is
interpreted as
as lower
lower tidal
tidal flat
flat or
or subtidal
subtidal (Ojakangas,
(Ojakangas,
interpreted
1983).
1983)
Thin sections
sections from
from this
this locality
locality show
show that
that the
the
Thin
sandstone beds
beds are
are feldspathic
feldspathicquartzite.
quartzite. Well-rounded
Well-rounded
sandstone
unit quartz
quartz grains
grains and
and feldspar
feldspar (mostly
(mostlyfresh
freshKKunit
feldspar) grains
grains are
are the
the dominant
dominant framework
framework
feldspar)
constituents, with
with chert
chert grains
grains aa poor
poor third.
third. Silica
Silica
constituents,
illitic clay
clay is
is present
present in
in many
many
cement is
is abundant
abundant and
and illitic
cement
samples as
as minor
minor to
to abundant
abundant matrix.
matrix. Illite
Illite is
is also
also
samples
the major
major constituent
constituent of
of the
the argillaceous
argillaceous beds
beds with
with
the
chlorite locally
locally prominent.
prominent. The
The coarser-grained,
coarser-grained,
chlorite
hematitic
sandstone
beds
are
really
thin beds
beds of
of ironironhematitic sandstone beds are really thin
formation,
consisting
of
granules
of
hematite,
chert,
formation, consisting of granules of hematite, chert,
and iron
iron silicates
silicates in
in addition
addition to
to the
the common
common quartz
quartz
and
grains.
grains.
Deformation of
of the
the Palms
Palms Formation
Formation has
has not
not been
been
Deformation
well
studied.
Basically,
it
is
part
of
a
large
well studied. Basically, it is part of a large
monocline in
in which
which folding
folding is
is rarely
rarely seen
seen except
except at
at
monocline
this
locality.
On
the
west
flank
of
the
hill,
several
this locality. On the west flank of the hill, several
open, round-crested
round-crestedfolds
folds exhibit
exhibit aa cleavage
cleavage that
that is
is
open,
flatter
than
the
bedding.
The
extent,
origin,
and
age
flatter than the bedding. The extent, origin, and age
of
these
folds
and
their
associated
cleavage
is
not
of these folds and their associated cleavage is not
known, but
but the
the folds
folds may
may have
have formed
formed when
when the
the beds
beds were
were
known,
approximately
flat-lying,
presumably
during
the
approximately flat-lying, presumably during the
Penokean orogeny.
orogeny. Alternatively,
Alternatively, the
the folding
folding might
might
Penokean
have
been
caused
by
drag
along
the
Sunday
Lake
fault, aa
have been caused by drag along the Sunday Lake fault,
northwest-trending
fault
that
passes
just
west
of
this
northwest-trending fault that passes just west of this
hill.
hill.
Stop 6.
6.
Stop
Archean Ramsay
Ramsav Formation
Formation (pillowed
(pillowed meenstone)
Archean
reenstone)
Location: Roadcut
Roadcut along
along U.S.
U.S. Hwy.
Hwy.
Location:
limits
of
Wakefield,
about
2.5
mi.
limits of Wakefield, about 2.5 mi.
of
Highways
2
and
28
in
Wakefield.
of Highways 2 and 28 in Wakefield.
at eastern
eastern city
city
22 at
SE
of
the
junction
SE of the junction
Archean Ramsay
Ramsay Formation
Formation forms
forms part
part of
of the
the
The Archean
The
Archean
basement
upon
which
the
Early
Proterozoic
rocks
Archean basement upon which the Early Proterozoic rocks
Prinz
and
others
(1975)
were
deposited.
According
to
were deposited. According to Prinz and others (1975)
metavolcanic rocks
rocks of
of Archean
Archean age
age extend
extend eastward
eastward for
for
metavolcanic
approximately
15
km
along
the
range
in
the
vicinity
of
approximately 15 km along the range in the vicinity of
Wakefield.
They
are
intruded
on
the
south
and
west
by
Wakefield.
They are intruded on the south and west by
the Puritan
Puritan Quartz
Quartz Monzonite,
Monzonite, and
and are
are bounded
bounded on
on the
the
the
east
by
gneissic
rocks
of
the
Puritan
Quartz
Monzonite.
east by gneissic rocks of the Puritan Quartz Monzonite.
They reported
reported that
that the
the eastern
eastern two-thirds
two-thirdsof
of the
the belt
belt
They
Archean
rocks
is
mainly
mafic
to
intermediate
flows
of
of Archean rocks is mainly mafic to intermediate flows
and
pyroclastic
rocks
with
sparse
felsic
volcanic
and pyroclastic rocks with sparse felsic volcanic
rocks. However,
However, intermediate
intermediate to
to felsic
felsic rocks
rocks are
are
rocks.
dominant
in
the
western
part
of
the
belt.
The
rocks
dominant in the western part of the belt.
The rocks
generally
strike
west
to
northwest
(roughly
parallel
to
generally strike west to northwest (roughly parallel to
26
the
the strike
strike of
of Early
Early Proterozoic
Proterozoic rocks)
rocks) and
and dip
dip steeply
steeply
to
to the
the south.
south. Pillows
Pillows indicate
indicate that
that the
the lava
lava flows
flows
generally
generally face
face southward.
southward.
Prinz
Prinz and
and others
others (1975)
(1975)suggested
suggested that
that the
the mafic
mafic
and
felsic volcanics
volcanics each
each have
have aa thickness
thickness of
of
and the
the felsic
approximately
approximately 3,000
3,000meters.
meters. Metamorphic
Metamorphic grade
grade
increases
from
greenschist
facies
increases from greenschist facies in
in the
the west
west to
to
amphibolite
amphibolite facies
facies along
along the
the eastern
eastern margin.
margin.
Rocks
Rocks exposed
exposed here
here are
are typical
typical pillowed
pillowed mafic
mafic
volcanic
rocks
that
have
been
metamorphosed
volcanic rocks that have been metamorphosed to
to
They are
are composed
composed of
of fine-grained
fine-grained
greenschist
fades. They
greenschist facies.
sodic
plagioclase,
quartz,
epidote-zoisite,
sodic plagioclase, quartz, epidote-zoisite,and
and
chlorite,
chlorite, with
with some
some pale
pale amphibole,
amphibole, carbonate
carbonate and
and
biotite
biotite (Prinz
(Prinzand
and others,
others, 1975).
1975). Pillows
Pillows with
with tops
tops
facing
facing southward
southward are
are well-exposed
well-exposed on
on the
the south
south side
side of
of
the
the highway.
highway.
Take
Take U.S.
U.S. Hwy.
Hwy. 22 east
east of
of Wakefield
Wakefield city
city limits
limits about
about two
two miles
miles to
to
Go
to
compressor
station
on
pipeline
and
turn
Go
to
compressor
station
on
pipeline
and
turn
Great Lakes
Lakes Road.
Road.
Great
left
mile in
in is
is aa "Y";
"Yn; keep
keep
left (north)
(north)on
on gravel
gravel road.
road. About
About 0.3
0- 3 mile
Walk west
west on
on logging
logging
1.5 miles
miles to
to end
end of
of road.
road. Walk
left and
and go
go about
about 1.5
left
road
road ±
2 270
270 paces,
paces, then
then go
go north
north through
through the
the woods
woods to
to the
the high
high
The outcrop
outcrop is
is an
an escarpment
escarpment along
along SW
SW side
side of
of hill
hill in
in the
the
hill. The
hill.
R.44W., (Wakefield
NE 7%Minute
%, Sec.
Sec. 18,
18, T47N., R.44W.,
(Wakefield NE
Minute
SE (,
SE
Quadrangle.)
Quadrangle.)
Stop 7.
Archean-Early
Archean-Earlv Proterozoic
Proterozoic unconformity.
unconformitv.
The
The unconformity
unconformity between
between the
the Archean
Archean Ramsay
Ramsay
greenstone
greenstone and
and the
the Early
Early Proterozoic
Proterozoic Sunday
Sunday Quartzite
Quartzite
is
is exposed
exposed along
along the
the southwest
southwest side
side of
of this
this hill.
hill. Note
Note
the
the schistosity
schistosity in
in the
the otherwise
otherwise massive
massive greenstone,
greenstone,
and
and the
the very
very thin
thin basal
basal conglomerate.
conglomerate. Just
Just westward
westward
from
from the
the unconformity,
unconformity, the
the Sunday
Sunday Quartzite
Quartzite at
at first
first
glance
glance appears
appears to
to be
be conglomeratic,
conglomeratic, but
but it
it is
is aa thin
thin
skin
skin along
along the
the face
face of
of the
the outcrop
outcrop and
and is
is probably
probably aa
fault
fault breccia.
breccia. This
This prominent
prominent exposure
exposure is
is probably
probably aa
fault
fault scarp.
scarp.
Moving
Moving along
along the
the base
base of
of the
the escarpment,
escarpment, note
note the
the
crosscross-bedding (some
(some of
of it
it herringbone
herringbone type,
type, with
with crosscross-bedding
beds in
in successive
successive beds
beds oriented
oriented at
at 180°
180' to
to each
each
beds
mudchip conglomerates,
conglomerates,
other), the
the numerous
numerous layers
layers of
of mudchip
other),
and mud-cracked
mud-crackedhorizons.
horizons. These
These are
are included
included in
in aa
and
series of stacked
stacked tidal
tidal channels
channels to
to about
about 22 mm thick,
thick,
series
each
each starting
starting with
with aa mud-cracked
mud-crackedhorizon.
horizon. These
These are
are
overlain by
by aa cross-bedded
cross-bedded unit
unit that
that includes
includes
overlain
herringbone
herringbone cross-beds,
cross-beds, and
and ends
ends with
with a
a parallel-bedded
parallel-bedded
unit. At this
this exposure,
exposure, 41
41 cross-beds
cross-beds yield
a broad
yield a
unit.
trimodal (but
(but largely
largely bimodal-bipolar)
the
trimodal
bimodal-bipolar) pattern with the
major mode
mode to
to the
the northwest
northwest and
and aa less
less prominent
prominent mode
mode
major
27
to
to the
the southeast.
southeast. All
All of
of these
these features
features are
are
characteristic
characteristic of
of aa tidal
tidal environment
environment of
of deposition
deposition for
for
the
the Sunday
Sunday Quartzite.
Quartzite. Thus
Thus aa sandy
sandy tidal
tidal environment
environment
transgressed
peneplained Archean
Archean craton.
craton.
transgressed onto
onto aa peneplained
Continue
Continue along
along escarpment
escarpment and
and note
note the
the nature
nature of
of
the
the transition
transition from
from the
the quartzite
quartzite into
into dolomite.
dolomite. This
This
is
is the
the Bad
Bad River
River Dolomite.
Dolomite. Note
Note also
also the
the abundance
abundance of
of
white
black chert
chert within
within the
the dolomite,
dolomiteIand
and the
the
white and
and black
Hanirnering
Please!
presence
stromatolites.
Hammering Please!
presence of
of stromatolites.
Because
Because the
the vertical
vertical facies
facies give
give the
the horizontal
horizontal facies
facies
relationships,
relationshipsl it
it seems
seems that
that the
the carbonate
carbonate was
was
deposited
deposited seaward
seaward of
of the
the terrigenous
terrigenous sands
sands that
that are
are now
now
the
the Sunday
Sunday Quartzite,
Quartzitel and
and this
this carbonate
carbonate environment
environment
then
then transgressed
transgressed over
over the
the quartz
quartz sands
sands as
as the
the sea
sea
transgressed
transgressed northward.
northward.
Stop
Stop 8.
8.
Mafic
Mafic phase.
phase, Emperor
Emperor Volcanics
Volcanics (with
(withcontribution
contribution by
by K.
K.
Licht)
Licht)
Location:
NE'(, NE
NE14•
%, Sec.
Sec. 24.
24, T.47N.,
T.47N., R.43W.
R.43W. on
Location: NE
Forest
Forest Service
Semice Roads
Roads 523
523 and
and then
then 8640
8640 approximately
approximately
6.5
6.5 miles
miles north
north of
of Marenisco,
MareniscotMichigan.
Michigan.
Rocks
Rocks exposed
exposed on
on this
this hill
hill illustrate
illustrate several
several
varieties
varieties of
of the
the mafic
mafic phase
phase of
of the
the Emperor
Emperor Volcanic
Volcanic
Complex.
Complex. The
The Emperor
Emperor Formation
Formation is
is at
at least
least 2,000
21000
meters thick
thick in
in this
this area
area but
but thins
thins rapidly
rapidly to
to the
the west
west
meters
and
10 miles
miles to
to the
the west
west in
in the
the
and is
is not
not recognized
recognized 10
Wakefield
Wakefield area.
area. The
The volcanics
volcanics are
are truncated
truncated on
on the
the
east
east by
by major
major faults.
faults. Therefore,
Thereforel the
the volcanic
volcanic rocks
rocks
form
foxm an
an eastward-thickening
eastward-thickeningwedge
wedge interbedded
interbedded with
with the
the
Ironwood Iron-Formation
Iron-Formationand
and the
the wedge
wedge is
is truncated
truncated by
by
Ironwood
faults
faults as
as shown
shown by
by Trent's
Trent's (1973)
(1973)mapping.
mapping.
The Emperor
Emperor Volcanic
Volcanic Complex
Complex consists
consists of
of aa mafic
mafic
The
and
and felsic
felsic phase.
phase. Dann
D a m (1978)
(1978)showed
showed that
that the
the rocks
rocks
range
range in
in composition
composition from
from basalt
basalt to
to dacite.
dacite. Mapping
Mapping by
by
LaBerge and
and J.
J. S.
S. Kiasner
Klasner in
in 1991
1991 indicates
indicates that
that the
the
LaBerge
mafic phase
phase Emperor
Emperor volcanics
volcanics are
are the
the products
products of
of
mafic
mainly subaqueous
subaqueous eruptions,
eruptionsland
and include
include massive
massive and
and
mainly
pillowed basalts,
basaltsI pillow
pillow breccias,
breccias! hyaloclastites,
hyaloclastitesland
and
pillowed
debris
debris flows.
flows.
Exposures near
near the
the base
base at
at the
the north
north end
end of
of the
the
Exposures
hill are
are pillowed
pillowed basalt
basalt with
with little
little or
or no
no interpillow
interpillow
hill
hyaloclastite.
hyaloclastite. Higher
Higher on
on the
the hill
hill and
and in
in the
the cliffs
cliffs
along
along the
the western
western side
side of
of the
the hill,
hilllsome
some exposures
exposures are
are
mainly hyaloclastite,
hyaloclastitelothers
others are
are hyaloclastite
hyaloclastite
mainly
containing
containing pillows
pillows and
and pillow
pillow fragments,
fragmentsIand
and some
some
exposures are
are mainly
mainly pillows
pillows(Figure
(Figure14).
14).
exposures
Figure 15
15 is
is aa possible
possible interpretation
interpretationof
of the
the
Figure
rocks
shallow
rocks exposed
exposedhere.
here. Lava
Lava fountaining
fountaining in
in aa shallow
28
Pillow breccia with pillow fragments
fragments in
in a
Jackknife is
hyaloclastite
cm long.
long.
hyaloclastite matrix.
matrix. Jackknife
is 88 cm
Figure
Figure 14.
14.
,.
,.t#
-
flSJ#%/%
fS ..F
— 'S1 fl
-
Water Surface
Hyaloclastite formed by
4,
shallow-water fountarning.
ii 41 fr1-1
I_4
4
I
':
4
*
a
A.
u
14
Ilebri S f (iw. ni p1 OW I,?It(. I
I—
end hyalocldst.itC.
Pillows and pillow breccia
i:.
formed by flows into —_...
• or onto hya1ocaSt1te
/
Li
)
4
.4 -.
Li • •fr,.•
01 1ii
Figure
Figure 15.
15.
'
iii t
Sketch showing
showing possible environment
environment of
of formation
formation
8.
of basalts exposed
exposed at
at Stop
Stop 8.
29
\
MARENISCO QUADRANGLE
QUADRANGLE
MICHIGAN—COGEBIC
MICHIGAN-GOGEBlC CO.
CO.
7.5 MINUTE
MINUTE SERIES
S E R I E S(TOPOGRAPHIC)
(TOPOGRAPHIC)
30
NW4
5' QUADRANGLE
N W ( 4 MARENtSOO
MARE&!SCO 15'
QGADRANGLE
subaqueous
subaqueous environment
environment may
may have
have produced
produced aa
Later eruptions
eruptions may
may have
have resulted
resulted
hyaloclastitedome.
dome. Later
hyaloclastite
in
in emplacement
emplacement of
of pillow
pillow lavas
lavas onto
onto (or
(orinto)
into) the
the
hyaloclastite. Additional
Additional hyaloclastite
hyaloclastite may
may have
have
hyaloclastite.
formed
formed on
on the
the surface
surfaceof
of the
the flow.
flow. Pillows
Pillows and
and pillow
pillow
fragments
fragments cascading
cascading off
off the
the flow
flow front
front may
may have
have became
became
mixed
mixed with
withhyaloclastite.
hyaloclastite. Debris
Debris flows
flowsof
of
hyaloclastite
hyaloclastite and
and pillow
pillow breccia
breccia may
may also
also have
have been
been
formed.
formed.
Rocks
Rocks collected
collected from
from this
this stop
stop were
were analyzed
analyzed in
in
Some textures
textures
thin-section
thin-sectionand
and by
by chemical
chemicalanalysis.
analysis. Some
and
and unique
unique findings
findings will
will be
be described
described in
in the
the following
following
paragraphs.
paragraphs.
Many
Many pillows
pillows within
within the
the hyaloclastite
hyaloclastite contain
contain
reaction
reaction rims
rims around
around the
the outer
outer margins
margins and
and along
along quench
quench
cracks. The
The thicknesses
thicknesses of
of these
these rims
rims vary
vary from
from aa few
few
cracks.
millimeters
millimeters to
to aa few
fewcentimeters.
centimeters. The
The reaction
reaction rims
rims
are
uniform highly
highly altered
altered
are characterized
characterized by
by aa fairly
fairly uniform
basaltic
texture
with
some
remnant
tabular
basaltic texture with some remnant tabularplagioclase.
plagioclase.
Vein
within the
the reaction
reaction rims
rims contain
contain
Vein fillings,
fillings) within
chlorjte,
recrystallized
quartz,
and
chlorite) recrystallized quartz) andepidote.
epidote.
. Jasper
Jasper occurs
occurs sporadically
sporadically and
and tends
tends to
to be
be
"molded"
between
the
pillows
and
in
fractures
l1mo1dedl1between the pillows and in fractureswithin
within
Si02 for
for the
the jasper
jasper may
may have
have been
been
pillows. Source
Source of
of Si02
pillows.
gelatinous
gelatinous ooze
ooze on
on the
the seafloor
seafloor at
at the
the time
time of
of
eruption. The
The source
source of
of the
the Fe
Fe was
was probably
probably the
the
eruption.
extruded basalt.
basalt.
extruded
The
The massive
massive basalt
basalt on
on the
the north
north end
end of
of the
the hill
hill
has
Si02content
contentofof62.48's.
62.48%. In
In
has an
an abnormally
abnormallyhigh
highSi02
thin-section,
thin-section)epidote,
epidote) recrystallized
recrystallized quartz,
quartz)chlorite,
chlorite)
and
and calcite
calcite are
are associated
associated with
with the
thealtered
alteredbasalt.
basalt.
The
texture) but
but
The hyaloclastite
hyaloclastite varies
varies greatly
greatly in
in texture,
varies little
little in
in composition.
composition. The
The included
included basalt
basalt
varies
fragments are
are highly
highly altered.
altered. The
fragments
The hyaloclastite
hyaloclastite matrix
matrix
is composed
composed of
of plagioclase,
plagioclasel calcite,
calcite)quartz,
quartz)chlorite,
chlorite)
is
amphiboleland
and epidote.
epidote.
amphibole,
9.
Stow 9.
Stop
Felsic phase.
~hase,
Em~erorVolcanics
Volcanics
Felsic
Emperor
Location: Outcrop
OutcropininSESE1A?4NE
NE1,?4 Sec.
Sec. 20,
20) T47N.,
T47N.) R.44W
R.44W
Location:
523
near Presque
Presque Isle
Isle River
River on
on Forest
Forest Service
Service Road
Road 523
near
approximately 66 miles
miles north
north of
of Marenisco,
MareniscoI Michigan.
approximately
Michigan.
This exposure
exposure contains
contains rocks
rocks representative
representative of
of
This
some varieties
varieties of
of the
the felsic
felsic phase
phase of
of the
the Emperor
Emperor
some
Volcanics. The
The dominant
dominant lithology
lithology at
at this
this locality
locality
Volcanics.
angular) tan,
tan) 22 to
to 10
10 cm,
cml
consists of
of relatively
relatively angular,
consists
monolithic clasts
clasts of
of microcrystalline
microcrystalline felsite
felsite in
in aa
monolithic
31
Whereas the
the small
small
finer hyaloclastite
hyaloclastite matrix.
matrix. Whereas
finer
hyaloclastite
hyaloclastite fragments
fragments typically
typically have
have aa whitish
whitish
alteration
rind! the
the larger
larger felsite
felsite clasts
clasts generally
generally
alteration rind,
lack
lack alteration
alterationrinds.
rinds. Prinz
Prinz and
and others
others (1975)
(1975) stated
stated
that
that the
the felsite
felsite clasts
clasts as
as well
well as
as the
the matrix
matrix have
have aa
In zones
zones of
of deformation
deformation the
the
dacitic
dacitic composition.
composition. In
hyaloclastite
hyaloclastite matrix
matrix tends
tends to
to be
be strongly
strongly foliated,
foliated!
whereas
whereas the
the felsite
felsite clasts
clasts tend
tend to
to show
show little
little
deformation
deformation (Figure
(Figure16).
16). Some
Some clasts
clasts contain
contain abundant
abundant
perlitic
perlitic cracks,
cracks! others
others are
are more
more massive
massive felsite.
felsite. This
This
lithology
lithology of
of the
the felsic
felsic phase
phase of
of the
the Emperor
Emperor extends
extends
westward
westward at
at least
least 55 km,
k m l where
where there
there are
are extensive
extensive
exposures
14 and
and 23
23 north
north of
of Wolf
Wolf Mountain.
Mountain.
exposures in
in Secs.
Secs. 14
In
In addition
addition to
to the
the variety
variety of
of rock
rock exposed
exposed here,
here! Sec.
Sec.
14
14 north
north of
of Wolf
Wolf Mountain
Mountain contains
contains extensive
extensive brecciated
brecciated
felsite
Although
felsite with
with scattered
scattered quartz
quartz phenocrysts.
phenocrysts. Although
the
the rock
rock is
is extensively
extensively brecciated,
brecciated, there
there appears
appears to
to
have
have been
been little
little or
or no
no movement
movement of
of adjacent
adjacent blocks
blocks with
with
respect
respect to
to one
one another.
another. In
In short,
short! the
the rock
rock appears
appears to
to
be
be aa felsic
felsic dome
dome that
that has
has been
been brecciated.
brecciated.
Surrounding
Surrounding the
the brecciated
brecciated felsite
felsite are
are numerous
numerous
exposures
exposures with
with breccia
breccia fragments
fragments of
of felsite
felsite in
in aa felsic
felsic
hyaloclastite matrix
matrix (Figure
(Figure17).
17). It
It is
is suggested
suggested that
that
hyaloclastite
the felsite
felsite breccia
breccia may
may be
be aa felsic
felsic dome
dome emplaced
emplaced into
into
the
felsic
felsic hyaloclastite
hyaloclastite produced
produced by
by an
an earlier
earlier eruption.
eruption.
Debris
Debris flows
flows off
off the
the dome
dome may
may have
have resulted
resulted in
in more
more
widespread
widespread units
units of
of the
the felsite
felsite breccia/hyaloclastite
breccia/hyaloclastite
as shown
shown in
in Figure
Figure 18.
18.
as
32
Figure
Figure 16.
16.
Photo
Photo of
of sawed
sawed slab
slab of
of Emperor
Emperor Volcanics
Volcanics from
from Stop
Stop
9. Larger,
Larger, angular
angular felsite
felsite clasts
clasts in
in aa finer
finer
9.
grained,
is
grained, foliated
foliated hyaloclastite
hyaloclastite matrix.
matrix. Slab
Slab is
18
18 cm
cm long.
long.
Figure
17.
Figure 17.
Photomicrograph
Photomicrograph of hyaloclastite
hyaloclastite from
from
Note
phase
of
tie
Emperor
Volcanics.
phase of the Emperor Volcanics. Note
cracks
cracks in
in the
the oval
oval clast
clast on
on the
the right
right
photo.
mm.
photo. Bar
Bar scale
scale is
is 22 mm.
the
the felsic
felsic
the
perlitic
the perlitic
side
side of
of
33
Water Surface
C a r a p a c e o f hyaloclastite formed
(IuI-~
e aI rIl ~
ier
Figure
Figure18.
18.
fountaininq.
Sketch
Sketch of
of possible
possible relationships
relationships within
within the
the felsic
felsic
Highly fractured
fractured
phase
phase of
of the
the Emperor
EmperorVolcanics.
~olcanics. Highly
felsite
felsite blocks
blocks become
become dispersed
dispersed in
in finer
finer grained
grained
felsic
f aa
felsic hyaloclastite
hyaloclastitein
indebris
debrisflows
flowsofoff
subaqueous
subaqueous lava
lavadome.
dome.
Stop
Stow 10.
10. Archean
Archeanmetacrravwacke
metaqrawacke
Location:
2, about
about 10.5
10.5 mi
mi SE
SE of
of
Location: Roadcuts
Roadcuts on
on Highway
Highway 2,
Marenisco,
'A,
X , Sec.
Sec. 30,
30, T.
T. 46
46 N.,
N., R.
R. 41
41W.
W. (Thayer,
(Thayer,
Marenisco,SWSW
MI
MI 15'
15' quadrangle).
quadrangle)
.
The metagraywacke
metagraywacke in
in these
these roadcuts
roadcutsis
is part
part of
of aa
The
km wide
wide and
and 20
20 km
km long.
long. The
The
northeast-trendingbelt
belt 44 km
northeast-trending
rocks
rocks are
are highly
highly folded
folded on
on tight
tight north-northeastnorth-northeasttrending
km
trending axes,
axes, and
and the
the unit
unit is
is probably
probably about
about 11km
thick.
thick. The
The graywacke
graywacke is
is aa fine-grained,
fine-grained,thinthin-to
to
thick-bedded
thick-beddedrock
rock with
with aa pervasive
pervasive northeast-trending
northeast-trending
schistosity(S2)
(S,) that
that is
is Penokean
Penokean in
in age;
age; an
an older
older
schistosity
(Si)that
that is
is Archean
Archean in
in age
age is
is nearly
nearly
schistosity(Si)
schistosity
parallel
parallel to
tobedding.
bedding. Graded
Graded beds
beds are
are common,
common,and
andtops
tops
at
at this
this outcrop
outcrop are
areto
tothe
thesouth.
south. Reversals
Reversals of
of grading
grading
indicate
apart.
indicate that
that the
the fold
fold crests
crests are
are about
about200
200mm apart.
34
The
The rocks
rocks were
were metamorphosed
metamorphosed to
to amphibolite
amphibolitegrade
grade
during the
the Archean
Archean and
and are
are totally
totally recrystallized;
recrystallized;most
most
during
of
of the
the garnet
garnet was
was retrograded
retrogradedto
to chlorite
chlorite++ muscovite
muscovite ++
epidote and
and opaques
opaques during
during the
the Penokean
Penokeanorogeny
orogeny(Sims
(Sims
epidote
The
nature
of
the
original
sediment
and others,
others,1984).
1984). . The nature of the originalsediment
and
is
is unknown
unknown because
because of
of the
the thorough
thorough recrystallization,
recrystallization,
but
CIA
indices
(chemical
indices
but CIA indices (chemical indicesof
of alteration)
alteration)
calculated
calculated for
for the
the graywackes
graywackes indicate
indicatean
anoriginal
original
immature
detritus
that
had
undergone
little
immature detritus that had undergone little if
if any
any
weathering
in
the
source
area
and
could
have
weathering in the source area and could have been
been
immature
immature volcaniclastics.
volcaniclastics. The
The area
area was
was mapped
mapped by
by
Cannon
Fritts
(1969),
Trent
(1973),
and
Prinz
(1981)
.
Cannon
Fritts (1969), Trent (1973), and Prinz (1981)
of
(1986b)
produced
a
structural
and
tectonic
map
(1986b) produced a structural and tectonic map of the
the
.
region.
region.
Ma)
Stoo11.
11. Watersmeet
Watersmeet Gneiss
Gneiss(3560
(3560Ma)
Stor
Location: Roadcuts
Roadcuts on
on Highway
Highway 45,
45, about
about 44 mi
mi NN of
of the
the
Location:
This junction
junctionis
is on
on the
the
junction
and 45.
45. This
junction of
of Highways
Highways 22 and
Roadcut location
location
edge of
of the
the village
village of
of Watersmeet.
Watersmeet. Roadcut
SS edge
is SSedge,
edge,NENE'A,
1/, Sec.
Sec. 4,
4,T.
T. 46
46 N.,
N., R.
R. 39
39 W.
W.
is
(Watersmeet,
(Watersmeet,MI,
MI, 15'
15' quadrangle).
quadrangle)
.
This
This exposure
exposure is
is located
located in
in the
the northern
northern part
part of
of
the
the Watersmeet
Watersmeet dome
dome which
which has
has an
an Archean
Archean core
coreabout
about25
25
km
km wide
wide in
inaaNNkm long
long in
in an
an E-W
E-Wdirection
direction and
and about
about 88 km
direction. The
The dome
dome is
is considered
considered to
to be
be aa mantled
mantled
SS direction.
gneiss dome.
dome. Sims
Sims (1990)
(1990) recognizes
recognizes considerable
considerable Early
Early
gneiss
Proterozoic
Proterozoic rocks
rocks in
in the
the core
core infolded
infolded with
with the
the
Archean. Deformation
Deformation of
of the
the Archean
Archean and
and adjacent
adjacent Early
Early
Archean.
Proterozoic
Proterozoic rocks
rocks occurred
occurred during
during the
the Penokean
Penokeanorogeny.
orogeny.
The rocks
rocks in
in the
the core
core were
were metamorphosed
metamorphosed to
to the
the
The
amphibolite
amphibolite fades,
facies,aa15
15km-wide
km-widezone
zoneof
of epidote
epidote
amphibolite
amphibolite facies
facies surrounds
surrounds the
the core,
core, and
and this
this in
in turn
turn
passes outward
outward into
into greenschist
greenschistfacies.
facies. (This
(This is
is the
the
passes
Watersmeet metamorphic
metamorphic node
node of
of James,
James,1955).
1955). The
The Early
Early
Watersmeet
Proterozoic
Proterozoic rocks
rocks to
to the
the north
north of
of the
the dome
dome have
have been
been
overturned toward
toward the
the northwest.
northwest. This
This area
area was
was studied
studied
overturned
Peterman and
and others
others
by Sims
Sims and
and others
others(1984)
(1984)and
andPeterxnan
by
(1980).
(1980)
The gray
gray gneiss
gneiss is
is aa tonalitic
tonalitic augen
augen gneiss
gneiss about
about
The
3560 Ma
Ma old
old (zircon
(zircondata)
data) (Sims
(Sims and
others, 1984);
1984); it
it
and others,
3560
is aa medium-gray,
medium-gray,mediummedium- to
to coarse-grained,
coarse-grained,biotitebiotiteis
rich, irregularly
irregularly layered
layered rock
rock with
with plagioclase
plagioclase augen.
augen.
rich,
The alternating
alternating layers
layers are
are feldspar-quartz-rich
feldspar-quartz-richand
and
The
biotite-rich. AA modal
modal analysis
analysis showed
showed 48%
48% plagioclase,
plagioclase,
biotite-rich.
30% quartz,
quartz, 8%
8% K-feldspar,
K-feldspar,and
and 13%
13%biotite.
biotite. AA possible
possible
30%
protolith
protolith was
was dacitic
dacitic volcanic
volcanic rock
rock (Sims
(Simsand
and others,
others,
1984). In
In this
this exposure,
exposure, biotite
biotite leucogranite
leucogranite veins
veins
1984).
Ma old
old (Rb/Sr
(Rb/Srand
and U-Pb
U-Pbmethods)
methods) cut
cut the
the gneiss.
gneiss.
2590 Ma
2590
Secondary whole-rock
whole-rock and
and mineral
mineral isochrons
isochrons give
give ages
ages of
of
Secondary
Ma and
and are
are thought
thought to
to approximate
approximate the
the time
time
1750 to
to 1800
1800 Ma
1750
span of
of the
the Penokean
Penokean event
event in
in this
this area
area (Sims
(Sims and
and
span
35
others, 1984)
others,
1984).
Whereas
the Archean
gneisses of
of the
the area
area may
may have
have
Whereas the
Archean gneisses
been folded
folded three
three times,
times, the
the cross-cutting
cross-cutting Late
Late Archean
Archean
leucogranite shows
shows only a single
leucogranite
single foliation,
foliation, that
that of
of the
the
Penokean
Penokean event.
event.
gneiss is
The gneiss
is comparable
comparable in
in age
age to
to those
those of
of the
the
Minnesota
Minnesota River
River Valley,
Valley, about
about 3500
3500 Ma
Ma old:
old: these
these are
are
parts
parts of
of the
the ancient
ancient gneiss
gneiss terrane
terrane that is
is separated
separated
granite-greenstone terrane
from the granite-greenstone
terrane to
to the
the north
north by
by the
the
GLTZ, the
GLTZ,
the Great
Great Lakes
Lakes Tectonic
Tectonic Zone
Zone of
of Sims
Sims and
and others
others
(1980).
(1980)
36
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Geological
Surveyl vol.
vol. 4, pp.
pp. 405-414.
405-414.
Sims,
Z. E.,
E e lPrinz,
Prinzl W. C.,
C e 1and
and Benedict,
Benedictl F.
F. C.,
Ce1
Simsl P. K.,
K a 1Peterman,
Petermanl Z.
1984, Geology,
Geology, geochemistry and age
Archean and
1984#
age of Archean
and Early
Early
Proterozoic
Proterozoic rocks
rocks in
in the
the Marenisco-Watersmeet
Marenisco-Watersmeetarea,
areal northern
northern
U.S. Geological
Michigan:
Michigan: U.S.
Geological Survey
Survey Prof.
Prof. Paper
Paper 1292-A,
1292-Al41
41 p.
p.
Sims,
Simsl P. K.,
K e 1Van
Van Schmus,
Schmusl W. R., Schulz,
Schulzl K.
K. J.,
J a 1and
and Peterman,
Peterman#Z.
Z.
E.,
Tectonostratigraphic evolution of the Early
E
e l1990,
19901 Tectonostratigraphic
Proterozoic
Penokean
Proterozoic Wisconsin magmatic terranes of the Penokean
Orogen:
Orogen:
Can. Jour.
Jour. Earth
Earth Sci.
Sci.
Southwick, D. L. and MoreyI
Morey, G. B
B.,
imbrication and
a 1 1991,
19911 Tectonic imbrication
Southwickl
foredeep
foredeep development
development in
in the
the Penokean
Penokean orogen,
orogenl east-central
east-central
Minnesota-An
Minnesota-An interpretation
interpretation based on
on regional
regional geophysics
geophysics and
and
results of
U.S. Geological
Geological Survey
results
of test
test drilling:
drilling: U.S.
Survey Bulletin
Bulletin
1904-C,
1904-Cl17
17 p.
p.
Suszekl T.
T. J.,
J a 11991,
1991, Petrography
Petrography and
and sedimentation
sedimentation of
of the
the Middle
Middle
Suszek,
Proterozoic
(Keweenawan) Nonesuch Formation,
Formationl Western
Lake
Proterozoic (Keweenawan)
Western Lake
Superior region,
region, Midcontinent
Midcontinent Rift
Rift System:
System: unpublished
unpublished M.S.
M.S.
Superior
thesis, University
University of
thesisl
of Minnesota,
Minnesotal Duluth,
Duluthl 198
198 p.
p.
e 1 1973,
19731 Geological
Geological Map of
of the
the Marenisco
Marenisco and
and Wakefield
Wakefield
Trentl
Trent, V. A
A.,
NE quadrangles,
quadranglesl Gogebic
Gogebic County,
Countyl Michigan:
Michigan: U.S.
U.S. Geological
Geological
Survey Open
Open File
File Report,
Reportl scale
scale 1:48,000.
1:481000.
Survey
Van
Hisel C.R.
C.R. and
and Leith,
Leithl C.
C. K.,
K a 11911,
19111 Geology
Geology of
of the
the Lake
Lake
Van Hise,
Superior
Superior Region:
Region: U.S.
Geological Survey
Survey Monograph
Monograph 52.
52.
U.S. Geological
40
KEWEENAWAN
SEDIMENTARY ROCK
OF THE
THE
SOUTH SHORE,
LAKE SUPERIOR
A.B. Dickas
Dickas
Department
Department of
of Geology,
Geology,
Wisconsin-Superior
University of Wisconsin-Superior
M.G. Mudrey,
Mudrey, Jr.
Jr.
Wisconsin
Wisconsin Geological and
and
Natural
Natural History
History Survey,
Sumey,
Madison, Wisconsin
INTRODUCTION
The sedimentary
sedimentary and igneous
igneous rock composing
composing the Precambrian
Precambrian (Middle
(Middle Proterozoic,
Roterozoic,
Keweenawan) section of the Lake Superior stratigraphic column
column have
have had
had aa long
long history
history of
of
study. Almost six decades ago (19331,
(1933), the geology of
of the
the "Lake
"Lake Superior Region,"
Region," constituting Excursion C-4, was presented as Guidebook
International Geological
Geological
Guidebook 27 of the XVI International
Congress. The
W. 0.
0. Hotchldss
The field
field guide
guide for
for this
this excursion was prepared by W.
Hotchlciss of
of the
the MichiMichigan College
College of Mining
Mining and
and Technology
Technology (now
(now Michigan
Michigan Technological
TechnologicalUniversity)
University)and
andwas
was
purchased
25 cents.
cents.
purchased for
for the
the sum
sum of
of 25
In his introduction to that guidebook,
guidebook, C. K. kLeith
i t h (University
(University of Wisconsin) stated:
The
The Lake Superior
Superior region
q i o n has been of special
special interest to students of pre-Cambrian
geology
because
it
presents
the
largest
and most varied pm-Cambrian
pre-Cambrian succession
succession that
geology because
has
has been
been definitely
definitely worked
worked out. Its
Itscontent
contentof
ofvaluable
valuableiron
imnand
andcopper
copperores
oreshas
has
made
made possible
possible more intensive
intensive and detailed studies
studies than have been accorded to
extensive
extensive pre-Cambrian
pre-Cambrian areas elsewhere. The
Thepre-Cambrian
pre-Cambriansuccession
successionnow
now known
known
(in the Lake Superior
Superior area)
area) represents a greater
mater thickness of sediments
sedimentsand
and aa larger
larger
time
time than
than all
all the
the post-Cambrian
post-Cambrianof
of North
North America.
America.
It is
k i t h , in a later section of this guidebook, presented the
is interesting
interesting to
to note how Charles
Charles Leith,
Keweenawan
Keweenawan sequence,
sequence, now
now known as
as the
the Midcontinent
Midcontinent Rift
Rift sequence:
sequence:
Keweenawan:
Keweenawan: Next
Next below
below (the
(the Cambrian)
Cambrian)is
is the
the nonfossiliferous
nonfossilifemusKeweenawan
Keweenawan
series,
consisting
of
an
immense
mass,
possibly
5
miles (8 km) thick,
thick, of sandstone,
sandstone,
series, consisting an immense mass,
miles
intercalated shales
shales and conglomerates, containing in its lower part large quantiwith intercalated
ties of
of extrusive
extrusive lavas
lavas and intrusive laccoliths and sills. In
In degree
degree of
of metamorphism
metamorphism
ties
is more
more like
like the
the Cambrian
Cambrian than the underlying Humnian
It has
has characterischaracterisitit is
Huronian series.
series. It
tic reddish,
reddish, yellowish,
yellowish, and
and purplish
purplish colors
colors and
and carries
cames various
various evidences
evidencesthat
that itit was
was
tic
essentially aa continental
continental deposit
deposit under semiarid conditions. Its
Its lower
lowerpart
part isis tilted
tiltedin
in
essentially
marked unconformity
unconformityto
tothe
theCambrian,
Cambrian,but
but its
itsupper
upperpart
part lies
liesnearly,
nearly,ifif not
notquite,
quite,
maiked
in an
an independent
independent
parallel to
to the Cambrian.
Cambrian. Obviously,
Obviously,ititwas
wasmainly
mainlydeposited
depositedin
parallel
basin before the incursion of the Upper Cambrian sea.
Although
the
Keweenawan is
sea. Although
Keweenawan
in the
the sense
senseof
of preceding
preceding the
the Upper
Upper Cambrian
Cambrian transgression,
transgression,having
having
pre-Cambrianin
pre-Cambrian
structural and
and igneous
igneousaffiliations
affiliationswith
with the
the pre-Cambrian,
pre-Cambrian, and
and being
being nonfossffiferous,
nonfossiliferous,
structural
may be
be Cambrian
Cambrianin
in the
the sense
sensethat
thatitit was
was being
being formed
formedatatthe
thesame
sametime
timeas
asMiddle
Middle
itit may
and Lower
Lower Cambrian
Cambriansediments
sedimentsin
in distant
distant Cambrian
Cambrianseas.
seas.
and
Today as
as we
we gather
gather for
for this
this 38th
38th Institute
Institute on
on Lake
Lake Superior
SuperiorGeology
Geologyfieldtrip,
fieldtrip,comparisons
comparisons
Today
1933 form
form distinct contrasts. In
In1992,
1992,with
with aasingular
singularexception,
exception,the
the
with conditions in 1933
copper mines
mines are
are closed
closed and
and the
the massive
massive iron
iron ore
ore fields
fields have
have fallen
fallen under
under different
different economic
economic
copper
Yet,aanew
new era
era of
of evaluation
evaluation and exploration has
climates. Yet,
has begun.
begun. The Lake Superior
Keweenawan section
section is
is widely
widely recognized
recognized as
as one
one of
of the
the fmest
fmest extant
extant examples
examples of
of continental
continental
Keweenawan
rifting. The
Theadvent
adventofofindustrial
industrialfield
fieldevaluation
evaluationof
of the
the hydrocarbon
hydrocarbon potential
potential of
of these
these rift
rift
rifting.
indhctly facilitated
facilitatedthe
thecollection
collectionof
ofapproxiapproxistrata, beginning
beginning in
in 1983,
1983,has
has directly
directly and
and indirectly
strata,
mately 4200
4200 km
km of
of seismic
seismic reflection
reflection data
data in
in the
the Lake Superior
Superior district
dismct alone,
alone, in
in addition
addition to
to
mately
supportive
supportive magnetic
magnetic and
and gravity
gravity surveys.
surveys.
This new
new period
period of
of exploration
exploration within
within the
the Lake
Lake Superior
Superior region has altered
altered our interpretation
interpretation of
This
its
geologic
development,
especially
during
Keweenawan
time.
m
y
of
these
updated
geologic
its geologic development, especially
Keweenawan
Many of these updated geologic
43
43
relationships will be examined during the process of this field mp.
trip. Our
Our principal
principal focus will be
on the geology and geologic history of the youngest Precambrian rock in the region, the
rifting,initiated
initiatedcirca
circa 1100
1100Ma.
Ma. The
The
Keweenawan. This
This rock
rock is
is a remnant of major continental rifting,
resultant Midcontinent
Midcontinent Rift System
System trends
trends from the Lake Superior
Superior region to
to as far
far southwest
southwest as
as
central
central Kansas
Kansas and
and as
as far
far southeast
southeastas
as southern
southernOhio.
Ohio.
As of this writing (March 2, 1992)
has been
been underway
underway for
for several weeks
weeks for
for
1992) site preparation has
the Terra/Patrick
TerraPamck #7-22 wildcat borehole,
borehole, a test
test of the
the hydrocarbon
hydrocarbon potential
potentialof
of the
theWisconWisconsin section of the Midcontinent rift of national significance and interest. Located
47N,
22,47N,
Locatedin
in 22,
this well
well will
will be
be operated
operated by
by Terra
Terra
6W, Keystone Township, Bayfield County, Wisconsin, this
Energy,
Energy, Ltd of Traverse
Traverse City, Michigan, in partnership with Patrick Petroleum Company of
arrangements with Amoco
Ammo Production
Production Company
Company(USA),
(USA),
Jackson, Michigan, under farmout arrangements
Houston, Texas. The
The target
target depth
depth will be 6,000 feet, permitting a test of the oil or gas
large dragdragpotential of the Oronto
Oronto Group
Group sequence
sequence of sedimentary
sedimentary rocks. The
The structure
structureis
is aa large
with north
north closure
closure against
againstthe
theDouglas
Douglasreverse
reversefault.
fault. The
The surface
surface expression
expression of
of
fold anticline with
60 square
square miles. Available
Available information
information
this structure
structure extends over an area of approximately 60
on this state
state record depth wildcat will be included in the 38th Institute on Lake Superior
Geology Proceedings as an abstract
abstract (Albert
(Albert B. Dickas).
The site of the TerraPatrick
Terra/Patrick #7-22 borehole is approximately 6 miles
miles north-northeast
north-northeast of
of an
Oronto
borehole
Amoco M
Production
Oronto Group
Group wildcat test W
h o l e announced in 1985 by Ammo
u c t i o n Company (USA).
This bborehole,
h o l e , planned for 13,
13,46N,7W,
46N,7W, and targeted for 15,000 feet, was never drilled for
economic
economic and
and legislative
legislative reasons.
44
GEOLOGY
GEOLOGYOF
OF THE
THEMIDCONTINENT
MIDCONTINENTRIFT
RIFTSYSTEM
SYSTEMALONG
ALONG THE
THESOUTH
SOUTH
SHORE
MICHIGAN
SHORE OF
OF LAKE
LAKE SUPERIOR,
SUPERIOR, WISCONSIN
WISCONSIN AND MICHIGAN
Reprinted
from Dickas,
Reprinted with
with permission /rom
Dickas, Albert B. (editor),
(editor),1989,
1989, Lake Superior
Superior basin
basinsegment
segment
of the Midcontinent Rift
Rjft System:
System: Field
Field trip
trip guidebook
guidebook T344.28th
T344, 28th International
International Geological
Geological
Congress,
Congress. 62
62 pages.
Introduction
Introduction
The
The Midcontinent
Midcontinent Rift
Rift System
System has been mapped over an axial length of in excess of 3,300
km.
structure are contained
krn.Portions
Portionsofofthis
thisPrecambrian
Precambrian(Ca.
(ca.1.1
1.1 Ga) extensional structure
contained in
in the
the states
states
of
of Kansas,
Kansas, Nebraska,
Nebraska, Iowa, Minnesota, Wisconsin, Michigan, and Ohio. This
Thisreport
report presents
presents
the
geology
of
this
rift
system
within
the
geographic
region
generally
defined
as
northwestthe geology of this rift system within the geographic region generally defined as northwestern
em Wisconsin
Wisconsin and
and the
the adjacent
adjacent upper
upper peninsula
peninsula of
of Michigan.
Michigan.
Wisconsin
Wisconsin Section
Section
The
The Midcontinent
MidcontinentRift
Riftsequence
sequenceof
of Precambrian
Precambriansedimentary
sedimentarystrata
stratain
inthe
thewestern
westernLake
Lake
Superior
Superior region
region constitutes
constitutes some
some of
of the
the oldest
oldest known
known and
and studied
studied such
such assemblages
assemblagesin
in the
the
United
United states.
states. These
Theseunits
unitswere
wereinitially
initiallydescribed
describedby
byHunt
Hunt(1873),
(1873),who
whonamed
namedthem
themthe
the
Keweenaw
Keweenaw Group
Group after
after the cupriferous
cupriferous strata exposed along the Keweenaw Peninsula of
Michigan. Thwaites
Thwaites(1912)
(1912)divided
dividedthis
this section
sectioninto
into the
the Oronto
Oronto (lower)
(lower) and Bayfield (upper)
(upper)
groups (Fig.
(Fig. 1). More
More recently
recently the
the U.
U. S.
S. and
and Ontario
Ontario Geological
Geological surveys
surveys have
have been
been inforinforgroups
mally
termKeweenawan
KeweenawanSupergroup.
Supergroup.
mally using
usingthe
theterm
The outcrop
outcrop belt
belt of
of the
theOronto
OrontoGroup
Groupisis traced
traced from
from northwestern
northwesternWisconsin
Wisconsin east
east and
and northnorthThe
east to
to the
the tip
tip of
of Keweenaw
Keweenaw Point,
Point, Michigan. The
TheOronto
OrontoGroup
Groupcrops
crops out
out principally
principally along
along
east
Ashland syncine
syncline(Fig.
(Fig.2),
2),which
whichisiscentrally
centrally located
located on the St. Croix horst. The
TheAshland
Ashland
the Ashland
the
syncline
is
asymmetrical
as
the
steeper
dips
are
found
within
the
southern
limb.
The
horst is
is
syncline is asymmetrical
steeper
limb. The horst
bounded by
by the
the reverse-throw
reverse-throwDouglas
Douglas and
and Isle
Isle Royale
Royale faults
faults to
to the
the north
north and
and the
the Lake
Lake
bounded
Owen and
and Keweenaw faults to the south. While
Whileseismic
seismicinformation
information suggests
suggests the
the presence
presence
Owen
of Oronto
Orontocomponents
componentsin
inthe
theflanking
flankingbasins,
basins,deep
deepdrilling
drillinghas
has yet
yetto
toconfirm
confirmthis
thisconjecture.
conjecture.
of
(FredaSandstone)
Sandstone)inin
However,on
on the
thebasis
basis of
ofaadrag
dragfold
foldexposure
exposureof
of Oronto
Orontotype
typestrata
strata(Freda
However,
Douglas County,
County,Wisconsin,
Wisconsin,White
White(1966)
(1966)supported
supportedthe
the concept
conceptof
of Oronto
Orontostrata
stratabeing
being
Douglas
found north
north of
of the
the Douglas
Douglas fault. Alternately,
Alternately,White
White (1966)
(1966) supported
supported the possibility of
found
post-Oronto(Bayfield
(BayfieldGroup)
Group)inliers
inliersbeing
beingfound
foundwithin
withinthe
thehorst
horstposition
positionas
assuggested
suggestedby
by
post-Oronto
heavy
heavy mineral
mineralanalyses
analysesconducted
conductedby
by Tyler
Tyleretetal.
al. (1940).
(1940).
The
OrontoGroup
Groupisis 40
40percent
percent sandstone
sandstoneand
and 60
60 percent
percent shale
shale (Thiel,
(Thiel, 1956)
1956) and
and constitutes
constitutes
The Oronto
aa maximum
maximum 6,100
6,100 m
m thick
thick sequence
sequence of reddish conglomerate,
conglomerate, sandstone, and mudstone. ItIt
shows
rela-.
shows aa general
general upward
upwardincrease
increasein
in maturity,
maturity, from
frompoorly
poorly sorted
sortedarkosic
arkosicto
toquartzose,
quartzose,relatively
tively well-sorted
well-sortedsedimentary
sedimentaryrocks.
rocks. Much
Muchofofthis
thisgroup
groupwas
waslocally
locallyderived
derivedthrough
through
erosionof
of Middle
MiddleKeweenawan
Keweenawanbasalts
basaltsand
andtransported
transportedonly
onlyaashort
shortdistance
distancetotoits
itsdeposideposierosion
tional
tionalbasin.
basin.
45
The
Thelowest
lowestunit
unitof
of the
theOronto
OrontoGroup,
Group, the
the Copper
CopperHarbor
HarborConglomerate,
Conglomerate, conformably
conformablyoveroverlies
liesbasement
basementlavas
lavasand
andlocally
locallyinterfingers
interfingerswith
with them
them (White
(White and
and Wright,
Wright, 1960).
1960). While
While
exposures
exposurescan
canregionally
regionallyexceed
exceed1,800
1,800m
min
in thickness,
thickness,as
asmuch
muchasas 610
610 m
m of
of this
this column
column may
may
be
be basic
basic volcanics
volcanics (Halls,
(Halls, 1966).
1966). Overall
Overallthis
thisunit
unitisisaafining-upward
fining-upwardaccumulation
accumulationof
of brown
brown
to
red,arkosic,
arkosic,pebble
pebble to
to boulder
boulder conglomerate,
conglomerate,dominantly
dominantlycomposed
composedof
of mafic
maficto
tofelsic
felsic
tored,
volcanics,
volcanics,with
with lesser
lesseramounts
amountsof
of rather
rather coarse,
coarse, cross-bedded,
cross-bedded, graywacke
graywackesandstone
sandstone(Daniels,
(Daniels,
1982).
Within
the
sands,
stratification
is
crude,
shallow
water
flow
regime
structures
1982). Within the sands, stratification is crude, shallow water flow regime structuresare
are
common
limited. Bedding
Bedding plane
plane dips
dips range from
from
common and
and lateral
lateral correlations
correlations for any distance are limited.
ten
degrees
to
overturned.
The
depositional
regime
of
the
conglomerate
facies
reflects
a
ten degrees to overturned. The depositional regime of the conglomerate facies
a
prograding,
prograding,alluvial
alluvialfan
fan complex
complex (Daniels,
(Daniels,1982),
1982),while the sandstone
sandstone units are
are more
more suggessuggestive
tive of
of shallow
shallow water.
water. AAlocalized
localized"redder"
"redder" arenaceous
arenaceousfacies,
facies,found
found along
along the
the Keweenaw
Keweenaw
Peninsula,
flood plain or
or standing-water
standing-waterdeposit
deposit by White
White and
and Wright
Wright
Peninsula,has
has been
been classified
classifiedas
as aa flood
(1960).
(1960). The
Thesame
sameenvironment
environmentisiselsewhere
elsewherelocally
locallysupported
supportedby
bythe
thelimited
limitedpresence
presenceof
of
algal
algal stromatolites
stromatolitesin
in the
thevicinity
vicinityof
of Copper
CopperHarbor,
Harbor, Michigan,
Michigan,the
thelocation
locationof
ofthe
thetype
type
section
sectionof
of this
thisformation.
formation.
11 —
F'1a.n1
anit
'I' I.
'I'
MuD—CO
z
-p
si o
rst
M I D - C O NTI
T I I'T
NENT
RIIF'
F'T
2
—orst
2
'I'
I.
A.
'I'
I
C
R.
.A.
P
II
Y
T
I
M
E
S
T
R
A
T
I
G
R
A
F
H
Y
3
u
.
r
'
e
nsu
a
3
—
tJ.
P
en
xiiixisu.
1la.
TIM
4 - L peninsula
- —-- i...
--
Iowa
I
Minn
I
1
Wisc
11
Freda.
Ss
1
NoneSuch
Fm
r
7
d
-Val s
, V A
i
"'I-
"
 ¥ 7 v ,.,"'I^^
7
7
A
."
V-01 s
>
< I
>"4
i
V < "
, ^ >tans_>
v
> < - ' < A<
7
Mich
Freda
ss
..
a!
NoneSuch
Tnor > St croik chenkwi.
d
pex1ins.iia.
- rt, a g e, >
l=>p
^
-
2
4
Lake
\ ' A . < > J 7 < L,,7,
VOlS',
* VO-1 s
+
Q
ffl
,
,
I-.
<
4
r
v
L
grap hic terminology
terminology and
and time
time relationships
relationships of Upper
FIGURE
FIGURE 1:
1: Strati
Stratigraphic
Upper Keweenawan
Keweenawan sedimentary
sedimentary
rocks
permission of
(1986), reprinted
reprinted by
by permission
of the
the
rocks along
along the
the Midcontinent
Midcontinent Rft
RiftSystem
System trend.
trend. After Dickas (1986),
American Association
Association of
of Petroleum
Petroleum Geologists.
Geologists.
American
46
In conformable and interfingering
interfingering contact with the Copper Harbor Conglomerate
Conglomerate is the
the
Formation, a hydrocarbon source rock Dickas, 1991)
Nonesuch Formation,
1991) and the center
center of industrial
industrial
interest focused on
on the
the Midcontinent
Midcontinentrift
riftsince
sincethe
theearly
early1980's.
1980's. These strata are gray to
distinguishable from enveloping
grain size
size ranges
ranges
black, easily distinguishable
enveloping red to brown formations. The grain
from medium sand to clay with
with shale subordinate to
to siltstone
siltstone and
and sandstone.
sandstone. This unit is
is the
the
from at least 140 m
m in
in northern
northern Wisconsin
Wisconsin to
to a maxithinnest of the Oronto Group, ranging from
mum 200 m in adjacent Michigan. Framework
Frameworkcomposition
composition emphasizes
emphasizesmafic
mafic rock
rock fragfragments, with analysis showing ranges
ranges of
of 15 to
to 30
30percent
percent (Hite,
(Hite, 1968).
1968). Of economic interest
is the cupriferous
nature
of
the
basal
10
m
of
the
Nonesuch
Formation,
especially in the
cupriferous
10
White Pine, Michigan area where an estimated 11 million tones of
of copper have been found
(Cox et al.,
aL, 1973), predominantly
predominantlyin
inthe
theform
formof
ofchalcocite
chalcociteand
andnative
nativecopper.
copper. The
The variety
variety of
of
sedimentary structures found in this formation have locally been employed
employed as
as supporting
supporting
environments. The best model appears to be that of
evidences for a variety of depositional environments.
reducing conditions
conditions within a standing
standing body of water (Hubbard, 1975), distinguished by
frequent variations
variations in water depths (Daniels, 1982), influenced by fluvial and deltaic associations (Barghoorn
(Barghoom et al., 1965),
1965), and marked by periodic salinity levels exceeding that of
Hieshima and
and Pratt
Pratt (1991)
(1991) offer
offer evidence
evidencesupsupgypsum precipitation. Recently,
Recently,however,
however, Hieshima
porting a marine embayment
embayment environment.
copper is found in direct lithologic
The copper
lithologic association
association with finely
finely disseminated
disseminated carbonaceous
carbonaceous
0.5% of
of the
the formation
formationby
byvolume.
volume. Kelly and Nishioka (1985) believe
matter which averages 0.5%
copper
copper precipitation was directly facilitated by the intimate presence of organic material.
The organic
organic matter, upon chemical extraction, shows remarkable structural features that have
been preserved by a bituminization, rather
rather than
than aa carbonization,
carbonization,process.
process. This material is
found both as matrix masses and as recognizable remains, such as bacteria cells, algal-like
units and fungal hyphae, with volume distribution being concentrated in the finest grained
Overall,individual
individuallaminae
laminaeare
are alternately
alternately rich and poor in organic components.
strata. Overall,
Moore et al. (1969), after thin-section analysis of the organic rich shale layers, concluded
these units arc
are characteristic of lake, swamp and tidal flat environments and that the majority
of preserved
preserved organic
organic material
material developed
developedin
in place.
place.
vugular
also of interest because, at the White Pine mine, associated vugular
The cupriferous zone is also
porosity and fractures often are
arc filled
filled with small amounts
amounts of solid
solid and liquid
liquid hydrocarbons.
hydrocarbons.
Analysis by Barghoom
Barghoorn et al. (1965) indicates this Nonesuch crude oil contains higher concentrations of alkanes
alkanes and
and lower
lower percentages
percentagesof
of aromatic
aromaticss than an average crude
crude oil, and
and
associated with Paleozoic production
production in the
the state
state of
of
resembles the paraffinic crude commonly associated
Pennsylvania. Further
Furtheranalysis
analysishas
has indicated
indicatedthe
the oil
oil is
is indigenous
indigenous to
to its
its host
host rock
rock (Eglinton
(Eglinton
et al.,
aL, 1964), is of organic origin (Moore et
et al.,
al., 1969), and
and was
was formed
formed under
under mild
mild formational temperature conditions (Barghoom
environment is
is also
also
(Barghoorn et
et al.,
a!., 1965). A low thermal environment
supported by Brown (197
I),
who
reported
the
presence
of
pink
bornite
and
djurleite
in
(1971),
reported
presence of pink bornite and djurleite in the
the
Formation, minerals
minerals which
which cannot
cannot exist
exist above
above respective
respective ranges
ranges of
of 75
75 degree
degreeCC
Nonesuch Formation,
and 95 degree C (167
(167 F
F and 203
203 F).
The Nonesuch crude presently represents one of the oldest known
known liquid
liquid hydrocarbon,
hydrocarbon, being
approximately
1,046
million
years
in
age.
This
was
determined
for
the
Nonesuch
host by
approximately
million years in age.
using rubidium-strontium ratios.
This date
date is verified by the work of Ruiz et al.
ratios. This
a!. (1984), who
obtained an age
age of
of 1,047 +I-35
cutting
+1-35million
millionyears
years by
by Rb-Sr
Rb-Sr dating
dating of
of calcite
calcite filled veins cutting
47
acrossthe
the cupriferous
cupriferous shale
shale of
of the
the Nonesuch
Nonesuch Formation in the White
As this
this
across
White Pine
Pine area. As
calcite
contains
primary
liquid
oil
inclusions,
Kelly
and
Nishioka
(1985)
refer
to
this
dateas
as
calcite contains primary liquid oil inclusions, Kelly and Nishioka (1985) refer to this date
the
time
of
oil
entrapment
and
thus
consider
it
as
a
minimum
age
for
White
Pine
hydrocarthe time of oil entrapment and thus consider it as a minimum age for White Pine hydrocarbon.
bon.
The upper
upper unit
unit of
of the
the Oronto
OrontoGroup,
Group,the
the Freda
FredaSandstone,
Sandstone,lies
liesconformably
conformablyover
overthe
theNoneNoneThe
such
Formation.
The
nature
of
the
Freda
Sandstone
upper
contact
is
not
known
for
nowhere
such Formation. The nature of the Freda Sandstone upper contact is not known for nowhere
is this
this formation
formation found
found in
in exposed
exposed (Myers,
(Myers, 1971)
1971)or
or drilled
drilled contact
contact with any
any unit
unit of
of the
the
is
Bayfield
Group
or
its
Michigan
correlative,
the
Jacobsville
Sandstone
(Fig.
1).
While
listed
Bayfield Group or its Michigan correlative,
Jacobsville Sandstone
While listed
by Hite
Hite (1968)
(1968)as
asexceeding
exceeding3,660
3,660m
min
inthickness,
thickness,only
only1,500
1,500 m
m is found
found on
on the
the north
north coast
coast
by
of
the
Keweenaw
Peninsula
(Halls,
1966),
near
the
type
section
at
Freda,
Michigan.
The
of the Keweenaw Peninsula (Halls, 1966),
Michigan. The
FredaSandstone
Sandstoneisiscomposed
composedof
ofbasal,
basal,subordinate
subordinateconglomerate
conglomeratefming-upward
fining-upwardtotosandstone
sandstone
Freda
color,aarepetition
repetitionof
ofthe
theCopper
CopperHarbor
Harborred
redto
tobrown
brownspectrum
spectrumisisobserved,
observed,
andsiltstone.
siltstone. InIncolor,
and
interrupted
by
spotty
and
laminar
unoxidized
zones
(Daniels,
1982).
With
an
analyzed
maturity
interrupted by spotty and laminar unoxidized zones (Daniels, 1982). With an analyzed maturity
indexof
of 0.63,
0.63, aafeldspar
feldsparrange
rangeof
of 10
10to
to24
24percent
percent (Hite,
(Hite,1968),
1968),and
andthe
thegrain
grainshape
shapebeing
being
index
angular
to
sub-rounded,
these
sandstone
units
arc
classed
as
immature.
Daniels
(1982)
considers
angular to sub-rounded,
are
immature. Daniels (1982) considers
theaverage
averagesand
sandunit
unitto
tobe
be aa fme-grained
fine-grained feldspathic
feldspathic lithic
lithic arenite.
arcnite. Sedimentary
Sedimentarystructures
structures
the
include
cross-bedding,
mud-cracks,
graded
beds,
ripple
marks,
and
a
wide
variation
include cross-bedding, mud-cracks, graded beds, ripple marks, and a wide variation inin
paleocurrentindicators.
indicators. The
Theimplied
implieddepositional
depositionalenvironment
environmentranges
rangesfrom
fromfluvial
fluvial(Daniels,
(Daniels,
paleocurrent
1982)
to
fluvial
plain
or
tidal
flat
(Hamblin,
1961).
1982) to fluvial plain or tidal flat (Hamblin, 1961).
Along the
theeastern
easternshore
shoreof
of Lake
Lake Superior,
Superior, in
in the
the Pt.
Pt. Mamainse
Mamainse region
region of
of Ontario,
Ontario, aa 60
60m
m
Along
section
of
gray
sandstone
is
found
unconformably
overlying
Keweenawan
basalts.
Work
by
section of gray sandstone is found unconformably overlying Keweenawan basalts. Work by
Hamblin (1961)
(1961)and
and Annels
Annels (1973)
(1973)allows
allows correlation
correlation of
of these
these strata,
strata, termed
termed the
the Mica
Mica Bay
Bay
Hamblin
sandstone,
to
the
Freda
sandstone
and
suggests
a
fluvial
environment
of
deposition.
sandstone, to the Freda sandstone and suggests a fluvial environment of deposition.
Overallthe
the Oronto
OrontoGroup
Groupcan
can be
be viewed
viewed as
as aa thick
thick sequence
sequence of
of clastic
clastic sedimentary
sedimentary rocks
rocks
Overall
that
are
stratigraphically
unified
by
similar
heavy
mineral
suites,
an
upward
and
distal
dethat are stratigraphically unified by similar heavy mineral suites, an upward and distal dethree formations
formations
creasing in
in grain
grain size,
size, and
and an
an upward
upward increase
increase in
The three
creasing
in lithologic
lithologic maturity.
maturity. The
of
this
group
were
derived
from
a
common
source
of
Keweenawan
igneous
material
(Tyler
of this group were derived from a common source of Keweenawan igneous material (Tyler
et at.,
al., 1940).
1940).While
Whileeach
eachformation
formationisisinterpreted
interpretedin
inlight
lightof
of slightly
slightlydifferent
differentdepositional
depositional
et
environments,
Elmore
and
Daniels
(1980)
view
the
group
environment
as
a
trangressiveenvironments, Elmore and Daniels (1980) view the group environment as a trangressiveregressivealluvial
alluvialfan,
fan,lacustrine
lacustrineand
andfluvial
fluvialsystem
systemthat
that filled
filledthe
the developing
developingMidcontinent
Midcontinent
regressive
Rift
basin
during
the
final
stages,
and
after
cessation,
of
volcanic
activities.
Rift basin during the final stages, and after cessation, of volcanic activities.
TheBayfield
BayfieldGroup,
Group,wherever
whereverstudied,
studied,isisfound
foundto
tobe
be in
in distinct
distinct contrast
contrast to
to the
the Oronto
Oronto
The
Group. The
TheBayfield
Bayfieldsequence
sequencedisplays
displaysaahigher
higherdegree
degreeof
of compositional
compositionalmaturity
maturityand
and
Group.
structurallythe
the bedding
bedding is
is more
more commonly
commonly subhorizontal.
subhorizontal. In
Interms
termsof
ofheavy
heavyminerals
mineralsthe
the
structurally
tourmaline-zircon
to
epidote
ratio
is
reversed
in
the
Bayfield
Group
as
compared
to
the
Freda
tourmaline-zircon to epidote ratio is reversed in the Bayfield Group as compared to the Freda
and Nonesuch
Nonesuch assemblages
assemblages (Tyler
(Tyler et
et a!.,
al., 1940).
1940). There
Thereisis aa question
question as
as to
to the
the type
type of
of contact
contact
and
between
these
groups.
Supporting
an
unconformable
contact
is
the
reported
change
in
between these groups. Supporting an unconformable contact is the reported change in
paleomagneticpole
poleposition
position(Du
(DuBois,
Bois,1962),
1962),the
thechange
changeinin accessory
accessoryminerals
minerals(Tyler
(Tylereteta!.,
al.,
paleomagnetic
1940)
and
the
immediate
change
from
steep
Oronto
dips
to
low
Bayfield
dips
(Morey
and
1940) and the immediate change from steep Oronto dips to low Bayfleld dips (Morey and
Ojakangas,1982).
1982).InInsupport
supportof
ofaaconformable
conformablecontact,
contact,or
orone
one of
of minimal
minimal hiatus,
hiatus, is
is the
the lack
lack
Ojakangas,
Orontoage
age in
in the
the basal
basal Orienta
Orienta sandstone
sandstone (Thwaites,
(Thwaites, 1912),
1912), and
and the
the nature
nature of
of the
the
of clasts
clasts of
of Oronto
of
previously
mentioned
Freda
Sandstone
drag
fold
exposure
in
Douglas
County,
Wisconsin
previously mentioned Freda Sandstone drag fold exposure in Douglas County, Wisconsin
(Tyler et
et at.,
al., 1940).
1940). From
Fromaatectonic
tectonicpoint
pointof
of view,
view, Morey
Morey and
and Ojakangas
Ojakangas (1982)
(1982) state
statethe
the
(Tyler
48
______
04
Isle Royale
J•)
Isle .o3rale1/
TI1iel P'a.u.lt
.Superior
ayfie 1 d
a.sixi
3rx1clir1e
-
MI
Jacosiile a.si
ICe-we eu aw P'a.u. it
I_i. Qre n P'a.tiit
..slilaxid
sfilandSy-ricilne
Sync line
Basalt
a.s a. it
L1 Cla.st].os
-Iiv-er F'ails $yucline l ] Ga.bbro
u
RIFT:
I F T : NORTHERN
N O R T H E R N SECTOR
SECT-?
OKm5O
o Mi 25
OMi 25
FIGURE2:2:Midcontinent
Midcontinent
Rifi
Systemtrend
trendand
andgeology
geologyfrom
fiomnorthwestern
northwesternMinnesota
Minnesotainto
intothe
the
FIGURE
Rift
System
Upper
Michigan. Projection
UpperPeninsula
PeninsulaofofMichigan.
ProjectionofofKeweenawan
Keweenawangeology
geology under
underwestern
westernLake
L d eSuperior
Superwr isis
shown
by hachured
hachuredpatterns.
patterns.From
FromDickas
D i c h(1986),
(19861,reprinted
reprintedby
bypermission
permissionofofthe
theAmerican
American
shownby
Association
AssociationofofPetroleum
PetroleumGeologists.
Geologists.
change
changefrom
fromOronto
OrontototoBayfield
Bayfield sedimentation
sedimentationmarks
marksaatransition
transition from
fkoman
an extensional
extensionalregime
regime
onecharacterized
characterizedby
byvertical
verticaltectonic
tectonicprocesses,
pracessesyi.e.,
i.eaY
theactivation
activationof
of the
theSt.
St.Croix
Croixhorst
horst
totoone
the
by
byuplifting
upliftingof
of4,600
4'600mm(Thiel,
(Thiely1956).
1956).
The
Thetype
typesection
sectionfor
forthis
thisgroup
p u pisisalong
alongthe
thesouthwestern
southwesternshore
shoreof
ofLake
LakeSuperior
Superiorfrom
fromthe
the
Apostle
Islands
to
just
east
of
Superior,
Wisconsin.
Reported
group
thicknesses
Apostle Islands to just east of SuperioryWisconsin. Reported group thicknessesrange
rangefrom
fiomaa
minimal
minimal815
815mmininoutcrop
outcrop(Craddock,
(Craddocky1972),
1972)'totoaamaximum
maximum2,100
2y100mmininthe
thesubsurface
subsurfaceasas
suggested
suggestedby
byseismic
seismicreview
review(Mooney
(Mmneyetetal.,
al.' 1970).
1970).
Because
Becauseofofthe
therelative
relativeuniformity
uniformityinincomposition,
composition,the
thethree
threeBayfield
Bayfieldsandstones
sandstonescan
canbe
bededescribedasasaaunit.
unit.Thiel
Thiel(l956)
consideredthe
theoverall
overallcomposition
compositiontotobebe99%
99%sandstone
sandstoneand
and1%
1%
scribed
(1956) considered
shale.
80%
shale.Peirographic
PetrographicstudIed
studiedbybyMyers
Myers(1971)
(1971)showed
showedthat
thatquartz
q mconstitutes
constitutesapproximately
approximately80%
ofofframework
frameworkgrains.
grains.The
TheOrienta
Orientaand
andChequamegon
Chequamegonsandstones
sandstonesand
andsiltstones
siltstonesare
arecommonly
commonly
red
dinincolor
colorand
andfeldspathic
feldspathicininmineralogy
mineralogyand
andcontain
containlayers
layersof
of shale
shaleand
and conglomerate.
conglomerate. The
The
Devil's
Devil'sIsland
Islandsandstone
sandstoneisisbuff
bufftotowhite
whiteinincolor
colorand
andso
soquartzose
quartzoseas
astotobe
beclassified
classifiedasasanan
almost
almostpure
p morthoquartzite.
o r t h o q d t e .An
Anascending
ascendingfluvial-lacustrine-fluvial
fluvial-lacustrine-fluvialenvironment
environmentofofdeposition
depositionisis
marks'and
andaareturn
return to
tocross-bedding
mss-bedding
documentedby
byrespectively,
respectivelyytrough
troughcross-bedding,
mss-bedding' ripple
ripplemarks,
documented
theChequarnagon
ChequamagonSandstone
Sandstone(Morey
(Moreyand
andOjakangas,
Ojakangas,1982).
1982).Myers
Myers(1971),
(1971)'states
statesthe
the
ininthe
49
Bayfleld
Keweenawan sediments
Bayfield Group
Group sedimentary
sedimentary rocks
rocks were derived mainly from older Keweenawan
sediments and
and
probably represent the reworking of
of Omnto
Oronto Group
Group units.
units. Ojakangas
O j h g a s and Morey (1982),
(19821, however, believe the Bayfield
were derived from an older granitic terrane.
Bayfleld Group clastics we=
2), the basic structure of
of Lake
Lake Superior
Superior is
is aa continuation
continuation of
of
West of the Thiel fault (Fig. 21,
northwestern Wisconsin Midcontinent Rift
Rift geology.
geology. The Ashland syncline is projected
offshore as
as the Lake Superior syncline. Evidence
Evidenceisislent
lentby
by northwest
northwest dips
dips of
of Oronto
OrontoGroup
Group
sedimentary
sedimentary rocks along the Keweenaw Peninsula (Daniels, 1982)
1982) and the southeast dips in
sedimentary rock of the same age which outcrop on the southwest shore of Isle Royale
(Wolff and Huber, 1973). This
This major
major syncline
syncline was
was later
later disrupted
disrupted by reverse
reverse faulting,
faulting,
forming
forming the offshore
offshore segment of the St. Croix horst (Fig. 2). Both
Both the
the Bayfield
Ba*eld basin
basin and
andthe
the
St. Croix
horst
offshore
extension
are
considered
to
be
floored
by
Middle
Keweenawan
floored Middle Keweenawan
Croix horst offshore extension are considered
clastics
clastics of the Oronto
Oronto and/or Bayfield groups (Green, 1982). Thus,
Thus, the
the western
western portion
portion of
of
Lake Superior
Superior is generally
generally portrayed to
to be immediately
immediately underlain by Keweenawan
Keweenawan
Supergroup
thickness (Ilinze
(Hinzeet
etal.,
al., 1982).
1982).
Supergroupclastics
clastics up to
to 10
10km in thickness
Michigan
Michigan Section
Section
Keweenawan-age outcrops
outcrops form the entirety
entirety of the Keweenaw Peninsula jutting into
into southsouthKeweenawan-age
Thekeel
keelof
of this
this peninsula
peninsulais
is formed
formed of
of Portage
Portage Lake
Lake
Superior (Fig. 2). The
central Lake Superior
which all
all Upper
Upper Keweenawan
Keweenawan sedimentary
sedimenmy rocks
rocksin
inthe
thearea
arearest
rest(Fig.
(Fig.1).
1).
volcanics, upon which
Those outcrops
outcropsnorth
north and
and northwest
northwest of
of the
the Portage
Portage Lake
Lake Volcanics
Volcanics form
form the
the classic
classic Oronto
Oronto
Those
Group. South
South and
and southeast
southeastof
of the
the Portage
Portage Lake
Lake volcanic
volcanic outcrop
outcropbelt
belt and
andin
in contact
contactwith
withthe
the
Group.
Keweenaw fault
fault is
is found
found aa thick
thick sequence
sequence of
of northerly
northerly dipping
dipping clastics
clastics named
named the
the
Keweenaw
Jacobsville
JacobsvilleSandstone
Sandstoneby
by Lane
Lane and
and Seaman
Seaman (1907)
(1907) from
from outcrops
outcropsnear
near Jacobsville,
Jacobsville,located
located
near
near the
the southeast
southeast base of the Keweenaw Peninsula. By
By geologic
geologic position these
these outcrops
outcrops
appear to
to be an
an easterly
easterly extension
extension of the River Falls syncline. However,
However, because
because the
the River
River
appear
axtnot
notinindirect
directcontact
contactdue
duetotothe
the
Falls syncline
synclineand
andthe
theJacobsvile
Jacobsvillesandstone
sandstoneoutcrops
outcropsare
Falls
exposureof intervening
interveningArchean
Archeangranitoid
granitoidrocks,
rocks,this
this area
area of
of Jacobsville
Jacobsvillesandstone
sandstonedeposition
depositionisis
exposure
herein
herein referred
referred to
to as
as the
the Jacobsville
Jacobsvillebasin
basin (Fig.
(Fig. 2).
2).
basin the
the Jacobsville
Jacobsvillesandstone
sandstone has
has aa maximum
maximum thickness
thickness of
of 867
867m
m by
by drill
drill hole
hole
In this basin
measurement
measurement (Kalliokoski,
(Kalliokoski, 1982),
19821,but
but on
on the
the Keweenaw
Keweenaw Peninsula
Peninsulaproper
proper its
its maximum
maximum
determined by geophysics to be approximately 3,000 m.
The formation
formationis
is
thickness has been determined
m. The
p~dominatelyquartzose
quartzoseand
andisis composed
composedof
of conglomerates,
conglomerates, sandstones,
sandstones, siltstones,
siltstones,and
and
predominately
shales, generally found
found in repetitive upward-fining sequences. The
The conglomerates
conglomeratesare
are
generally
generally basal,
basal, up
up to
to 100
100m
m in
in thickness
thickness and
and contain
contain clasts
clasts composed
composed of
of iron
iron formation,
formation,vein
vein
quartz, volcanics,
volcanics, and metamorphic rocks. The
Thesandstones
sandstonesare
arefine
fine to
to coarse
coarsegrained,
grained, light
light
quartz,
reddish
reddish to
to purple
purpleto
tomottled
mottledcream-white
cream-whitein
in color
colorand
andcontain
containstructures
structuresranging
rangingfrom
fromcrosscrossbedding to oscillation
oscillation and
and current
current ripple-marking. Shales
Shalesand
andmudstones
mudstones appear
appearto
to be
be in
in the
the
The overall
overall
minority in terms of thickness and are found interbedded with laminated sands. The
section
section varies
varies from
from sub-arkose
sub-arkose to
to aa quartz
quartz arenite,
arenite, with
with plagioclase
plagioclasefound
found more
more commonly
commonlyin
in
the
the younger
younger layers
layers(Kalliokoski,
(Kalliokoski,1982).
1982).
The
The Jacobsville
Jacobsvillesandstone
sandstonewas
was derived
derived from
from highlands
highlands situated
situated to
to the
the southwest
southwestand
and southsoutheast
fluvial,
east of the
the depositional
depositional basin. The
Thedetermined
determinedenvironments
environmentsof
of deposition
deposition include
includefluvial,
50
lacustrine, and alluvial fans. Because
Because the
the Jacobsville sandstone is nowhere found in direct
dkct
contact with either
either the
the Oronto
Oronto or
or Bayfield
Bayfield Group
Group formations,
formations, its
its exact
exactrelationship
relationshipwithin
withinthe
the
Keweenawan Supergroup
Supergroup is in some doubt. As
As itit overlies
overliesPortage
Portage Lake
Lake volcanics
volcanics and
and
underlies the Late Cambrian
Cambrian Munising
Munising sandstone
sandstone there seems
seems little doubt
doubt regarding
regarding its
its
Keweenawan age. Considering
Consideringthat
thatTyler
Tyleret
et al.
al. (1940)
(1940)noted
noted aa strong
strong similarity
similaritybetween
between the
the
Jacobsvile
JacobsvilleSandstone
Sandstoneand
andthe
the Orienta
Orientasandstone
sandstone heavy
heavy mineral suites and that Kalliokoski
Kalliokoski
(1982)
(1982) related the Jacobsville
Jacobsville Sandstone
Sandstone to
to the
the Fond
Fond du Lac
Lac Formation
Formation of Minnesota
Minnesota on
on the
the
basis of sediment,
sediment, mineral,
mineral, and
and clay
clay matrix comparison,
comparison, most
most workers
workers today
today consider
consider the
the
Jacobsville Sandstone
Sandstone to be a Michigan
Michigan equivalent
equivalent to the Bayfield Group
Group of neighboring
neighboring
Wisconsin
Wisconsin (Fig.
(Fig. 1).
1).
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B-i to
53
____
_____
CENTRAL
RWf DEVELOPMENT
CENTRAL NORTH
NORTH AMERICAN CASE FOR SEGMENTED RIFT
DEVELOPMENT
Reprinted with
with permission
permission from Dickas,
Dickas, A. B.
Reprinted
B. and
and M.
M. G.
G. Mudrey,
Mudrey,Jr,
Jr. 1989,
1989, Abstracts,
Abstracts, 28th
InternationalGeological
GeologicalCongress,
Congress,vol.
vol.1,1,p.
p. 1-396
1-396toto1-397.
1-397.
International
The
The Midcontinent
MidcontinentRift
Rift System
System (MRS)
(MRS) is
is aa major Middle Proterozoic
Proterozoic intracontinental,
intracontinental,
thermotectonic
therrnotectonic structure
structure that
that has
has been
been traced by regional
regional gravity
gravity and
and magnetic
magnetic data,
data, subsursubsurface
face drilling,
drilling, and
and outcrop
outcrop control over a length of 3,300 km in the central United States. In
In
the Lake
outcrop is
is known,
known, this
this structure
structurewas
was
Lake Superior
Superior region,
region, the
the only
only area
area in
in which
which MRS outcrop
infilled by plateau lavas and sedimentary rock comprising
Keweenawan Supergroup.
Supergroup.
infihled
comprising the Keweenawan
About 1140
rift. After
1140 Ma, basalt was extruded along the rift.
After cessation
cessation of
of volcanic
volcanic activity,
activity,
generally
elastic, sedimentary rock
rock was
was deposited.
deposited. Geophysical
Geophysical analysis
analysis in
generally maturing upward, clastic,
the 1950's
1950's (Thiel,
(Thiel, 1956)
1956)suggested
suggestedaa broad
broad graben
graben with a central horst separating
separating wedgeshaped
rocks. These
shaped thick accumulations
accumulations of low-density sedimentary rocks.
These sedimentary
sedimentary sections
sections
were identified by negative gravity anomalies. Since
Sincethen,
then, models
models and
and geologic
geologic maps
maps of
of the
the
rift have
been
consistent
in
demonstrating
an
early
developed
symmetrical
extensional
have
consistent demonstrating
developed symmetrical extensional
crustal
crustal basin filled
filled with
with extrusive
extrusiveand
and volcaniclastic
volcaniclastic rock, subsequently
subsequently modified
modified by
by latelatephase
compressional
faulting
forming
the
presently
identified
central
horst
(Fig.
2,
previous
phase compressional faulting forming the presently identified central horst (Fig. 2, previous
paper).
rN
% 14
w
IL
NE
MO
KANSAS
KS
@
Gravity High
High
Gravity
o
ioo
#11
o
OH
I
io
- 1
Gravity Low
Low
Gravity
r///,2 Accommodation
AccommodationStructure
Structure
FIGURE 3:
3: Gravity
FIGURE
Gravityexpression
expressionofofthe
theMidcontinent
Midcontinent
Rift System,
R4ft
System,showing
showingthe
thelocation
location and
and extent offirstofflrstorder rift
riftzones.
zones.
order
54
Recent
Recent acquisition
acquisition of over
over 4200
4200
km of onshore
onshore and
and offshore
offshore
reflection
reflection seismic
seismic data
data and
and analyanalysis
sis of deep
deep mineral
mineral exploration
exploration
drill
drill holes
holes from
from northern
northern WisconWisconsin
sin suggest
suggest aa different
differentgeometry
geometry
for
the
MRS.
This
geometry,
for
This geometry,as
as
proposed here, is similar
similar to
to strucstructures attributed
attributed to
to the Gregory
Gregory rift
rift
in East Africa. Bosworth
al.
Bosworth et
et al.
(1986)
(1986) suggest
suggest the
the Gregory
Gregory rift
rift
structure
structure formed
formed through
through the
the
development of deep, crustal,
crustal,
opposing detachment systems. As
these detachments intersect at
depth,
depth, one
one side
side locks,
locks, resulting
resulting in
in
half-graben development.
development. With
lateral rift propagation,
propagation, aa subresubrelateral
gional
gionalseries
seriesof
oftaphrogeosynclines
taphrogeosynclines
(haif-graben)
(half-graben) are formed,
formed, with
adjacent
adjacent grabens
grabens displaying
displaying
igneous
igneous and sedimentary
sedimentary infill
infill
packages of opposing
geometry.
opposing geometry.
The initial rift is thus unable
unable to
extend
extend laterally
laterally in uniform
uniform strucstructural patterns but rather forms a
linear
linear pattern of individualized,
individualized,en
en
echelon, asymmetric
asymmetric basins, each
each
history and
and geometry
geometry and each
each separated by a type of cross-structure,
cross-structure,
with its own infihling
infilling history
generally referenced as "accommodation" faulting.
faulting.
On a regional
regional basis, the fmal
final structural configuration of a rift may also be dependent upon total
extension developed at right angles to rift propagation. Schuepbach
(1980) suggest
Schuepbachand
andVail
Vail(1980)
suggest that
with such differential
differential rift development, and even though structural extension along rift strike
may be synchronous, basin formation and filling
fifing could
(1984) supcould be diachronous. Quenneil
Quennell(1984)
ported
ported this
this evolution
evolution by
by mapping
mapping the
the Dead
Dead Sea
Sea rift as divided
divided into three
three unique segments,
segments,each
each
operating
operating independently in geologic development. Viewed
Viewed as
as an
an entirety,
entirety, a rift may thus display
first-order,
representing coeval stages
first-order, regional-size
regional-size rift
rift segments
segments(Dickas,
(Dickas,1986),
1986), representing
stages of differing
differingrift
rift
maturity. In
Inturn,
turn,these
thesesegments
segmentsmight
mightbe
be divided
divided into
into subregional-sized,
subregional-sized, second-order
second-order subbasins separated
separatedby
by "accommodation"
"accommodation" faulting.
faulting.
Viewed
Viewed from
from the
the perspective
perspective of this
this geometry,
geometry, the
the extent
extent of the MRS is
is here
here interpreted
interpreted to
to
be composed
(1987). Clockcomposed of
of five
five first-order
first-order rift
rift segments,
segments, termed
termed "zones" by
by Rosendahi
Rosendahl(1987).
Clockwise from
are here named Kansas,
Kansas, Iowa,
from the
the southwestern
southwestern end
end of the
the MRS.
MRS, these zones are
Superior,
identified on the basis of major
major interrupinterrupSuperior, Mackinaw,
Mackinaw, and Maumee (Fig. 3), and are identified
tions
tions in gravity
gravity and
and magnetic
magnetic patterns,
patterns, seismic
seismic review,
review, terrane
terrane compositions,
compositions, and
and borehole
borehole
analysis.
analysis.
function of
of overall
overall rift
rift extension,
extension, zones
zones may
may be
be further
further divided
divided
Locally, and
and developing
developing as
as aa function
Locally,
into
into a series
series of structurally
structurallyindependent
independent basins. These
These second-order
second-order sub-basins,
sub-basins, termed
termed
by Rosendahi
Rosendahl(1987),
Withinthe
theSupeSupe"units" by
(1987), are separated by "accommodation" faulting. Within
rior
rior zone
zone of
of the
the MRS,
MRS, four
four units
units are
are now
now recognized
recognized on
on the
the basis
basis of
of seismic
seismicinterpretation,
interpretation,
interrupted geopotential
geopotential trends, and core analysis. These
These sub-basins
sub-basinsare
are here
here named
named the
the
interrupted
These units appear
appear to
to have
have underChisago, Brule, Ontonagon, and Manitou units
units (Fig.
(Fig. 4).
4). These
similar history of structural
structural development
development during the early phase of MRS evolution.
evolution.
gone a similar
On
On the
the basis
basis of differential
differential listric
listric movements
movements along
along axial-oriented,
axial-oriented,rift
rift fault
fault systems,
systems,igneous
igneous
and
and sedimentation
sedimentationwedges
wedges of alternating
alternating isopach
isopach patterns
patterns distinguish
distinguish juxtaposed zones.
patterns of
of these
these wedge
wedge geometries
geometriesdiffer
differ by
by being
being symmetrical
symmetricalparallel,
parallel, but
but alteralterIsopach patterns
nating asymmetrical
asymmetrical perpendicular,
perpendicular, to
to the
the MRS
MRS axis.
axis.
nating
In light
light of this suggested
suggested model, future
future stratigraphic
stratigraphic and
and seismic
seismic correlations
correlations can no longer
longer be
be
the framework
frameworkof
of aa symmetric
symmetric rift
rift model
model existing
existing along
along the
the entirety
entiretyof
of the
the
interpreted within the
bedependent
dependentupon
upon consideration
considerationof
of isolated
isolated igneous
igneous
MRS axis.
axis. Instead,
Instead,such
suchcorrelations
correlationsmust
mustbe
MRS
and sedimentary
sedimentary packages
packages and isopach geometries created in response
response to
to first- and
and second-order
second-order
and
be directed
directed to
to basebasedegree of tectonic development.
development. Exploration
Explorationphilosophies,
philosophies, whether
whether they
they be
degree
metal
or
hydrocarbon
evaluation,
must
be
constrained
so
as
to
recognize
related
regional
and
metal or hydrocarbon evaluation, must be constrained so as to recognize related regional and
subregional
subregional alterations
alterationsin
in structural
structuralstyle.
style.
55
Central Structure
(St. Croix Horst)
SCHEMATIC SECTION
FIGURE
FIGURE4:
4:Structural
Structuralrift
riftunits
unitscomposing
composingthe
theSuperior
Superiorrift
riftzone,
zone,Midcontinent
MidcontinentRift
Rift System,
System, as
defined by accomodation
accomodation structures
structures(A.S.)
(AS.) and isopach
isopachthickening
thickening trend,
trend,
References
References
1986, A new look at Gregory's Rift:
Rift: the
the strucstrucBosworth, W., J. Lambiase, and R. Keisler, 1986,
tural style
582-583.
style of continental
continental rifting: Eos, v. 67, p. 557,
557,582-583.
Dickas, A.B.,
1986, Seismologic analysis of arrested stage development
A.B., 1986,
development of the Midcontinent
Rift, in M.G. Mudrey, Jr. (ed), Precambrian petroleum potential, Wisconsin and Michigan: Geoscience Wisconsin, Wisconsin Geological and Natural History Survey, v. 11, p.
45-52.
Quennell, A.M.,
1984, The western Arabia rift-system, in J.F.
A.H.F. Robertson
Robertson
A.M., 1984,
J.F. Dixon and A.H.F.
Society Special
Special
(eds.), The geological
geological evolution of the eastern Mediterranean: Geological
Geological Society
Publication 17,
Publication
17, p. 775-788.
Rosendahl, B.R.,11987,
B.R.,11987, Architecture of continental
special reference
reference to
to East
EastAfrica:
Africa:
Rosendahi,
continental rifts with special
Annual Review Earth
Earth and
and Planetary
Planetary Science
ScienceLetters,
Letters,v.
v. 15,
15, p. 445-503.
1980, Evolution of outer highs on divergent
divergent continental
continental
Schuepbach, M.A.
M.A. and P.R.
P.R. Vail, 1980,
margins, in Continental
Continental tectonics:
tectonics: National Academy of Science,
Science, 197
197 p.
p.
56
1956, Correlation of gravity
gravity anomalies
anomalies with the Keweenawan geology
geology of
of WisconWisconThiel, E., 1956,
sin and Minnesota: Geological Society of America Bulletin, v. 67, p. 1079-1100.
1079-1100.
LAKE
FRONTIER
LAKE SUPERIOR
SUPERIORBASIN
BASIN AS
AS A HYDROCARBON FRONTIER
Mod(fied
from aa series of requested articles written by Albert
Albert Dickas
Dickasfor
for the daily print
print media
Modified from
of
January, 1992, during site preparations
preparations for the
the Terra/Patrick
TerrdPatrick#7-22,
#7-22,
of northwest Wisconsin,January,
Bayjield County,
County,Wisconsin
Wisconsin exploratory
exploratory wildcat.
wildcat.
Since
Since the initial discovery of crude oil at a depth of 21.2 m beneath the rolling hills of
Titusville,
Titusville,Pennsylvania
Pennsylvania in
in 1859,
1859,hundreds
hundreds of
of thousands
thousands of
of boreholes
boreholeshave
havesought
soughtthis
this
natural resource in the United States. Wisconsin
Wisconsin has played a small role in this 133
133 year old
"hydrocarbon revolution." Between
49 boreholes
Between 1865
1865and
and 1992,
1992,49
boreholes have sought economic
volumes of oil
oil or
or gas
gas in the
the Badger state. Not
Notone
oneof
ofthese
thesewells
wellshowever,
however,has
hasfound
foundanyanything
thing other
other than
than fresh
fresh and
and salt
salt water.
water.
The
The majority
majority of
of exploration
exploration in
in Wisconsin
Wisconsin has
has taken
taken place
place close
close to
to the
the Lake
LakeMichigan
Michiganshoreshoreline
(Fig. 5).
5). Here
line between Milwaukee and Door County (Fig.
Here the rocks form the western edge of
the Michigan
Michigan basin,
basin, one
one of
of the
the most
most prolific
prolific producers
producers of
of hydrocarbon
hydrocarbonin
in the
theUnited
UnitedStates.
States.
Unfortunately,
Unfortunately, this
this productivity
productivity is
is basically confined to the deeper
deeper portions
portions of the basin, and
and
not
not along
alongits
its edges.
edges. The
Theremaining
remainingarea
areaof
ofWisconsin
Wisconsinhas
haslong
longbeen
beenconsidered
considerednon-producnon-productive
because
of
the
great
geologic
age
and
physical
nature
of
much
of
its
bedrock.
tive because of the great geologic age and physical nature of much of its bedrock.
In
In the
the early
early 1980s,
1980s,pessimism
pessimism turned
turned to
to guarded
guarded optimism.
optimism. Petroleum
Petroleumgeologists
geologistsbegan
begantoto
recognize
recognize that
that rocks
rocks deposited
deposited during
during early
early chapters
chapters of earth
earth history--the so
so termed
termed PrecamPrecambrian
brian Eon--possesses,
Eon--possesses, contrary
contraryto
to conventional
conventionalwisdom,
wisdom,those
thoseseven
sevento
toeight
eightcharacteristics
characteristics
necessary
necessary for
for the
the development
development of
of an
an oil
oil or
or gas field.
field. Studies
Studies suggested
suggested these characteristics
characteristics
were often
often contained
containedwithin
within rift
rift structures,
structures,formed
formed by the
the dissection
dissection of pre-existent
pre-existent conticontias Africa
Africa is
is presently
presently being
being torn
tom into
into portions along
along the
the African
African Rift
Rift Zone.
Zone.
nents, much as
has been
been known
known for
for 35
35 years
years that
that northwestern
northwestern Wisconsin
Wisconsin occupies
occupies aa portion of
of the
the worldworldItIt has
krn from
from Kansas
Kansas to
to Ohio
Ohio by way
way of Lake
Lake
class Midcontinent
Midcontinent Rift
Rift System,
System, extending
extending3300
3300 km
class
Superior. With
Withthe
therecognized
recognizedassociation
associationof
of ancient
ancientrift
rift rocks
rocks with
with economic
economichydrocarbon
hydrocarbon
Superior.
reserves, that
that portion
portion of
of the
theMidcontinent
Midcontinentrift
rift lying
lyingbetween
between the
the upper
upperpeninsula
peninsulaof
of Michigan
Michigan
reserves,
and Kansas
Kansas was
was quietly
quietly invaded
invaded by the United States oil and gas industry. Initial
Initialinterests
interests
and
included Mobil,
Mobil, Texaco,
Texaco, Standard
Standard of California, and Amoco, among others. Field
Fieldcrews
crews
included
1980's and
andspecial
special "frontier
"frontier region"
region" analysis
analysisteams
teams of
of geologeolobegan operating
operating in the early
early 1980's
began
gists, geophysicists,
geophysicists, paleontologists,
paleontologists, and geochemists were formed. Studies
Studiesranged
ranged from
from
gists,
hand investigation
investigationof
of rock
rock samples
samplesto
to sophisticated
sophisticated computer
computer reviews
reviews of
of newly
newly collected
collecteddata
data
hand
pertaining
pertainingto
tothe
theseismic,
seismic,magnetic
magneticand
andgravity
gravitycharacter
characterof
ofthe
therift.
rift.
In 1984
1984Texaco
Texacomade
madethe
thefirst
firstmove
moveby
bydrilling,
drilling,ininnortheast
northeastKansas,
Kansas,their
their#1
#1Noel
Noel Poersch
Poersch
In
well
3444 m.
m. InIncertain
certainaspects
aspectsthis
thiswas
wasaaguinea-pig
guinea-pigborehole.
borehole.
well to
to aa record
record state
statedepth
depth of
of 3444
Because
Because the
the rift
rifthad
had never
never been
been drilled
drilledbeyond
beyond normal
normal water-well
water-welldepths
depthsof
ofseveral
severalhundreds
hundreds
of
of feet,
feet, industry
industry was
was probing
probing into
into the geologic darkness. Expectations
Expectationswere
were based
based upon
upon
comparison
comparisonof
of Kansas
Kansasdata
datato
tothe
thenear-surface
near-surfacegeology
geologyof
ofDouglas
Douglasand
andBayfield
BayfieldCounties,
Counties,
Wisconsin, aa region
region where rift rocks are exposed at the surface and
The
Wisconsin,
and thus
thus easy
easy to
to study.
study. The
Texacodrilling
drilling plan
plan envisioned
envisionedseveral
several thousands
thousands of feet
feet of sandstone
sandstone and organic
organic shale
shale
Texaco
overlyingan
an even
even thicker
thicker sequence
sequence of layered
layered lava. Under
Underideal
idealconditions
conditionseither
eitheroil
oil or
or gas
gas
overlying
would
have
formed
within
the
organic
shales
and
subsequently
moved
upward
into
the
would have formed within the organic shales and subsequently moved upward into the
sandstones
sandstones until
until trapped
trapped and
andconcentrated
concentratedinto
into an
an economic
economic "pool."
"pool."
57
Drilling
results were
were not
not released
released until
until mid-1988.
mid-1988. This
Drilling ceased in early 1985
1985 but detailed results
three year delay made sense when information became
becamepublic.
public. The drill sequence
sequence appeared
reversed when compared to expectations, with lava and related igneous rock overlying
overlying thick
thick
sandstone layers. Shale
Shalewas
was found
found only
only in trace
trace amounts
amounts and that shale present was not
not possible.
possible. The
organic. Without
Without organic
organic shale
shale an oil or gas field is not
The Texaco
Texaco well was
classed as "dry and
and abandoned"
abandoned" and
and exploration
exploration geologists
geologists began
began to
to reinterpret
reinterpretthe
the geologic
geologic
history of the Midcontinent
Midcontinent rift.
rift.
Amoco
Amoco Production
Production Company
Company was
was the
the second
second major
major player
player to
to enter
enter the
thesearch
searchfor
forPrecamPrecambrian-age rift hydrocarbon. Disenchanted
Disenchantedby
by the
the Kansas
Kansas results,
results, Amoco
Amoco geologists
geologistschose
chose
west-central Iowa as their preferred drilling site. In
1987their
their#1
#1Eischeid
EischeidboreboreIn March
March of
of 1987
hole was set into operation and seven months later reached
5,441 m.
m.
reached a state
state depth
depth record
record of
of 5,441
This
This borehole was shrouded in secrecy, protected by fence and guard. Under
Underfavorable
favorable
weather
weather conditions
conditions a nearby hill was covered
covered with curious
curious observers
observers and
and industry
industry scouts
scouts and
and
skimpy reports were issued daily
daiiy by the media.
media. Even
Even after
after the
the projected
projected depth
depth was
was reached
reached
Amoco,
Amoco, following
following the lead established by Texaco in Kansas, maintained secrecy. With
With the
the
release
1990 encouragement was given by the fact a normal
release of information
information in the spring
spring of 1990
sequence of rock colunm
column was drilled--shales were found
found and they were organic.
organic.
The
The high
high risks
risks associated
associatedwith
with the
the oil
oil and
and gas
gas business
business is
is largely
largely attributed
attributed to
to the
the fact
fact that
that while
while
the
the proper
proper conditions
conditions for
for an
an economic
economicreservoir
reservoir are
are often
often found,
found,the
the sought
soughtafter
afterproduct
productisisoften
often
absent. The
Iowa
venture
was
high
risk
in
planning
and
execution
and
the
end
result
was
The Iowa venture was high risk planning and execution and the end result was
labeled
labeled "dry and
and abandoned."
The
The third
third borehole
borehole to
to test the Midcontinent rift was initiated in late 1987 and completed two
months later. The
Theoperator
operatorwas
wasagain
again Amoco
Amoco Production
Production Company,
Company, but now the geographic
geographic site
months
finaltotal
totaldepth
depthwas
was
near the upper peninsula community
community of Munising, Michigan. The
Thefmal
was near
2210 m. While
Whilecomplete
completeinformation
informationhas
has not
not yet
yet been released, it has been reported that all
2210
formations composing
composing the
the target Oronto
Oronto Group
Group were drilled--but not in the
the normal
three of the formations
order
order as
as known
known from
fromWisconsin
Wisconsingeology.
geology.
It is
is interesting
interesting to
to note
note this
this borehole
borehole is
is located
located in
in aa region
region of
of historic,
historic, little
little known,
known, exploraexploraIt
In1865
1865the
theOntonagon
OntonagonPetroleum
PetroleumCompany,
Company, financed
financed by
by aa capital
capital stock
stock
tion for crude oil. In
$500,000, was
was formed
formed for
for the
the purpose
purpose of "engaging in
in and
and carrying
carryingon
on the
thebusibusioffering of $500,000,
offering
mining~petroleum."By
ByDecember
Decemberof
of that
that year
yearthe
the first
first and
and only
only venture
venture of
of this
this
ness of mining---petroleum."
company was
was underway,
underway, fueled
fueledby
by aa rapid
rapid increase
increase in
in the
the price
price of
of oil
oil stock
stockand
and an
an outbreak
outbreak
company
While microfilm
microfilm records indicate "two
Thefinal
finaldrilling
drilling depth
depth was
was 38 m. While
of "oil fever." The
(of oil)
oil) have
have been
been hoisted
hoisted up,"
up," there
there isis no
no record
record of
of the
the Ontonagon
OntonagonPetroleum
PetroleumComCombarrels (of
pany
pany beyond
beyond the
the year
year of
of its
its incorporation.
incorporation.
Since
Since 1929
1929 active
active underground
underground oil
oil seeps
seeps have
have been
been reported
reported from
from the
the White
White Pine
Pine mine
minein
in
Ontonagon County,
County, Michigan. Analyses
Analysesshows
showsthis
thiscrude
crudeto
tobe
be very
very similar
similarchemically
chemicallyto
to
Ontonagon
high grade
grade oil of Phanerozoic
Phanerozoic age produced in the state of Pennsylvania. Radiometric
Radiometricageagedating techniques
techniques suggest
suggestthe
the White
White Pine
Pine product
product may
may be
be the
the oldest
oldest known
known crude
crudeoil
oilin
inthe
the
dating
Whilethese
thesevolumes
volumesare
aremodest,
modest,their
theirpresence
presencekeeps
keeps alive
alive the
the possibility
possibility of
of "black
"black
world. While
gold"
gold" production
productionin
in northern
northern Wisconsin.
Wisconsin.
58
As
As of
of early
early March,
March, 1992,
1992,site
sitepreparation
preparation was
was underway
underway for
for the
the fourth
fourthtest
testof
ofthe
thehydrocarhydrocarbon potential
potential of
of this
this rift.
rift. Located
Located in
in Bayfield
Bayfield County,
County, Wisconsin,
Wisconsin, this
this wildcat
wildcat has
has been
been
programmed
programmed as
as aa 1825
1825m
m (6000
(6000ft.)
ft.) test
test of
of the
the Oronto
Oronto Group
Group in its position
position overlying
overlying the
the
central St. Croix horst. Available information on this TerraPatrick
Terra/Patrick #7-22 borehole will be
presented
presented in
in the
the Proceedings
Proceedings volume
volume of this 38th meeting of the Institute on Lake Superior
Superior
Geology.
Geology.
59
60
Wisconsin.
in degrees 75 and areas, River Isle Presque and Pine White the in grees
de- 10 about to area Mountain Porcupine the in overturned from varies tude
atti- Structural
rn/km. 4 than more at basinward thickens Nonesuch the Park,
State Isle Presque of east km 40 about area Pine White the In
southwest. the
area.
to thins generally and thickness, in m 180 about averages Nonesuch The
Pine White the in petroliferous is and material organic in rich is rock The
dikes. clastic as such features dewatering and bedding graded show many and
microscale, in common are alternations grain—size rhythmic Thin, thick. mm 1
than less laminae many with laminated evenly to massive—appearing from ranges
bedding The color. in darkest the is rock grained finest The sandstone.
medium to shale black to gray medium a is Nonesuch the Generally, quences.
se- bed red overlying or underlying the than maturity compositional and tural
tex- greater of generally and color in drab more is Nonesuch The
Description:
1968 Hite, from (modified
1982) Daniels, and
Jr. Daniels, P.A. and Jr. Mudrey, M.G.
Authors:
1956). quadrangle,
topographic 15—minute (Thomason Michigan County, Gogebic W., 45 R.
N., 50 T. 30, and 19 Sec. River, Isle Presque the of Mouth Location:
Formation Nonesuch — Park State Isle Presque Title:
Both
The
The Nonesuch
Nonesuch possesses
possesses aa great
great variety
variety of
of sedimentary
sedimentary features.
features. Both
Laminated
stratifiLaminated
stratifihorizontal
stratification
and
cross
stratification
occur.
horizontal stratification and cross stratification occur.
irregular, and
and ripple
ripple stratifistratification
lenticular, wavy or irregular,
cation is
is the
the most
most conmion;
common; lenticular,
60 cm
cm in
in thickness,
thickness, and
and the
the thicker
thicker
cation also
also occur.
occur. Units
rarely exceed
exceed 60
Units rarely
cation
beds are
are coarser
coarsersandstone
sandstoneand
and conglomerate.
conglomerate. Alternating
Alternatinglaminae
laminae of
of fine
fine
beds
common.
gray sandstone
sandstoneand
and black
black shaley
shaley ailtstone
siltstoneless
less then
then 0.5
0.5 cm
can thick
thickare
are
common.
gray
The cross—stratification
cross-stratification is
is of
of two
two types,
types, planar
planar cross
cross stratification
stratification and
and rib
rib
The
and furrow.
furrow. Both
Both of
of these
these types
types are
are abundant
abundant on
on the
thePresque
Presque Isle
IsleRiver
River in
in
and
Michigan. Shrinkage
Shrinkage cracks
cracks and
and disturbed
disturbed bedding
bedding occur
occur locally
locallyand
and sometimes
sometimes
Michigan.
abundantly. Sediment
Sediment and
and current
current transport
transport data
data support
support aa dominant
dominant flow
flow
abundantly.
in the
regime to
tothe
the
west-southwest during
during deposition
deposition of
of the
theNonesuch
Nonesuch in
thePresque
Presque
regime
west—southwest
Islearea.
area.
Isle
Discussion: The
The sedimentary
sedimentary structures
indicate that
that the
the depositional
depositional envienvistructures indicate
Discussion:
ronment
ronment of
of the
the Nonesuch
Nonesuch Formation
Formation was that of
of a standing
standing body of
of water, with
with
was at
at least
least high
high
perhaps significant
significant variation
variation in
in water
waterdepth.
depth. Salinity
Salinity was
perhaps
enough to
to precipitate
precipitate
gypsum.
gypsum.
enough
initial
-
The initial formation
formation of
of this
this water
water body
body could
could have
have occurred
occurred either
either due
due
The
to
creating a closed topographic
to subsidence
subsidence along
along the
the rift, creating
topographic low
low that would
as local
local base level, or perhaps, more likely, the
the disruption
disruption of
of rethen act
act as
then
gionaldrainage
drainagepattern
pattern
sometype
type
damming.
gional
bybysome
ofof
damming.
61
Title: Parker
Parker Creek
Creek —- Oronto
Oronto Group
Group
IR 47/O1E/30
47/01E/30
Location:
Location: Approximately 300
300 a
m upstream to
to the
the southeast
southeast from
from the
the northwest
northwest
corner
Sec. 30,
30, T.
T. 47
47 N.,
N., H.
R. 1 F.,
E., Iron
Iron
corner of
of section
section 30.
30. NWl/4,
NW1/4, Sec.
County, Wisconsin (Oronto
(Oronto Bay
Bay 77 1/2—minute
1/2-minute topographic
topographic quadrangle,
quadrangle,
1980).
Author:
Author:
M.G. Mudrey,
Mudrey, Jr.
Jr. (modified
(modified from
from Myers,
Myers, 1971
1971 and
and Rosenberry,
Rosenberry, 1924)
1924)
M.G.
Description:
Description: Approximately
Approximately 600
600 am of
of continuous
continuous exposure
exposure from
from the
the last
last Kewee—
Keweenawan
nawan lava
lava flow,
flow, through
through the
the Copper
Copper Harbor
Harbor and
and Nonesuch
Nonesuch Formations
Formations into
into the
the
lower
of the
the Freda
Freda area
area exposed
exposed along
along Parker
Parker Creek
Creek (also
(also known
known as
as Davis
Davis
lower 300
300 in
m of
Creek).
Creek). Because
Because the
the upper
upper reaches
reaches of
of the
the creek
creek tend
tend to
to run
run parallel
parallel to
to the
the
bedding,
bedding, the
the Copper
Copper Harbor
Harbor appears
appears to
to be
be much
much thicker
thicker than
than normal.
normal.
All
E., 85
85degrees
degreesNW.
NW. There
There are
are aa
All of
of the
the units
units trend
trend N.
N. 60
60 degrees
degrees F.,
few,
very minor
minor flexures
flexures and
and faults
faults in
in the
the wall
wall of
of the
the valley,
valley, but
but the
the sec—
secfew, very
tion
tion appears
appears to
to be
be continuous
continuous with
with no
no repetition
repetition by
by faulting.
faulting.
Approximately
Approximately 200
200 mm of
of Copper
Copper Harbor
Harbor Formation
Formation are
are exposed
exposed in
in the
the upper
upper
reaches
reaches of
of the
the creek.
creek. The
The unit appears
appears to
to consist
consist of
of five
five main
main conglomeritic
conglomeritic
zones
60 am in thickness,
thickness, interbedded
zones about
about 30
30 to
to 60
interbedded with coarse,
coarse, sandy
sandy
62
conglomerate. Both
Both lithologies
lithologies are
are clast—supported
clast-supported conglomerate.
conglomerate. The
The Copper
Copper
conglomerate.
Harbor is
is relatively
relativelytypical,
typical, consisting
consistingof
of rounded
rounded cobbles
cobblesof
of Keweenawan
Keweenawan volvolHarbor
canic
canic rock
rock and
and granite
graniteand
and iron
iron formation
formationfrom
from the
the underlying
underlyingEarly
Early Protero—
Proterozoicand
and Archean
Archeansuccession.
succession.
zoic
About310
310inm of
of Nonesuch
Nonesuch Formation
Formationis
isexposed.
exposed. The
The Nonesuch
Nonesuch consists
consists of
of
About
The lower
lower
several dark
dark colored
colored slate
slate units,
units, interbedded
interbedded with
with sandy
sandyslate.
slate. The
several
contact
contact of
of the
the Nonesuch
Nonesuch and
and the
the Copper
Copper Harbor
Harbor is
is gradational
gradationalin
inthat
thatthe
the
nonconglomerate
asthe
theupper
upper
nonconglomerateinterbeds
interbeds become
become finer
finer grained
grained and
and more
more abundant
abundantas
Bedding is
is clearly
clearly defined
defined in
in the
the
contact with
with the
theNonesuch
Nonesuch isisapproached.
approached. Bedding
contact
Nonesuch,
Nonesuch, and
and local
local cross
cross bedding
bedding and
and deformation
deformation structures
structures can
can be
be recogrecognized. The
The upper
upper contact
contact of
of the
the Nonesuch
Nonesuch with
with the
the Freda
Freda is
is also
also gradational
gradational
nized.
in that
thatsand
sand
beds
become
more
commonand
and slate
slatebeds
beds less
lessso.
so.
beds
become
more
common
in
of the
the Freda
Freda Formation
Formation is
is aa thick
The lower
lower member
member of
thickbedded,
bedded,fine—
fine- to
to
The
Local
pebble
Local
pebble
is
strongly
stained
with
iron.
mediwgrained
sandstone
that
is
strongly
stained
with
iron.
medium—grained sandstone that
The middle
middle Member
Member of
of the
the Freda
Freda is
is seen
seenonly
onlyat
at the
the
zones are
are common.
common. The
zones
It
is
much
better
exposed
in
Spoon
Creek
It
is
much
better
exposed
in
Spoon
Creek
creek.
northernmost
outcrop
in
the
creek.
northernmost outcrop in the
and
very
fine—grained
sandsiltstone
This
member
consists
of
siltstone
and
very
fine-grained
sandto
the
west.
This
member
consists
of
to the west.
stone with
with occasional
occasional shale
shale and
and cross—stratified
cross-stratified fine—grained
fine-grained sandstone
sandstone
stone
to
compared
beds.
Bedding
is
more
pronounced
in
the
middle
member
Bedding
is
more
pronounced
in
the
middle
member
compared
to the
the lower
lower
beds.
member. The
The upper
upper member
member of
of the
the Freda
Freda is
is best
best seen
seen at
at the
the mouth
mouth of
of the
the
member.
The
Montreal
Montreal River
River to
tothe
theeast,
east, particularly
particularlyat
at Superior
SuperiorFalls
FallsininMichigan.
Michigan. The
upper member
member consists
consists of
of thick
thick bedding
bedding units
units of
ofmicaceous
micaceous siltstone,
siltstone, very
very
upper
fine—grained,
fine-grained, laminated
laminated sandstone
sandstone with
with abundant
abundant scours
scours and
and and
and current
current direcdirection
tion indicators,
indicators, and
and slightly
slightly conglomeritic,
conglomeritic, fine—grained
fine-grained sandstone
sandstone with
with bedbedding poorly
poorly developed.
developed. The
The White
White River
River location
location is
is probably
probably in
in the
the upper
upper memmemding
ber.
ber.
Discussion: Clearly,
Clearly, the
the Oronto
Oronto Group
Group represents
represents aa single
single depositional
depositional epiepiDiscussion:
sode, the
the individual
individual formations
formations representing
representing various
various environments
environments within
within that
that
sode,
episode. All
All of
of the
the units
units appear
appear to
to be
be gradational.
gradational. The
The Freda
Freda appears
appears to
to
episode.
thicken from
from its
its type
type locality
locality near
near Freda,
Freda, Michigan,
thicken
Michigan, to
to the
the Montreal
Montreal River
River
and west.
west. The
The Nonesuch,
Nonesuch, conversely,
conversely, appears
appears to
to thin
thin toward
toward the
the west
west from
from
and
Presque Isle
Isle State
StatePark.
Park. At
At Copper
Copper Falls
Falls State
State Park
Park the
the dominant
dominant shale
shale unit
unit
Presque
the shale-bearing
shale-bearing interval
interval is
thick; however,
however, the
appears to
to be
be less
lessthan
than2020inm thick;
is
appears
thick.
about140
140inm thick.
about
All along
along the
the Gogebic
Gogebic Range,
Range, the
the Early
Early and
and Middle
Middle Proterozoic
Proterozoicunits
units dip
dip
All
northwest at
at aa steep
steepangle.
angle. Reconstruction
Reconstruction of
of the
the thickness
thicknessof
of the
thesection
section
northwest
results in
in an
an estimate
estimate that
that is
is too
too thick,
thick, 20
20 km
km of
of section
section from
from Hurley
Hurley to
to
results
Lake Superior.
Superior. In
Keweenaw and
and
In Michigan
Michigan this
iseasy
easyto
tounderstand
understand as
as the
the Keweenaw
this is
Lake
associated faults
faults clearly
clearly repeat
repeat parts
parts of
of the
the section;
section; however,
however, in
in Wisconsin
Wisconsin
associated
because of
of poor
poor exposure,
exposure, the
the faults
faults are
are not
not readily
readilyrecognized.
recognized. As
As will
will be
be
because
seen and
and discussed
discussed at
at White
White River,
River, there
there is
is at
at least
least one
one major
major anticline
anticline
seen
known in
in outcrop
outcrop at
at Marble
Marble Point,
Point, and
and at
at least
syncline with
with the
the
least one
one major
major syncline
known
Freda. AA more
more reasonable
reasonable estimate
estimate for
for the
the total
total thickness
thickness of
of the
the Oronto
Oronto
Freda.
km.
Group would
would be
be 55 km.
Group
63
NE 25
0
Distance
NE25
D i s t a n c e ffrom
rom N
E 2 5 tto
o El/4
E 11425
25
== 1/2
112 mile
mile
ml conglomerate
conglomerate
Lo.QI
sandy
s a n d y conglomerate
conglomerate
sandstone
•:.:1...sandstone
sandstone
s a n d s t o n e and
a n d shale
shale
-.A.
shale
slate
conglomerate
ale
glomerate
;hale
late
on glomerate
andy
conglomerate
ate
0
E '/425
Sketch of
o f geology
geology in
i n Parker
Parker Creek
Creek from
from Rosenberry
Rosenberry (1924).
(1924).
Sketch
64
70
::.'.)
Silistone ona
shole,grOy to
block, alter-
noting and
laminated to
40
very thin bedoccasional
•35
medium- to
coorse-grained
sandstone units
3 to 6" thick
o few frne, thin-
bedded sand(UNIT 4) 65 ---tones
Unit 2
:=2-:- SttsIns.
—--=.
ded (1/4— I");
Ss, some Os
-- lTt.oi
alternating and
::_l
laminated, with
tUN IT 7)
90
alternating, some
Unit I.
irregular
irregular bosal
basal
contocis,
contocfs, generolly
generally
(UNIT 3)
.;)5ac.irflobIW.
61
— sm.— to c.- grain—
sole-morks
Sandstone,
Sandstone, blactc
block
medium—
to coarsemedium- to
coorsegroined.
groined. some
some
Ss. blk., vTto c.—groined
thin to thick
85
thick—bedded
thick-bedded
(UNIT
( U N I T 8)
8)
bedded
(UNIT 6
(UNIT I)
30
Stratigraphic
Stratigraphic section
section showing
showing 100
100 feet of
of typical
typical Nonesuch
Nonesuch Formation
Formation (figure
(figure
20 of
of Rite,
Hite, 1968).
1968).
20
Stratigraphic
Stratigraphic section
section showing
showing 100
100 feet
feet of
of typical
typical Lower
Lower Freda
Freda Formation
Formation
(figure
(figure 30
30 of
of Rite,
Hite, 1968).
1968).
65
Stratigraphic
section showing
Stratigraphic section
showing 100
100 feet of typical
typical Middle Freda
Freda Formation
Formation
(figure
33 of
of Hite,
Hite, 1968).
1968).
(figure 33
Siltstone, red
micoceous and
Siltstone, red
rnicoceous,lOm-
laminated, 1terbedded with
irioted with
sholes ond
cross-bedded
siltstones, inter—
21
70
C\
stratified
(UNIT 3)
I
Sandstone, red
very fine— to tine
groined, 10mm—
65
oted,w,th
40
U.
:.—•
...
—.....-
.:
bedding which
becomes more
abundant upword
in the section
distorted bedding
rn central
60
part of the
unit.
6iltstone, red
micaceous,
irregular lam—
motions with
micro (ripple)
cross—stab
(UN IT 4)
in upper part.
interbedded St
red, fine-grained
(UNIT 2)
_30
-:;.-:
micro cross-
—
Sandstone, red
fmne—groined,
:
55
Sandstone,
same as
95
Unit4,but
sandstone, white to
green, finegroined
cross—bedded
(UN I T 8)
larger scale
Cross—strata.
—U.LNLL6)-.
90
Shale and
fine silistone,
brick red,
micaceous,
laminated. 85
interbedded
coorse—grouned
cross—bedded
sandstone ocair
6 feet below
the top of the
unit
(UNIT 5) 80
Sandstone red
fine-groined,
bedding poorly
developed
shale pebbles
common in
upper part
(UNIT 7)
laminated,
rib and furrow
StruClurC
(UNIT I)
Stratigraphic
section
feet
ofFreda
typical
Upper Freda Formation
Stratigraphic
section
showing showing
100 feet of 100
typical
Upper
Formation
(figure
42
of
Hite,
1968).
(figure 42 of Hite, 1968).
66
- Oronto
Oronto Group
Group
Title:
Title: Copper
Copper Falls
Falls State
State Park
Park —
AS
AS 45/02E/17B
45/02E/17B
Location:
Northwest of
Mellen on
Location: Northwest.
of Mellen
on Highway
Highway 169.
169. SE1/4,
SE1/4, Sec.
Sec. 17,
17, T.
T. 45
45 N.,
N.,
B.
R. 22 E.,
E., Ashland
Ashland County,
County, Wisconsin. (Mellen
(Mellen and
and High
High Bridge
Bridge
77 1/2—minute
1/2-minute topographic
topographic quadrangles,
quadrangles, 1967
1967 and
and 1984,
1984,respectively).
respectively).
Author:
M.G. Mudrey,
Mudrey, Jr.
Jr.
Author: M.G.
Copper Falls
Falls is
is formed
formed where
where the
the Bad
Bad River
River cuts
cuts through
through the
the rereDescription:
Description: Copper
sistant ridge
ridge of
of Keweenawan
Keweenawan lava
lavaflows.
flows. Downstream,
Downstream, 0.6
0.6 km
km from
from Copper
Copper Falls
Falls
sistant
at the
north, the
the contact between basalt to the south and rhyolite to the north,
Tyler
Tyler Forks
Forks River
River joins
joins the
the Bad
Bad River
River at
at Brownstone
Brownstone Falls,
Falls, which
which occurs
occurs at
at
the
the contact
contact between
between basalt
basalt to
to the
the south
south and
and rhyolite
rhyolite to
to the
the north.
north. The
The conconglomerate of
of the
the Copper Harbor Formation is exposed in the lower
glomerate
gorge at
lower gorge
Devils Gate
Gate about
about 200
200 mm downstream
downstream from
from Brownstone
Brownstone Falls.
Falls. The
Devils
The conglomerate
conglomerate
is
Montreal River
is 129
129 m thick;
thick; at the
the Montreal
River the
the conglomerate
conglomerate is
is 515
515 mm thick.
thick. NorthNorthwest
Nonesuch Formation;
west of
of the
the conglomerate
conglomerate is
is the
the black shale
shale of
of the
the Nonesuch
Formation; northnorthwest
west of
of the
the Nonesuch
Nonesuch is
is the
the sandstone
sandstone of
of the
the Freda
Freda Formation.
Formation. There
There is
is black
black
shale distributed
distributedthrough
through141
141inm of
of section,
section, but
but only
only2020inm are
are considered
considered
shale
Nonesuch Formation.
Formation. Above the
the interval
interval with black shale,
the sediment
sediment is
is red
red
Nonesuch
shale, the
shaley
shaky arkose.
arkose.
67
The name Copper
Copper Falls comes
comes from a
a small
small copper
copper prospmt
prospect in a small
small raravine
working was
south of
of the
the concession
concession building.
building. The working
was begun by
vine about 200 m south
the Ashland
Mining Company
CompanyininAugust
August1864
1864and
andclosed
closed in
in February 1866.
Copthe
Ashland Mining
1866. Cop-
per in
quartzvein
veininindiabase
diabasewas
was prospected.
prospected.
per
in aa quartz
reopened
at the
the turn
turn of
reopened at
of the
the century.
century.
Apparently
the property
Apparently the
property was
was
Discussion:
Discussion: Although faults
faults complicate
complicate the
the local
local geology, the
the essentially
essentially
conformable relationship between the Oronto Group
Group and Keweenawan
conformable
Keweenawanflows
flows can
can be
seen.
lowerparts
parts of
seen. The
the
Oronto Group,
Group, the
the Copper Harbor and the
The lower
of the
theOronto
Nonesuch,
Nonesuch, are significantly
significantly thinner
thinner here
here than
than at
at localities
localities to
to the
the east.
east.
68
Title:
Title:
South
South Fish
F i s h Creek
Creek
Exposures
Exposures in
i n banks
banks of
of South
South Fish
F i s h Creek beneath bridge
b r i d g e on
on north—south
north-south
secondary
m i l e s south
south of
of U.
U. S.
S. Highway
Highway 22 on
on the
t h e east
e a s t line
l i n e of
of the
t h e SE-,
SE*,
secondary road
road 1.2
1.2 miles
SE,
S E ~NE*,
, NE*, Sec.
Sec. 20,
20, T.47N.,
T.47N., R.5W.,
R.5W., Bayfield
Bayfield County
County (Moquah
(Moquah 7.5
7.5 minute
minute topographic
topographic
quadrangle, 1964).
1964).
Location:
Location:
Author:
Author:
M. E.
E. Ostrom
Ostrom (modified
(modified from
from Myers, 1971)
1971)
M.
Exposures of
of steeply—dipping
steeply-dipping Freda
Freda Sandstone
Sandstone exhibit
e x h i b i t the
t h e lithologic
lithologic
Description: Exposures
and mineralogic character
c h a r a c t e r of
of the
t h e formation.
formation. A description
d e s c r i p t i o n of
of the
t h e strata
s t r a t a downdownand
is:
stream from
from the
t h e bridge
b r i d g e is:
stream
PRECAMBRIAN
SYSTEM
PRECAMBR
IAN SYSTEM
Keweenawan
Keweenawan Series
Series
Oronto Group
Group
Oronto
Freda Sandstone
Sandstone Formation
Formation (11.0
(11.0 feet)
feet)
11.0'
11.0'
Sandstone, grayish
g r a y i s h red
r e d to
t o reddish
r e d d i s h brown,
brown, uniformly fine—grained,
fine-grained,
Sandstone,
hard,
h a r d , cross—bedded
cross-bedded with
w i t h parting
p a r t i n g lineation.
l i n e a t i o n . Much
Much leaching.
leaching.
deformation. Current ripple
r i p p l e marks found in
in
Penecontenporaneous deformation.
float.
float.
69
BASE OF EXPOSURE
Significance:
Significance: Provides
Provides evidence
evidence of
of environmental,
environmental, geologic
geologic and
and structural
structural
history. Examine
Examine lithology
lithology and
and mineralogy.
mineralogy. What
What do
do they
they signify?
signify? What
What direcdirechistory.
tion
tion is
is the
the top
top of
of the
the beds?
beds? Measure
Measure dip
dip and
and strike
strike of
of beds.
beds. What do
do these
these
mean
mean in
in terms
terms of
of structural
structural history?
What
What is
is the
the origin
origin of
of the
the red
red color?
color?
References:
References:
70
From
From what
what direction
direction did
did the
the sand
sand come?
come?
Thwaites,
Thwaites, 1912;
1912; Myers,
Myers, 1971.
1971.
Chequamegon Formation
Formation
Title:
U
l e : Washburn
Washburn Harbor
Harbor --- Chequamegon
BA 48104W/33
48/04W/33
Location: Outcrop on northside of
J,ocatton~
of Washburn Harbor,
Harbor! SW1/4,
SW1/4, SEll
SEl/
Sec.
33,
T48N,
R4W,
Bayfield
County,
Wisconsin
(Washburn
4
!
33, T48Nr R4Wr Bayfield Countyf Wisconsin (Washburn
4,
7.5—minute quadrangle,
7.5-minute
quadrangle! topographic,
topographic! 1975).
1975).
thor: M.G. Nudrey,
Mudrey, Jr.
Jr. (1992)
(1992)
Author:
Description:
Description:
This low outcrop of Chequamegon
sandstone of
of the
the
Chequamegon sandstone
Bayfield Group
Group consists
consists of
of thick,
thick, massively
massively bedded,
bedded! fine
fine —- to
Bayfield
to
medium-grained! grayish—red,
grayish-red! quartzose
quartzose sandstone.
sandstone. The
The grains
grains
medium-grained,
are subangular
subangular to
to subrounded.
subrounded. The bedding surfaces
surfaces are
are gritty
gritty
to coarse
coarse grained,
grained, but
but are
are thin
thin and
and discontinuous.
discontinuous. Some
Some
bedding planes
display scattered
scattered small
small quartz
quartz pebbles.
pebbles. Here!
planes display
Here,
bedding dips gently to the southeast;
southeast; however, at Big Rock
Wayside Park
Park at
at the
the east
east quarter
quarter corner
corner of
of Section
Section 24,
24! T48N,
T48Nf
R5W, crossbedded
R5Wf
crossbedded sandstone
sandstone dips
dips 15
15 degress
degress to
to the
the northwest,
northwest!
suggesting that
that small,
small! low-amplitude
low-amplitude folds
folds with
with aa
suggesting
northeasterly
northeasterly trend
trend may
may occur
occur in
in the
the region.
region.
Discussion:
The Chequamegon
Qiscussion~
Chequamegon sandstone
sandstone is
is similar
similar to
to the
the
underlying Orienta sandstone,
sandstone! which is found in fault contact
with
with Keweenawan
Keweenawan volcanics
volcanics and
and Oronto
Oronto group
group strata.
strata. Bayfield
Bayfield
Group strata
strata are
are not
not known
known to
to have high
high dips,
dipsI and
and on
on this
this
Group
basis have
have been
been distinguished
distinguished from
from the
the upper
upper parts
parts of
of the
the
basis
underlying
underlying Freda
Freda sandstone.
sandstone. The
The Douglas
Douglas Fault
Fault intervenes
intervenes
between
between this
this locality
locality and
and South
South Fish
Fish Creek
Creek where
where the
the Oronto
Oronto
Group
A petroleum
petroleum
Group dips
dips south
south at
at aa high
high angle
angle to
to the
the south.
south. A
test
test well
well has
has been
been drilled
drilled between
between these
these two
two locations
locations
immediately
immediately adjacent
adjacent to
to the
the Douglas
Douglas Fault.
Fault. At
At the
the time
time of
of
this
this writing,
writingI no
no further
further information
informationwas
was available.
available.
72
REFERENCES
General geology,
in Wisconsin
Wisconsin Geological
Aldrich,
Aldrich, H.R.,
H.R., 1925,
1925,VI.
VI. General
geology, &
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and
History Survey
Natural History
Survey Township
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T. 45
45 N.,
N., R.
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W. (1924),
(19241,p.
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36—69.
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Daniels, P.A.,
P.A., Jr.,
Jr., 1982,
1982, Upper Precambrian
Precambrian sedimentary rocks:
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the Lake
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Memoir 156,
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Dickas,
Comparative Precambrian
Precambrian stratigraphy
Dickasy A.B., 1986,
1986, Comparative
stratigraphy and
and structure
structure along
along
the
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the Mid-continent
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Bulletin, v. 70,
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no, 3,
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227.
Bulletin,
1982, The sandstone architecture
architecture of the
Eckert, K.B., 1982,
the Lake
Lake Superior
Superior region:
region:
Michigan
Michigan State
State University,
University, Lansing,
Lansing, unpublished
unpublished Ph.D.
Ph.D. dissertation,
dissertation,
504
p.
504 p.
Green,
Keweenawan plateau
plateau volcanism
Green, J.C., 1977,
1977, Keweenawan
volcanism in
in the
the Lake
Lake Superior
Superior region,
region,
Bara,dr, W.R.A.,
W.R.A., ed., Volcanic regimes
in Bara&dr,
regimes in Canada: Geological
Geological
Association
.16, p
p.
407—422.
Association of
of Canada
Canada Special
Special Paper
Paper.16,
. 407422.
Hainblin,W.K.,
W.K., 1965,
1965, Basement
Basement control
control of
of Keweenawan
Keweenawan and
and Cambrian
Cambrian
Hamblin,
Association of
of
sedimentation
American Association
sedimentation in
in the
the Lake
Lake Superior
Superior region:
region: American
Petroleum
Geologists Bulletin,
49,p.
p. 950—959.
950-959.
v.,49,
Petroleum Geologists
Bulletin, v.
Hatch,
J.R., and Morey,
Morey, G.B.,
G.B., 1985,
1985, Hydrocarbon
Hydrocarbon source
source rock
rock evaluation
evaluation of
of
Hatchy J.R.,
Proterozoic Solor Church
Church Formation,
North American Mid-Continent
Mid—Continent
Middle Proterozoic
Formation, North
System, Rice County,
County, Minnesota:
Minnesota: American Association
Association of Petroleum
Rift System,
Geologists Bulletin,
v. 69,
69, no.
no. 8,
8, p.
p. 1208—1216.
1208-1216.
Geologists
Bulletin, v.
Hite,
D.M., 1968,
1968, Sedimentology of the Upper Keweenawan
Keweenawan sequence
sequence of northern
Hite, D.M.,
University of
Wisconsin, Madison,
Wisconsin and
Wisconsin
and adjacent
adjacent Michigan:
Michigan: University
of Wisconsin,
Madison,
unpublished Ph.D. thesis,
p.
thesis, 217
217 p.
unpublished
Hubbard,
H. A., lW5,
1975, Keweenawan
Keweenawan geology
geology of
of the
the North
North Ironwood,
Ironwood, Ironwood
Ironwood and
and
Hubbard, H.A.,
Little Girls
Girls Point
Point quadrangles,
Little
quadrangles, Gogebic
Gogebic County,
County, Michigan:
Michigan: U.S.
Geological Survey
Geological
Survey Open
Open File
File Report
Report OF
OF 75—152,
75-152, 23
23 p.
p.
or copper—bearing
system, $J
in Geology
Irving, R.D.,
1880, The
The Keweenawan
Keweenawan or
Geology of
of
Irving,
R.D., 1880,
copper-bearing system,
Wisconsin,
p. p185,
205—206.
Wisconsin, v.
v. III.,
III.,
. 185,
205-206.
frontier oil
oil province:
Kerr, S.D.,
Lee, C.K.,
C.K., and
and Kerr,
S.D., 1984,
1984, Midcontinent—a
Midcontinent-a frontier
province: Oil
Oil
Journal, August
August 13,
and Gas
Gas Journal,
13,1984,
1984,p.p.144—150.
144-150.
Superior regions
region, U.S.
A., and
and its
its
Morey, G.
B., 1978,
in the
G.B.,
1978,Metamorphism
Metamorphism in
the Lake
Lake Superior
U.S.A.,
crustal evolution,
evolution, in Fraser,
J.A., and
edseS
relation to crustal
Fraser, J.A.,
and Heywood,
Heywood, W.W.,
W.W., eds.,
relation
Metamorphism
Metamorphism in
in the
the Canadian
Canadian Shield:
Shield: Geological
Geological Survey
Survey of
of Canada
Canada Paper
Paper
78-10,
p. 283—314.
283-314.
78—10, P.
.,
,
Morey,
and 0
Ojakangas,
R.W.W.,
1982,Keweenawan
Keweenawan sedhentary
sedimentary rocks
rocks of
Morey, G. I3B., and
jakangas R.
, 1982,
eastern
Minnesota and
eastern Minnesota
and northwestern
northwestern Wisconsin,
Wisconsin, in
$J Wold,
Wold, R.J.,
R.JeSand
and Hinze,
Hinzes
W.F.,
W.F., eds., Geology
Geology and
and tectonics
tectonics of
of the
the Lake
Lake Superior
Superior Basin:
Basin: Geological
Geological
Society of
135-146.
Society
of America
America Memoir
Memoir 156,
156, p.
p. 135—146.
73
Mudrey,
Mudrey, M.G.,
M.G., Jr.,
Jr., 1979,
1979, Geologic
Geologic summary
summary of
of the
theAshland
Ashland 2°
20 quadrangle:
quadrangle:
Wisconsin
WOFR
Wisconsin Geological
Geological and
and Natural
Natural History
History Survey
Survey Open—file
Open-file Report
Report WOFR
79—1,
79-1, 39
39 p.
p.
Myers,
Myers, W.D.,
W.D., II,
11, 1971,
1971, The
The sedimentology
sedimentology and
and tectonic
tectonic significance
significance of
of the
the
Bayfield
Bayfield Group
Group (Upper
(Upper Keweenawan?),
Keweenawan?), Wisconsin
Wisconsin and
and Minnesota:
Minnesota: University
University
of
of Wisconsin,
Wisconsin, Madison,
Madison, unpublished
unpublished Ph.D.
Ph-D.thesis,
thesis,269
269p.
p.
Ostrom,
Ostrom, M.E., 1967,
1967, Paleozoic
Paleozoic stratigraphic
stratigraphicnomenclature
nomenclatureof
ofWisconsin:
Wisconsin:
Wisconsin
Wisconsin Geological
Geological and
and Natural
Natural History
History Survey
Survey Information
Information Circular
Circular 8,
8,
chart
chart and
and text.
text.
Ostrom,
ME., and
Ostrom, M.E.,
andSlaughter,
Slaughter,A.E.,
A.E., 1967,
1967,Correlation
Correlation problems
problems of
of the
the Cambrian
Cambrian
and
and OrdOvician
Ordovician outcrop
outcrop areas
areas of
of the
the Northern
Northern Peninsular
Peninsular fsicJ
[sic] of
of
Michigan:
Michigan: Annual
Annual Field
Field Excursion,
Excursion, Michigan
Michigan Basin
Basin Geological
Geological Society,
Society,
p. 1—5.
1-5.
p.
Paull,
Paull, R.K.,
R.K., and
and Paull,
Paull, R.A.,
R.A., 1980,
1980, Field
Field Guide
Guide to
to Wisconsin
Wisconsin and
and Upper
Upper
Michigan:
Kendall/Hunt Publishing
Publishing Co.,
Co., Dubuque,
Dubuque,Iowa,
Iowa,260
260p.
p.
Michigan: Kendall/Hunt
Rosenberry,
Rosenberry, S.C.,
S.C., 1924,
1924,VI.
VI. General
General geology,
geology,in:
&: Wisconsin
Wisconsin Geological
Geological and
and
Natural
Natural History
History Survey
Survey Township
Township Report
Report T.
T. 47
47N.,
N., R.
R. 11 E.
E. (1924),
(1924),
p. 17—28.
17-28.
p.
Thwaites,
Thwaites, F.T.,
F.T., 1912,
1912, Sandstones
Sandstones of
of the
the Wisconsin
Wisconsin coast
coast of
of Lake
Lake Superior:
Superior:
Wisconsin
p.
Wisconsin Geological
Geological and
and Natural
Natural History
History Survey
Survey Bulletin
Bulletin 25,
25, 117
117 p.
Weiblen,
Weiblen, P.W.,
P.W., and
and Morey,
Morey, G.B.,
G.B., 1980,
1980, A summary
summary of
of the
the stratigraphy,
stratigraphy,
petrology and
and structure
structure of
of the
the Duluth
Duluth Complex:
Complex: American
petrology
American Journal
Journal of
of
Science,
Science, v.
v. 280—A,
280-A, pt.
pt. 1,
1, p.
p. 88—133.
88-133.
Wold, R.J.,
R.J., and
and Hinze,
Hinze, W.F.,
W.F., eds.,
eds., 1982,
1982, Geology
Geology and
and teconics
teconics of
of the
the Lake
Lake
Wold,
Superior Basin:
Basin: Geological
Geological Society
Society of
of America
America Memoir
Memoir 156,
156, 280
280 p.
p.
Superior
74
GEOLOGY
OF
GEOLOGY
OF
KEWEENAWAN
KEWEENAWAN
SUPERGROUP
ROCKS
SUPERGROUP
ROCKS
NEAR THE
NEAR
PORCUPINE MOUNTAINS,
PORCUPINE
MOUNTAINS,
GOGEBIC
ONTONAGON &
& GOGEBIC
ONTONAGON
COUNTIES,
MICHIGAN
COUNTIES,
MICHIGAN
William F. Cannon,
Cannon,
William
Suzanne W. Nicholson,
Nicholson,
Suzanne
Cheryl A. Hedgman,
Laurel
Laurel G.
G.Woodruff,
Woodruff,
and Klaus J. Schulz
U.S..Geological
Geological Survey,
U.S
Survey,
Reston,
Reston, Virginia
5
5
5
LiL trip stops.
1r:-1 Figure 1.
CONTOUR INTERVAL 50 FEET
0
0
10
ii
Statute MiIe
j Kilometers
________
WAN SUPERGROUP
SUPERGROUPROCKS
ROCKSNEAR
NEAR THE
THE PORCUPINE
PORCUPINE MOUNTAINS,
MOUNTAINS,
GEOLOGY OF KEWEENA WAN
ONTONA GONAND
AND GOGEBIC
GOGEBIC COUNTIES.
COUNTIES, MICHIGAN
MICHIGAN
ONTONAGON
William F. Cannon,
Suzanne W.
W. Nicholson,
Nicholson, Cheryl
Cheryl A.
A. Hedgman,
Hedgman, Laurel
LaurelG.
G.Woodruff,
Woodruff, and
Cannon, Suzanne
J. Schulz
Klaus J.
Schulz
U.S. Geological Survey,
Survey, Reston,
Reston, VA
U.S.
INTRODUCTION
INTRODUCTION
This field trip
trip examines
examines the
the geology
geology of
of rocks
rocksof
ofthe
theKeweenawan
KeweenawanSupergroup
Supergroup(1
(1.1
.IGa)
Gal and
and
related intrusive rocks
rocks of
of the Midcontinent rift
rift system
system (MRS)
(MRS) in the western part of the
the northern
northern
peninsula of
of Michigan. The
The combination
combination of
of stops
stopsincludes
includesall
all formations
formations of
ofthe
theKeweenawan
Keweenawan
Supergroup in
in this
this region. Examination
Supergroup
Examination of
of all
alldescribed
described localities
localitiesrequires
requires more
more than
than aa single
single day
day
and participants are
are encouraged
encouragedto
to use
usethis
this guidebook
guidebookon
ontheir
theirown
own to
to supplement the
the localities that
will
will be
be visited
visited on
on our one-day trip. Because
Because of uncertainties
uncertainties of weather,
weather, road
road conditions,
conditions, and
and
remaining snow
snow pack
pack in
in early
earlyMay
Mayininthis
thisregion
regionofofvery
veryheavy
heavysnowfall,
snowfall,the
thestops
stopsthat
thatwe
we will
will visit
visit
will
will not
not be
be known
known until
until the
the date
date of
of the
the trip.
trip. Stops
Stopsare
arenumbered
numberedin
in stratigraphic
stratigraphic order,
order, from
from oldest
oldest
to youngest,
youngest, not in the order in which they will
will be
be visited.
GENERAL
GENERALGEOLOGY
GEOLOGY
Supergroup is
is aa very
very thick
thick sequence
sequence of
of volcanic and sedimentary rocks
The Keweenawan Supergroup
that was deposited
deposited during and shortly
shortly after
after an
an episode of continental
continental rifting
riftingat
atabout
about11.1
.IGa,
Ga, when
when
formed within the
the MRS formed
the Proterozoic
Proterozoic craton. The
TheKeweenawan
KeweenawanSupergroup
Supergroup was
was deposited
deposited in
in and
and
marginal to
to rift
rift graben
graben and in a post-rift
post-rift thermal
thermal basin
basin more or less centered on the axis of the
the
of the field trip (figs. 11 and
former rift. In the area
area of
and 2),
21, the
the Keweenawan
Keweenawan stratigraphic section (fig.
(fig. 3)
3)
Quartzite)that
thatis
is overlain
overlainby
by aa great
greatthickness
thickness of
of
consists of a thin
thin basal
basal quartzite (Bessemer
(Bessemer Quartzite)
subaerial flood
flood basalt flows and
and felsite
felsite flows (Powder
subaerial
and lesser
lesser andesite and
(Powder Mill
MillGroup
Groupand
andPortage
Portage
Lake Volcanics).
Volcanics). The
progressively thicker
thicker toward
toward the rift
Lake
The volcanic section
section becomes progressively
riftaxis
axisand
and
exceeds 20
20 km
and others,
others, 11988;
988; Cannon and others,
exceeds
km in
in places
places beneath
beneath Lake Superior (Behrendt and
1
990).
1990).
In the vicinity
vicinity of
of the
thePorcupine
Porcupine Mountains,
Mountains, the
the volcanic
volcanic sequence
sequence includes, at the
the top,
top, aa
stratovolcano
stratovolcano composed mostly of andesite
andesite and rhyolite flows
flows and
and sub-volcanic
sub-volcanic felsic
felsic intrusions
intrusions
Volcanics). These
(Porcupine Volcanics).
These late
late intermediate and felsic volcanic rocks
rocks are
are atypical
atypical of
of the
theMRS
MRSas
as
a whole and are limited to aa few
few felsic
felsic volcanic
volcanic centers.
centers.
fluvial sedimentary
sedimentary rocks
rocks and
and lesser
lesser lacustrine
lacustrine
Conformably overlying the volcanic rocks
rocks are
are fluvial
sedimentary rocks (Oronto
(Oronto Group), which
which are
are as
as much
much as
as 88 km
km thick
thickbeneath
beneathLake
LakeSuperior
Superior(Cannon
(Cannon
and others,
others, 11990)
990) and
and at
at least 5 km thick on shore in the field
field trip
trip area.
area.
Post-rift reverse faulting
faulting has caused
caused block
block rotation
rotation on a very large scale
scale so
so that
that the
Post-rift
Keweenawan Supergroup
Supergroupsection
sectionisissteeply
steeplytotovertically
verticallydipping.
dipping. More than 13
13 km
km of
of volcanic
volcanic
rocks and 5 km
generally north-facing
north-facing section. This
km of
of sedimentary
sedimentary rocks
rocks are exposed in a generally
This
thick section constituted at least half of the crustal
enormously thick
crustal thickness by
by the
the close
close of
of rifting
riftingand
and
thus our field trip
trip provides
provides aa traverse through the upper half of the Middle
Middle Proterozoic
Proterozoic crust.
The initial
initial subsidence of the
the MRS
MRS is
is recorded
recorded by deposition
deposition of
of the
theBessemer
Bessemer Quartzite
Quartzite(stop
(stop
1),
1
), a blanket of relatively
relatively pure, fluvial
fluvial quartzite
quartzite as
as much
much as 100
100m
mthick
thick(Ojakangas
(Ojakangasand
and Morey,
Morey,
It formed in a broad basin,
basin, more
more or
or less
less centered
centeredon
onthe
thesite
Siteofofthe
thefuture
future deep
deeprift.
rift. The
1982).
1982). It
The
age of
of deposition has an
an upper
upper bound
boundof
of about
about 1109
1109 Ma,
Ma, the age
age of
of the oldest overlying basalt
age
flows (Davis
(Davis and Sutcliffe, 1985).
1985).Following
Followingdeposition
depositionofofthe
theBessemer,
Bessemer,the
thearea
areabecame
became volcanivolcanias 15 km) of basalts and lesser andesites
andesites and
and rhyolites
rhyolites
cally active and a great thickness (as much as
in only about 1155 m.y.
m.y. The
accumulated in
The earliest
earliest basalts
basalts constitute
constitutethe
the Siemens
Siemens Creek
Creek Formation
Formation of
975a) (stops
(stops 11 and
and 2).
2), which lies
the Powder Mill
Mill Group (Hubbard, 1
1975a)
lies conformably
conformably on
on the
the Bessemer
Bessemer
Quartzite. Soft
flow (seen
Quartzite.
Softsediment
sediment deformation
deformation ofofthe
theBessemer
Bessemer by
by the overriding
overriding basal basalt flow
(seen at
flow is
stop 1)
1) indicates a very short interval between the units. Locally,
Locally, as at Stop
stop 1, the basal flow
is
pillowed, but
but all
allsucceeding
succeedingflows
flowsare
aresubaerial.
subaerial.
77
00
1
,00730"
10G130
2
—
•••_o___
o
—-
—
'KtOMETERS
UE8
\'
________
EXPLANATION
EXPLANATION
Intrusive rock.
rocks
Intrusive
O.bbrc or dleb...—grancphyn. ifitrunin. cOrgia000
I, f.rr.d f.I.la .tcob
Volcanic and ..dimsntery rock.
Jccob.vllla S.nd.ton.— •d end bran, f.ld.p.thln and aantoo..
scndstone. all (atcn.. nunglanut.
O r o n l oGroup
Group
SAUGL— Ornate
Fr.do Fcnn.tlc,,— r.d .cndst.n. end nu.,dsl,n.
Noc.aoh Shale— gre—bl eat .llt.tone. shela. hoe needol000
Capper Herbs,
Cnnqlansn.te— tears. nod—bins ..nglnr.te
sad
Ccpp.r Harbor Ccnglanr.te basalt ,m,H.r
Bergland Grasp
Perapie. Vclo.nlo. rhyollt. m..H.r— .phyr a to gaunt. and f.ldoper
in abner 1.1 don... fleas end d.brl. floe.
pitynlo
rhy.lit*
Poroopin. Voloeni.. basalt cod end., it. .e.,*.r thin
°F
ft...
of and.. It.
and miner tholelillo ba.clt
ParIng. 1.1.. Vnl.noloe—nmotly nphltlo IlicleIltI. bdeelt floe..
naHasIt.. nhy.llt. utid intart lee t.ngla,arnt.
nil,,,
Po.dennsi II
I
I
iii
1!
Creep
Fc.d.nniIl Creep (endlvld.d)— Thnl.iltic kooelt flea.. .nlncç
II
end.slt.
end rhyc lit.
anacin
Leear Prat.ronuie cod Arch.., rack. (cndlvldad)
Tr.ndofpronlnanl •.ran.gn.liu lin..n.nl.
Mountainsarea
area showing
showing
PorcupineMountains
Figure
Figure 2.
2, Generalized
Generalizedgeologic
geologicmap
mapand
andsection
section of
of the
thePorcupine
the
location
of
field
trip
stops.
the location of field trip stops.
79
STOP j#
12a
12
Jocobsville
Jocob,ville Sandstone
Lj
Sandstone
Freda Sandstone
Nonesuch Shale
Copper Harbor
Horbor Conglanerate
Conglamrote
rnmber)
(bosoitk rnerther)
(bosa;
Porcupine Volcanic,
Volcanics
1o
I
Portage
Volcanics
Portage Lake Volconic,
4
5
Kollander
Kal
lander Creek Formation
3
-
2
1
0
Sernena
Siemens Creek Formation
Bess-r
Quartzite
Bessner Quartzite
KM
Early Preterozoic
Eorly
Proterozoic rocks
rocks
Figure
Stratigraphicsection
sectionof
ofKeweenawan
Keweenawan Supergroup
Supergroup rocks in the field
field trip
trip area
area
Figure 3. Stratigraphic
showing the stratigraphic
position
of
stops.
Thicknesses
are
approximate
average
stratigraphic position of stops. Thicknesses are approximate average
thicknesses for
for the area.
area. Some
Some formation
formation thicknesses
thicknesses vary significantly
significantlyacross
across the
the region.
region.
80
Overlying the Siemens
Creek Formation
Formationisisaathick
thick section
section of
of more felsic
felsic rocks
Siemens Creek
rocks (stop
(stop 3)
3)
of rhyodacite,
rhyodacite, tra~hyandesite~
trachyandesite, andesite,
andesite, and
andbasalt.
basalt. These rocks constitute
constitute the
composed of
the Kallander
Kallander
Creek
(Hubbardl 1975a).
Powder Mill
Mill volcanic
volcanic rocks
rocksare
are
Creek Formation
Formation of
of the
the Powder Mill Group (Hubbard,
1 975a). The Powder
distinguished from overlying
overlying basalts
basalts of similar
similar character
character largely on the
the basis
basis of
of their
theirreversed
reversed
to the normal polarity
magnetic polarity as opposed to
polarity of
of younger
younger units
unitssuch
suchas
asthe
thePortage
PortageLake
Lake
Volcanics. In Michigan,
Michiganl the Powder Mill
Mill Group crops out in
in aa belt
belt that
that isisseparated
separated from
from the
the
Volcanics by
by the
the Jacobsville
Jacobsville Sandstone,
Sandstone, which
which lies with
outcrop belt of the
the overlying
overlying Portage
Portage Lake Volcanics
subsurface, however, the Powder Mill
Mill rocks. In the subsurfacef
Mill and
and
angular unconformity
unconformity on Powder Mill
Lake probably
probably form
form a continuous
Portage Lake
continuousdepositional
depositionalsequence.
sequence.
Supergroup range
rangein
incomposition
compositionfrom
from olivine
olivine tholeiite
tholeiite to
Volcanic rocks of the Keweenawan Supergroup
rhyolite. By
high-Al olivine
olivine tholeiite
tholeiite (A1203
(Al2 03==15-19
wt%) followed by
rhyolite.
By far
far the
the dominant
dominant rock type is high-A1
15-19 wt%)
lesser high-Fe
high-Fetholeiite
tholeiite and
and rocks
rocks of
of intermediate and
and felsic
felsic composition
composition (Green,
(Green, 1982; Brannon,
lesser
Brannon,
1984; Paces,
Paces, 1988).
1988). The
Theolivine
olivinebasalts
basaltscommonly
commonly are
are ophitic in
in texture
texture and
and the
the dominant
dominant
plagioclase. The most
are geochemically
geochemically similar
similar to
to
phenocryst is plagioclase.
most primitive
primitive Keweenawan
Keweenawan basalts are
ridge basalts.
basalts. However,
primitive midocean ridge
However, incompatible
incompatible trace elements in most Keweenawan
are enriched
enriched compared
comparedto
to depleted
depletedor
or primitive
primitive mantle.
mantle. Radiogenic
basalts are
Radiogenic isotope
isotope analyses
analyses (Sr.
(Sr, Nd,
Pb) of
of the
the main stage
stage high41
high-Al olivine
olivine tholeiites
tholeiites suggest
suggest that
that aa likely
likely source
source of
of the voluminous
and Pb)
(Pacesand
andBell,
Bell,1989;
1989; Nicholson
Nicholson and
and Shirey,
Shirey, 1990).
1990).
basalts is a mantle plume (Paces
Some of
of the
the oldest flowsl
flows, such
Some
such as
as those in
in part
part of
of the
the Siemens
Siemens Creek
Creek Formation,
Formation, have
distinctly different
distinctly
different chemical
chemical compositions
compositions than the younger basalts. These
These basal
basal rocks
rocks are
are transitransitional to weakly
characterized by
by low
low A1203
A1203 content
content and clinopyroxene
weakly alkaline
alkaline olivine basalts characterized
phenocrysts. Locally,
phenocrysts.
Locally, in other parts of the rift, basal
basal flows
flows are
are picritic
picritic but
but such
such flows
flows have
havenot
notbeen
been
identified in
in this
this area.
area.
Flows
Lake Volcanics
Volcanics north
north of the Keweenaw
Flows near the base of the exposed Portage Lake
Keweenawfault
fault
were erupted at about 1096
near the
the top
top of the formation
1096 Ma
Ma and those near
formation at
at about
about 1094
1094Ma
Ma(Davis
(Davis
and Paces,
Paces, 1990).
1 990). Thus,
Lake Volcanics,
Volcanics, at
at least 8 km in this
Thusl the great thickness of Portage
Portage Lake
this area,
areaf
was erupted in only aa few
few million
millionyears.
years. The
Theestimated
estimatederuption
eruptionrate
ratefor
forPortage
PortageLake
LakeVolcanics
Volcanicsinin
the present eruption
eruption rate
the western
western Lake
Lake Superior region must have approximated the
rate of
of the
the Hawaiian
Hawaiian
hot spot,
spotl the
the most
mostvigorous
vigorous volcanic
volcanic center
center of
of the
themodern
modernearth
earth(Cannon,
(Cannonfin
in press).
press).Because
Because
synchronous volcanism
volcanism occurred
occurredalong
alongthe
theentire
entiretrend
trendofofthe
therift,
rift, the
the rift
rift system
system as
as a whole was
was
producing
producing basalt at
at aa rate
rate unrivalled
unrivalled by
byany
any modern
modernanalog.
analog.
area of
of the
the Porcupine
Porcupine Mountains
Mountains contains
contains aafelsic
felsicto
to andesitic
andesitic volcanic
volcanic center
center that
that
The area
became
active late
late in
in the
the volcanic
volcanic history
history of
of the
the regionl
region, at
at about
about 1090
1090 Ma. The
became active
ThePorcupine
Porcupine
called the
the "unnamed
"unnamed formation")
formation") (Cannon and
and Nicholson,
Nicholson, 1992) were erupted
erupted
Volcanics (previously called
from
from that
that center
center and
and accumulated
accumulated as much as 5 km of
of andesite,
andesite, rhyolite,
rhyolite, and
and basalt
basalt in
inaalarge
large shield
shield
deposited on top of the
the Portage
Portage Lake
Lake Volcanics lava field and centered near the Porcupine
Mountains. The
Thepresent
presentarcuate
arcuate shape
shape of the
the mountains
mountains and
and the
the unusual
unusual hook-shaped map pattern
reflection of the
of the Porcupine
Porcupine Volcanics is partly a reflection
the original
original shape
shape of the
the volcanic
volcanic shield.
shield.
The Porcupine Volcanics consists of aa sequence of subaerially deposited andesite,
andesite, basalt,
felsite, and quartz-porphyry
quartz-porphyry lava flows, and
and minor
minor interbedded
interbedded volcaniclastic
volcaniclastic lithic
lithic sandstone,
sandstone,
siltstone,
1 975b). The abundance
siltstonel and
and conglomerate (Hubbard,
(Hubbardl 1975b).
abundance of felsic rocks
rocks and the predomipredominance of
of andesite over basalt clearly distinguish the Porcupine Volcanics from the
nance
the underlying
underlying
Portage Lake
LakeVolcanics.
Volcanics. Felsite
Felsite is
is most
most common
common near
nearthe
thetop
topof
of the
the formation
formation where it
it occurs
occurs as
Portage
both flows
flows and
and domes
domes (stops
(stops 55 and
and 6).
6).
The major element chemistry of basalt,
basalt, basaltic
basaltic andesite,
andesite, and
and andesite
andesite of
of the
thePorcupine
Porcupine
Volcanics and the Portage
Volcanics are
are very
very similarl
similar, but the Porcupine
Portage Lake Volcanics
Porcupine Volcanics are
are distinctly
distinctly
enriched in rare earth elements WEE)
(REE)and
andTh
Thcompared
comparedtotothe
thePortage
PortageLake
LakeVolcanics.
Volcanics. The two
two
formations differ more profoundly in their rhyolite chemistry. Rhyolite
Rhyolitethat
that occurs
occurs most
mostcommonly
commonly
in the Portage
Volcanics is
is aphyric
aphyric or
or may
may contain
contain sparse
sparse quartz
quartz phenocrysts.
phenocrysts. Rhyolites
Rhyolitesof
of the
the
Portage Lake Volcanics
Portage Lake
Lake Volcanics
Volcanics on
on Keweenaw Peninsula
Peninsulahave
havevery
verylow
low abundance
abundance of
of incompatible trace
elements (such as
as REE,
REE,Zr,
Zr,Y,Y,Hff
Hf, and
andThIl
Th), whereas
whereasthe
the rhyolite
rhyolite body
body near
near Bergland
Bergland (stop
(stop 41,
4), one
one of
of
the few
few rhyolite
rhyolitebodies
bodies with
withthe
thePortage
PortageLake
LakeVolcanics
Volcanics in
in the
the Porcupine
Porcupine Mountains
Mountainsarea,
area, has
has
moderate abundances of incompatible
incompatible trace
trace elements.
elements. In contrast, the numerous rhyolite bodies in
Volcanics range
rangefrom
fromrhyolites
rhyolitesthat
that are
areaphyric
aphyricto
to those
those with
with abundant quartz andlor
and/or
the Porcupine Volcanics
81
81
The thrust faulting
faulting and
and folding can be indirectly dated in the interval of approximately 1060
1060
and othersl
others, 1984; Bornhorst and
andothers,
others, 1988;
1988; Cannon
Cannonand
andothers,
others, 11990).
to 1040
1040 Ma
Ma (Ruiz
(Ruiz and
990). The
The
cause of
of the regional compression
compressionisisnot
not known
known with certainty
cause
certainty but
but isislikely
likelylinked
linkedininsome
somemanner
manner
orogeny, which was in progress at that time.
with the
the Grenville
Grenville orogeny,
STOPS
FIELD TRIP STOPS
Stop
.--BessemerQuartzite
Quartziteand
andbasalt
basaltat
at the
the base
base of
of the
the Powder Mill
Mill Group
Group
Stop 11.--Bessemer
A low
east of
of the county road (see
(seefig.
fig. 4)
4) shows quartzite of the
low rock
rock knob
knob in
in aa pasture
pasture east
upper part
part of
of the Bessemer
Quartziteinincontact
contactwith
with the
the overlying
overlying basal
basalbasalt
basaltflow
flow of
of the
the Powder
Powder
upper
Bessemer Quartzite
Mill Group. The
975a).
Thebasalt
basaltisisthe
the lowermost
lowermostflow
flowofofthe
theSiemens
SiemensCreek
CreekFormation
Formation(Hubbard,
(Hubbard,11975a).
The Siemens
Creekisiscomposed
composedpredominantly
predominantlyofofthin
thinflows
flows of
of basalt and
and minor
minor andesite.
andesite. Average
Siemens Creek
Average
thickness of
of flows is about 3 m, but the basal flow
flow is about 50 m thick at this locality. Here,
Here, the
the
basal flow
flow is nonporphyritic
holocrystalline. Unlike
basal
nonporphyritic and holocrystalline.
Unlike some of the basal
basal flows along
along strike
strike to
to the
the
flows do
west, these flows
do not
not contain
contain clinopyroxene
clinopyroxene phenocrysts. Chemical
Chemical analyses
analyses (table 1) of the
flows are similar to the
basal flows
the younger
younger high-alumina
high-alumina main-stage basalts of the Portage
Portage Lake
Volcanics. The
Creekflow
flow is
is pillowedl
pillowed, which
which is very
very unusual for
for basalts of the
Volcanics.
The basal
basal Siemens
Siemens Creek
Supergroup, nearly all of which
Keweenawan Supergroupl
which are
are subaerial.
subaerial. In places,
placesl stringers of the
the quartzite
quartzite
been injected
injected up
up into
into rubbly
rubbly material
material at
at the
the base
baseof
of the
the flow,
flow, indicating
have been
indicating that
that the
theBessemer
Bessemer was
was
unconsolidated at the time of eruption.
eruption.
that dips here
here are
areabout
about65O
65° N.
N. Contrast
Note that
Contrast these
these with
with steeper
steeper to slightly
slightly overturned
overturned
dips at stops 2 and 3, higher in the Powder Mill section. This
Thisupdip
updipfanning
fanningrelationship
relationship commonly
commonly
Hubbard (1
(1975a) to propose that they
they were
were deposited
deposited in
in aa
shown by the Powder
Powder Mill
Mill flows
flows led
ledHubbard
basin that was centered
centered south (updip)
(updip) of the
the present
present outcrop
outcrop belt
belt and
and the
the volcanics
volcanics originally
originally
thickened in that direction.
direction. If that interpretation
interpretationis
is correct,
correctDthere
there must
musthave
havebeen
been aa rapid
rapid change
change in
in
depocenters at the outset
outset of
of volcanism
volcanism because
because most
most paleocurrent
paleocurrent directions
directionsin
inthe
theBessemer
Bessemer are
are
northerly
Morey, 1982).
(Ojakangas and Morey,
1982).
northerly (Ojakangas
Stop 2.--Basalt
2.--Basalt flows
flowsofofSiemens
SiemensCreek
CreekFormation
Formation
Roadcuts along
along Powderhorn Road
Road(fig.
(fig.41,
4), which
which follows the
the valley
valley of
of Powder
PowderMill
MillCreek,
Creek,
provide a convenient cross section of the Powder Mill
Mill Group in its
its type
type area
area as defined by
by Hubbard
Hubbard
975a). At
exposed and
and at
at stop
stop 33
(1975a).
At stop
stop 22typical
typicalbasalt
basaltflows
flowsofofthe
theSiemens
SiemensCreek
Creek Formation
Formation are exposed
Creek Formation
Formationcan
canbe
beseen.
seen. Steeply
Steeply north-dipping
north-dipping basalt
basalt flows
flows of the
the Kallander Creek
the Siemens
Siemens
Creek
Formationare
arewell
wellexposed
exposedininoutcrops
outcropson
onthe
thehill
hillwest
westof
of the
the road
road and
and in
in low
low roadcuts. The
Creek Formation
The
meters or less thick.
Commonly these
these fine-grained flows have
have
basalt flows
flows are thin,
thin, typically a few meters
thick. Commonly
amygdaloidal and
and rubbly
rubbly flow
flow tops,
seen locally
locally at
at the
the base
base of
of some flows.
amygdaloidal
topsl and
and pipe vesicles can be seen
show patchy alteration to
to chlorite and
and have
have abundant
abundantchlorite
chloritein
inthe
the matrix,
matrix. This
The basalts show
This relarelatively high
high degree of metamorphism is typical of basalts
basalts of the
the Siemens
Siemens Creek Formation and is one
distinction between
distinction
betweenthem
them and
and less
less metamorphosed
metamorphosed basalt
basalt of the
the overlying
overlyingPortage
PortageLake
Lake Volcanics.
Volcanics.
Some
flows are porphyritic
Some flows
porphyritic with
withsmall
smallplagioclase
plagioclase phenocrytsts.
Stop 3.--Kallander Creek
Creek Formation
Formation
About 2 km
km north
north of
of stop
stop2,
2, aaroad
roadimprovement
improvementproject
projecthas
has produced
produced freshly
freshly blasted
blastedoutoutCreekFormation.
Formation. Because
Becauseall
allflows
flowsare
aredipping
dippingnearly
nearly90Â
90° in
in this
this area,
the
crops in the Kallander
Kallander Creek
area, the
passes up
upthrough
through about
about 22 km
km of
of section,
section, an impressive
impressive pile
pile of
of basaltl
basalt, yet
yet this
drive between stops passes
erupted into
into the
the deepest
deepestparts
partsofofthe
therift.
rift. The
is only 10% of the total thickness of basalt erupted
The flows
flows in
in
the Kallander
Creek Formation
Formationare
arebasaltic
basalticto
toandesiticD
andesitic,more
morerarely
rarelyrhyoliticl
rhyolitic, and
and generally
generally aa few
few
Kallander Creek
Creekisismore
morefelsic
felsicthan
thanthe
the Siemens
SiemensCreek.
Creek. At this
this stop
stop the
the
meters thick. Overall,
Overall, the Kallander Creek
flows include chloritized
chloritized amygdular basalt,
basalt, fine-grained
fine-grained basalt
basalt with
with plagioclase phenocryts
exposed flows
stone"),non-porphyritic
non-porphyriticandesite,
andesite,andesite
andesitewith
with pla~ioclase
plagioclasephenocrysts,
phenocrysts, and tuff
tuff
(locally a TMdaisy
-daisy stonew),
breccias containing
containing both
both basalt
basalt and
andandesite
andesitefragments.
fragments. Although
Although rhyolite occurs higher in the
breccias
formation, none
flows are
none is
is exposed
exposed here. The
TheKallander
Kallander Creek
Creek flows
are generally
generally less metamorphosed than
the Siemens
Siemens Creek flows.
83
asv
c'RokFor
48N.
47N.
Falls
° fPuritSch
Figure4.
4. Location
setting foi
for stops 1, 2, and
Figure
Location and geologic setting
and 3.
3. Geology
Geologygeneralized
generalized and
modified from Hubbard (1975a).
975a). Note
scale between
between the
the two
two adjacent
Note change
change in scale
quadrangles.
quadrangles.
84
TABLE 1:
1: CHEMICAL
CHEMICALANALYSES
ANALYSESFOR
FORSOME
SOMEVOLCANIC
VOLCANICUNITS
UNITSININTHE
THEKEWEENAWAN
KEWEENAWANSUPERGROUP
SUPERGROUP
TABLE
ELEMENT
ELEMENT
1
2
N=2
N=4
Si02
Si02
52.02
Ti02
Ti02
1.75
14.32
1.85
9.42
0.17
6.93
8.18
3.57
1.57
0.23
A1203
A1203
Fe203
Fe203
FeO
FeO
MnO
Ph0
MgO
MgO
Cao
CeO
Na20
Na20
K20
K20
P205
SUM
SUM
Cr
Cr
Ni
Ni
Nb
Nb
Rb
Rb
Sr
Sr
Zr
Y
V
La
La
Sm
Sm
Yb
Hf
Ta
Ta
Th
Th
3
N=5
77.23
0.12
12.91
0.60
1.19
0.03
0.30
1.43
1.57
4.62
0.01
99.99
100.00
233
2.9
4
N=1
48.66
1.69
16.51
1.83
10.38
0.18
7.37
10.59
2.29
0.32
0.18
100.00
71.79
0.42
12.84
1.74
3.49
0.10
1.18
0.23
0.76
7.38
0.06
99.99
188
204
2.5
50
212
8
7
470
123
18
281
16
19.0
4.9
1.67
3.12
1.04
1.94
112
76.9
12.9
10.00
10.24
3.92
39.48
252
100
23
32
219
53
1130
103
273
27.3
94
20
35
1
11.6
4.0
2.08
2.65
0.58
1.04
<2
10.4
22.2
2.96
35.70
5
6
N=2
N=1 1
47.60
2.26
17.03
2.04
12.30
0.19
7.60
5.11
4.09
1.26
0.53
100.00
168
95.5
17
1.70
16.99
1.65
10.90
0.22
8.16
6.50
2.86
2.53
0.20
100.00
159
188
8.7
61
37.2
7.1
3.55
4.475
0.86
2.63
1 Average analysis of
of Siemens
Siemens Creek
Creek basalts
basalts
Average analysis
analysis of
of rhyolite
Average
rhyolite from quarry
quarry near
near town
town of
of Bergland
Bergland
Average
Average analysis of basalts
basalts of
of the
thePortage
PortageLake
LakeVolcanics
Volcanicsnorth
northofofcrossroads
crossroadsatatMerriweather
Merriweathernear
nearBergland
Bergland
Analysis
Analysis of rhyolite
rhyolite at
at quarry
quarry near
near White
White Pine
Pinemine
mine
Average
Average analysis of basalts
basalts at
at Lake
Lake of
of the
theClouds
Cloudsoverlook
overlookininPorcupine
PorcupineMountains
MountainsWilderness
Wilderness State
StatePark
Park
analysis of basalts
6 Average
Average analysis
basalts of the
the Portage
Portage Lake
LakeVolcanics
Vofcanics on
on Keweenaw
Keweenaw Peninsula
Peninsula
6
7
Average analysis
analysis of
of basalts
basalts from
from the
thePorcupine
PorcupineVolcanics
Volcanics
7 Average
33
44
5
5
48.29
490
1
2
2
7
N=8
126
26
11.9
4.0
2.12
2.65
0.59
1.20
50.68
2.47
14.36
2.11
11.94
0.23
5.32
6.46
2.70
2.89
0.86
100.00
64
42
18
62
407
294
56
55.7
11.1
5.11
7.10
1.32
5.94
Stop4.--Portage
4.--PortageLake
LakeVolcanics
Volcanics
Stop
This
Thisstop
stopillustrates
illustratesrhyolite,
rhyolite,basalt,
basalt,and
andinterfiow
interflowsandstone
sandstoneofofthe
thePortage
PortageLake
LakeVolcanics.
Volcanics.
Exposures
and quarry
quarry along
alongthe
thenorth
north side
sideof
of Michigan
MichiganHighway
Highway 28
28 (fig.
(fig. 5). The
The
Exposures are in a roadcut and
prominent
expression
prominentsouth-facing
south-facingscarp
scarpthat
thatruns
runsparallel
paralleltotothe
thehighway
highwayusually
usuallyisisthe
thegeomorphic
geomorphic
expression
ofofthe
the
Portage
Lake
theKeweenaw
Keweenawfault,
fault,but
buthere
herethe
thefault
faultlies
liesmore
morethan
thana amile
miletotothe
thesouth,
south,and
and
the
Portage
Lake
Volcanics
underlie
the
lowlands
surrounding
the
north
end
of
Lake
Gogebic
as
well
as
the
highlands
Volcanics underlie the lowlands surrounding the north end of Lake Gogebic as well as the highlands
northofofthe
thescarp.
scarp.
north
The
are a thick
thick pile of dominantly basaltic, subaerial flows,
flows, with
ThePortage
Portage Lake
Lake Volcanics are
with
lesser
lesser rhyolites
rhyolites and
and interflow
interflow sedimentary
sedimentary rocks. At
Atthis
thislocality,
locality,however,
however,rhyolite
rhyoliteisisthe
thedominant
dominant
rock
rocktype,
type,although
althoughgood
goodexamples
examplesof
ofbasalt
basaltare
are in
inthe
theroadcut.
roadcut.Another
Anotherunusual
unusualfeature
featurehere
hereisisthe
the
local
localdevelopment
development of
of pillows
pillowsalong
along the
the toe
toe of
of aaflow
flowwhere
whereititapparently
apparentlyentered
enteredaabody
bodyof
ofwater
waterinin
whichsandstone
sandstonewas
wasbeing
beingdeposited.
deposited.
which
The
Thestratigraphy
stratigraphy in
inthe
the quarry
quarryarea
area consists of an ophitic
ophitic basalt
basalt flow
flow overlain
overlain by
by aa thin
thin interinterflowsandstone
sandstonetotosiltstone
siltstonewhich
whichininturn
turnisisoverlain
overlainby
byan
anamygdaloidal
amygdaloidalbasalt.
basalt.The
Theamygdaloidal
amygdaloidal
flow
basalt
basaltexhibits
exhibitswell
welldeveloped
developedpipe
pipevesicles
vesiclesatatroad
roadlevel.
level. Rhyolite
Rhyoliteoverlies
overliesthe
theamygdaloidal
amygdaloidalbasalt
basalt
and
andisiswell
wellexposed
exposedin
inthe
the quarry
quarry floor
floor and
and walls
walls and
and is
is overlain
overlain by
by ophitic
ophitic basalt
basalt flows
flows on
onthe
the
hillside
compositionally to
to low Ti02
hillsideabove
above the quarry.
quarry. The
Thebasalts
basaltshere
here are
are identical compositionally
Ti02basalts
basaltson
on
1).
KeweenawPeninsula
Peninsula(table
(table1).
Keweenaw
ItItisisnot
notclear
clearwhether
whetherthe
therhyolite
rhyoliteisisaasingle
singleshallow
shallow intrusive
intrusivebody
bodyor
oraaseries
series of
of flows.
flows. ItIt
contains
containsseveral
severallenticular
lenticular masses
masses of
of basalt
basalt and
and an
an unusual
unusual greenish sandstone. Are
Arethese
thesexenoliths
xenoliths
ininan
anintrusion
intrusionor
ordiscontinuous
discontinuousinterbeds?
interbeds?Several
Severalfeatures
featuressuggestive
suggestiveof
of extrusive
extrusiveorigin
origincan
canbe
be
Foldsare
arepresent
presentnear
near the
the contact
contactof
of the
therhyolite
rhyolitewith
withthe
theunderlying
underlyingbasalt.
basalt. In
Inthe
theupper
upper
noted.
noted. Folds
part
partof
ofthe
thequarry,
quarry, the
therhyolite
rhyoliteshows
showswell-developed
well-developed columnar
columnar jointing, some
some of
of which
whichisisbent,
bent,
following
followingaaramp
rampstructure
structureininthe
therhyolite.
rhyolite.The
Thepossibility
possibilityofofaaseries
seriesofofflows
flowsisisenhanced
enhancedby
bythe
the
distribution
distributionof
ofspherulitic
spheruliticrhyolite.
rhyolite.Spherulites
Spherulitesare
arecommon
commonthroughout
throughoutthe
thequarry
quarrybut
butare
areconcenconcentrated
tratednear
nearthe
the base
base of
of the
the rhyolite,
rhyolite, near
near the
the contact
contactwith
withaasedimentary
sedimentary lens,
lens, and
and near
near the
the top
top of
of
the
thequarry.
quarry.More
Moremassive
massiveororstony
stonyrhyolite
rhyoliteoccurs
occursbetween
betweenthe
thespherulitic
spheruliticzones.
zones.
Alteration
Alterationrelated
relatedto
tocopper
coppermineralization
mineralizationis
is widespread
widespread in
in the
the quarry.
quarry. Epidote
EpidoteisisdissemidissemiSecondarycopper
copperminerals
mineralsare
are
nated
anunusual
unusualgreenish
greenishcolor.
color.Secondary
natedin
inmuch
muchof
of the
therhyolite,
rhyolite,giving
givingititan
abundant on
on joint
joint surfaces.
surfaces.
abundant
Stop
Stop5.-Porcupine
5.-Porcupine Volcanics:
Volcanics:Rhyolite
RhyoliteatatSummit
SummitPeak
Peak
Summit
one of
of the highest
Summit Peak
Peak is the highest
highest point
point in
in Porcupine
Porcupine Mountains Park and one
highest points
points
in
in the
the state.
state. The
Theobservation
observationtower
toweratatthe
thesummit
summitprovides
providesaapanoramic
panoramic view of
of the
the field
field trip
triparea
area
(fig.
highlands
are
6).ToTothe
thesouth,
south,the
the
highlands
areunderlain
underlainbybythe
thePortage
PortageLake
LakeVolcanics
Volcanicsand
andPorcupine
Porcupine
(fig.6).
Volcanics
Volcanics along
along the
the main
main monocline
monocline of volcanic
volcanic rocks
rocks of
of the
the Keweenawan
KeweenawanSupergroup.
Supergroup. The
The
lowlands
OrontoGroup
Groupininthe
theeasteastlowlandsimmediately
immediatelyto
to the
thesoutheast
southeastare
are underlain
underlain by
by rocks
rocks of
of the
the Oronto
plunging
plunging Iron
IronRiver
River syncline.
syncline. Looking
Lookingeast
eastalong
alongstrike,
strike, the
thesmelter
smelteratatWhite
WhitePine
Pinecan
canbe
beseen
seenin
inthe
the
distance.
distance. To
Tothe
thenorth,
north,the
theinterior
interiorofofthe
thepark
parkextends
extendsover
overthe
therugged
ruggedtopography
topography in
in the
the foreforeground
ground to
to Lake
Lake Superior
Superior in
in the
the distance.
distance. The
Theinterior
interiorofofthe
thepark
parkisismaintained
maintainedas
asaawilderness
wildernessarea
area
contains the
the largest
largest stand
stand of
of virgin
virgintimber
timberin
inthe
thestate.
state.
and access
access is
is only
only by
byhiking.
hiking. ItItcontains
and
Most
Mostof
ofthe
thepark
parkinterior
interiorisisunderlain
underlainby
byaathick
thickunit
unitofofrhyolite
rhyolitecomposed
composedofofaaseries
seriesofof
flows
flowsand
anddomes
domestypified
typifiedby
bythe
therocks
rocksseen
seenat
atSummit
Summitpeak.
peak. Good
Goodexposures
exposuresof
ofcoarse
coarserhyolite
rhyolite
breccia
the trail leading to
to the summit and at
at the overlook platform west of
breccia are along the
of the
the summit.
summit.
Thisbreccia
brecciaisisprobably
probablythe
thecarapace
carapaceof
of aarhyolite
rhyolitedome.
dome.
This
Excellent
Excellent exposures
exposures of typical
typical intermediate
intermediateand
and felsic
felsic units
unitsofofthe
thePorcupine
PorcupineVolcanics
Volcanicscan
can
be seen
seen on the north
north side
side of the
the hill
hill (549
(549m
melevation)
elevation)near
near the center of Sec.
Sec. 31.
31. Take
Takethe
theBeaver
Beaver
be
Creek
Creek Trail about 0.5 mi
mi from
from the
the Summit
Summit Peak
Peak parking lot. The
Theunits
unitsdip
dipto
tothe
thesouth
southand
andinclude,
include,
in
in stratigraphic
stratigraphic order,
order, sparse
sparse outcrops
outcrops of
of intermediate
intermediateto
to mafic
maficrocks
rocksininthe
thecreek
creekbed,
bed,overlain
overlainby
byaa
vesicular
vesicular siliceous
siliceous andesite,
andesite, which is
is in
in turn
turn overlain
overlain by
by aacoarse
coarse rhyolite
rhyolite breccia
breccia or
or debris
debris flow.
flow.
The
in size
size from
from nearly
nearly aa meter
meterto
to less
lessthan
than 11 cm.
cm. The
The breccia
breccia is
is
The breccia
breccia contains clasts ranging in
clast
clast supported
supported and
and some
some clasts
clasts are
are subrounded. Overlying
Overlyingthe
thebreccia
brecciaisisaamedium-grained
medium-grainedbasalt
basalt
flow.
flow. Capping
Cappingthe
thehill,
hill,and
andoverlying
overlyingthe
thebasalt,
basalt, isisan
anaphanitic
aphanitic massive
massive rhyolite that is
is
microspherulitic.
microspherulitic.
86
—H-- -j
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SCALE1:25
1:25000
OW
SCALE
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1
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KILOMETERS
1
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1000
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1
7
1000
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-
-
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00
1000
1000
00
-
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.?OM-
-
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METERS
4000
3000
3000----4000
-
-
2000
2000
1000
1000
00
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MILES
MILES
-5000
5000
-
6000
6000
1000
7004
8000
8000
9000
WOO
10000
10!00
itt:
I
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CONTOUR INTERVAL
INTERVAL55METERS
METERS
CONTOUR
Figure 5. Location
Locationand
andgeologic
geologic setting
setting of
ofstop
stop4.
4.Geology
Geologygeneralized
generalizedfrom
fromunpublished
unpublished
Figure
compilation by
by Walter
Walter White.
White.
compilation
87
88
(unpublished). mapping
cooperative Survey Geological Corp./Michigan Range Copper from generalized Geology
marked. well are all but shown, not are peak Summit on tower and platform observation
and improvements, trail and road Recent 5. stop of setting geologic and Location 6. Figure
METERS
5
INTERVAL CONTOUR
FEET
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
Oa0
0
MILES
2000
1001)
METERS
0
1
KILOMETERS
0
51N.
R.44W.
1000
.5
1
Stop
Stop 6.--Porcupine Volcanics: Rhyolite
Rhyolite quarry
quarry near
near White
White Pine
Pine
here are
are part
part of
of a small rhyolite body near the top
The rocks exposed here
top of
of the
thePorcupine
Porcupine
Volcanics. The
beyond the
the hill
hill into
into which the quarry is
Volcanics.
The body
body probably
probably does not extend much beyond
is cut
cut
(fig. 7). The
Thequarry
quarryprovides
providesaacross
crosssection
sectionthrough
throughpart
partof
ofaasubaerial
subaerialagglutinate
agglutinatedeposit.
deposit.
form at vents by spatter of erupting magma and
and buildup
buildup of
of mounds of
of hotl
hot,
Agglutinate deposits form
viscous material.
material. The mound of erupted material
materialeventually
eventuallyflows
flowsoutward
outward under
underits
its own
own weight
weight
resulting in large
large flow
flow folds such as seen
seen in
in this
this quarry.
quarry. The
The near-vent
near-vent nature of this deposit
deposit is
is
deduced
from the
the presence
presenceof
of lithic
lithic fragments
fragments within
within the
the rhyolite,
rhyolite, large flow
flow folds and
deduced from
and stratification
stratification
of rhyolite
rhyolite (light
(light and dark units). At
Atthe
theedges
edges of
of agglutinate
agglutinate deposits,
deposits, flowage typically has
has
homogenized the
the magma and
and such
such stratification
stratification and
and folding
foldingare
are generally
generally not
notpreserved.
preserved.
feldspar phenocrystsl
phenocrysts, which
which are
are typically
typically aligned parallel to
to the
The rhyolite here contains feldspar
It is enriched substantially in such incompatible
stratification
incompatible trace
trace elements
elements as Zr, Ba,
Ba,
stratification and
and foliation. It
Hf,
Hfl and
and the
the light
light rare
rareearth
earth elements
elements compared to the
the rhyolite
rhyolitein
inthe
thePortage
PortageLake
Lake Volcanics
Volcanics near
near
Bergland
(table 1).
1). These
and sources
sources for
for the
the two
two
Bergland (table
These differences suggest different petrogenesis and
rhyolites in spite of their physical
physical proximity.
proximity.
Stop 7.--Basalt
7.--Basalt flows within
withinthe
theCopper
CopperHarbor
HarborConglomerate
Conglomerate(Lake
(Lake of the
the Clouds
Clouds overlook)
overlook)
Along the road
road leading
leading to Lake
Lake of the
the Clouds
Clouds overlook
overlook are several exposures of conglomerconglomerate of the Copper
Conglomerate. To the north
Copper Harbor Conglomerate.
north is
is aa good
good view
view of
of Lake
LakeSuperior
Superior and
and the
the lowlowlands underlain
underlain by
by sedimentary
sedimentaryrocks
rocksofofthe
theOronto
OrontoGroup.
Group. From
From the
the overlook
overlook parking
parking lotl
lot, aa short
short
hike leads to
to the overlook and a spectacular view of Lake
Lake of the
the Clouds
Clouds and
and the
the Porcupine
Porcupine MounMountains Wilderness State
State Park.
Park.
The overlook is along the south escarpment of
of a high ridge supported by a series of northdipping lava flows
flows within
the
Copper
Harbor
Conglomerate
(fig. 88).
) . The
The low
low area
area south
south of
of the
the ridge,
ridgel
within the Copper
Conglomerate (fig.
including Lake
Lake of
of the
the Cloudsl
Clouds, is underlain
underlain by
by sandstone
sandstoneand
andsiltstone
siltstone and
andaafew
few basalt
basalt flows
flows which
which
Conglomerate. The higher regions farther
farther south are
are
constitute the lower
lower part
part of
of the
the Copper
Copper Harbor Conglomerate.
by volcanic
volcanic rocksl
rocks, mostly rhyolite
underlain by
rhyolite of the
the Porcupine
Porcupine Volcanics.
Toward
surface shows
shows a series
series of
of thin
areal a large glaciated surface
Toward the
the east
east end of the
the overlook
overlook area,
basalt flowsl
flows, which
average
a
few
meters
thick.
Individual
flows
can
be
readily
identified
by
which average few
can be
identified by chilled
chilled
vesicular
bases,
in
places
containing
inclusions
of
older
flows,
and
by
rubbly
or
vesicular
tops.
vesicular basesl in places containing inclusions of older flows, and by
Abundant epidote alteration and
and vesicle
vesiclefillings
fillingsalso
alsoimpart
impartaadistinctive
distinctivegreenish
greenishcast
castto
toflow
flow
margins. Hubbard
the flows
flows in
as mostly andesite with
with minor
(1975b) described the
in the Copper
Copper Harbor as
minor
margins.
Hubbard (1
basalt,
but
chemical
analyses
of
two
samples
from
this
locality
(table
1)
indicate
that
they
are
basalt
basaltl
chemical analyses of two samples from this locality (table 1) indicate that
similar to an
an average basalt from the
the Porcupine
Porcupine Volcanics. Compared
Comparedto
toPortage
PortageLake
LakeVolcanics,
Volcanics,
these basalts are enriched in incompatible
incompatible trace
trace elements.
Stop 8.--Upper part
part of
ofCopper
CopperHarbor
HarborConglomerate
Conglomerateat
atUnion
UnionBay
BayCampground
Campground
Good exposures of reddish sandstone containing
containing thin conglomerate
conglomerate beds are abundant along
Superior at
at this
this locality
locality (see
(seefig.
fig. 91.
9). The
is noted
noted for
for
The Copper
Copper Harbor Conglomerate
Conglomerate is
the shore of Lake Superior
volcanogenicconglomerates,
conglomerates,which
whichform
formmost
mostof
of the
the lower
lower part
part of the section
coarse volcanogenic
section throughout
throughout
much of its outcrop
finer grained
grained sandstones.
sandstones. But,
outcrop belt
belt and which grade up into finer
Butl north of the
the
Porcupine Mountains
Mountains there
there is
is aa different
different facies
faciesrelationship.
relationship. The lower part of the
the formation
formation isis
lava flows;
flows; conglomerate
is very
very subordinate.
subordinate. These
mostly sandstone,
sandstonel siltstone and lava
conglomerate is
These rocks underlie
south of the highway. The
the high hills immediately south
Thecoarse
coarse conglomerate
conglomerate facies is less abundant
and is higher in the section. The
Theexposures
exposureshere
here at
at Union
Union Bay
Bay are near the base of the upper
upper unit
unit
are probably
probably about
about 1,000
1,000 m above the
the base
base of
of the
the formation.
formation. The
and are
The sandstones
sandstones at
at Union
Union Bay
Bay dip
dip
10-20° to
10-20Â
to the
the north.
north.They
Theyare
arevolcanogenic
volcanogenicand
and quartz-poor.
quartz-poor. They
Theyshow
showexcellent
excellentexamples
examples of
of
generally indicating
indicating aa northeastward
northeastwardcurrent
currentvectorl
vector, and
and aa variety
variety of
of other
trough cross bedding,
bedding, generally
sedimentary features
features including
including desiccation cracksl
cracks, rip-up
rip-up clasts,
clasts, oscillation and current ripples,
ripplesl and
sedimentary
swash marks.
marks.
Conglomerate north
north of
of the Porcupine
The exposures of Copper Harbor Conglomerate
Porcupine Mountains
Mountains are
are the
the
from the
the source
source highlands
highlandsto
tothe
thesouth
southof
of any
any part
part of
of the
the unit
unit on land.
land. The
farthest removed from
The
compared to
to exposures
exposuresfarther
farther south
south probably
relative scarcity of thick units
units of
of coarse
coarse conglomerate
conglomerate compared
89
89
ft 41 W.
\/
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Figure 7.
7.Location
Locationand
andgeologic
geologicsetting
settingofofstop
stop6.6.Geology
Geology
generalizedfrom
fromJohnson
Johnsonand
and
Figure
generalized
White
(1
969).
White (1969).
90
_______
R. 44 W.
51 N.
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Figure8.8.Location
Location
andgeologic
geologicsetting
settingofofstop
stop7.7 Geology
. Geologygeneralized
generalizedfrom
fromHubbard
Hubbard
Figure
and
(1
975b).
(1 975b).
91
R. 43 W.
51N.
SCALE 1:24 000
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1000
2000
3000
5000
4000
6000
7000
8000
9000
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Figure9.9.Location
Location
andgeologic
geologicsetting
settingof
ofstop
stop8.8.Geology
Geologygeneralized
generalizedfrom
fromHubbard
Hubbard
and
Figure
(1
975b).
(1 975b).
92
reflects the distal nature of these rocks and may
may be
be aa good
good representative
representativeof
of much
much of
of the
the Copper
Copper
Harbor beneath Lake
Lake Superior.
Superior. The unit here
here shows
shows aa crude coarsening-upward
coarsening-upward trend as
as opposed
proximal parts
parts of
of the
the unit.
unit. This
to the fining-upward trend typical of the more proximal
This relationship
relationship is
is consistent with
with northward
northwardprograding
progradingalluvial
alluvialplain
plain deposition.
deposition.
Several large
large boulders
boulders are
are distributed
distributed along the
the beach and are
are composed
composed of conglomerate
conglomerate
typical of the lower
lower part
part of
of the
the formation
formationelsewhere.
elsewhere. These
These conglomerates
conglomerates contain
contain almost excluexcluKeweenawan Supergroup
Supergroupvolcanic
volcanicrocks
rockstypes
typescommon
commonininthe
theregion.
region. An
An interestsively clasts of Keweenawan
boulders. Although
ing question is the source of these boulders.
Althoughthe
theCopper
Copper Harbor
Harbor does
does contain
contain some
some
conglomerate beds of this aspect
aspect nearby,
nearby, none
none of
of the
thestreams
streamsentering
enteringLake
LakeSuperior
Superiornear
nearhere
here
seem capable
capableof
of transporting
transporting such
such large
largeboulders.
boulders. Most have low
low gradients,
seem
gradients, especially
especially near the
lakeshore, and
and the
the streambeds
streambedsdo
donot
notcontain
containsuch
suchlarge
largeboulders.
boulders. No outcrops
outcrops of coarse conglomlakeshore,
erate would be expected nearby to the north
north beneath
beneath the lake;
lake; so
so ifif the
theboulders
bouldersare
areglacial
glacial erratics,
erratics,
they must
perhaps from
from the vicinity of
must have
have been transported for a long distance,
distance, perhaps
of Isle
Isle Royale.
Royale.
Stop 9.--Nonesuch Shale
Shale at
at Bonanza
Bonanza Falls
Falls
The most
most complete exposure of the Nonesuch
Nonesuch Shale in the region is along the Big
Big Iron
Iron River
River
BonanzaFalls,
Falls,although
althoughaccess
accesstotoparts
partsofofititisisdifficult
difficult and
and dangerous
dangerousat
at times
times of
of high water
near Bonanza
(see fig.
fig. 10).
10). The
TheNonesuch
Nonesuchisisexposed
exposed nearly
nearly continuously in
in aa gently
gently southeast-dipping
southeast-dipping section
section
Fallsto
to the
the sharp
sharp bend
bendin
inthe
the river
river near
nearthe
the northeast
northeast corner
corner of
of
from just upstream
upstream of Bonanza
Bonanza Falls
(1991)
section 13. AAdetailed
detailedmeasured
measuredsection
section is
is presented
presented by Suszek
Suszek (1
991 ) and fig.
fig. 11
11isisgeneralized
generalized
from that section. The
Theexposed
exposed rocks total 226
226 m
m of
of section,
section, which
which includes
includes nearly
nearly all of the
Nonesuch, although neither the
the upper nor lower contact
Nonesuch,
contact is
is directly
directlyexposed.
exposed.
The Nonesuch Shale
Shale is
is distinguished
distinguished from
from other
other sedimentary units of the
the Keweenawan
Keweenawan
by the
the predominance of
of gray,
gray, green or
or black,
black, fine-grained
fine-grained sediments.
sediments. The
Supergroup by
TheBig
BigIron
IronRiver
River
predominanceof
ofsiltstones
siltstones and
andfine
finesandstones
sandstonesover
overtrue
trueshales.
shales. Many rocks show
section has aa predominance
trough cross
asymmetrical ripples,
ripples, rib
rib and
and furrow
furrow structures, and
cross bedding, symmetrical and asymmetrical
and parting
parting
lineations. The
shales, which
which are most abundant lower in
lineations.
The finer-grained
finer-grained rocks
rocks include
include well-laminated shales,
in
the section. The
shaleyunits
unitscommonly
commonlyhave
haveball
balland
and pillow
pillowstructures
structuresand
andcalcareous
calcareousconcretions.
concretions.
Theshaley
The Nonesuch displays coarsening-upward
at scales ranging from aa few
coarsening-upward sequences at
few meters
meters
to
to the
the entire
entire thickness
thickness of
of the
the unit.
unit.On
Ona asmaller
smallerscale,
scale,normally
normallygraded
gradedsequences
sequences are
are common
common in
in
units from a few centimeters
centimeters to aa few
few meters
meters thick.
thick.
concentrations of
In the lower 10
10m
m of
of the
thesection
sectioncopper
copper mineralization
mineralizationcan
can be
be seen as concentrations
chalcocite, bornite
bedding planes.
planes. The mineralization is
is cogenetic
cogenetic with the
bornite and malachite along bedding
the
copper mineralization
mineralizationat
atthe
the White
White Pine
PineMine
Minewhere
wherethe
thedowndip
downdip extension
extension of
of this unit
major copper
unit is
is
mined just
just to
to the
the south
south and
and east.
east.
A good
good exposure
exposure of
of the
the mineralized
mineralized base of the Nonesuch
Nonesuch Shale and the top of
of the
theCopper
Copper
Harbor Conglomerate
Conglomerate is
is along
alongthe
the Little
Little Iron
Iron River
River near
nearthe
thecenter
centerof
of the
the SW
SW 114,
1/4, Sec. 13,
13, but
but
It is an easy walk
requires aa walk
walk of about 1 mi south from Highway
Highway 107.
107. It
walk along
along an
an unmaintained
unmaintained
trail on the east bank of the river
river for those
those who
who can
can spend
spend more time in the area. Remains
Remains of
of early
early
mining efforts for
there, as
as well
well as
as "ore"
"ore" specimens from
from old dumps.
for native
native silver
silver can
can be seen there,
White Pine Mine
Mine
White
The trip will
will not
not visit
visitthe
theWhite
WhitePine
Pine Mine;
Mine; but
butbecause
because of the
the importance
importance of
of this
thisorebody,
orebody,
the following
following brief
briefsummary
summary is
is provided.
provided.
copper and
andsilver
silverfrom
from aa very
very large
large strataform
strataform orebody in the base of
White Pine recovers copper
Shale and
and in
in places
places from
from the upper few meters
the Nonesuch Shale
meters of
of the
the Copper
Copper Harbor
Harbor Conglomerate.
Conglomerate.
Excellent summaries
summariesof
of the
the geology
geology and
and origin
origin of
of the
the orebody
orebody are
are provided
provided by
by White and Wright
(1966),
and others
others(1
(1968),
White (1
(1971),
and Brown
Brown(1
(1971).
mostly of
of
(1966), Ensign
Ensign and
9681, White
971 1, and
971 1. Ore,
Ore, composed
composed mostly
copper, grades about
about 1
.1%%Cu
Cuand
and99glt
g/t Ag
Ag over a mining height
height
chalcocite and lesser native copper,
1.1
typically about 5 m.
m. Some
Somebeds
bedswithin
withinthat
thatinterval
intervalconsistently
consistentlycontain
containgreater
greater than
than 3%
3%Cu.
Cu.
Reservesare
areabout
about200,000,000
200,000,000 tt of extractable ore.
ore. Mining
Mining of
of the
the gently
gentlydipping
dippingorebody
orebody is
is by
by
Reserves
and pillar
pillar method.
method. The mine workings underlie about
about 35
35 km2
km2 of
of the low country
room and
country south
south and
and
town of
east of the town
of White
WhitePine.
Pine.
93
1
w
-
1
7
4
-
-
i
I
00
55
I
+
1MILE
MILE
00
47
7
I KILOMETER
; KILOMETER
-
CONTOUR
CONTOUR INTERVAL
INTERVAL 20
20FEET
FEET
Figure 10.
10. Location
Locationand
andgeologic
geologicsetting
settingof
ofstop
stop9.9.Geology
Geologygeneralized
generalizedfrom
fromCopper
Copper
Figure
Range Corp./Michigan
CorpJMichiganGeological
GeologicalSurvey
Surveycooperative
cooperativemapping
mapping(unpublished).
(unpublished).
Range
94
STRATIGRAPHIC SECTION
SECTION OF NONESUCH
NONESUCH SHALE
SHALE AT
AT BONANZA
BONANZA FALLS
FALLS
STRATICRAPHIC
Generalized from
from Susiek
Suszek (1991)
(1991)
Sandstone,
Sandstone, siltstone,
siltstone.rnudstone.
mudstone. Fineto
Fine— to
coarse—grai
ned. coarsening-upward
coarsening—upward trend.
coarse-grained,
trend.
200—
Reddish-brown
increase in
in top
top 50 rn
Reddish—brown units increase
m
of unit.
unit.
8
3
SiItstone,
sandstone. muastone.
mustone. Fine—
Siltstone. sandstone.
Fine- to
to
medi.rn—grained, general
general coarsening
mediim-grained,
ccarsening upward
upward trend.
trend
Sndstcne.
Sandstone, shale.
shale.
Coarse- to
to fine—
fine- grained.
grained.
Coarse—
Fine— tornediun—grained.
Sandstone. shale.
Sandstone,
shale. Fineto mediim-grained.
CContains
o n t a i n s some
some red—brown
red-brown units.
units.
oc
Shale, sandstone.
sandstone, mudstone.
mudstone. Black
B l a c k to
to dark
dark gray.
gray.
Lominites.
Lcminites. trough
trough cross
c r o s s bedS,
beds. and
end undulatory
undulatory
bedding.
Chioritic and henatitic
Chloritic
hematitic beds and
and
lenses. Calcareous concretions.
lenses.
concretions.
Sandstone, siltstone.
siltstone. mudstone.
mudstone.
Light to
to dcrk
dark
gray wwith
gray
i t h some
s o m e beds
b e d s of
of red
red hematitic
hematitic mudstone
mudstone
and siltstone.
Fine—grained. Contains
siltstone. Fine-grained.
Contains
carbonate lominite
iominita units, ball
boll and pillow
pillow
structures, and colcareous
concretjons.
structures,
calcareous concretions.
Fissil. shcie,
Fissile
shale. mmudstone,
u d s t o n e , siltstone.
siltstone. Block
to
Black to
laninites with
w i t h interbeds
interbeds
dark gray. Carbonate
Carbonate lanintes
of siltstone
siltstone and
and sandstone.
sandstone.
SSiltstone,
i l t s t o n e , sandstone,
sandstone. shale.
shale. Fine—grained.
Fine-grained.
Dessication
Dessication cracks,
cracks, ball
ball and pillow
structures, floating
floating clasts.
clasts.
Figure
Measuredstratigraphic
stratigraphicsection
sectionof
ofNonesuch
Nonesuch Shale
Shale along Big Iron River near
Figure 11. Measured
Falls, generalized from Suszek
991).1.
Bonanza Falls,
Suszek (1
(1991
95
Copper
Copper was introduced
introduced to
to the
the Nonesuch
Nonesuch Shale
Shale mostly
mostly during
during early
early diagenesis,
diagenesis, probably
probably by
by
upward
upwardcirculating
circulatingconnate
connate water
water which
whichdissolved
dissolvedcopper
copper from
from the
the underlying
underlyingredbeds.
redbeds. Chalcocite,
Chalcocite,
largely
of diagenetic
diagenetic pyrite.
pyrite. AAlater
largely of
of submicroscopic
submicroscopic size,
size, formed by the replacement of
laterphase
phaseofof
others
(in
copper
mineralization,
documented
by
Mauk
and
others
(1989)
and
Mauk
and
copper mineralization,
Mauk
(1989) and Mauk and others (inpress),
press),
introduced
introduced native
native copper, mostly
mostly in
in structurally
structurallydisturbed
disturbed zones.
zones. This
Thissecond
secondstage
stagemineralization
mineralizationisis
probably
probably cogenetic with
withthe
theclassic
classicnative
nativecopper
coppermineralization
mineralizationof
of the
theKeweenawan
KeweenawanSupergroup
Supergroup
basalts.
basalts.
Stop
O.--Nonesuch Shale and
Stop110.-Nonesuch
and Freda
Freda Sandstone
Sandstoneat
at Presque
PresqueIsle
IsleRiver
River
The
The upper
upper portion
portionof
ofthe
theNonesuch
NonesuchShale
Shaleand
andthe
thebase
baseof
ofthe
theFreda
FredaSandstone
Sandstoneare
arewell
well
exposed
exposed in
in the
the gorge
gorge of
of the
thePresque
Presque Isle
Isle River
River near its mouth
mouth and
and along the shore
shore of
of Lake
Lake Superior
Superior
west
west of
of the
theriver
river(fig.
(fig.12).
12).Continuous
Continuousexposures
exposuresalong
alongthe
thepicturesque
picturesquegorge
gorge of
of the
theriver
riverextend
extend
from
fromjust
justupstream
upstreamof
ofNawadaha
NawadahaFalls
Falls to
to the
the lakeshore.
lakeshore. Exposures
Exposures continue
continue in
in bluffs
bluffsalong
alongthe
the
lakeshore
for about
about half
half aamile
milewest
westof
ofthe
theriver
rivermouth.
mouth. In the
the interest of
of time,
time, we will limit
limit our
our
lakeshore for
examination
near the
the river
river mouth
mouth and
and aa short
short distance to
to the west along
examination to exposures near
along the
the shore.
shore.
This
This requires
requires a round trip
trip hike
hike of
of nearly
nearly aa mile, mostly
mostly on
on well-maintained
well-maintained trails
trails and
and stairways.
stairways.
The
The rocks
rocks exposed
exposed here
here are on the northeast
northeast limb
limb of
of the
the Presque
Presque Isle
Isle syncllne,
syncline, aa gentle
gentle
10ÂSW.
SW.The
TheNonesuch
NonesuchShale
Shalehere
hereisis
northwest-plungingfold.
fold. Dips
Dips range
range from nearly
nearly flat to
to about
about 100
northwest-plunging
very
for stop 9 on the Big Iron River
Riverabout
about30
30 km
km to
to the
the east.
east. The
very similar
similar to that described
described for
The lower
lower
part
with copper
part of
of the
thesection
sectionin
inthis
thisarea
area (not
(notexposed)
exposed) is
is also
also strongly
strongly mineralized
mineralized with
copper and
and contains
contains aa
fine-scale
with the stratigraphy at the white
fine-scale stratigraphy directly correlatable
correlatable with
White Pine
Pine mine,
mine, indicating
indicating
that
thatsedimentary
sedimentary conditions
conditions were
were very
very uniform
uniformover
overthe
theentire
entireregion
regionsurrounding
surroundingthe
the Porcupine
Porcupine
Mountains
Mountainsduring
duringNonesuch
Nonesuchdeposition.
deposition.
The
The upper
upper part
partof
ofthe
theNonesuch
Nonesuchexposed
exposedhere,
here, as
asdescribed
describedin
inmore
moredetail
detailby
bySuszek
Suszek
(1991),
consistsof
offining-upward
fining-upwardsequences
sequencesand
andlaminites
laminitesinterbedded
interbeddedwith
withgraded
gradedsandstones
sandstonesand
and
(1991 1, consists
siltstones. Most
Mostrocks
rocksare
aredark
darkgray
graywith
withaafew
fewreddish
reddishlenses
lensesand
and beds.
beds.
The
upward to the
through a zone in which
The Nonesuch
Nonesuch grades upward
the Freda
Freda Sandstone through
which dark
dark gray
gray
laminated
siltstone and sandy mudstone
mudstone is interbedded
laminated and small-scale cross-bedded siltstone
interbedded with
withmediummediumto
to coarse-grained
coarse-grained reddish brown sandstone. The
The upper
upper contact
contact of
of the
theNonesuch
Nonesuchisisplaced
placedwhere
where
the
the reddish
reddishsandstone
sandstonebecomes
becomes dominant.
dominant.
.--
Stop
FredaSandstone
Sandstonealong
alongPresque
Presque Isle
IsleRiver
River
Stop 11.-11 Freda
This
This stop,
stop, near
near the
the axis
axis of
of the
thePresque
Presque Isle
Isle syncline (fig. 12),
121, shows
showsreddish
reddishcross
crossbedded
bedded
sandstone typical of the lower
lower part
part of
of the
the Freda
Freda Sandstone.
Sandstone. The
The gently
gently southwest
southwest dipping
dipping beds
beds
exposed here are
are probably
probably 100-200
100-200 m above the
the base
base of
of the
the formation and slightly
slightly higher
higher stratistratigraphically than those seen
seen at
at stop
stop 10. They
They are
are mostly
mostly lithic
lithic sandstone
sandstone and
and are somewhat
somewhat micaceous in places. The
The Freda
Freda marks the return
return to
to fluvial
fluvial redbed
redbed deposition
deposition following
followingthe
thelacustrine
lacustrine
deposition of
of the
theunderlying
underlyingNonesuch
NonesuchShale.
Shale.
deposition
The
The Freda
Freda is a very thick unit
unit in
in much
much of
of the
therift
riftininthe
theLake
LakeSuperior
Superiorregion
regionand
andisisvolumetvolumetrically the dominant unit
unit of
of the
the post-rift
post-riftsedimentary
sedimentary fill.
fill. Along
Along the
the Montreal
Montreal River,
River, about 30
30 km
km to
to
the
is even
even thicker
the west,
west, about
about 4,000
4,000 mmofofFreda
Fredaare
are exposed.
exposed. Seismic
Seismic sections
sections indicate that itit is
beneath the
the Lake.
Lake.
beneath
The
generally becomes
becomesfiner-grained
finer-grainedand
andmore
moremature
matureupward.
upward. Excellent
The Freda
Freda Sandstone generally
Excellent
exposures of the upper part of the
seen near
nearthe
the mouth
mouth of
of the Montreal
the Freda
Freda can be seen
Montreal River, in
in bluffs
bluffs
along the
the lakeshore.
lakeshore.
along
Stop 12.-12.- Jacobsville
JacobsvilleSandstone
Sandstone
Stop
The roadcut along the west side of the road is a fairly typical exposure of the
the conglomerate
conglomerate
found abundantly in the Jacobsville Sandstone
Sandstone in
in this
this region.
region. Although
Although exposures
exposures are rare,
rare, west of
of
Lake Gogebic the Jacobsville appears to be dominated by
by conglomerate
conglomerateand
and coarse
coarse sandstone
sandstonein
in
contrast
it is
to mature quartzose,
quartzose, feldspathic,
feldspathic, and lithic
is a submature to
contrast to
to areas
areas farther east where it
arenite with
with minor
minor shale,
shale, siltstone,
siltstone, and
and conglomerate
conglomerate layers (Kalliokoski, 1982).
1982).
At
strikes east
eastand
anddips
dips9O
9° to
to the
the north,
north, but only about 500 m
At this
this stop
stop the
the conglomerate
conglomerate strikes
to the
the north
north ititdips
dips steeply
steeply to the south
south (62°)
(62O)as
as aa result
result of drag
drag along the Keweenaw
The
to
Keweenaw fault.
fault. The
96
96
17
45W.
R.
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11CENW4ETER
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INTERVAL 55 METERS
METERS
CONTOUR
Locationand
and geologic
geologic setting
setting of
of stops
stops 10
10and
and 11.
1 1. Geology
Geologygeneralized
generalized from
from
Figure 12.
12. Location
Figure
Copper
Range
CorpJMichigan
Geological
Survey
cooperative
mapping
(unpublished).
Copper Range Corp./Michigan Geological Survey cooperative mapping (unpublished).
97
R. 45 W.
405
21
:
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THE MAP
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METERS ON ThE
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Figure
Figure 13.
13. Location
Locationand
andgeologic
geologic setting
setting of
of stop
stop12.
1 2.Geology
Geologygeneralized
generalized from
from Copper
Copper
Range
CorpJMichigan
Geological
Survey
cooperative
mapping
(unpublished).
Range Corp./Michigan Geological Survey cooperative mapping (unpublished).
98
is poorly
poorly consolidated and
and weathers
weathers to
to a material that superficially
conglomerate is
superficially resembles
resembles glacial
till or
or outwash, which covers
covers most of the roadcut. From
From time to time parts
parts of this
this material
material slump
till
away to
of fresh conglomerate
to expose
expose less
less weathered conglomerate,
conglomerate, and small exposures of
conglomerate are
usually exposed in the drainage
drainage ditch
ditch beside
beside the road.
road.
conglomerate is
is clast-supported.
clast-supported. Coarse
sandy matrix
matrix surrounds pebbles.
pebbles. Where
The conglomerate
Coarse sandy
Where
weathered, it is generally unconsolidated
unconsolidatedbut
buton
onfresher
freshersurfaces
surfacesisisfairly
fairlywell
wellcemented.
cemented. Clay fills
weathered,
interstices and
and coats
coats all
all grains,
grains. It
It is uncertain whether the clay is a product of diagenetic
diagenetic
most interstices
mechanicalinfiltration
infiltration resulting
resulting from
from seepage
seepageof
of muddy
muddy water
water through coarse
cement or is an early mechanical
alluvium.
are subrounded
subroundedto
to well
well rounded,
rounded, range
rangein
in size
sizefrom
from 3-1
3-10
cm, and consist mostly
Clasts are
0 cm,
mostly of
of
iron-formation (63-77%)
iron-formation
(63-77%)and
and quartzite
quartzite (8-20%)
(8-20%) derived
derived from
from Early
Early Proterozoic
Proterozoic rocks
rocks such
such as
as those
those
Range. Small amounts of altered rhyolite (1-5%)
(1-5%) are
are believed
believed to be
be
now exposed in the Gogebic Range.
from Keweenawan
Keweenawan Supergroup
Supergrouprocks.
rocks. Vein
Vein quartz
quartz (9-1
(9-10%),
litholoderived from
0%), chert (1%),
(1%I, and other lithologies (1 %
% or
or less)
less) derived
derivedfrom
from Archean
Archean rocks
rocks are
are minor.
minor. Several
Several other conglomerate
conglomerate outcrops
outcropsto
to
the west have
have similar compositions,
compositions, but outcrops
outcrops less
less than 2 km
km to
to the
thesoutheast
southeastalong
alongJackson
Jackson
Creek
(shown by X on the map) are dominated
dominated by
by vein
vein quartz
quartz (73-82%)
(73-82%) with
Creek (shown
with lesser
lesser amounts
amounts of
of
(11-13%)
and iron-formation
iron-formation (9-1
(9-12%).
suggest that
that most
most of
of the
quartzite (1
1-13%) and
2%). These
These clast lithologies suggest
area to
to the
the south had been
been stripped
stripped of
of Keweenawan
Keweenawanvolcanics
volcanicsby
bythe
the time
time of
of Jacobsville
source area
deposition. Although
Althoughthe
thesurface
surfaceconsisted
consistedmostly
mostlyofofEarly
EarlyProterozoic
Proterozoicsedimentary
sedimentaryrocks,
rocks, the
the
presence of
of large quantities of vein
vein quartz
quartz pebbles
pebbles indicates
indicates at
at least
leastsome
some drainages
drainageswere
wereeroded
eroded
Archean crystalline
crystalline rocks.
down into
into Archean
In a few places
appear to
to be imbricated
imbricated but
but otherwise bedding within
within
places clasts found in
in place appear
conglomerate units
units is
is not
not apparent.
apparent. Clay
nearly all
all clasts
clasts so
so that
that the
the conglomerate
Clay and iron coatings cover nearly
clasts are
are broken
brokenopen.
open. Many
various lithologic types are not apparent unless clasts
Many clasts
clasts have
have aa highly
highly
like desert
desert varnish
varnish and
and some
somehave
havedistinctly
distinctly striated
striated
shiny surface or patina that looks
looks remarkably like
surfaces of
of unknown origin.
origin. AAfew
fewclasts
clastshave
havebeen
beenfound
foundthat
thathave
haveshapes
shapes and
and surface textures
surfaces
resembling ventifacts.
east of
of the
the road,
road, slightly higher in the section, the
In the streambanks east
the conglomerate
conglomerate is
is interinterwith coarse sandstone.
sandstone. The
are poorly
poorly sorted,
sorted, discontinuous
discontinuous irregular
layered with
The sandstone
sandstone layers are
approximately 5-30
5-30 cm thick. The
layers approximately
Thesandstone
sandstoneconsists
consists of
of subangular
subangularto
tosubrounded,
subrounded, medium
medium
to coarse
coarse sand (.25-2 mm)
mm) and
and gravel (2-30
(2-30 mm)
mm) with
with larger
larger cobbles
cobbles scattered
scattered throughout.
throughout. In thin
thin
section the sand is framework
framework supported
predominantly of
of quartz and feldspar with
with
supported and composed predominantly
minor lithic fragments, biotite, and
and muscovite.
muscovite. Pores
Poresare
are filled
filled with
withclay
clayand
andmany
manygrains
grainsare
are coated
coated
with
with iron
iron minerals.
minerals.
99
CITED
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1171-1 190.
90.
102
GEOLOGY OF ThE
GREAT LAKES
TECTONIC
TECTONICZONE
ZONE
INTHE
IN THE
MARQUE
TIE AREA,
MARQUETTE
AREA,
MICHIGAN
MCHIGANA
ARCHEAN
ALATE
LATE
ARCHEAN
-
PALEOSUTURE
PALEOSUTURE
P.K.
P.K. Sims
Sims and
and Z.E.
Z.E. Peterman
Peterrnan
U.S .Geological
Geological Survey,
Survey, Federal
Federal Center,
Center,
U.S.
Denver,
Denver, Colorado
Colorado
the geology
geology of
tectonic zone in the
Guide to the
of the
the Great Lakes tectonic
Marquette area,
area, Michigan--A
Michigan--A Late Archean paleosuture
P.K.
and Z.E. Peterman
P.K. Sims and
SUMMARY
length
The Great Lakes tectonic zone (GLTZ) is
is an Archean crustal boundary of subcontinental length
that separates a greenstone-granite terrane (southern part of
of Superior
Superior province of Canadian
Shield) on
on the
the north
north from a partly
partly older
older gneiss
gneissterrane
terrane on
on the south.,
south. The
TheGLTZ
GLTZisisgenerally
generally
Shield)
collision. The tectonic zone is
paleosuture resulting
resulting from continent-continent collision.
interpreted as a paleosuture
covered in most of the Lake
covered
Lake Superior
Superior region
region by
by Proterozoic
Proterozoic rocks
rocks or
orPleistocene
Pleistoceneglacial
glacialdeposits,
deposits,
and its position and
and characteristics
characteristics were
were previously
previously determined
determined mainly
mainlyby
by geophysical
geophysical data.
data.
Geologic mapping
mappingin
in the
the Marquette,
Marquette, Michigan
Michigan area
area provides
provides for
for the first time direct
Geologic
direct
observations of
of the structure.
observations
In the
the Marquette
Marquette area,
area,the
theGL1'Z
GLTZisischaracterized
characterized by
by aa zone
zone of
of mylonite
mylonite (orthomylonite)
(orthomylonite) that
that
formed at brittle-ductile
brittle-ductile transition
transition conditions;
conditions; this
this was superposed on
on previously
previously deformed
deformed rocks
rocks
of both the Archean
Archean greenstone-granite
greenstone-graniteterrane
terraneand
andthe
theArchean
Archeangneiss
gneissterrane.
terrane. The
W. Foliation in the mylonite strikes about
boundaries of the GLTZ
GLTZ trend
trend about
aboutN.
N. 600
60' W.
N. 70'
70° W.
W. and
and dips
dips steeply
steeply southwest.
southwest. A pronounced
pronounced stretching lineation and tight fold hinges
N.
about42°
42' S.
S. 430
43O E. The attitude
attitude of
of the
the stretching
stretching lineation (line
(line of
of tectonic
tectonic transport),
transport),
plunge about
of movement sense, indicate that collision
together with asymmetric structures indicative of
collision at
at this
this
was oblique.
oblique. This resulted in dextral-thrust
locality was
dextral-thrust shear along
along the
the boundary,
boundary, northwestward
northwestward
vergence, and
and probable
probable overriding
overriding of
ofthe
the greenstone-granite
greenstone-graniteterrane
terrane by
by the
the gneiss
gneiss terrane.
terrane.
vergence,
Transmittal of the dextral shear stress across a large area of
of the
the greenstone-granite
greenstone-granite crust
crust
(Superior province) to the
the north
north may
may have
have formed
formed the
thenearly
nearly east-west
east-west foliation,
foliation, upright
upright folds,
folds,
and northwestdextralfaults
faultsand
andshear
shearzones
zones at
at least
least as
as far north as the
northwest- to east-west-trending
east-west-trending dextral
the
Quetico fault,
fault, in
in southern
southern Ontario,
Ontario, a distance
distance of
of about
about 250
250 krn.
km. These structures in Superior
province rocks are superposed on
on older
older recumbent
recumbent folds.
folds.
angular bends
bends that alternately trend
As a whole, the GL1'Z
GLTZ is characterized by systematic
systematic angular
trend westwestnorthwestward,
as in
in the
the Marquette
Marquette area, and northeastward.
northeastward. This
northwestward, as
This zigzag
zigzag pattern probably
probably
reflects relict irregularities in the continental margin (Superior province)
province) composed
composed of
of
greenstone-granite crust.
crust. Late
Archean convergence
convergence along this margin resulted in aa variable
variable
greenstone-granite
LateArchean
trajectory of stress into the
the greenstone-granite
greenstone-granite crust
crust and
and probably
probably in
in along-strike
along-strike diachroneity
diachroneity of
of
orogeny. The
orogeny.
The major
major deformation
deformation resulted
resulted from
from oblique
oblique compression
compression at
at promontories,
promontories, which
which
which compressive
compressivestress
stresswas
wasdirected
directedinto
intothe
thecrust.
crust. In addition to
to
acted as buttresses against which
the dominant
strike-slip faults
faults also
also formed,
formed, such
such as
as the
the
dominant foliation,
foliation, major brittle-ductile
brittle-ductile to brittle strike-slip
Vermilion fault
fault system
system in
in northern
northern Minnesota
Minnesota and the Quetico and Rainy
Vermilion
Rainy Lake-Seine
Lake-Seine River
River
faults in southern Ontario; these resulted from a more brittle continuum of the
the transcurrent
transcurrent
shear caused by collision along the GLTZ.
INTRODUCTION
km
The Great Lakes tectonic zone (GLTZ) is an Archean crustal boundary more than 1,000
1,000 krn
long that separates a greenstone-granite
terrane (southern part of Superior province)
greenstone-granite terrane
province) on the
north from a gneiss
gneiss terrane
terrane on
on the
thesouth
south(Sims
(Sims and
andothers,
others,1980;
1980;Sims
Sims and
and Peterman,
Peterman, 1981;
1981;
Peterman, 1979). The
The GLTZ
GLTZisiscovered
covered throughout
throughout most of
of the
the Great
GreatLakes
Lakesregion
region by
by younger
younger
105
glacial deposits,
deposits,but
but recently
recentlyitithas
has been
been delineated
delineated and studied
Proterozoic rocks or Pleistocene
Pleistocene glacial
in outcrop in an area
area south
southof
of Marquette,
Marquette,Michigan
Michigan (fig.
(fig. 1).
1).
The
The boundary
boundary was
was first
first recognized
recognized in
in Minnesota
Minnesota (Sims
(Sims and
andMorey,
Morey,1973;
1973; Morey
Morey and
andSims,
Sirns,1976)
1976)
from regional geologic relations,
relations, which
which indicated
indicated that
that the two adjoining basement terranes
terranes had
had
different geologic histories
histories and probably had evolved
separately. Regional
evolved separately.
Regional magnetic
magnetic and
and gravity
gravity
data were utilized to determine the
the position
position of
of the
the boundary.
boundary. Later
Later(Sims,
(Sims,1980),
1980),the
the boundary
boundary
approximately delineated
delineated in the western part of
was approximately
of Upper
UpperMichigan
Michigan(Sims,
(Sims, 1980;
1980; Sims and
others, 1984)
1984) and northwestern Wisconsin (Sims and others, 1985),
1985), east of the
the Middle
Middle
Proterozoic Midcontinent rift system,
system, and itit was
was inferred by
by indirect
indirect evidence
evidence to
to extend
extend
eastward through the Sudbury
the Middle
Middle Proterozoic
Proterozoic
Sudbury structure, where it is truncated by the
Grenville tectonic zone (Sims
(Sims and others,
others, 1980).
1980).
Recent geologic
geologic mapping in the Sands
Sands and
and Palmer
Palmer7½-minute
7%-minutequadrangles,
quadrangles, Michigan
Michigan (fig.
(fig. 1),
I),
previously mapped
mapped by
by Gair
Gair and Thaden (1968)
(1968) and Gair (1975),
(1975), has delineated this Archean
outcrop for
for the
thefirst
firsttime
time(Sims,
(Sims,1991).
1991). It is exposed
exposed on the south side
side of the
the Early
Early
boundary in outcrop
Proterozoic
Proterozoic Marquette
Marquette synclinorium,
synclinorium, and its northwestern projection
projection into the
the trough
trough coincides
coincides
with a major Early
Early Proterozoic
Proterozoic fault,
fault,the
the Richmond
Richmondfault.
fault. In the Marquette area,
area, the
the GLTZ
GLTZisis
wide that is overprinted
krn wide
overprinted on rocks of both the
the greenstone-granite
greenstone-granite
a mylonite zone about 2 km
terrane and
Inthis
this area,
area,the
theGL1'Z
GLTZisisinterpreted
interpreted as
asaacontinentcontinentand the
the Archean
Archean gneiss terrane. In
continent collision
zone. The
resulting in
in dextral
dextral wrench
wrench shear
shear on
on the
the
collision zone.
The collision
collision was oblique, resulting
60ÂW.-trending
the
N. 60°
W.-trending boundary
boundary and
and northwestward
northwestward vergence
vergence of
of the
the gneiss
gneiss terrane
terrane against the
greenstone-granite terrane.
The purpose of this field guide is to examine
examine the exposed
exposed GLTZ
GLTZ in the context of
of the regional
geology, to
to discuss
discuss genetic
geneticrelationships
relationshipsbetween
betweenconvergence
convergencealong
alongthe
theboundary
boundaryand
and structural
structural
geology,
features in the Archean rocks to the north, and to present
present aa new
new interpretation
interpretation of
of the
theevolution
evolution
of the GLTZ throughout
throughout the
the Lake
Lake Superior
Superior region.
region.
GEOLOGIC
GEOLOGIC SETI1NG
SETTING
in the Sands and Palmer
moderately well exposed in
The Great
Great Lakes
Lakes tectonic
tectonic zone (GLTZ)
(GLTZ) isis moderately
7%-minute quadrangles in Upper
Upper Michigan
Michigan (fig.
(fig. 1).
1). It separates
separates the
thetwo
two distinctive
distinctive Archean
7½-minute
terranes in the area.
area. The
Thenorthern
northerngreenstone-granite
greenstone-granite terrane
terraneisiscomposed
composed largely
largely of Late
granitoid rocks,
rocks, with
with subordinate
subordinate approximately
approximately coeval metavolcanic and
Archean granitoid
metasedimentary
metasedimentary rocks
rocks of
of greenstone
greenstone affinity.
affinity. The layered rocks and most of the granitoid
rocks were metamorphosed
metamorphosed (mainly to greenschist
greenschist facies)
facies) and deformed during Late Archean
orogeny.
The southern
southernArchean
Archeangneiss
gneissterrane
terraneisiscomposed
composedmainly
mainlyof
oflayered
layered gneiss,
gneiss, migmatite,
migmatite,
orogeny. The
and amphibolite--rocks
that are
are distinct
distinct from
from those
those in
in the
the greenstone-granite
greenstone-granite terrane.
terrane. Except
amphibolite--rocks that
Except
post-tectonic, generally
for late-tectonic to post-tectonic,
generally small(?)
small(?)granitoid
granitoidbodies,
bodies,the
therocks
rocksofofthe
the terrane
terrane are
metamorphosed,
metamorphosed, mainly to
to amphibolite
amphibolite facies.
facies. The rocks exposed within
within the two
two terranes in
Upper Michigan
Michigan are
are closely
closely similar to those in Minnesota (Morey
(Morey and
and Sims,
Sims, 1976;
1976; Sims and
1980), thus
others, 1980),
thus establishing
establishingthe
the identity
identity of
ofthe
the GLTZ
GL1'Z in
in the
the Marquette area.
are overlain
overlain in
in the Marquette
the Republic
The Archean rocks in Michigan are
Marquette synclinorium,
synclinorium, the
trough,
River, Clark
Clark Creek,
Creek, and
and Baraga
Baraga basins
basins by
by shelf and foredeep deposits
deposits of
of
trough, and the Dead River,
Cannonand
andGair,
Gair,1970;
1970;Barovich
Barovich
the Early Proterozoic
Proterozoic Marquette
Marquette Range
RangeSupergroup
Supergroup(fig.
(fig. 1;1;Cannon
and others, 1989).
1989).
106
Bayley(1897)
(1897)introduced
introducedthe
thenames
names"northern
northerncomplex"
complef for the Archean rocks
rocks
Van Hise and Bayley
north
of
the
Marquette
synclinorium
and
southern
complef
for
the
Archean
rocks
south
north
synclinorium and "southern complex" for the Archean rocks south of the
syndilnorium.
synclinorium.
relationships in
A Late Archean age
age for
for the
the GL1'Z
GLTZisis now
now established
established by regional geologic relationships
north-central United
United States.
States. The Archean rocks in
in the
the greenstone-granite
greenstone-granite terrane
terrane in
in northern
northern
north-central
Minnesota (Hudleston and others,
1988)
and
northernmost
Michigan
(fig.
1;
Sims,
1991)
are
others, 1988)
Michigan (fig. 1; Sirns,
are
mainly by
by ductile
ductileand
and brittle
brittle structures
structures formed
formed in
in response to dextral
dextral shear,
shear, which
which
characterized mainly
accords with
with the
the deformation
deformation pattern
pattern observed
observed in
in exposures
exposures of
ofthe
the GLTZ
GLTZ south of Marquette.
west-trending steep foliation and upright
These structures include a generally
generally west-trending
upright folds,
folds, widespread
indicative of dextral shear; the
Z-shaped folds, and northwest- to west-trending dextral faults, indicative
the
(Johnson and Bomhorst,
Bornhorst, 1991).
structures are superposed on recumbent folds (Johnson
1991). In
In contrast,
contrast, Early
Early
Proterozoic Penokean
Penokean deformation
deformation in
in the
the Marquette
Marquette area had little effect on the Archean
Proterozoic
basement (Cambray,
1984). Cambray (1984)
(1984)proposed
proposed that
that the Penokean deformation was
basement
(Cambray, 1984).
produced by
by horizontal
horizontal compression
compressionthat
thatwas
wastransmitted
transmittedfrom
fromthe
thebasement
basement to
to the folded
folded cover
cover
produced
rocks
by
narrow
ductile
shears
in
the
basement.
Readjustment
of
rigid
basement
blocks
along
rocks by narrow ductile shears in the basement.
weaknesses,which
whichresulted
resultedininsome
someshortening
shorteningbut
but not
not folding
folding of
of the
the
these shears utilized old weaknesses,
basement rocks.
proposed a
rocks. For
Forthe
theEarly
EarlyProterozoic
ProterozoicMarquette
Marquettesyndlinorium,
synclinorium, Cambray (1984) proposed
nearly north-south compressional
which initially
initially produced
produced reverse dip slip
axis, which
slip on
on faults
faults
compressional axis,
bounding
the
syncline
and
compressed
the
Early
Proterozoic
sedimentary
rocks
within
the
bounding
and compressed the Early Proterozoic sedimentary
within the
syncline into
into west-trending
west-trending folds.
folds. Subsequently, resistance
resistance to this movement within the
the trough
trough
resulted in sinistral
sinistral strike
strike slip
slip motion and
and the
thedevelopment
development of
ofF2
F, folds
folds with
with northwest-trending
northwest-trending
axial surfaces and variable
variable plunge.
plunge.
Gair and Thaden (1968)
applied the
the name "Compeau
"Compeau Creek
Creek Gneiss"
Gneis? to both the
(1968) applied
the foliated
foliated
rocks of
of the Archean
Archean greenstone-granite
greenstone-granite terrane
terrane (northern complex)
granitoid rocks
complex) and the
the layered
layered
gneisses and
and massive
massive intrusions
intrusions of
of the
the gneiss
gneiss terrane
terrane (southern complex),
gneisses
complex), and subsequent
investigators extended
extended this
this terminology
terminologyto
to the
the western
western parts
parts of
of the southern
investigators
southern complex
complex (Cannon
1973). To apply this
this name
name to rocks in
in both
both Archean terranes, however,
and Simmons,
Simmons, 1973).
however, is
inappropriate, because the two
two terranes
terranes consist
consist of
of distinctive
distinctive rock types of different origins.
origins.
Accordingly, in
in this
this guide
guide informal
informal lithologic
lithologicnames
namesare
are used
used to
to describe the
Accordingly,
the crystalline
crystalline rocks
rocks in
in
the two terranes.
ARCHEAN GREENSTONE-GRANITE TERRANE
TERRANE
greenstone-granite terrane
terrane in the Marquette area
The greenstone-granite
area (fig.
(fig. 1)
1) consists of a several thousandmeter-thick succession
succession of metamorphosed
metamorphosed subaqueous
subaqueousmafic
mafictotofelsic
felsicflows,
flows,pyroclastic
pyroclastic rocks,
rocks,
volcanogenic sedimentary
sedimentaryrocks
rocksthat
that is
is intruded
intruded by
by small
smallbodies
bodies of
of gabbro
gabbro and ultramafic
and volcanogenic
rocks and by large plutons of granitoid rocks (Bornhorst,
Johnson and Bornhorst,
(Bornhorst, 1988;
1988; Johnson
Bomhorst, 1991).
1991).
The metamorphosed volcanic rocks
rocks have
have been
been named
named the
the Ishpeming
Ishpeming greenstone
greenstone belt
belt (Morgan
and DeCristoforo, 1980);
they represent
represent the southwestern
extension of
of the Wawa
1980); they
southwestern extension
(Shebandowan)
subprovince of
of the
the Superior province
province of
of Canada (Card and
(Shebandowan) subprovince
and Ciesielski,
Ciesielski, 1986).
1986).
volcanicrocks
rocksadjacent
adjacenttotoan
an ultramafic
ultramaficbody
bodynorth
northofofIshpeming
Ishpemingare
are host
host to
to the gold
Felsic volcanic
deposits of the
the Ropes
Ropes mine
mine(Brozdowski
(Brozdowski and
and others,
others,1986;
1986;Brozdowski,
Brozdowski, 1988,
1988, 1989), and the
ultramauic body
bodyhosts
hostsadditional
additionalmineral
mineralprospects
prospects(Bodwell,
(Bodwell,1972).
1972). Foliated
Foliated tonalite from the
ultramafic
the
1978) in
in the
the greenstone-granite
greenstone-granite terrane,
terrane, has a U-Pb
northern complex (Hammond, 1978)
U-Pb zircon
zircon age
age of
of
2,703±16
by Zell
Zell E. Peterman), and associated rhyolite has aa U-Pb
2,7032 16 Ma (recalculated
(recalculated by
U-Pb zircon
zircon age
age
byZeil
ZellE.
E. Peterman).
Peterman). These ages are consistent with
with more
± 69 Ma (recalculated
of 2,780
2,780269
(recalculated by
precise U-Pb ages in the Wawa
subprovince
in
adjacent
Canada
(Corfu
and
Stott,
Wawa subprovince in adjacent Canada
Stott, 1986).
1986).
107
Figure 1.
46°45
Xmsç
MICH
—- A
0
0
-r
5
5
•F )1f
8P30'
10
1''•
Marc1uette
IAKU SIJPURIOR
10 MILES
15 KILOMETERS
(h'
—
Geologic map of part of Marquette 1°x2° quadrangle, Michigan. After Sims (1991). Palmer 7½-minute quadrangle is
immediately west of Sands quadrangle. Marquette 7½-minute quadrangle is immediately north of Sands quadrangle.
880 00
________
_________
CORRELATION
CORREIATION OF
O F MAP
MAP UNITS
Archean
Archean
terrane
Greenstone-granite terrane
Reany Creek
Creek Formation
Formation
.-
\
J Proterozotc
Middle
Unconformity
Unconforrnity
Granitoid rocks—
Dominantly granodiorite
lie but
Granitoid
rocks-bminantly
grandiodteand
andtonal
tonalite
Including granite; generally
including
generalb foliated
foliated
Mafic
Mafic to felsic
felsic flows
flows and
and pyrodastic
pyrodasticrocks
r o c kand
and
volcanogenic
sedimentary rocks
wkanogenic sedimentaq
rocks
Ultramafic
Utramafiirocks
rocks
>
1-1'a
m
i
i:
Penokean orogeny
Penokean
orogeny deformation
deformation and
and metamorphism
metamor~ism
xm1L..... 1
+1
\\\\'' )
\ .\I.
,-!t~yt
p
W}Mzgy 1
1
Unconformity
Unconforrnily
Xmn
Menominee
Group
J
—
Marquette
Marquette
Range
Range
Supergroup
suwrgrOup
Xc j
granite
and massive granite
Contact
Contact
-
J
a
A
1
.
I
Late
Unconformity
I
-15
—15
kf5
J
DESCRIPTION OF MAP UNITS
DE!XRPTION
UNnS
Middle
Mlddle Proterozoic
Proierowic
.
Jacobsvllie
Jacobsville Sandstone
Sandstone
Early Proterozoic
Alkali granite
33 ±25
Alkali
granite (1.7
(1,733
*25 Ma)
Ma)
,,
Marquette Range SUPWSTOUP
Supergroup
, Marquette
j
Vertical
Bearing and
lineation—
Bearing
andplunge
plungeofof
lineation-May
Maybebecombined
combinedwith
with
foliation symbol
foliation
symbol
Direction
Direction of stratigraphic tops
tops
--15
} Ahean
y\.;;3,77,
dip of foliation
Strike and dip
foliation
Inclined
Inclined
4—
+
I
F,
Transcurrent
fault—
Transcurrent
fault-Showing
Showingrelative
relathhorizontal
horizontalmovement
m-mt
A Thrust
Thrustfault—Sawteeth
fault-Swteeh on upthrown
upthrownside
side
50
Continent-continent
collision ((=2,690
Cmtinentantinent colliskm
~ 2 , 6 9Ma)
Ma)
0
South of
North of
ofGLTZ
GLTZ
South
of GLTZ
GLTZ
North
Gnelss terrane
terrane
Gneiss
Greenstone-granitetterrane
emne
Greenstone-granite
!'zá'_1
+ _±I
- - ".1 e:w;aJ
, 4 1 - wuI
Gneiss, migmatite,
amphibolite—lncludes
Gneiss,
migmatite,and
and
amphibolide-Includes foliated
foliated
7High-angie
Wih-anglefault—Bar
fault-& and
andball
ballon
ondownthrown
downthrownside
side
Chocolay
Group
I
3
Ago
Early
h
dy
Proterozolc
Proterozoic
I
Unconformity
unconformity
Gneiss terrane
terrane
Tilden (Hammond,
Granite near Tilden
(Hammond, 1978)
1978)
[\-.\' -"i
Baraga
Group
Group
Bearing
of minor
minor fold
Bearins and plunge of
fold
MyloniteMylonlte— Dominantly
Dominantly orthornylonite
orthomyionite
CRSZ
Carp River shear zone
CRFSZ
Carp River Falls
Falls shear zone
zone
DRSZ
DRSZ
Dead River
River shear
Dead
shear zone
zone
GLTZ
GLTZ
tectonic zone
Great Lakes tectonk
zone
GGT
GGT
R
217
Greenstone-granite terrane
terrane
Ropes mhse
Ropes
mhe
J.
Michigamme
MkhlgammeFormation—Dominantly
Fonnatbn-Domlmntlysiaty
slatyrocks
rocksIn
Inlower
lowerpart
part
88'
89'
Ciarksburg
C
l a h b u r g Volcanica
Volcnnlcs Member
Member of
of Michigamme
Mkhigamm Formation
Fonnatbn
Goodrich
Quartzite
G
o d r i c h Quartzite
41'
map
Menominee
Group, undlvlded,
undivided, in
trough
Menominee Group,
i n Republic
Republic tmugh
1
Xmn
1
w
1
1
Xch
Edge of
Siamo
Ajibik Quartzite,
S i m o Slate and Ajibik
Quartzite, undivided
undivided
L_
eozoicNç
Negaunee
Iron-formation
Negaunee Iron-formatbn
cover
o
o
Cisocolay
Chocolay Group, undivided
undivided
4OKIWMETERS
46'
INDEX MAP
4OMLIS
Granitoid Rocks
Rocks
The Late Archean granitoid
granitoid rocks
rocks in the
the greenstone-granite
greenstone-granite terrane
terrane (fig.
(fig. 1)
1)are
aredominantly
dominantly pink
pink
to grayish pink,
pink, generally
generally medium
medium grained,
grained, porphyritic,
porphyritic,foliated,
foliated,and
and homogeneous
homogeneous tonalite
tonalite or
granodiorite (Gair
P. 18-23).
xenoliths and
and schlieren of
of biotite
(Gau and
and Thaden,
Thaden, 1968,
1968, p.
18-23). They contain xenoliths
schist and amphibolite.
amphibolite. In
foliated granite is also a
In the
theSands
Sandsquadrangle
quadrangle(fig.
(fig. 1),
I), weakly foliated
common rock
rock type (table 1),
I), but its age relation to the
the tonalite-granodiorite
tonalite-granodiorite was
was not determined.
determined.
The bimodal composition of the granitoid rocks suggests,
however, that
that the
the granite represents
suggests, however,
represents aa
event.
discrete magmatic event.
The granitoid rocks of
of the greenstone-granite
greenstone-granite terrane
terrane in the Marquette area
area exposed
exposed on
on both
both
sides of the Marquette
altered (table 1). Plagioclase
Marquette synclinorium
synclinorium are strongly altered
Plagioclase (oligoclase)
(oligoclase) is
saussuritized and albite
albite twinning
twinning is largely obscured, and biotite is largely changed to
to chlorite
chlorite
and opaque oxides
(locallyrutile).
rutile). The rocks are highly
fractured and
and fracture
fracture surfaces are
are
highly fractured
oxides (locally
coated by chlorite and other propylitic
minerals.
Along
the
southern
margin
of
the
Marquette
propylitic minerals. Along the southem margin of
syndilnorium,
the granitoid
granitoidrocks
rocksare
are exceptionally
exceptionallyrich
richininquartz
quartz(table
(table1).
1). The granitoid rocks
synclinorium, the
in the Sands
Sands quadrangle also are
are protomylonites.
protomylonites. The
Theprotomylonite
protomylonite isis characterized
characterized by
by
recrystallization of quartz to
shes and
and localized
localized shear-induced
shear-induced recrystallization
recrystallization of
of
to fine
fine grain
grain sizes
plagioclase and potassium
potassium feldspar
feldspar to fie-grained
fine-grained polycrystalline
polycrystallineaggregates
aggregates(type
(type IP,
1P, llP,
liP, 1M,
lM,
and 111M
1M structures of Hanmer,
Hanmer,1982).
1982).
The relative homogeneity of the foliated granitoid rocks, the
occurrence of
of sharp-walled
sharp-walled
the occurrence
xenoliths, and
and the general absence of a lithologic layering
layering that
that could represent relict
xenoliths,
relict sedimentary
sedimentary
or volcanic layering
layering suggest
suggestthat
thatthe
the granitoid
granitoid rocks
rocksof
ofthe
the greenstone-granite
greenstone-granite terrane
terrane are of
magmatic origin.
origin. Their pervasive foliation
foliation is
is attributed
attributed to
to deformation
deformation subsequent
subsequent to primary
magmatic
crystallization, as
as discussed
discussedfollowing.
following.
Structure
The rocks in the greenstone-granite
greenstone-granite terrane
terrane (northern complex),
complex),on
onthe
the north
north side
side of
of the
the GLTZ
(fig. 1),
I), record early recumbent
recumbent folding
folding (F1)
(FJ of metavolcanic rocks of the Ishpeming
Ishpeming greenstone
greenstone
(fig.
Superposeddeformation
deformation(D2)
(D2)produced
produced northwestnorthwest- to
to west-trending
west-trending upright,
upright, upwardupward- and
belt. Superposed
An
downward-facing
1991). An
downward-facingfolds
folds(F2)
(F2)that
thatare
are Z-shaped
Z-shaped in
in plan
plan view
view (Johnson
(Johnson and
and Bomhorst,
Bornhorst, 1991).
axial plane
The Z-symmetry
Z-symmetry of
of the
theF2
F2folds
folds is
is consistent
consistent with
with
plane foliation was developed during Fp
F2. The
their development
dextral shear component.
component. Associated
development in a deformation
deformation regime with a dextral
Associated
by D2. Younger,
Younger, northwestnorthwest- to
to west-trending
west-trending faults,
faults, some
someof
of
granitoid rocks also were deformed by
which
have demonstrable
demonstrable dextral
dextral movement,
movement, transect
transect and
and offset
offset the
the folded
folded rocks.
rocks. Commonly,
which have
Commonly,
these faults
faults separate
separatedomains
domainsof
ofvolcanic
volcanic rock
rock that
that have
have opposite
oppositestratigraphic
stratigraphicfacing
facingdirections.
directions.
of fig.
fig. 1)
1) is
is
Foliation and lithologic layering
layering in
in the
the vicinity
vicinityofofthe
the Ropes
Ropes mine
mine (R,
(R, near center of
puzzling with
respect
to
the
dominant
regional
structure.
Here,
foliation
and
lithologic
layering
with respect to
Here, foliation and lithologic layering
strike
strike about
aboutN.
N.700
70' E. and are
are nearly
nearly vertical (Brozdowski,
(Brozdowski, 1988,
1988,p.p.A-44).
A-44). The published
area (Negaunee
(Negaunee SW
SW quadrangle;
quadrangle; Clark
Clark and
and others,
others,
geologic map
map that
that includes the Ropes mine area
hte
1975) also
also shows
showsaa steep
steep (stretch?)
(stretch?) lineation that plunges southeastward in unit Wkf of the Late
Archean Kitchi Schist (included in unit
unit Wv
Wv on
on fig.
fig. 1).
1). Possibly,
the
northeast-trending
foliation
Possibly, the
large-scaleD2
D2Z-fold.
Z-fold.
the Ropes
Ropes mine
mine area
arearepresents
representsthe
theeast-northeast-trending
east-northeast-trendinglimb
limbofofaalarge-scale
in the
110
greenstone-granite terrane
terrane of
of the
the Marquette area
Table 1.
1. Approximate
Approximatemodal
modal content
content of
of granitoid
granitoid rocks
rock in Archean greenstone-granite
['Fr.,
pr., trace; blank, absentj
absent]
Rock
Rock
Constituent
Constituent
Sample
Sample number
136R
l36R
lilA
177A
17Th
177B
179A
179A
179B
179B
182A
18%
182B
182B
183A
183A
186A
186A
199
199
200
202
208
208
21LA
21lA
136-1
136-1
203
1
1
22
33
Plagioclase
Plagioclase
37
37
35
35
60
60
66
54.5
54.5
48
52.5
52.5
38
54.5
36
36
56.5
60
60
39
39
41
41
273
27.5
33
33
61.2
58.5
58.5
50.6
50.6
Quartz
Quartz
30
30
32.5
24
24
21.5
28.5
28.5
45
36
34
24
24
27.5
27.5
37.5
28
28
30
30
27
39
39
23
28
26
26
22.2
373
37.5
Potassium
Potassium
feldspar
feldspar
30
31
31
10
10
5.5
2.5
00
24.5
14.5
14.5
31
31
33
11
30
24
32
32
373
37.5
3.8
12.8
1.4
1.4
Biotite
Biotite
Ti.
Tr.
Tr.
Ti.
Tr.
Tr.
10
10
Ti.
Tr.
Tr.
Tr.
Ti.
Tr.
0.2
Ti.
Tr.
11
66
77
Ti.
Tr.
66
4
Tr.
Chlorite
Chlorite
1.3
1.3
33
Epidote
Epidote
0.5
Tr.
Tr.
63
6.5
7
7
55
Ti.
Tr.
10
10
0
Ti.
Tr.
Ti.
Tr.
Ti.
Tr.
Ti.
Tr.
3
3
Ti.
Tr.
Tr.
Tr.
88
Ti.
Tr.
Tr.
Tr.
2.5
13
1.5
03
0.5
0.6
00
Ti.
Tr.
Sphene
Sphene
Opaque
Opaque oxides
oxides
Tr.
Tr.
Accessory
minerals
minerals
Tr.
Tr.
Ti.
Tr.
0.2
Muscovite
Muscovite
Tr.
Tr.
Ti.
Tr.
Ti.
Tr.
Ti.
Tr.
0.5
3.6
3.6
Ti.
Tr.
0.5
4
Tr.
Ti.
Tr.
11
Tr.
0.5
Tr.
Tr.
Tr.
Tr.
11
11
Ti.
Tr.
Tr.
Tr.
Tr.
Ti.
Tr.
Tr.
Tr.
1
1
Tr.
Tr.
0.2
0.2
Tr.
'Fr.
23
2.5
23
2.5
Ti.
Tr.
1.8
Ti.
Tr.
0
0
Ti.
Tr.
Tr.
Tr.
Ti.
Tr.
1.3
1.3
7
DESCRIPTIONS
SAMPLE DESCRIF'TXONS
136R.
corner of
Pink, fineof biotite
biotite schist. Sands
W., 675 ft N. of the
the SE.
SE. corner
of
f i e - to
to medium-grained,
medium-grained,slightly
slightly porphyritic,
porphyritic, altered granite;
granite; contains
containswispy
wispy inclusions of
Sandsquadrangle,
quadrangle,1,800
1,800 ft W.,
sec.
3, T. 47 N., R. 25 W.
sec.3,T.47N.,R.ZW.
lilA.
T. 47
Pink, medium-grained,
porphyritic, massive,
massive,altered
altered granite;
granite; cut
cut by
by cldorite-coated
chlorite-coatedfractures.
fractures. Sands
5., 2
2,150
E. of
of the
the NW. corner of sec.
sec. 16, T.
medium-grained, porphyritic,
Sands quadrangle, 775 ftft S.,
,l50 ftft E.
47 N.,
N.,
R. 25W.
25 w.
17Th.
Pink,
porphyritic, massive,
massive,altered
altered granite.
granite. Sands
P
i i fine-grained,
fie-grained, porphyritic,
Sands quadrangle.
quadrangle. Same
Samelocality
localityas
as177A.
177A.
179A.
26,
Pinkish-gray, medium-grained,
medium-grained,porphyritic,
porphyritic,altered
altered tonalite;
tonalite;contains
containsoriented
oriented layers
layersof
ofbiotite
biotite schist.
schist. Palmer quadrangle,
S., 2
250
W. of
of the
the NJ2.
NE. corner of sec.
50 ftft W.
see 26,
quadrangle, 650 ft S.,
T. 47 N.,
N.,R. 26
26 W.
w.
179B.
182A.
foliated, altered
altered tonalite.
tonalite. Biotite
Gray, medium-grained, foliated,
Biotite is
is fresh. Palmer
Palmerquadrangle.
quadrangle.Same
Samelocality
localityasas179A.
179A.
W.
Pinkish-gray, medium-grained
medium-grained altered
altered tonalite gneiss.
gneiss. Sands
quadrangle,
1,350
ft
E.,
100
ft
N.
of
the
SW.
Sands quadrangle, 1,350 E.,
N.
SW. coiner
corner of
of sec.
sec. 31,
31, T. 47 N., R. 25 W.
182B.
Pinkish-gray
altered tonalite gneiss. Sands
Piish-gray to medium-gray,
medium-gray, altered
Sands quadrangle.
quadrangle. Same
Samelocation
locationasas182A.
182.4.
183A.
Pinkish-gray,
fine- to medium-grained
Pinkish-gray, fmemedium-grained granite
granite gneiss.
gneiss. Sands
Sandsquadrangle,
quadrangle,675
675ftftN.,
N.,300
300ftftE.of
E.of the
theSW.
SW.corner
wrnerofofsec.
sec.31,
31,T.T.47
47N.,
N.,R.R.2525 W.
W.
186A.
Pinkish-gray,
medium-grained, altered
altered grandorite
granodiorite gneiss. Palmer
Pinkish-gray, medium-grained,
Palmerquadrangle,
quadrangle,2,500
2,333ftftN.,
N., 2,200
2,200ftft E.
E. of
of the
theSW.
SW.corner
wrnerofofsec.
sec.36,
36,T.
T.47
47N.,
N.,R.R.2626 W.
W.
199.
Pinkish-gray, medium-grained
medium-grained granite
granite gneiss. Sands
Sandsquadrangle,
quadrangle,1,000
1,000 ftft S.,
S.,2,000
2,000 ft W. of the
the NE.
NE. corner
mrnerofofsec.
sec.16,
16,T.
T.46
46N.,
N., R.
R.25
25W.
W.
200.
Pinkish-gray,
fine- to
to medium-grained
medium-grained altered
altered tonalite gneiss.
gneiss. Sands
N., 2,075
2,075 ftft E.
E. of
of the SW.
SW. corner
corner of
of sec.
sec. 9, T.
T. 46
46 N.,
N.,R. 25
Piish-gray, fmeSands quadrangle,
quadrangle, 775 ft N.,
25 W.
W.
202.
Gray, fine-grained
chlorite-coated fractures.
fractures. Sands
E. of
of the SW. wrner
corner of sec. 5,
5, T.
T. 46
46 N.,
tine-grained biotite tonalite
tonalite gneiss cut by chlorite-mated
Sandsquadrangle,
quadrangle, 850
850 ft N.,
N., 2,350
2,350 E.
N.,R.
R. 25
25 W.
W.
208.
Pink, medium-grained,
2,150 ftftN.,
N., 2,000
2,WO ftft W.
W. of
of the
theSE.
SE.corner
wrnerofofsec.
sec.8,8,T.T.46
46N.,
N.,R.R.2525 W.
W.
medium-grained, leucocratic
leucocratic granite
granite gneiss.
gneiss. Sands
Sandsquadrangle,
quadrangle,2,150
211A.
Pink, medium-grained, altered granite
W. of
of the SE. corner
granite gneiss. Sands
Sandsquadrangle,
quadrangle,2,550
2,550 ft S., 150 ft W.
wrner of
of sec.
sec. 7, T. 46 N.,
N.,R. 25 W.
W.
136-1.
Pink, medium- to coarse-grained, foliated
foliated granite
granite gneiss.
gneiss. Fractures mated
coated by chlorite.
chlorite. Sands
as136R.
l36R.
Sands quadrangle. Same
Samelocation
locationas
203.
Pink, medium-grained, leucwratic
leucocratic granite
granite gneiss.
gneiss. Sands
2,300 ftft W.
W. of
of the
the SE.
SE. wrner
corner of sec. 5, T. 46 N., R. 25 W.
Sands quadrangle, 200 ft N.,
N., 2,300
W.
1.
Average of 14 samples of
of foliated tonalite. Marquette
Marquettequadrangle,
quadrangle,north
northofofthe
theMarquette
Marquettesyndlinorium
syncborium(Gair
(Gauand
andThaden,
Thaden,1968,
1968,table
table6,6,no.
no.1).
1).
2.
Average of five samples of
of foliated grandorite.
granodiorite. Marquette
Marquettequadrangle,
quadrangle,north
northofofthe
theMarquette
Marquettesynclinorium
synclinorium(Gair
(G& and
andThaden,
Thaden,1968,
1968,table
table6,6,no.
no.9).
9).
3.
Average of 11
11 samples of quartz-rich, foliated tonalite. Sands
Sandsquadrangle,
quadrangle,south
southof
of the
theMarquette
Marquettesynclinorium
syncborium (Gair
(G& and
andThaden,
Thaden,1968,
1968, table
table 6,
6, no.
no. 6).
6).
The
The steep,
steep,northwestnorthwest- to
to west-trending
west-trending shear zones and faults in the
the greenstone-granite
greenstone-graniteterrane,
terrane,
as exposed
in the northern complex,
are tens
tens of
of meters
meters to
to aa few
fewhundred
hundred meters
meters wide.
wide. They are
exposed in
complex, are
highly
highly schistose zones characterized by an intense,
intense, close-spaced
close-spaced foliation, a steep
steepstretching(?)
stretching(?)
ilneation,
1991). The faults are of
lineation, and strong retrograde alteration (Johnson and Bornhorst, 1991).
of
both
ductile
and
brittle-ductile
types.
Major
structures
include
the
Dead
River
shear
zone
both ductile
types. Major structures include the Dead River shear zone
(fig. 1, DRSZ) (Puffett, 1974),
whichforms
formsthe
the northern
northern boundary
boundaty of
of the Dead River
19741, which
River basin;
basin; the
the
Carp
Carp River
River shear
shear zone
zone (CRSZ),
(CRSZ),east-northeast
east-northeastof
of the
theRopes
Ropesmine,
mine,which
which separates
separates two
two different
different
blocks of Archean volcanic rocks;
rocks; and the Carp River
River Falls
Falls shear
shear zone
zone(CRFSZ),
(CRFSZ),which
whichforms
forms
the
in this area. The
the northern
northern margin
margin of
of the
the Marquette
Marquettesynclinorium
synclinorium in
TheCarp
CarpRiver
RiverFalls
Fallsshear
shear
zone is reported
becut
cutby
byrelatively
relatively undeformed
undeformed mafic
mafic dikes of presumed
presumed Late
Late Archean
Archeanage,
age,
reported to
tobe
indicating an Archean age for the
the shear
shearzone
zone(Baxter
(Baxter and
andBornhorst,
Bornhorst,1988).
1988).
In the
the area
areanorth
northof
ofthe
thePalmer
Palmerfault
fault(fig.
(fig. 1),
I), foliated
foliated and fractured Late
Late Archean
Archean granitoid
granitoid
rocks
form
partly
fault-bounded
domes
surrounded
by
Early
Proterozoic
rocks
of
the
Marquette
rocks
of
Marquette
Range Supergroup. The
large
Archean
granitoid
body
in
the
Sugarloaf
Mountain
area,
88 km
km
The large Archean granitoid body the Sugarloaf
north
north of
of Marquette,
Marquette,isisalso
alsoaadome;
dome;foliation
foliationin
inadjacent
adjacentmetavolcanic
metavolcanic rocks
rocks (Puffett,
(Puffett,1974)
1974) dips
dips
gently to moderately
moderately away
away from the granite
granite contact.
contact.
Late-Tectonic
Late-Tectonic Conglomerate
Conglomerate (Archean)
(Archean)
The
is the
the Reany
Reany Creek
Creek Formation
Formation
The youngest Archean unit in the greenstone-granite terrane is
(Puffett, 1969; 1974).
1974). On the basis of a reinterpretation
reinterpretation of
of age
age and
and structural
structuralrelationships,
relationships, the
the
Reany Creek Formation is no longer included
included as
as part
part of
of the
the C
Chocolay
Group
or
the
Marquette
h w l a y Group or the
Range Supergroup
Supergroup (Sims, 1991). ItIt isis aa heterogeneous
heterogeneousbody
body of
of conglomerate,
conglomerate, arkose,
arkose,chloritic
chloritic
slate,
slate, graywacke,
graywacke, and
and boulder-bearing
boulder-bearingslate.
slate. It
It transects
transectsstructures
structuresin
in the
theolder
oldervolcanic
volcanicrocks,
rocks, isis
less deformed than
than the
themetamorphosed
metamorphosedvolcanic
volcanic rocks,
rocks, and is bounded along
along its
its southern
southern
margin by the northwest-trending
northwest-trending Dead River
River shear
shear zone
zone (fig.
(fig. 1).
1). Its age
age has been uncertain
uncertain
(see Puffett,
of basin
basin fill,and
and relationship
relationship to
to the
Puffett, 1974),
19741, but its penetrative
penetrative foliation, asymmetry
asymmetry of
Dead
Dead River
River shear
shearzone
zonestrongly
strongly suggest
suggest that itit developed
developed concurrently
concurrently with dextral shear along
the
similartoto"Timiskaming-type"
"Timiskaming-typensequences,
sequences, such
such as
as the
theSeine
Seine
the Dead
Dead River
Rivershear
shearzone.
zone.ItItis issimilar
Group,
Group, in northern
northern Ontario,
Ontario,now
now commonly
commonly interpreted as forming in Archean analogs to
modern
modem pull-apart
pull-apartbasins
basins(Poulsen,
(Poulsen,1986).
1986).
fa
ARCHEAN
A R C H W GNEISS
GNE3SS TERRANE
TERRANE
The Archean gneiss
gneiss terrane
terrane in the Marquette area (fig. 1) constitutes the
the greater
greater part of the
southern complex,
complex, as defined
defined by
by Van Hise
Hise and
andBayley
Bayley (1897).
(1897). ItItconsists
consistsofofgneiss,
gneiss,migmatite,
migmatite,
southern
amphibolite, substantial amounts of
and amphibolite,
of deformed and undeformed granite pegmatite, and
Cannon and
and Simmons
Simmons (1973)
(1973) have
have described
described the
the
massive to weakly
weakly foliated
foliated granite
granite plutons.
plutons. Cannon
as Late
Late Archean.
Archean.
general rock types in much
much of
of the southern complex.
complex. One rock has been dated as
sample of
of gray gneiss (called Compeau Creek Gneiss by Gair, 1975)
1975) collected in the
A sample
SEY&W% ofofsection
SE¼SWV4
section36,
36,T.T.4747N.,
N.,R.R.27
27W.
W.(Hammond,
(Hammond,1978)
1978)has
hasaaU-Pb
U-Pb zircon
zircon age
age of
of
2,779521
802576 Ma (recalculated
(recalculated by Zell E.
E. Peterman).
Peterman).
2,779
±21 Ma
Ma and
and a lower intercept age of 802±76
Gneiss
Gneiss and
and Associated Granitoid
Granitoid Rocks
Rocks
Compositiondly layered,
Compositionally
layered, medium-grained
medium-grainedgneiss
gneissand
andmigmatite
migmatiteare
are the
the dominant rock types in
the gneiss
gneiss terrane in
in the
the Marquette
Marquettearea.
area.Layered
Layeredfelsic
felsicgneisses
gneissesranging
rangingin
in composition
composition from
from
the
to granite
granite predominate.
predominate. Smaller
Smalleramounts
amountsof
of massive
massive to
to layered
layered amphIbolite
amphibolite are
tonalite to
113
intercalated
intercalated with
with the
the felsic
felsic gneiss, but amphibolite constitutes layers several tens of meters thick
thick
at
be seen
seenon
onthe
thegeologic
geologicmap
map of
of the
thePalmer
Palmer7½-minute
7%-minutequadrangle
quadrangle(Gair,
(Gair,1975,
1975,
at places,
places, as
as can
can be
p1.
1), which
whichisisimmediately
immediatelywest
westofofthe
the Sands
Sands quadrangle.
quadrangle. The
pi. I),
The felsic
felsic gneisses
gneisses are
are gray
gray to
to
pinkish gray; typically,
compositionallayering
layeringisisexpressed
expressedby
bydifferent
differentproportions
proportions of
of the
the major
typically, compositional
silicate minerals, as
plagioclase-quartz-biotite-microperthite,(2)
(2) microperthitemicroperthiteas for
for example,
example, (1)
(1) plagioclase-quartz-biotite-microperthite,
quartz-plagioclase-biotite,
and (3) biotite-quartz-plagioclase-microperthite. Textural differences
quartz-plagioclase-biotite, and
at
layering. Pink aplitic granite and granite
at places
places emphasize the compositional
compositional layering.
granite pegmatite
pegmatite
commonly
transect the gneiss and
and amphibolite
amphibolite and
and locally
locallyform
formmigmatite.
migmatite. Metasedimentary
commonly transect
rocks such as iron-formation, form layers in the felsic
of the Republic
felsic gneisses in the vicinity
vicinity of
trough (Cannon and
Simmons,
1973);
they
were
not
observed
in
the
Palmer
and
Sands
and Simmons,
they were not observed in the Palmer
Marquette area.
area.
quadrangles, in the Marquette
Pinkish-gray to pink, medium-grained, massive
massive to weakly
weakly foliated,
foliated, homogeneous
homogeneous granite
granite (table
(table 2)
2)
intrudes
intrudes the
the gneisses
gneisses in several places.
places. Hammond
Hammond (1978)
(1978) delineated
delineated aa body
body of
of massive
massive granite
granite
about 44 km2
km2in areal
areal extent
extent south
south of
of Ishpeming
Ishpeming (unit
(unit Wgt,
Wgt, fig.
fig. 1),
I), which
which he
he informally
informally called the
the
"Tilden granite."
granite." ItIt isisaagray
porphyritic, and massive granite
grayto
topink,
pink,medium-grained,
medium-grained, locally
locally porphyritic,
that locally
gneiss. ItIt isis cut
locally contains oriented xenoliths of mafic gneiss.
cut by
by pink
pink pegInatite
pegmatite and
andisishighly
highly
fractured. The
Thefractures
fractureshave
haveslickensided
slickensided surfaces and a thin coating of chlorite and other
propylitic
propylitic alteration minerals.
minerals. U-Pb
U-Pb isotopic
isotopic data
data on
on aa sample
sample of
of the
thegranite
granitenear
nearTilden
Tilden
(Hammond, 1978)
1978) indicate an age
age of
of 2,585±15
2,5855 15 Ma
Ma (R.E.
(R.E. Zartman,
Zartman,oral
oralcommun.,
commun.,1991).
1991).
fig. 1) about
about 2 km in diameter occurs about 3
A nearly
nearly circular
circular body
body of alkali granite (unit Xga, fig.
A
km south of Humboldt (Schulz
(Schuiz and others, 1988). The
The granite
graniteisislight
light red
red to
tobrick
brickred,
red,generally
generally
massive,
fineto
medium-grained,
and
equigranular
to
hypidiomorphic
granular.
The
granite
massive, fine- to medium-grained, and equigranular to hypidiomorphic granular.
granite is
is
the Arabian
Arabian Shield
Shield
similar compositionally to Sn-W mineralized
mineralized alkali feldspar
feldspar granites of the
(Jackson and Ramsay,
Ramsay, 1986)
1986) and the
the Nigerian
Nigerian younger granite province
province (Kinnaird
(Kinnaird and
and others,
others,
interpreted as a
Thegranite
granitehas
hasaaRb-Sr
Rb-Srwhole-rock
whole-rockage
ageofof1,733
1,733±525
25 Ma, which
which is interpreted
1985). The
crystallization
crystallization age (Zell E. Peterman,
Peterman, written
written commun.,
commun., 1988);
1988); it is aa post-tectonic
post-tectonic intrusion.
intrusion.
Structure
Structure
the gneiss
gneiss terrane
terrane (southern
(southern complex)
complex) form aa northwest-trending
northwest-trending antiformal
antiformal
Archean gneisses in the
structure
by Paleozoic
Paleozoic rocks
rocks to
to the
theeast
east(Sims,
(Sims,1992).
1992).
structure that
that closes
closes to
to the
thewest
west and
andisisoverlapped
overlapped by
An infolded belt of Early Proterozoic (Marquette Range Supergroup) rocks indents the Archean
the Republic
Republic trough (fig. 1). In
Inthe
thearea
areawest
westofofthe
theRepublic
Republictrough,
trough,Taylor
Taylor(1967)
(1967)
fold nose in the
determined two
two principal
principal phases
phases of
of deformation:
deformation: (1)
(1)early,
early,probably
probablyflat-lying
flat-lyingfolds
foldswith
withaxial
axial
determined
trending northeastward,
northeastward, and
and(2)
(2)younger
youngerupright
upright folds
foldswith
with steep
steepnorthwest-trending
northwest-trendingaxial
axial
planes trending
surfaces. The
Theyounger
younger folds
foldsmainly
mainly control the
the distribution
distribution of
of the
the rock
rock units.
units.
In the
the Marquette
Marquettearea,
area,early
earlygently
gently inclined
inclined to recumbent
recumbent folds that trend
trend northwestward
northwestward and
and
plunge gently
gently northward
northward (fig.
(fig.2)
2) are
are the
the dominant
dominant structure in the gneisses.
gneisses. These
Thesefolds
folds deform
deform
an older
older foliation
foliation (Si).
(S,). Boudinage
Boudinageaccompanied
accompaniedthe
thefolding;
folding; the
theboudins
boudinsplunge
plungesubparallel
subparallelto
to
gently inclined
inclined fold
fold hinges.
hinges. A later steep, northwest-oriented
foliation, that
that is axial planar
planar to
northwest-oriented foliation,
northwest-trending folds, occurs at least locally, but
but it does not significantly
affect the
significantly affect
distribution of the rock
stop 3.
3.
rock units. These
Theseyounger
younger structures
structures will
will be examined at stop
114
Table 2. Approximate
Approximate modal
modal content
content of
of granitoid
granitoid rocks
rocks in the
the Archean
Archean gneiss
gneiss terrane
terrane
of the Marquette
Marquette area
area
[Tr., trace; blank, absenti
absent]
Rock
Constituent
Constituent
Sample
Sample No.
153A
153A
146-88
146-88
11
226A
226B
226B
Plagioclase
Plagioclase
34.7
33.5
25
263
393
Quartz
28
23.8
25
38.2
32.7
Potassium
feldspar
28
28
34.2
40
40
33.0
25.0
9
8.0
8.0
33
2.2
3
3
Biotite
Biotite
Chlorite
Chlorite
Muscovite
Muscovite
Tr.
Tr.
Tr.
Tr.
Tr.
Tr.
22
Tr.
Tr.
Tr.
Tr.
Tr.
Epidote
Epidote
Tr.
Sphene
Sphene
Opaque
Opaque
oxides
oxides
Tr.
Tr.
Accessory
minerals
minerals
03
0.5
Tr.
0.3
Tr.
DESCRU'TIONS
SAMPLE DESCRIPTIONS
153A.
Pinkish-gray, medium-grained,
medium-grained, foliated
foliated granite,
granite, SE%NE!4
SE¼NE¼ of section
T. 46
46 N.,
N.,
Pinkish-gray,
section 7, T.
R. 26 W.
W. Biotite
R.
Biotiteisisweakly
weakly altered.
146-88.
"Tilden granite"
granite"ofofHammond
Hammond(1978).
(1978). Quartz
Quartz and biotite are recrystallized in
Tilden
shears; biotite is slightly altered to chlorite.
1.
"Tilden granite"
granite"of
ofHammond
Hammond (1978,
(1978,p.p.63).
63). Potassium-feldspar
Potassium-feldspar is microperthite.
Tilden
has concentric zoning.
zoning. Highly
Plagioclase has
Highly fractured.
fractured.
226A.
Light-gray,
mediuni-grained,foliated
foliatedgranite.
granite. Cut
Cut by
by fractures.
fractures. Biotite
Light-gray, medium-grained,
Biotite isis highly
highly
N., R. 26 W.
altered to chlorite.
chlorite. Quarry,
Quarry,SE¼SW½
SE%SW?hof
of section
section 21, T. 46 N.,
226B.
Pale-reddish-brown,
mediuin-grained foliated
foliated granite.
granite. Cut
Pale-reddish-brown, medium-grained
Cut by
by shears,
shears,some
somewith
with
mylonite. Biotite
mylonite.
Biotite is
is highly
highly altered to chlorite and calcite. Same
Samelocality
locality as
as 226A.
226A.
115
GREAT
GREATLAKES
LAKES TECTONIC
TECTONIC ZONE
ZONE
is characterized
characterized in
in the
the Marquette area
The GLITZ
GLTZ is
area by
by a mylonite
mylonite zone about 22 km
km wide that
that has
has
The
been
been superposed
superposedon
onpreviously
previously deformed
deformed layered rocks and granitoid rocks of the
the northern
northern
Archean
Archean greenstone-granite
greenstone-granite terrane
terraneand
andpreviously
previously deformed rocks of the southern
southern Archean
Archean
gneiss
rocks of
of the
the two
two terranes.
terranes. The
gneiss terrane. Therefore,
Therefore,the
themylonite
mylonite overprints rocks
The great
great width
width of
this
this shear
shear zone
zone and
and the
thepreponderance
preponderanceof
of mylonite
mylonite distinguish
distinguish it from the other,
other, much
much narrower
narrower
shear
shear zones
zones and
and faults
faults in
in the
the region.
region. The
Themylonite
myloniteabruptly
abruptlygrades
gradesnorthward
northward into
into
protomylonite
and
highly
fractured
and
altered
rocks
along
the
northeast
wall
of
the GLTZ
protomylonite and highly
GLTZ
(fig.
(fig. 3).
3).
Boundaries
Boundaries(walls)
(walls) of
of the
theGLTZ
GLTZare
aresubparallel
subparalleland
andtrend
trendN.N.55°-60°
55O-600 W.,
W., subparallel
subparallel to
to the
the
contact
between
the
Archean
greenstone-granite
and
gneiss
terranes
(fig.
3).
Foliation
in
contact between the
greenstone-granite
gneiss terranes (fig. 3). Foliation the
the
750
mylonite
Thus,the
thefoliation
foliationstrikes
strikes10°100mylonite has
has an
anaverage
averageattitude
attitudeofofN.N.7Q0
700W.,
W., 75O SW
SW (fig.
(fig. 4). Thus,
15°
more westward
westwardthan
than the
the trend
trend of
of the shear zone walls.
walls. The
15O more
The angular
angular relationship
relationshipbetween
between the
the
orientation
orientationof
of the
thefoliation
foliationand
andshear
shearzone
zoneboundaries
boundariessuggests
suggestsdextral
dextralwrench
wrench shear
shearalong
alongthe
the
walls
1986,fig.
fig.2b).
2b). A
A pronounced rodding (stretching) lineation in the
walls (Simpson, 1986,
the mylonite
mylonite has
has
an
average
plunge
of
42°
and
an
average
bearing
of
S.
43°
E.
an average plunge of 42' and an average bearing of S. 43O E.
Hinges
to the stretching lineation
lineation (fig.
(fig. 4).
4). The
Hinges of tight to open
open folds
folds plunge subparallel to
The folds
folds
have
asymmetricalpatterns.
patterns. Z-shaped folds
have both symmetrical and asymmetrical
folds are
are most
most common,
common, but SSshaped
and both
both may
may occur
occur in
in the
the same outcrop area, as on the
shaped folds coexist and
the west
west shore
shore of
of
Powell
covers outcrops
outcrops and
and prevented
prevented determination of
Powell Lake (fig. 3). Nearly
Nearly pervasive
pervasive lichen covers
0
t
0
•
0
+
++ +
+
++
0
+
++
+
++
+ ++
Lower
Lower hemisphere
hemisphere
Equal-areaprojection
projectionof
ofpoles
polesto
tofoliation,
foliation,lineations,
lineations, and
and fold
fold axes
axes in gneisses of
Figure 2. Equal-area
Archean gneiss
gneiss terrane.
terrane.
Archean
116
fold symmetries
symmetriesover
overlarge
largeparts
partsof
ofthe
the GLTZ.
GLTZ. Ridley
have demonstrated
demonstrated that
fold
Ridley and Casey (1989)
(1989) have
folds, with
with axes
axes subparallel
subparallelto
to the
the maximum
maximum extension
extension direction
direction (X finite strain
strain
symmetrical folds,
axis), are produced by wrench shear,
axis),
shear, whereas
whereas asymmetric
asymmetric folds, with axes close to the
the extension
extension
form in
in a regime of
of combined
combinedwrench
wrenchand
andthrust
thrustshear.
shear. In wrench shear, because the
direction, form
fold axis does
does not
not lie
lie initially
initiallywithin
withinthe
theslip
slipplane
planeofofshear,
shear,the
the axis
axisrotates
rotates toward
toward the
the direction
maximum extension
extensionwith
withthe
theaddition
addition of
ofprogressive
progressivestrain.
strain. In
of shear displacement and maximum
combined wrench
wrench and thrust shear, at high
high strain
strain the
the fold
fold axial
axial plane and
and both
both limbs
limbs approach
approach
parallelism
with the
the shear plane.
plane. Ridley
parallelism with
Ridley (1986)
(1986) has further shown that dextral wrench shear and
sinistral wrench shear can result from strain inhomogeneity in
in the same shear-parallel
shear-parallel extension
within a single deformation
regime.
deformation regime.
Mylonite
Mylonite
The mylonite
(1984),
mylonite in the
the GLTZ
GLTZisismainly
mainly orthomylonite,
orthomylonite, as
as defined
defined by
by Wise
Wise and
and others
others(1984),
inasmuch as
as surviving
survivingmegacrysts
megacrystscompose
compose10-20
10-20
percent
rock.
theterminology
terminolor of
of
inasmuch
percent
of of
thethe
rock.
InInthe
Hanmer
because the
the porphyroclasts have
Hanmer (1987),
(1987), the
the mylonites
mylonites are
are mainly
mainly "heteroclastic"
"heteroclastic" because
mylonite having
havingmore
more uniform
uniform textural
variable size ranges, but include
include "homoclastic"
"homoclastic" mylonite
characteristics. Ultramylonite
Ultramyloniteisis absent
absent except
except on
on aa scale
scaleof
of aa few
few centimeters.
centimeters.
The mylonite typically
typicallyhas
has aa pronounced
pronounced planar
planar foliation
foliation that
that differs markedly
markedly from
from the
the more
irregular foliation in
in rocks
rocks outside
outside the
the GLTZ.
GLTZ. In
In this
this respect,
respect, the
the mylonite
mylonite fits the pattern of
of
most mylonite zones in comprising a "straight zone."
zone." Quartz
recrystallized into
Quartz typically
typically is recrystallized
"ribbon quartz,"
quartz,"yielding
yieldingaapronounced
pronouncedrodding
rodding (stretch)
(stretch) lineation.
lineation. The
"ribbon
Therelatively
relatively stiff
stiff minerals,
minerals,
recrystallized to
to finer grain sizes, with the
plagioclase and potassium feldspar, are generally recrystallized
development of core-mantle structures (White, 1976) or type 1P
IP and 1M
1M structures
structures (Hanmer,
(Hanmer,
these structures
structures form
form oriented
oriented aggregates
aggregatesof
ofquartz
quartz and
and feldspar
feldspar that
that produce a
. 1982);
1982); these
prominent stretch
stretch lineation.
lineation. Accompanying
Accompanying biotite
biotite is
is mainly
mainly recrystallized
recrystallized in planar or irregular
irregular
shears. The
Thequartz-poor
quartz-pooramphibolite
amphibolitein
in the
theGLTZ
GLTZhas
hasaaconspicuous
conspicuous lineation
lineation given
given by elongate
aggregates of
of actinolite and
and chlorite.
chlorite. The
in rocks
rocks in
in the GLTZ are
The mafic
mafic mineral assemblages in
are
upper greenschist
greenschist facies,
fades, indicating retrogressive alteration of amphibolite
amphibolite to
to actinolite
actinoliteschist.
schist.
Interpretation
The Great Lakes
Lakes tectonic zone has been interpreted (Gibbs and others, 1984)
1984) as a paleosuture
that juxtaposed
juxtaposed the
the Archean gneiss and greenstoneresulting from continent-continent coffision
collision that
greenstoneInthe
theMarquette
Marquettearea,
area,the
thestretching
stretchinglineation,
lineation, which
which represents the line of
granite terranes. In
tectonic transport
transport (Schackleton
(Schackleton and
and Ries,
Ries,1984),
1984),indicates
indicates that
that collision
collision was oblique,
oblique, resulting
resulting in
in
dextral wrench-thrust shear along the N. 60°
W.-trendingboundary
boundary (paleosuture).
(paleosuture). Asymmetric
60ÂW.-trending
Asymmetric
structures, mainly microscopic, indicate
indicate northwestward
northwestwardvergence
vergence and
and probable
probable overriding of the
structures,
Archean greenstone-granite
greenstone-granite terrane
terrane by
by the
the Archean
Archean gneiss
gneiss terrane.
collisionwould
wouldbe
beexpected
expectedtotoproduce
producedextral
dextralshear
shearacross
acrossaalarge
largeregion
regionnorth
north of
of
The oblique collision
of the area affected by
by this dextral
dextral transcurrent
transcurrent shear is not
the GLTZ
GLTZ (fig.
(fig. 5).
5). The extent of
known, however,
however,because
because dextral
dextral shear
shear was
was the
the dominant mechanism of deformation
definitely known,
deformation
throughout most of the Superior
Superior province (Card, in press).
Structures in the greenstone-granite
greenstone-granite rocks of northern Minnesota (Wawa subprovince) are
similar to
to those
those in
in northern
northern Michigan.
Michigan. In northern Minnesota,
remarkably similar
Minnesota, deformed
deformed and
and
metamorphosed
sedimentary rocks of the
the Vermilion
Vermilion district
district(Sims,
(Sims,1976;
1976;Sims
Sims and
and
metamorphosed volcanic and sedimentary
117
_
1r30.
:
=
—.
(n1
-
31
32
.
,
/
\
1
.
/
)
-
N
6
\'\
4
fl \
:.'
/
I
SS
I
...
S
..•...
12
S
' \ $"za'
I
,L
'-S
R.26W.I R.25W.
lUtE
I
-
o0
I
I
p
1 KILOMETER
Figure
Figure 3.
3. Structure
Structuremap
mapofofsouthwestern
southwesternpart
partofofSands
Sands7½-minute
7%-minutequadrangle
quadrangleshowing
showing the
the
Great
GreatLakes
Lakestectonic
tectoniczone.
zone. Modified
Modifiedfrom
fromSims
Sims(1991).
(1991).
118
________
_________
_______
D(PLANATION
EXPLANATION
Figure 3--Continued
1
Wsg
Archean
Greenstone-granite terrane
tenane
Biotite schist and
and granitoid rocks
[
e
l
Wggm
I[
[wgn
Mylonlte—Protollth
MylonitÑProtolit dominantly
dominantly granttoid
granitoid rocks
rocks but
but includes
includes
blotite schist
sdilst and
biotite
and amphlbolite
amphibolite
Gneiss terrane
Gneiss, migrnatite,
migmatite, and
andamphibolite-includes
amphibolitdncludeifoliated
foliatedand
and
granite
massive
massive granite
Mylonfte—Oomlnantly mylonitic
Mylonite-Dominantly
myloniticquartzofeldspathic
quarlzofeldspathicgneiss
gneiss
wgmj
[Â¥Wgm
Silkified
.
Silicified rocks
rocks
:.'I...
.. .. .
--— — — Boundary
between
rocks
of of
greenstone-granite
Boundary
between
rocks
greenstone-granite terrane
terrane
.
------
25
+
-
—ts
F45
20
and gneiss
gneiss terrane
terrane within
wIthin the Great
and
Great Lakes tectonic
(GLTZ )
zone (GLTZ)
Approximate outer limit
limit of
of orthomylonite
orthomylonite in
in Great
Great Lakes
tectonic zone
Strike
Strike and dip of foliation
foliation
Inclined
Indined
Vertical
Bearing and
and plunge
plunge of
of minor fold
Bearing
fold
Bearing and
andplunge
plungeof olineation—May
f lineation-Maybebe
combinedwith
with
Bearing
combined
symbols
Ioliatlon
foliatton symbols
+
+
+ ++ +-
0
•.
++
.
.
Lower
Lower hemisphere
hemisphere
4. Equal-area
Figure 4.
Equal-areaprojection
projection of
of poles
polesto
to foliation
foliation (crosses),
(crosses), stretching
stretching lineation (dots),
(dots), and
and
fold hinges (open circles) in mylonite
mylonite of
of the
the Great Lakes
Lakes tectonic
tectonic zone,
zone, Sands
Sands and
and
Palmer 7½-minute
7%-minute quadrangles, Marquette area,
area, Michigan.
Michigan.
119
\
''
I
95°
Complex continental-margin sequences
Plutonic and volcanic rocks
Middle Proterozoic
Volcanic, sedimentary, and plutonlc rocks of
Mldcontinent rift system (i.1,100 Ma)
Early Proterozoic
Turbid,tes overlying shelf deposits
96°
940
E
930
cover
)" ,°.
Rocks of gnelss terrane
Rocks of greenstone-granite terrane
(Sutienor province)
ean
EXPLANATION
(ntad
A
91°
f No
PalOoZOiC(o>,A>dI>c(1,
92°
-,
s
89°
89°
A
A
Trace of Great Lakes tectonic zone I3LTZ)
Thrust fault— Sawteeth on overthrust block
— — — Trend of magnetic anomaly
A
movement
I
± Transcurrent fault— Showing relative horizontal
High-angle fault
4
owisconsin magmat(c terranes
,__a_rv•r r.a.cntyJ
pFAULLII
,' ',a
900
Figure 5. Simplified tectonic map of the Lake Superior region showing Great Lakes tectonic zone and adjacent Archean terranes.
After Sims (1991). Terminology of Early Proterozoic rocks modified from Southwick and Morey (1991).
I
97°
Southwick,
1985)compose
composean
an east-trending
east-trendingbelt
belt between
between higher
higher grade
grade rocks
rocks of
of the Late Archean
Southwick, 1985)
Archean
Vermilion Granitic
and the Giants
Granitic Complex
Complex (Quetico
(Quetico subprovince,
subprovince, fig. 5) (Southwick, 1972) and
Giants
Range batholith to the south. The
Themeasured
measuredstrain,
strain,aacleavage,
cleavage,upright
upright folds,
folds, and a mineral
mineral
lineation in this belt have been attributed to the
the main"
"main"phase
phaseof
ofdeformation
deformation(D2)
(D,) that
thatfollowed
followed
an early nappe-forming event
event (Dl)
(D1) (Bauer,
(Bauer, 1985).
1985). The nappes show little
little evidence of
of aa
(D,)
penetrative fabric (Hudleston, 1976).
1976). Hudleston and others
others (1988)
(1988) attributed
attributed the
the (D2)
deformation to regional dextral transpression,
transpression, as
as the
the strain pattern requires
requires aa northeastnortheastsouthwestcomponent
componentofofshortening
shorteningininaddition
additiontotoshear.
shear. They
They further
further proposed
proposed that
that major
major
southwest
dextral faults, such
such as the Vermilion fault (fig.
(fig. 5),
5), are
are later, more brittle, expressions of this
D2transpressive
transpressive deformation resulted from
from oblique
oblique
shear regime. They
Theyconcluded
concluded that
that the
theD2
compression between
between the two more rigid crustal blocks
blocks to
to the
the north (Quetico
(Quetico subprovince)
subprovince) and
south (Giants Range batholith). A
Asimilar
similar tectonic
tectonic regime
regime has been recognized
recognized in the
the Rainy
Rainy
Lake area
area (fig.
(fig. 5;
5; Poulsen
Poulsen and
andothers,
others,1980;
1980;Day
Day and
andSims,
Sims,1984;
1984; Wood,
Wood, 1980),
1980), where
where early
early
recumbent folding
by upright folding and dextral strike-slip
folding was followed by
strike-slip faulting.
Recent precise isotopic analyses of
of zircon,
zircon,titanite,
titanite, and
and rutile
rutile from the Rainy Lake area,
area, Canada
Canada
(fig. 5; Davis and others,
others, 1989),
which lies
lies between
between the
the Quetico
Quetico and
1989), which
and Wabigoon
Wabigoon subprovinces,
subprovinces,
provided time
time constraints
constraints on
on these
these structural events.
events. The
have provided
The major
major deformation,
deformation, including
including
nappe emplacement,
emplacement, thrusting, and local doming, took place between 2,696 Ma and 2,692 Ma;
followed shortly
shortlyby
bytranscurrent
transcurrent faulting
faulting and
and simultaneous
simultaneous deposition
deposition of
this deformation
deformation was followed
conglomerate/arenite (Timiskaming-type
rocks, as
as in
in Seine
Seine Group),
Group), which
which occurred
occurred in
in the
the
conglomerate/arenite
(Tirniskaming-type rocks,
2,692-2,686Ma.
Ma. Late (Algoman) granitic plutons were emplaced about 2,686 Ma,
interval 2,692-2,686
and
although some are older. InInthe
theWawa
Wawasubprovince,
subprovince,west
west of
of Thunder
Thunder Bay
Bay (fig.
(fig. 5), Corfu and
found that
that the D1
deformation occurred
occurred during
duringor
or before
before the
the intrusion
intrusion of
of the
Stott (1986) found
Dl deformation
Shebandowan Lake
Lake pluton
pluton at
at 2,696±
2,696±2 Ma.
Ma. Deformation
Shebandowan
Deformation D2
D2 in this area occurred between
2,689+3/-2
+3/-2 Ma
Ma and
and 2,684+
2,684+6/-3
by Davis and others (1989) for
2,689
61-3 Ma, similar to the age
age suggested
suggested by
D2
in
the
Rainy
Lake
wrench
fault
zone.
These
ages
are
compatible
D, in
Rainy Lake wrench fault zone.
compatible with the less precise
on rocks in
in northern Minnesota and Michigan (Peterman,
isotopic ages on
(Peterman, 1979),
1979), and it seems
seems
probable that the
the rocks
rocks and
and structures
structuresthroughout
throughout the
the Wawa
Wawa and
and Quetico
Quetico subprovinces
subprovinces are
approximately coeval
coeval (Percival,
(Percival, 1989).
1989). Although
Although convergence
convergencealong
alongthe
the GLTZ
GLTZ undoubtedly was
approximately
probably occurred
occurred in the approximate
diachronous, collision probably
approximate interval
interval2,692-2,686
2,692-2,686 Ma (Davis and
others, 1989),
1989), to yield the transcurrent
transcurrentfaults
faultsand
andTimiskiming-type
Timiskiming-typerocks.
rocks.
Kinematic Analysis
Kinematic
Analysis
attitude of
of the
the stretching
stretchinglineation
lineation (line
(line of
of tectonic
tectonic transport)
transport) in
in the
themylonite
mylonite exposed
exposed south
south
The attitude
of Marquette together
micro-structures that
that reveal
reveal sense of
of
together with
with asymmetric
asymmetric meso- and micro-structures
movement, indicate that
that the
theoblique
obliquecollision
collision resulted
resulted in:
in: (1) dextral-thrust shear along the
the
GLTZ, and
and (2)
(2) northwestward
northwestward vergence and probable overriding of the Archean
Archean greenstonegreenstonegranite terrane
terrane by
by the
the Archean
Archean gneiss
gneiss terrane.
terrane. Kinematic
Kinematicindicators--rotated
indicators--rotatedmica
micagrains
grainswithin
within
narrow compositional layers, asymmetric porphyroclasts
porphyroclastswith
withtails
tails (o
(a type; Simpson, 1986), and
and
asymmetric microfolds in
in mylonitic
myloniticlayering-demonstrate
layering--demonstratenorthwestward
northwestwardvergence.
vergence. This
information implies southward subduction of
(Wawa
of the
the Archean
Archean greenstone-granite
greenstone-granite terrane
terrane (Wawa
subprovince) beneath
beneath the Archean gneiss
gneiss terrane
terrane (fig.
(fig. 1).
1).
121
Evolution
The northwest direction of tectonic transport during suturing of the Archean terranes,
terranes, as
as
ascertained from the Marquette area,
area, provides
provides a means for determining the evolution
evolution of the
GL1'Z
GLTZ and the variable trajectory of stress into the
the Superior
Superior province
province crust.
The GL1'Z
bends that
GLTZ in the Lake
Lake Superior
Superior region
region is
is characterized
characterized by systematic angular bends
alternately
(fig.5).
5). Presumably this zigzag
zigzag pattern
pattern
alternately trend northeastward and west-northwestward
west-northwestward (fig.
reflects relict
relict irregularities
irregularities in
in the
the margin
margin of
of the
the Archean
Archean greenstone-granite
greenstone-graniteterrane
terrane (or Superior
reflects
was aa continental margin before the
province) crust, which was
the convergence
convergence and
and coffision
collision with the
southern Archean gneiss
terrane.
gneiss terrane.
segmentsof
ofthe
the GLTZ
GLTZ have
have different
different
The northeast-trending and west-northwest-trending
west-northwest-trending segments
structural styles.
styles. Deformation
Deformation along
along the
the northwest-trending
northwest-trending segments of the GLTZ,
GLTZ, as
as
by data
data from the
particularly shown by
the Marquette
Marquettesegment,
segment,was
wasprincipally
principally caused
caused by
by dextral
dextral
transpression resulting from oblique
oblique collision.
coffision. Transmittal
Transmittal of
of this
this transcurrent
transcurrent shear into rocks
rocks
north of
of the
theGLTZ
GLTZyielded
yieldedaawidespread,
widespread,pervasive
pervasivewest-northwestwest-northwest- to
towest-striking
west-striking foliation,
foliation,
subparallel upright
upright folds, and northwestnorthwest- to west-trending
west-trending dextral
dextral faults
faults and
and shear zones in the
the
Archean greenstone-granite terrane.
northwest-trending segment
segment of
of the
the GLTZ in northwestern Wisconsin
The similarly-oriented northwest-trending
Wisconsin has
structural features in common
common with
withthe
the Marquette
Marquette segment.
segment. Foliation and
many structural
and upright
upright folds
folds in
low amphibolite-facies
amphibolite-facies rocks
rocksof
of the
the Archean
Archean greenstone-granite
greenstone-granite terrane
terrane (unit
(unit Wga,
Wga, fig.
fig. 4, Sims
low
and others,
others, 1985)
1985) strike
strike west-northwest and mineral lineations and fold hinges mainly plunge
gently
southeast.
The
gently southeast. The boundary
boundary between the two terranes is not exposed
exposed because of a glacial
cover, but
but is presumed
presumed to lie along
along the
the south
south edge
edge of
of unit
unit Wga.
Wga. Numerous northwest-trending
cover,
faults, some of
of which
which reactivated
reactivatedin
in Early
EarlyProterozoic
Proterozoictime,
time,have
havebeen
been mapped
mapped in
in the
dextral faults,
area (Sims
(Sims and
and others,
others,1985;
1985; fig.
fig. 1).
1).
Coffisionalong
alongthe
the northeast-trending
northeast-trending segments
segmentsofofthe
the GLTZ,
GLTZ, on
on the
the other hand, produced
Collision
northeast-trending structures
structures of
of apparently
apparentlymore
morerestricted
restrictedareal
areal extent.
extent. In the northeastnortheast-trending
northeastthe boundary is covered by Early
trending Marenisco segment (fig. 5; Sims and others, 1984), the
lithologic layering
layering and
and foliation
foliation in rocks of
Proterozoic sedimentary
sedimentary and
and volcanic
volcanic rocks, however, lithologic
Archean greenstone-granite
greenstone-graniteterrane
terrane near
near the boundary
boundary trend
trend northeastward
northeastward and are
the adjacent Archean
45°-50°
SW. These
deformed into upright, moderately tight northeast-trending folds that plunge 45'
-50' SW.
subparallel to
to the
the covered
covered Archean
Archean GLTZ
GLTZ boundary.
boundary. Archean
structures are presumably
presumably subparallel
metamorphism has been overprinted
overprinted by
by Early Proterozoic Penokean nodal
nodal metamorphism
metamorphism
centered on
on the
theWatersmeet
Watersmeet dome
dome(Sims
(Simsand
andothers,
others,1985;
1985;Sims,
Sims, 1990);
1990); the presence of
of relict
relict
garnet at aa few
rocks of
of the
the greenstone-granite
greenstone-granite terrane
terrane near the boundary
few places in the Archean rocks
suggests that
that these rocks were metamorphosed
metamorphosed to
to at least upper greenschist facies in Archean
time. Similarly,
Similarly, north-verging
north-verging Penokean deformation in the
the boundary
boundary zone
zone overprinted
overprinted Archean
Archean
structures (Sims
and others, 1984).
plane S,
2 (Penokean)
(Sims and
1984). An axial plane
(Penokean) penetrative
penetrative cleavage,
cleavage, striking
northeast
SE., was
was superposed
superposed on
on the previously
previously folded
folded rocks.
rocks. Apparently
northeast and
and dipping
dipping 45°-70°
45'-70' SE.,
Apparently
the Archean rocks were not refolded, however,
as aa result
result of
of the Penokean deformation.
however, as
In the northeast-trending
segment of
of the
the GLTZ (fig.
(fig. 5),
5), neither
neither the terrane
terrane boundary
northeast-trending Minnesota segment
nor adjacent Archean rocks on either side are exposed. These
These rocks
rocks are
are covered
covered in
in west-central
west-central
by thick
thick Quaternary glacial deposits
deposits and
and in
in central
central Minnesota by
by Early Proterozoic
Minnesota by
sedimentary and volcanic rocks
rocks of
of the
the Animikie
Animikie basin
basin (Southwick
(Southwickand
andothers,
others, 1988).
1988). The GLTZ
GLTZ
122
has been investigated,
computer-generated
investigated, however,
however, by a detailed
detailed aeromagnetic
aeromagneticsurvey,
survey, by computer-generated
the second
second vertical
vertical derivative
derivative of
of the
the gravity
gravity field,
field, and
and by
by shallow
shallow test drilling
drilling (see
(see
mapping of the
Southwick and
Southwick
and Sims,
Sims,in
inpress);
press);the
the boundary
boundary has
has been
been located
located rather
rather accurately on
on the basis of
these data. The
Thedrilling
drillinghas
hasshown
shown that
that the
therocks
rocks on
on the
thenorthwest
northwestside
sideare
arevolcanogenic
volcanogenic
sedimentary and mafic to intermediate
rocks,which
whichare
aremetamorphosed
metamorphosed to
to upper
upper
intermediate volcanic
volcanic rocks,
greenschist facies
facies and
and intruded by
by Archean
Archean tonalite (Southwick
(Southwickand
and Chandler,
Chandler, 1983).
1983). These
greenschist
of the
the Archean
Archean greenstone-granite
greenstone-granite terrane
terrane in exposed
exposed parts
parts of
of the Lake
rocks are typical of
region. A
by COCORP
COCORP
Superior region.
A seismic
seismic reflection profile in central Minnesota acquired by
(Consortium for Continental Reflection
Profiling)has
hasbeen
been interpreted
interpreted to indicate that the
Reflection Profiling)
the
GL1'Z
north-dipping tectonic
tectonic feature
feature (Gibbs and others,
this area
area isisaashallow
shallow(—30°)
(-30° north-dipping
others, 1984).
1984).
GLTZ in this
In east-central Minnesota, the
the GLTZ
GLTZ isis covered
covered by
by Proterozoic
Proterozoic rocks of the
the Animikie
Animikie basin.
The structural
structural style
style in the
the Proterozoic
Proterozoiccover
cover indicates
indicates north-verging
north-verging tectonism
tectonism (Southwick
(Southwick and
others,
others, 1988),
1988), and as in the Marenisco
Marenisco segment, the Archean crustal
crustal boundary had aa role
role in
in
defining Penokean deformation.
deformation.
Deformation along both of the northeast-trending
of the
northeast-trending (Minnesota and Marenisco) segments of
GL1'Z
resulted
mainly
from
northwest-southeast
crustal
shortening,
probably
dominantly
by
GLTZ resulted mainly from northwest-southeast crustal shortening, probably dominantly by
flattening strain. The
Thedirection
directionofoftectonic
tectonictransport
transportduring
duringconvergence
convergencewas
was virtually
virtually
perpendicular to the
the juncture of
of the
the two
two terranes
terranes at
at these
theselocalities.
localities.
The origin of the
the zigzag
zigzag pattern of the
the south
south edge
edge of
of the
the Superior
Superiorprovince,
province, now
now marked
marked by
by the
the
GL1'Z, is uncertain.
uncertain. The
is one
one of
of a sequence
GLTZ,
The prevailing
prevailing thought is that the Wawa subprovince is
of stacked island arcs that formed
from north
north to south above north-dipping
formed progressively
progressively from
subduction zones
zones as
as the
the continental mass
mass to
to the south of the GLTZ
GLTZ (i.e.,
(i.e., Archean gneiss
terrane)
terrane) migrated
migrated to the
the north
north (Card,
(Card,1990).
1990). With
Withthis
thisinterpretation,
interpretation,possible
possible modern
modem analogs
analogs
of the Superior
province
are
the
convergent-plate
boundaries
of
the
western
Pacific,
as
for
Superior province are the convergent-plate
of the western Pacific, as for
example those of the
the Indonesian
Indonesian region
region (Hamilton,
(Hamilton,1979).
1979).
The physical
of the southern margin of the Superior
physical resemblance of
Superior province to the
the AppalachianAppalachianOuachita Paleozoic
orogenic
belt
(Thomas,
1977),
however,
suggests
a
possible
alternative
Paleozoic orogenic belt (Thomas, 1977), however,
alternative
margin. In this interpretation, the
interpretation for the origin of the Superior margin.
the Superior
Superior margin
margin. Two interpretations have been
been made for the origin of the
was a rifted continental margin.
margin: (1)
Paleozoic continental margin:
(1)rift
riftsegments
segmentswere
were offset
offset by
by transform
transform faults,
faults, as
as suggested
suggested by
by
1983),or
or (2)
(2) intersections
intersections between
between active
activerift
rift arms
arms at
at triple junctions
junctions (Rankin,
Thomas (1977,
(1977, 1983),
with the
1976). Of these
these two
two suggestions,
suggestions, the rift-transform mechanism seems the more
more likely, with
5) and
and the northwestern
northwestern
Minnesota and Marenisco
Marenisco segments
segmentsbeing
being the
the rifted
rifted segments
segments(fig.
(fig. 5)
Wisconsin and
and Marquette segments being highly
highlymodified
modifiedtransform
transformfaults.
faults. Regardless of the
Wisconsin
the
by which
whichthe
the zigzag
zigzagArchean
Archeancontinental
continentalmargin
marginoriginated,
originated,the
thesubsequent
subsequent trace
trace of
of
mechanism by
the orogenic belt was probably
probably inherited
inherited from
from the
the shape
shape of
of the earlier margin.
CONCLUDING
CONCLUDING REMARKS
REMARKS
Convergence along the
the irregularly shaped
shaped margin
margin of
of the
the Archean greenstone-granite
greenstone-granite terrane
terrane
(GLTZ) resulted
resulted in
in aa variable
variable trajectory
trajectory of
of stress
stress into the
the continental
continentalcrust
crust and
and probably
probably in
in
diachroneityofoforogeny.
orogeny. Structural
Structuraldata
data from
fromthe
the Marquette
Marquette area, in particular, as
along-strike diachroneity
well as
as elsewhere
elsewhere along
alongthe
the GLTZ,
GLTZ, suggest
suggestthat
thatthe
the major
major direction
direction of
of tectonic
tectonic transport
transport was
well
northwestward. Accordingly,
promontories such as those along the concave parts of the
northwestward.
Accordingly, promontories
the
Marquette and
and Wisconsin
Wisconsin segments of the GLTZ
GLTZ (fig.
(fig. 5), where the zone bends from a
northeast orientation
orientation to
to aa northwest
northwest one,
one, must
must have
have projected
projected as
as buttresses
buttressesagainst
against which
which
123
compressive
compressive stress was directed into the continental crust. Oblique
Obliquecompression
compression at
atthese
thesepoints
points
produced
produced dextral
dextral shear across
across the region north of
of the
the suture,
suture, probably
probably at least
least as
as far
far northward
northward
as
km. This
as the
the Quetico
Quetico fault
fault (fig.
(fig. 5), a distance
distance of about 250 km.
This shear
shear imposed
imposed aa roughly
roughly easteastwest,
west, steep
steep structural
structural fabric
fabric on the
the rocks
rocks and, also as a late, more
more brittle
brittle expression
expression of the
the shear
shear
regime
regime (Hudleston
(Hudleston and
andothers,
others,1988),
1988),the
thenorthwestnorthwest- to
towest-trending
west-trending dextral
dextraltranscurrent
transcurrentfaults.
faults.
Along
Along the
the Marenisco
Mareniscoand
andMinnesota
Minnesotasegments
segmentsof
ofthe
theGLTZ
GLTZ(fig.
(fig.5),
5),where
whereconvergence
convergencewas
was
more
more perpendicular
perpendicular to
to the
theancient
ancientcontinental
continentalmargin,
margin, aanortheast-trending
northeast-trendingstructural
structuralfabric
fabricwas
was
imposed
imposed on
on the
therocks
rocksimmediately
immediately cratonward
cratonward from
from the
thesuture.
suture.
We
We suggest
suggest that the
the main
main structural
structural fabric
fabric (D2)
(D2) in rocks of the Archean greenstone-granite
greenstone-granite
terrane
5)resulted
resulted
terraneininthe
thenorth-central
north-centralUnited
UnitedStates
States(Wawa
(Wawaand
andQuetico
Queticosubprovinces;
subprovinces;fig.
fig. 5)
from
along the
the GLTZ.
GL1'Z. The
from the
the coffision
collision along
The predominance of orthomylonite rather than
ultramylonite
ultramylonite and
and the
thenearly
nearly pervasive
pervasive retrogressive
retrogressive alteration
alteration (greenschist
(greenschist facies)
fades) in
inrocks
rocksof
of
the
the greenstone-granite
greenstone-granite terrane
terranesuggest
suggest that
that the
theexposed
exposedcollision
collision zone was developed at aa
moderately
level (at
(at brittle-ductile
brittle-ductile transition
transition conditions).
conditions). As discussed in a
moderately shallow
shallow crustal level
previous
report
(Sims
and
others,
1980),
the
Archean
structures
previous report (Sims and others, 1980),
structures in
in this
this regime
regime also
also played
played aa
strong
strong role
role in
in subsequent
subsequenttectonism,
tectonism, especially
especially in the
the Early
Early Proterozoic
Proterozoic north-verging
north-verging
deformation.
deformation.
We
We further
further suggest
suggest that the
the late-tectonic
late-tectonic granite bodies in the Archean gneiss terrane are
are
possibly
related to the collision
along the
the GLTZ
GLTZ and presumed
presumed southward
southward subduction.
subduction. The
possibly related
collision along
available
available age data on these
these granites
granites are
are compatible
compatible with a presumed 2.69
2.69 Ga age for the
coffision.
collision. The
The "Tilden
"Tilden granite"
granite" of Hammond (1978)
(1978) in Michigan
Michigan has a Late
LateArchean
Archeanage
age
(2,585±15
both the U-Pb and Rb-Sr
Rb-Sr systems
systemsare
are disturbed.
disturbed. In the
(2,5852 15 Ma), although
although both
the Minnesota
Minnesota
River
southwestern Minnesota,
Minnesota, aa large
large pluton
pluton of
of late-tectonic
late-tectonic granite
granite (Sacred
(Sacred Heart
River valley, in southwestern
Granite)
and a Rb-Sr age of
Granite) has
has aaPb-Pb
Pb-Pb age
ageof
of about
about2,605
2,605 Ma (Doe
(Doe and
and Delevaux,
Delevaux, 1980) and
about
andand
Delevaux
(1980)
have
shown
thatthat
207Pb-204Pb
about 2.7
2.7 Ga
Ga(Goldich
(Goldichand
andothers,
others,1970).
1970).Doe
Doe
Delevaux
(1980)
have
shown
^Pb-^Pb
values in the Sacred Heart
Heart Granite
Graniteare
arecharacteristic
characteristicof
of ensialic
ensialic environments,
environments, as contrasted
with the
the ensimatic
ensirnatic (arc)
(arc) granitoid
granitoid bodies
bodies in the
the Superior
Superior province (greenstone-granite terrane).
with
The
The ensialic
ensialic environment indicates that the Archean gneiss terrane had been cratonized
cratonized prior
prior to
to
emplacement
of the Sacred
Sacred Heart Granite.
Granite. Precise
emplacement of
Precise ages
ages are
are required
required to
to test
test the
thehypothesis
hypothesis that
that
the
the Late
Late Archean
Archean granites
granites south
south of
of the
theGLTZ
GLTZwere
wereindeed
indeed formed
formedduring
duringcontinent-continent
continent-continent
collision.
collision.
Cumulative
Cumulative data on
on the
the Archean
Archean Superior
Superiorprovince
province (see
(see Hoffman,
Hoffman, 1989,
1989, for review)
review) indicate
consists of
of generally east trending belts
that itit consists
belts of
of island
island arc
arc and related rocks that were
assembled progressively from
from north to south
south (Card,
(Card, in
in press),
press),before
beforefmally
finallycoffiding
colliding with the
Archean
Ma. This pattern of
Archean gneiss terrane (continent) on the south at about 2,690
2,690 Ma.
of accretion
accretion as
well as the
the tectonic
tectonic style,
style, is not unlike
unlike that in
in modern
modem plate-tectonic
plate-tectonic regimes,
regimes, indicating
indicating that
that
well
plate-tectonic mechanisms existed
existed in
in the Archean as well
well as in the Proterozoic
Proterozoic and
and Phanerozoic.
Phanerozoic.
124
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region: U.S.
Contributions to Precambrian geology
geology of
U.S. Geological
Geological Survey
Bulletin 1904-C,
1904-C, 17
17 p.
D.L., Morey, G.B.,
G.B., and McSwiggen,
P.L., 1988,
1988,Geologic
Geologicmap
mapof
of the
the Penokean
Penokean
McSwiggen, P.L.,
Southwick, D.L.,
text: Minnesota Geological
orogen, central and eastern Minnesota, and accompanyingtext:
Report of
p., scale
scale1:250,000.
1:250,000.
Survey Report
of Investigations
Investigations37,
37,25
25p.,
Taylor, W.E.G.,
W.E.G., 1967, The
The geology
geologyofofthe
the lower
lowerPrecambrian
Precambrian rocks
rocks of
of the
the Champion-Republic
area of Upper
Upper Michigan:
Michigan: Evanston,
Evanston,Illinois,
Illinois,Northwestern
Northwestern University
University Report
Report 13,
13,33
33 p.
p.
Thomas, WA., 1977,
of Appalachian-Ouachita salients
salients and
and recesses
recesses from
from reentrants
1977, Evolution of
and promontories
promontories in
in the
the continental
continentalmargin:
margin: American
AmericanJournal
JournalofofScience,
Science,v.
v. 277,
277,
p. 1233-1278.
1233-1278.
1983,Continental
Continental margins,
margins, orogenic
orogenicbelts,
belts,and
and intracratonic
intracratonic structures:
structures: Geology,
1983,
Geology, v.
v. 11,
11,
p. 270-272.
270-272.
Van Hise, C.R.,
W.S., 1897,
1897,The
TheMarquette
Marquetteiron-bearing
iron-bearingdistrict
district of
of Michigan:
Michigan: U.S.
C.R., and Bayley,
Bayley, W.S.,
Geological Survey Monograph
608 p.
Monograph28,
28,608
p.
129
White,
of strain on the
White, S.H.,
S.H., 1976,
1976, The effects of
the microstructures,
microstructures, fabrics, and deformation
deformation
mechanisms
mechanisms in quartzites:
quartzites: Royal
RoyalSociety
Societyof
of London
LondonPhilosophical
PhilosophicalTransactions,
Transactions,ser.
ser.
A283,
p.
69-86.
A283, p. 69-86.
Wise,
Wise, D.U., and
and seven
sevenothers,
others,1984,
1984,Fault-related
Fault-relatedrocks--Suggestions
rocks-Suggestions for terminology: Geology,
Geology,
v. 12,
12,p.p.39391-394.
v.
1-394.
Wood,
of the Hewitt Lake area,
Wood, John,
John, 1980,
1980, Geology of
area, District
District of
of Kenora,
Kenora,Patricia
Patriciaportion:
portion:
Ontario
OntarioGeological
GeologicalSurvey,
Survey,Report
Report186,
186,122
122p.
p.
130
Field stops and road log
Field stops
stops shown
shown on figure
figure 66
Miles
0
0
3.8-6.3
Depart from
from Ramada
RamadaInn
Inninindowntown
downtown Marquette
Marquetteon
onMichigan
Michiganhighway
highway28
28 (M28)
(M28)
and proceed
proceed west.
west.
of pillow
pillowbasalt
basaltalong
alongM28.
M28. Basalt
Basaltcomprises
comprisesaamajor
majorpart
part of
of
Numerous outcrops of
Archean greenstone-granite terrane
terrane (Wawa
(Wawa subprovince)
subprovince) in the
the northern
northern complex
complex
of Marquette
Marquette district.
district.
Proterozoic rocks of Marquette
Early Proterozoic
Marquette syncline
syncline lie to the
the south
southof
ofhighway.
highway.
7.2
Turn left (south)
(south)from
from M28
M28 onto
ontoM35.
M35.
10.8
M35 intersects Marquette County road
road 480
480 (Co.
(Co. 480).
480). Continue
Continue south
south on
on M35.
M35.
11.2
Outcrops
Outcrops of Early Proterozoic
Proterozoic Negaunee
Negaunee Iron-formation.
Iron-formation.
14.1
o'clock, Empire
Empire iron
iron mine
mine waste
waste rock.
rock. Pass through Palmer.
High dumps at 12 o'clock,
16.9
Turn right
right (west)
(west)on
onCounty
Countyhighway
highway 565.
565.
18.1
At road
road fork
fork turn
turnright
rightonto
ontoCounty
Countyhighway
highway 476.
476.
19.5
Stop 1.
1. Roadcut
Roadcutinin"Tilden
Tilden granite"
granite" (Hammond,
(Hammond, 1978).
1978).
The "Tilden granite"
granite" is aa pinkish-gray
pinkish-gray to
to pink,
pink, medium-grained,
medium-grained,locally
locally porphyritic,
porphyritic,
massive to weakly
weakly foliated,
foliated, homogeneous
homogeneous granite
granite that
that locally
locally contains
contains oriented
oriented
of mafic
mafic gneiss.
gneiss. It intrudes
xenoliths of
intrudes gneisses
gneisses and in turn
turn isis cut
cut by
by pink
pink pegmatite.
pegmatite.
surfaces and a thin
The granite is
is highly fractured; the fractures have slickensided surfaces
of chlorite
chlorite and
and other
other propylitic
propyliticalteration
alterationminerals.
minerals. The granite has a U-Pb
coating of
concordia intercept age of 2,585±15
Ma
(R.E.
Zartman,
2,585Â15
Zartman, oral
oralcommun.,
commun., 1991).
1991).
Chemically,
the granite has characteristics of post-orogenic, within
within plate
plate granite on
Chemically, the
on
Bornhorst, written
written commun.,
commun., 1990),
1990), which is consistent
consistent
discrimination diagrams
diagrams (T. Bornhorst,
with its
its post-tectonic
post-tectonic field
field relationships.
relationships.
Continue southwest
Continue
southwest on
on Co.
Co. 476.
476.
19.7
Turn around and return to
to M35.
M35.
22.4
Junction of Co. 476 and M35.
M35. Proceed
Proceed southeast
southeaston
onM35.
M35.
23
gneiss terrane).
terrane).
Stop 2. High
High roadcut
roadcut on
on M35
M35 in
in Archean
Archean gneiss (of Archean gneiss
Road shoulder is narrow. Be
Becareful.
careful.
Excellent outcrop
outcrop of
of layered
layered gneiss
gneiss(of
(ofArchean
Archeangneiss
gneissterrane)
terrane) on
on the
the northeast
Excellent
side of road. Amphibolite
Amphibolite and
and quartzofeldspathic
quartzofeldspathic gneiss
gneiss are
are interlayered, and are
by red pegmatite. Pegmatite
cut by
Pegmatite and
and aplite
aplite are
areboudined
boudined and
andbrecciated;
brecciated; boudins
boudins
131
plunge
plunge parallel to
to recumbent
recumbent fold
fold axes.
axes. Tight
Tightfolds
foldswith
with axial
axial planar
planar foliation
foliation
plunge
25' with a strike
strike of
of N.
N. 50
5
' W.,
W., as
as does
does aa mineral
mineral lineation.
lineation.
plunge about
about 25°
This
This outcrop
outcrop is
is typical
typical of the gneiss of the Archean gneiss terrane, but amphibolite
amphibolite
is
abundant here. Tight
is uncommonly
uncommonly abundant
Tight recumbent
recumbent folds
folds that
thatplunge
plunge gently
gently northward
northward
are
are the
the dominant
dominant structures
structures in
in this
this part
part of
of the
thegneiss
gneiss terrane.
terrane.
Return
M35.
Return to
tovehicle
vehicle and
and continue
continue southeast
southeaston
onM35.
23.7
M35 intersects private road
road (Cleveland
(ClevelandCliffs
Cliffs Iron
Iron Company).
Company).
24.2
Stop 3.
area. Outcrop
3. Low
Low outcrop
outcrop on
on northeast
northeast side of road in grassy
grassy area.
Outcrop has
has been
been
covered
covered by
by loose sand
sand and
and isislichen
lichen free.
free.
This outcrop contains
contains northwest-trending
northwest-trending folds
folds with an
an axial
axial planar
planar foliation
foliationvisible
visible
600
on polished
polished surfaces
surfaces that
that strikes
strikesN.
N. 65°
65' W.,
W., and
and dips
dips 60' NE.
NE. This structure
structure is
is
superposed on recumbently folded
folded gneiss,
gneiss, and
and therefore
therefore is younger.
younger. Thick
Thick layers
layers of
felsic gneiss retain
retain a dominant older
older foliation
foliation that
that strikes
strikesN.
N. 250
25' W.
W.
The steep
foliation and
and associated
associated folds
foldsare
are parallel
parallel to
to the
the trend
steep northwest-trending
northwest-trending foliation
of the
(of Marquette district).
district). Although
the antiformal
antiformal southern complex
complex (of
Although minor
minor in
in this
this
area,
area, upright
upright northwest-trending
northwest-trending folds are the dominant
dominant structures
structures in
in the
theRepublic
Republic
area,
30 km
km to
to the
thewest.
west.
area, 30
M35.
Continue southeast
southeast on
on M35.
Continue
132
24.3
Turn around
around at
atCommunity
Community Club
Club and
and proceed
proceed northwest
northwest on
on M35
M35 toward
toward Palmer.
Palmer.
24.9
M35 intersects private
private road
road (CCI).
(CCI). Continue
Continue north
north on
on M35
M35 to
to intersection
intersection with
with
480.
Co. 480.
32.3
Intersection of M35 and Co. 480. Turn
Turn right
right (east)
(east) on
onCo.
Co.480.
480.
35.6
Turn right (south) on secondary road.
road. Roads
Roads for
for remainder
remainder of
of field
field excursion
excursion are
are
unmarked; accordingly,
we have
have arbitrarily
arbitrarily assigned
assigned letter
letter designations A through L
accordingly, we
L
(see fig. 3) to specific
segmentsas
asan
anaid
aidininlocation.
location. The main
main road into area
specific segments
(road A)
A) is
is aa graded
graded sandy
sandy dirt road; other
other roads
roads followed
followed on the trip are
are not
not
and may
may be
be rutted
rutted or
orsoft,
soft,requiring
requiringsome
somecare
careinindriving.
driving.
graded and
39.6
Road A
A crosses
crosses Chicago
Chicago and
and Northwestern
Northwestern railway
railway near
near Gentian.
Gentian.
39.7
(B) and
andcontinue
continuesouthwest
southwest on
on B.
B.
road fork
fork (B)
Take right road
40.6
Junction with east-west
east-west road
road (C).
(C). Proceed
Proceed west
west on
on CC for
for0.3
0.3 mi.
mi.
40.9
40.9
Stop 4. Rock
Rockknob
knobnorth
northside
sideof
of road
road C.
C.
Stop
This
This outcrop
outcrop isis typical
typical of
of the
the Archean
Archean greenstone-granite
greenstone-granite terrane.
terrane. ItIt consists
consists of
of
granite
granitegneiss
gneiss (or
(or foliated
foliated granite)
granite) that
thatcontains
containslenses
lensesof
ofbiotitic
biotiticand
andchioritic
chloritic
amphibolite.
amphibolite. These
Theserocks
rocksare
arecut
cutby
bygranite
granitepegmatite
pegmatiteand
andaplite,
aplite,which
whichcrosscut
crosscut
the
the foliation
foliation in
in the
the granitoid
granitoid rocks.
rocks. Note
Notethat
thatcrosscutting
crosscutting aplite
aplite and
and pegmatite
pegmatite are
are
not
70ÂW.
W.and
anddips
dips65°
65' Sw.
SW.
not sheared
shearedout.
out.Foliation
Foliationstrikes
strikesN.N.70°
Although
Although this outcrop lies near the
the north
north margin
margin of
of the
theGreat
GreatLakes
Lakestectonic
tectoniczone
zone
and
although
it
has
a
foliation
subparallel
to
that
in
the
GL1'Z,
the
outcrop
and
and although it has a foliation subparallel
the GLTZ, the outcrop and
those
those to
tothe
thewest-northwest
west-northwest are
areexcluded
excluded from
from the
theGLTZ
GLTZbecause
becausethe
thecrosscutting
crosscutting
pegmatite
pegmatite and
and aplite
aplite are
are not
not sheared
sheared out
out and
and the
the rocks
rocks are
are protomylonite.
protomylonite. The
The
northern
northern boundary
boundary isis placed
placed at
at the
theabrupt
abrupttransition
transitionfrom
from"straight-banded"
"straight-banded"
mylonite in the GL1'Z
GLTZ to
to rocks
rocks that
that retain
retainrecognizable
recognizable igneous
igneous structures,
structures, such
such as
as
here. The
Therocks
rockstotothe
thenorth
northofofthe
theGLTZ
GLTZare
areprotomylonite.
protomylonite.
Return
Return totovehicle
vehicleand
andcontinue
continuewest
weston
onCCfor
for0.2
0.2mile.
mile.
41.1
41.1
Intersection
Intersection of roads C and D. Turn
Turnleft
left on
onDDand
andcontinue
continuefor
for0.4
0.4 mi.
mi.
41.5
41.5
Stop 5. Outcrops
Outcropsofofmilky
milky quartz
quartz and
andaltered
alteredArchean
Archeanrocks
rocksabout
about50
50yards
yardswest
west of
of
Stop
D.
road D.
road
Milky
Milky quartz and
and silicified
silicified country rock, such as those exposed here,
here, characterize
characterize
the
the hifis
hills to the west. These
Theseoutcrops
outcrops comprise
comprise the
the most
most eastern
eastern exposed
exposed part
part of
of aa
quartz
km
long. The quartz
1.4 l
un long.
quartz body
body
quartz lense, oriented about
about east-west,
east-west, that is about 1.4
may
may occupy a major tension fissure zone related to
to dextral
dextral wrench
wrench shear
shear within
within the
the
GLTZ.
GLTZ.
Turn
Turn around
around and
and proceed
proceed north
north on
onDDtotointersection
intersectionwith
with C.
C.
Then
Then follow
follow C and D to
to road
road fork
fork at
atGentian
Gentiancrossing.
crossing.
42.8
42.8
sharply right from road B
B onto
onto road
road E;
E;continue
continuesouth
southfor
for1.2
1.2ml.
mi.
Turn sharply
44
44
Road
Road fork;
fork; continue
continue south
south on
on road
road F.
F.
44.35
44.35
Road
fork (G),
(G), then turn left at road
Road fork;
fork; continue south on right fork
road fork
fork onto
onto H
Hand
and
east on
on H
Hto
tolocked
lockedgate.
gate.
continue east
45.0
45.0
and walk
walk on private road east
east and
and northeast
northeast to
towest
west shore
shoreof
ofPowell
Powell Lake
Lake
Park and
(distance 0.3
mi). walk
0.3 mi).
Walkisiseasy.
easy.
Stop
Thisisisan
anexceptionally
exceptionally good
good series
series of
of outcrops
outcrops of
of mylonite
mylonite within the
Stop 6. This
GLTZ
clean. The
dominantly granite
granite
GLTZ because
because they
they are
areperiodically
periodically washed
washed clean.
Therock
rockisisdominantly
gneiss containing pods
pods of
of migmatized
migmatized amphibolite
amphiboliteand
and hornblende
hornblende schist.
schist. The
The
protolith is
is granite and
and amphibolite
amphibolite of the Archean greenstone-granite terrane.
133
variable. A
Foliation in the
the gneiss
gneiss strikes
strikes about
about N.
N. 75°
75' W.
W. and
and dips
dips 75°
75' SW.,
SW., but is variable.
conspicuous stretching
stretching lineation
lineation plunges
plunges uniformly
uniformly about
about550
55' and bears
bears S.
S. 45°
45' E.
Both dextral and sinistral asymmetric folds plunge
plunge subparallel
subparallel to
to the lineation;
dextral folds predominate.
predominate. Porphyroclasts
Porphyroclasts are
are common,
common, but they
they do
do not
not clearly
clearly
have internal monodlinic
symmetly and
and elongate
elongate tails,
tails, necessary
necessary to
to determine
determine
monoclinic symmetry
1986).
movement sense
sense (Simpson,
(Simpson,1986).
Return to parked vehicle
at locked
lockedgate.
gate. Proceed
Proceed west
west on
on road
road H
H to road fork
vehicle at
(distance 0.25
mi), then
then turn right on H to
0.25 mi),
to junction of
of H
H with
with JJ (distance
(distance0.15
0.15 mi).
mi).
45.4
Junction of
of roads
roads H and J. Turn
Junction
Turnsharply
sharply left
left on
on JJ and
and continue
continue for
for 0.2
0.2 mi.
mi.
45.6
45.6
Road fork. Continue
Continueon
onright
rightfork
fork(K)
(K)for
for0.1
0.1ml.
mi.
45.7
45.7
northwest, a distance
distance of about
about 750
750 ft, to outcrops.
outcrops.
Park and walk on trail to northwest,
Stop 7. Low
Low outcrops
outcropsof
of pink
pink mylonitic
mylonitic gneiss with lenses of biotite schist.
Protolith is considered to be felsic
felsic gneiss
gneissof
ofArchean
Archeangneiss
gneissterrane.
terrane. Foliation
N. 750
75' W. and dips
dips 65°
65' SW.
A quartz
quartzrodding
rodding lineation
lineationplunges
plunges50°
50' and
and
strikes N.
strikes
SW. .A
S. 45'
45°E.
E. Note abundant ribbon
ribbon quartz.
quartz. This outcrop is in the southern part
bears S.
of the GLTZ.
Return to road. Turn
Turnaround
aroundand
andfollow
follow roads K and J to road
road fork
fork with
with H.
Proceed south on H
H to
to junction
junction (1.5
(1.5 mi).
46.2
Continue south on L.
47.3
Road L intersects power line. Park
Parkand
andwalk
walk750
750ftft southwest
southwest to
tolow
low outcrops
outcrops
along power line.
Stop 8. Outcrops
Outcropscomprise
comprise pink
pink granite
granitegneiss
gneiss with amphibolite layers and lenses,
cut by
Foliation strikes
strikes N. 40°
40' W.,
by pink
pink granite
granite pegmatite.
pegmatite. Foliation
W., and
and dips
dips 80'
80°NE.
NE. Rocks
are not sheared.
sheared. Two
Twoparallel
parallel diabase
diabasedikes
dikes cut
cut the
theArchean
Archean gneiss.
gneiss.
These outcrops are
are within the Archean gneiss terrane, a distance of about 0.65
0.65 km
south of the south boundary
boundary of the (3L1'Z.
GLTZ. Thus,
Thus, we have crossed from the
Archean greenstone-granite terrane, through the GLTZ, into the
the Archean
Archean gneiss
gneiss
terrane
terrane to
to the
the south
south of
of the
the GLTZ.
GLTZ.
Suggest returning to Co.
Co. 480 via
End of field trip. Suggest
via dirt roads L, G, F, E, and A.
134
U)
w
-J
In —
U)
w
I
0— -0
Figure 6. Field
Fieldtrip
tripstops.
stops.Marquette
Marquetteand
andmost
mostofofMichigan
Michiganhighway
highway 28 are
are just north
north of
of the
the
Figure
map area.
area.
135
Issues of the Proceedings of this meeting may
may be ordered
ordered from
from
M.G. Mudrey,
Mudrey Jr.,
M.G.
Jr., Secretary-Treasurer
Secretary-Treasurer
do
c/oWisconsin
WisconsinGeological
Geologicaland
and Natural
Natural History
History Survey
Survey
3817
38 17 Mineral
Mineral Point Road
Road
Madison,
Madison, Wisconsin
Wisconsin 53705-5
53705-5100
100
1: Program and Abstracts: $7.00 U.S.
Part 1:
Part 2: Field Trip Guidebook:
Guidebook: $7.00 U.S.
Orders
Orders will
will be filled while supplies last.
last.
All volumes back to 1955
1955 are
are available
available for
for photocopying
photocopying at
at the
the prevailing
prevailing rate,
rate, from
from the
the Michigan
Michigan
Technological
Technological University
University Library,
Library, through
through Mr.
Mr. M.S.
M.S. Spence,
Spence, Archivist.
Archivist.
Telephone
Telephone (906)
(906) 487-2505
487-2505