STRUCTURAL RELATIONSHIPS AND CHRONOLOGIES OF
TWO CHERT TO CLASTIC ROCK SUCCESSIONS,
WESTERN SIERRA NEVADA, CALIFORNIA
by
Lynette Diane Wethington, B.S.
A THESIS
IN
GEOSCIENCES
Submitted to the Graduate Faculty
of Texas Tech University in
Partial Fulfillment of
the Requirements for
the Degree of
MASTER OF SCIENCE
Approved
Accepted
August, 1976
A
90S
ACKNOWLEDGMENTS
I am profoundly grateful to Dr. S.E. Cebull
guidance and counsel were invaluable in the
of this t h e s i s .
Appreciation
preparation
is expressed to Professor
D.H. Shurbet for reading the m a n u s c r i p t .
I especially
thank David W e t h i n g t o n , my brother, for enduring
field work with me.
11
whose
the
TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS
ii
ABSTRACT
v
LIST OF ILLUSTRATIONS
vii
CHAPTER
I.
INTRODUCTION
1
Purpose and Scope
1
Location and Geography
1
Techniques of Study
7
Previous Work of Special Relevance
II.
III.
REGIONAL GEOLOGY
10
12
Geologic Setting--Sierra Nevada
12
Geology of the Western Sierra Nevada....
12
Interpretations of the Geologic
History
18
DESCRIPTION OF ROCK TYPES AND MESOSCOPIC
STRUCTURES IN THE MERCED RIVER AREA
21
LithoStrati graphi c Relationshi ps
Within the Rock Succession
22
Cherts
23
Pel itic-Sil iceous Rocks
34
Slates
40
Limestones
43
Subareas
45
Geometric Relationships of Mesoscopic
Structures
45
111
IV.
V.
STRUCTURAL INTERPRETATION OF THE MERCED
RIVER AREA
Structural Relationships
50
Metamorphic Events
55
Deformational Chronology
56
DESCRIPTION OF ROCK TYPES AND MESOSCOPIC
STRUCTURES IN THE LITTLE BALD MOUNTAIN
AREA
62
Subarea I:
Subarea II:
Cherts
Pelitic-Siliceous Rocks..
Subarea III:
VI.
Clastic Rocks
64
70
76
STRUCTURAL INTERPRETATION OF THE LITTLE
BALD MOUNTAIN AREA
VII.
50
82
Structural Relationships
82
Metamorphic Events
88
Related Work in Adjacent Areas
89
Deformational Chronology
COMPARISON OF THE MERCED RIVER AND LITTLE
BALD MOUNTAIN AREAS: THEIR POSSIBLE
TECTONIC RELATIONSHIP
91
94
Penecontemporaneous Deformation
94
Comparison of Rock Successions
97
Comparison of Mesoscopic Structures...
98
Correlation of Deformational
Chronologies
99
Regional Interpretation
VIII. SUMMARY OF CONCLUSIONS
LIST OF REFERENCES
1 V
103
105
106
ABSTRACT
In the western Sierra Nevada, C a l i f o r n i a , an east to
west rock succession of layered chert,
pelitic-siliceous
rock, and fine-grained clastic rock characterizes part of
the Paleozoic Calaveras Formation.
Along the Merced
River, west of Y o s e m i t e , and at Little Bald M o u n t a i n ,
west of Lake T a h o e , mesoscopic structures in these lowgrade metamorphic rocks are chiefly folds and
fracture
cleavage in the c h e r t s , folds and poorly defined
cleavage
in the pelitic-siliceous rocks, and slaty or crenulation
cleavage in the fine-grained clastic rocks; lineations
(mainly c r e n u l a t i o n s ) occur in all
lithologies.
In the Merced River area, layering ( S Q ) mainly
strikes northerly and dips steeply east; a metamorphic
alignment of micas (S,) parallels S^;
cleavages
(S2)
strike west-northwest and dip steeply south; and folds
(F^) and lineations ( K ) are parallel, mainly
plunge
steeply to the southeast, and are dispersed in S^parallels F^ axial planes.
The subvertical
disposition
of S^ may be a product of tilting or isoclinal
(D,).
Later, F^ f o l d s , S^, S^,
S^
folding
and 1^ developed during D^
under conditions of greenschist facies
metamorphism.
In the Little Bald Mountain area, layering ( S Q )
strikes mainly northeast and dips steeply south; a metamorphic alignment of micas (S^) parallels S^; F^ folds
plunge shallowly with diverse trends; cleavages
(Sp)
strike north-northwest and dip subvertically; F^ folds
plunge steeply to the south; and lineations ( K )
steeply and are dispersed in S Q .
S2 transects F^ folds
and may be axial plane cleavage of Fp folds.
parallel to ?2 ^^^^
plunge
^^^ ^2 " ^0 "intersections.
1, is
F, folds
and S^ developed in association with tilting of isoclinal
folding (D,) which was accompanied by low-grade metamorphism.
Later, Sp, F^ f o l d s , and 1, formed under conditions
of greenschist facies metamorphism during Dp.
The two areas are similar with respect to absence of
significant penecontemporaneous deformation, rock succession, mesoscopic s t r u c t u r e s , and proposed deformational
chronologies.
The first (D^ and D.) and second (D- and
Dp) deformational events of the two areas tentatively are
correlated with the Sonoma and Nevadan orogenies,
respectively.
VI
LIST OF
ILLUSTRATIONS
Figure
Page
1.
Generalized map showing structural and
lithologic units of the Sierra Nevada,
central California
2.
Generalized description of structural and
lithologic units of the western Sierra
Nevada metamorphic belt
3
3.
Map of Merced River study area
5
4.
Map of Little Bald Mountain study area
6
5.
Types of mesoscopic structures measured
for each major rock type
6.
Photomicrograph of chert sample from Merced
River area
25
Typical stream-polished outcrop of folded
cherts along Merced River
27
Idealized fold shapes to which folds in
the Merced River area were compared and
t a b u l a t i o n s of 66 such comparisons
28
Chevron fold in cherts along the Merced
River
29
Fold with bulbous hinge and with limb
intruded into crest; cherts of Merced
River area
30
Apparent pinch-and-swel1 structures in
cherts of Merced River area
31
Fracture cleavage in c h e r t s , Merced
area
33
7.
8.
9.
10-
11.
12.
13.
14.
15.
Elongate clasts in pelitic-siliceous
along Merced River area
River
rocks
35
Photomicrograph of sample of pelitic-si 1iceous rock, Merced River area
37
P h o t o m i c r o g r a p h of a sample of silty-slate
from Merced River area
42
VI 1
16.
17.
18.
19.
20-
21.
22.
23.
24.
25.
26.
27.
28.
29.
L o w e r - h e m i s p h e r e , equal-area p r o j e c t i o n s ,
for Merced River area, of l i n e a t i o n s , fold
a x e s , and poles to axial p l a n e s , c l e a v a g e ,
"unidentified" s^-planes, layering, and
joints for each subarea
47
Synoptic l o w e r - h e m i s p h e r e , equal-area
plots of fold a x e s , l i n e a t i o n s , layering,
c l e a v a g e , and axial planes of Merced
River area
52
Schematic outline of deformational
chronology of Merced River area
55
Schematic outline of alternative deformational chronologies for Merced River area..
60
L o w e r - h e m i s p h e r e , equal-area p r o j e c t i o n s ,
for Little Bald Mountain area, of poles to
l a y e r i n g , poles to c l e a v a g e , l i n e a t i o n s ,
fold a x e s , and poles to axial planes for
each subarea
63
"Lumps" in chert l a y e r s . Little Bald
Mountain area
67
Fracture cleavage in cherts at Little Bald
Mountain
67
Shallowly plunging fold in cherts of
Little Bald Mountain area
69
Photomicrograph of slate interlayer in
cherts of Little Bald Mountain area
71
Photomicrograph of sample of peliticsiliceous rock. Little Bald Mountain area..
74
Steeply plunging fold in subarea II,
Little Bald Mountain area
77
Photomicrograph of sample of clastic rock
from Little Bald Mountain area
79
Sketch map showing Little Bald Mountain
area, study area of M o r a h a n , and study
area and subarea III of Redmond
83
Synoptic equal-area plot of layering,
c l e a v a g e , l i n e a t i o n s , and fold a x e s . Little
Bald Mountain area
83
viii
30.
31.
32.
33.
Synoptic equal-area plot of data from
subarea III of Redmond
84
Synoptic equal-area plot of data from
Paleozoic rock succession of Morahan's
study
84
Schematic outline of deformational
chronology of Little Bald Mountain area....
88
Tentative correlation of the deformational
chronologies of the Merced River and
Little Bald Mountain areas
1X
100
CHAPTER I
INTRODUCTION
Purpose and Scope
An east to west succession of layered chert, pelitic
siliceous rock, and fine-grained clastic rock crops out
i n t e r m i t t e n t l y along the length of the western
Nevada metamorphic belt, California.
Sierra
These rocks of the
Paleozoic Calaveras Formation have undergone
low-grade
m e t a m o r p h i s m and at least one period of deformation.
Folds in the cherts and cleavage in the clastic rocks are
the r e s p e c t i v e predominant mesoscopic s t r u c t u r e s .
The
central purpose of this study is to describe the structures and petrographic character of these rocks and to
determine the nature of their d e f o r m a t i o n ( s ) in two areas
in the western Sierra Nevada (Fig. 1 ) .
Important ques-
tions include interpretations concerning the
penecontem-
poraneous and/or tectonic origin of folds in the c h e r t s ,
the r e l a t i o n s h i p between structures in the cherts and
those in the clastic r o c k s , and the number and age(s) of
deformation(s).
Location and
Geography
The first area to be discussed is located along the
Merced River and a parallel portion of California
State
•
^
'"^•^^''-iim
^v^.i
Metomorphic rocks cost of
western metomorphic belt
rrrrrrrrn piuton'ic rocks of Slerro Nevodo
Linittl
bothoiith
A
r
.Slerro
rNevodo
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lai®
0
50
I
100 Kilometers
Figure 1.--Generalized map showing struc tural and lithologic units of the Sierra Neva da, c entra 1 California,
Dot and triangle s how location s of the t wo areas, of
this study; modifi ed from Schw ei ker t and Cowan ( 1 9 7 5 ) .
Units shown are de scribed in t he te xt an d Figure 2.
Figure 2.--(opposite pa ge) Generali zed d e s c r i ption of structural and lltholog ic units of the w e s t e r n Sierra Nevada
metamorphic belt, The rocks a re ch i e f l y metamorphic,
but many retain th eir original text u r e s . Hence, here
they are referred to in a mixe d sed i m e n t ary, volcanic,
and metamorphic te rminology. Descr i p t i o ns for each
block are after: Clark (1964; bloc ks A , B, and E ) ,
Duffield and Sharp (1975; bloc k C ) . Schw eikert and
Cowan (1975; block D ) , McMath (1966 . biock E ) , and
Hietanen (1973; bl ock F ) .
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Yosemite National
Park (Fig. 1 ) .
The area extends up-
stream from B r i c e b u r g , for 9.5 miles (15.2 k m ) , to a
point k mile (.4
3).
km) south of a Geological Exhibit
(Fig.
Good exposures in the area are in road cuts and in
stream polished o u t c r o p s .
The second area is located around Little Bald Mountain, a p p r o x i m a t e l y 20 miles (32 km) west of Lake T a h o e ,
between the North and Middle Forks of the American
(Fig. 1 ) .
River
It is situated along the crest of Little Bald
M o u n t a i n , a portion of Duncan Creek, and the road to
French Meadows Reservoir (not shown in Fig. 4) and is
accessible from Foresthill
by the Mosquito Ridge Road
and the Forest Hill Divide Road (Fig. 4 ) .
The prominent
cultural landmark in the area is Duncan Peak Lookout,
on the western crest of Little Bald Mountain.
Good out-
crops are mainly on the crests of ridges, in stream
c h a n n e l s , and at road cuts.
Both study areas are located in the western Sierra
Nevada metamorphic belt, a region of predominantly metasedimentary and metavolcanic rocks lying chiefly to the
west of the Sierra Nevada batholith.
This region ranges
in elevation from 300 to 7000 feet and is characterized
by a gently rolling upland surface that is commonly
---;:! ._.>
J^/J^.
?i^
• >V^^. ,il-\
) '^i . k ' ^ N i .
M
'.^•'Hn'' '
Entrance
Bf
Figure 3.--Map of Merced River study area showing c r o s s sectional traverse along State Highway 140 and the
subareas into which the traverse is divided.
From
the El P o r t a l , C a l i f o r n i a , 15 minute q u a d r a n g l e , 1 9 4 7 .
m
J 1 MILE
J 1 KILOMETER
— - • — Batholith contact
Contour Interval 8 0
feet
Figure 4.--Map of Little Bald Mountain study area showing subareas into which c r o s s - s e c t i o n a l traverses
are divided and batholith contact mapped by Redmond
( 1 9 6 6 ) ; from Duncan Peak, C a l i f o r n i a , 1 9 5 2 , and
Granite C h i e f , C a l i f o r n i a , 1 9 5 3 , 15 minute quadr a n g l e s . For recent road and r e s e r v o i r d e v e l o p m e n t s , which are not shown on these q u a d r a n g l e m a p s ,
the reader is referred to t h e Forest Service m a p :
Tahoe National Forest, C a l i f o r n i a , M t . Diablo
Meridian, 1974.
deeply weathered or covered by vegetation or more recent
deposits.
Deep youthful canyons dissect the upland sur-
face and provide good rock exposures.
Within these
canyons westwardly flowing rivers carry a seasonably
variable runoff which is greatest during the early
summer melt.
Annual precipitation ranges from 20 inches
at lower elevations on the western edge of the region
to 60 inches at higher elevations to the east.
Most
precipitation occurs during the fall, winter, and spring
m o n t h s , and much falls as snow which may remain in
sheltered areas at high elevations until mid-July.
Average temperature ranges in the summer are from 45° to
75° F in the uplands and from 55° to 90° F in the canyons.
Winter average temperature range is from 20° to 40° F.
Vegetation in the region includes g r a s s e s , shrubs (manzanita, poison oak, and c h a m i s e ) , o a k s , and pines.
Extensive grasslands occur at low elevations on the
western edge of the region.
As elevation increases scat-
tered deciduous forests become common.
Above 2000 feet
mixed conifer forests predominate, except in areas of
recent fire d a m a g e , which are characterized by grasses
and shrubs.
Important economic activities in the region
include lumbering, tourism, and mining.
Techniques of Study
Structural data were collected along cross-sectional
8
traverses in each area during June and July of 1975 (Figs
3 and 4 ) .
Types of mesoscopic structures measured are
listed in Figure 5 by rock types in which they occur,
and their relative abundance in each study area is indicated.
Where outcrops are continuous, a minimum of one
measurement of each type of structure present was made
ewery
100 feet.
Five or more measurements of the same
type were often made in a single 100-foot interval.
Measurements were made with a Brunton compass, and
locations of data were plotted on the following
7.5
minute topographic sheets:
Merced River area--Feliciana Mountain, California, 1947;
Kinsley, California, 1947;
Little Bald Mountain area--Bunker Hill, California, 1953;
Duncan Peak, California, 1952;
Greek Store, California, 1952;
Royal Gorge, California, 1953.
Fold shapes in the Merced River area were recorded by
comparing fold profiles to the idealized fold forms
developed by Hudleston ( 1 9 7 3 ) .
Schmidt nets are used to
display data and to determine structural
geometries.
Thin-sections from portions of the rock succession were
examined to determine mineral compositions, analyze the
expression of mesoscopic structures at the microscopic
scale, and interpret metamorphic histories.
Mineral
STRUCTURES
CHERTS
PELITIC-SILICEOUS
ROCKS
Minor fold hinge
lines
M, 1
m, 1
1
Lineations
(crenulations,
M, L
M, L
M, L
Layering
'M, L
M, L
L
Fracture cleavage
M, L
L
CLASTIC
ROCKS
long axes of
elongate clasts,
"ridges", intersections of
layering and
cleavage )
Cleavage
L
Axial planes of
minor folds
M, 1
Penetrative surfaces not readily
indentifiable
in the field
Joints
M, L
m, 1
L
M, L
M
M
M
Figure 5.--Types of mesoscopic structures measured for
each major rock type. The following symbols indicate study area and relative abundance of
measurements:
M,
m,
L,
1.
common measurement in Merced River area
less common measurement in Merced River area
common m e a s u r e m e n t in Little Bald Mountain area
less common measurement in Little Bald Mountain
area.
10
p e r c e n t a g e s listed in this thesis were estimated by
visual m e a n s .
Previous Work of Special
Relevance
S c h w e i k e r t and Wright (1975) deal in part with
cherts and clastic rocks of the C a l a v e r a s Formation along
the S t a n i s l a u s and T u o l u m n e R i v e r s .
In this study they
d e s c r i b e the C a l a v e r a s Formation as a melange which
consists of blocks of folded chert in a chaotic mixture
of clastic rocks and report a regional penetrative deformation which "produced a regionally consistent
lineation
of chert rods and minor folds whose axial surfaces parallel the regional f o l i a t i o n " (Schweikert and W r i g h t , 1 9 7 5 ) .
They suggest that much of the melange is a product of
disruption of sediments before c o n s o l i d a t i o n , and that the
p e n e t r a t i v e d e f o r m a t i o n occurred during the Late Paleozoic Sonoma
orogeny.
The Merced River area is included in an extensive
report on the western Sierra Nevada metamorphic belt by
Clark ( 1 9 6 4 ) .
He d e s c r i b e s folded cherts and clastic
rocks in the Calaveras Formation as well as s t r u c t u r e s ,
such as m i n o r f o l d s , s c h i s t o s i t y , slip c l e a v a g e , chert
r o d s , and c r e n u l a t i o n s , and concludes that most of the
s t r u c t u r e s are related to two d e f o r m a t i o n s .
He associates
shallowly plunging or horizontal minor f o l d s , which are
11
uncommon in the Calaveras Formation, with the first
deformation and slip cleavage and steeply plunging
minor folds with the second.
Upper Jurassic rocks.
Both deformations affect
The first ended before the
emplacement of Upper Jurassic granitic r o c k s , but the
second may have continued into the period of major batholith emplacement during the Middle Cretaceous
(Clark,
1960).
In the Little Bald Mountain area Redmond
(1966)
reports cherts and clastic rocks in the Calaveras
Formation.
For a study area centered about the Middle
Fork of the American River between the Sierra Nevada
batholith and Melones fault zone, he describes structures
such as f o l d s , strain-slip c l e a v a g e , l i n e a t i o n s , and
slaty cleavage which is axial plane to minor folds.
He
suggests that his data support an interpretation of two
periods of d e f o r m a t i o n , one of which apparently occurred
during the Late Jurassic Nevadan orogeny.
At present, W.J. Nokleberg (Fresno State University,
C a l i f o r n i a ) is directing student studies of the folded
cherts along the Merced River (personal
1975).
communication,
CHAPTER II
REGIONAL GEOLOGY
Geologic S e t t i n g - - $ i e r r a
Nevada
The Sierra Nevada is a large, tilted crustal
block,
primarily composed of plutonic igneous and metamorphic
rocks.
It is bounded on the east by the Sierra Nevada
fault system, along which it was tilted westward, and on
the south by the Garlock fault.
It is onlapped on the
west by Upper Cretaceous and Cenozoic sedimentary rocks
of the Great Valley and on the north by Cenozoic volcanic
rocks extending southward from the Cascade Range.
South-
ern and eastern portions of the Sierra Nevada are composed
mainly of plutonic igneous rocks of Mesozoic age, collectively referred to as the Sierra Nevada batholith.
These
plutons were intruded into deformed, m e t a m o r p h o s e d , sedimentary and volcanic rocks of Paleozoic and Mesozoic age.
In the southern third of the Sierra Nevada, these metamorphic rocks occur only as scattered roof pendants.
In
the central and northern t h i r d s , such rocks comprise most
of the western Sierra Nevada and are referred to as the
western m e t a m o r p h i c belt (Bateman, 1 9 6 8 ) .
Geology of the Western Sierra Nevada
Stratigraphy
Rocks of the western Sierra Nevada are divided into
12
13
two g r o u p s , a superjacent series and a bedrock complex.
The superjacent s e r i e s , which is not considered in this
paper, is composed of nearly flat-lying alluvial and
volcanic d e p o s i t s .
The bedrock complex consists of
Paleozoic and Mesozoic m e t a s e d i m e n t a r y and metavolcanic
rocks, fault-related r o c k s , and Mesozoic plutonic rocks
of gabbroic to granitic composition
(Clark, 1 9 6 4 ) .
The m e t a s e d i m e n t a r y and metavolcanic rocks of the
western m e t a m o r p h i c belt generally are separated from
metamorphic rocks in the eastern Sierra Nevada by the
Sierra Nevada batholith, but in a few localities, such as
the North Fork of the American River (Clark and others,
1962) and the area north of Taylorsville (McMath, 1 9 6 6 ) ,
an unconformity separates these rocks (Fig. 1 ) .
The
belt is divided, here, into six structural blocks (A, B,
C, D, E, and F) which are bounded by faults of the Foothills fault system (modified after Schweikert and Cowan,
1 9 7 5 ) ; the Melones and Bear Mountains fault zones are the
most conspicuous components of this fault system (Fig. 1)
D e s c r i p t i o n s of the individual blocks are presented in
Figure 2.
Blocks A and B are considered to be island arc
sequences by Schweikert and Cowan ( 1 9 7 5 ) .
Block C is
interpreted by Clark (1964) as a portion of the Calaveras
Formation, whereas Duffield and Sharp (1975) interpret it
14
as a tectonic m e l a n g e which is unrelated to the Calaveras
Formation.
Block D is considered by Schweikert and Cowan
(1975) to be a partial o p h i o l i t e s e q u e n c e .
The rocks of
block E are viewed as marginal
basin deposits by
S c h w e i k e r t and Wright
Most rocks in block E are
(1975).
referred to the C a l a v e r a s F o r m a t i o n , following the conventional
usage of the name for all Paleozoic rocks in
the Sierra Nevada (Schweikert and Cowan, 1 9 7 5 ) .
However,
in the north the Shoo Fly Formation and an upper Paleozoic p y r o c l a s t i c
sequence are recognized but at present
are not d i r e c t l y correlated with the Calaveras
Formation.
S c h w e i k e r t and Cowan (1975) suggest that block F is a
m e l a n g e and that portions of block E are also m e l a n g e s .
U l t r a m a f i c r o c k s , commonly s e r p e n t i n i t e s , are associated with the faults of the Foothills fault
system.
With few e x c e p t i o n s ultramafic plutons are aligned
fault zones or bounded on one side by f a u l t s .
along
These
e l o n g a t e and commonly slickensided bodies range in size
from feet to miles and in texture from blocky to
s t r o n g l y foliated.
Because these bodies are related to
faults which are a p p a r e n t l y of Late Jurassic a g e , their
emplacement
is c o n s i d e r e d to have occurred during the
Late J u r a s s i c
(Clark,
1964).
The gabbroic to granitic rocks crop out as small
large isolated plutons and as plutons of the
Sierra Nevada b a t h o l i t h .
Gabbroic to dioritic
composite
rocks
to
15
occur in isolated plutons and on the m a r g i n s of less mafic
plutons.
These plutons are commonly associated with ser-
p e n t i n i t e s or s m a l l e r mafic b o d i e s .
Some appear to be
the products of in situ r e p l a c e m e n t or r e c r y s t a l 1 i z a t i o n
of m e t a v o l c a n i c r o c k s , whereas others probably are intrusive igneous b o d i e s .
Granodi critic to granitic rocks
comprise larger isolated plutons and most plutons of the
Sierra Nevada b a t h o l i t h .
These plutons are characterized
by g r e a t e r h o m o g e n e i t y than the more mafic plutons and by
contact m e t a m o r p h i c effects (Clark, 1 9 6 4 ) .
Radiometric
age dating shows that these plutons were emplaced at
r e l a t i v e l y shallow depths during as few as two
(Lanphere
and Reed, 1 9 7 3 ) to as many as five (Evernden and Kistler,
1 9 7 0 ) intrusive epochs which range in age from Middle
T r i a s s i c to Late C r e t a c e o u s .
Structure
The western m e t a m o r p h i c belt is located in the
w e s t e r n part of a regional structural c o n f i g u r a t i o n which
resembles a complexly faulted synclinorium whose axial
region is occupied by the Sierra Nevada batholith
man, 1968).
(Bate-
Blocks A, B, C, D, E, and F comprise the
western flank of the " s y n c l i n o r i u m ; " the internal
ture of each block is described in Figure 2.
struc-
The Foot-
hills fault system is an a s s e m b l a g e of subparallel
which s e p a r a t e these b l o c k s ; it is discussed below.
faults
Also,
16
the m e s o s c o p i c structures of the metamorphic belt are
described because they provide data useful in determining
the deformational
history of the western Sierra Nevada.
As described by Clark (1960, 1 9 6 4 ) , the Foothills
fault system in the southern portion of the belt consists
of the northwest trending Melones and Bear Mountain fault
z o n e s , whereas in the north it also includes several
b r a n c h i n g , northwest and north trending fault zones
(Fig. 1 ) .
"Shear planes" and schistosity within the
steeply east-dipping fault zones commonly dip eastward at
more than 8 0 ° .
Lineations plunge steeply down
schistosity
surfaces and are characterized by both elongated
and aligned m i n e r a l s .
clasts
Because the age of blocks A, B,
C, D, E, and F generally increases to the east, displacement along the faults is considered to be in part reverseslip, and because steeply plunging lineations in the
fault zones appear to be b - 1 i n e a t i o n s , displacement is
considered to have a strike-slip component (Clark, 1 9 6 4 ) .
The faults are interpreted by some workers as roots of
westward driven thrusts (Davis, 1969; Russell and Cebull,
1974) and as large-scale tectonic features juxtaposing
structural blocks of varied histories by others (Schweikert and Cowan, 1 9 7 5 ) .
Because the youngest rocks
(Mariaposa F o r m a t i o n ) cut by the fault system are of Late
17
J u r a s s i c a g e , and because a portion of the fault
system
is t r u n c a t e d by a lobe of the b a t h o l i t h dated as Early
Cretaceous
(Evernden and K i s t l e r , 1 9 7 0 ) , motion along the
fault system is i n t e r p r e t e d as having begun in Late
Jurassic time and ceased by Early C r e t a c e o u s time.
M e s o s c o p i c features described by Clark
(1964)
include b e d d i n g , f o l i a t i o n s , l i n e a t i o n s , and minor folds.
Bedding is less well preserved in the eastern portion of
the belt than in the west and commonly is parallel to a
foliation.
Two f o l i a t i o n s are commonly o b s e r v e d .
One is
a c l e a v a g e in the west and a s c h i s t o s i t y in the east;
the other is a slip cleavage which is well developed in
the east but only locally developed in the west.
t i o n s , such as e l o n g a t e c l a s t s , mineral
alignments,
i n t e r s e c t i o n s of foliation and b e d d i n g , and
of f o l i a t i o n s , occur in two o r i e n t a t i o n s .
intersections
The most
one is steeply p l u n g i n g ; the other is shallowly
Minor folds are found in both o r i e n t a t i o n s .
steeply plunging folds parallel
Linea-
plunging.
Axes of
steeply plunging
linea-
t i o n s , and axial planes of minor folds are parallel
slip c l e a v a g e and s c h i s t o s i t y .
common
The shallowly
to
plunging
folds appear to be products of a major folding
event,
w h e r e a s slip c l e a v a g e and steeply plunging folds and
l i n e a t i o n s a p p e a r related to m o v e m e n t along the
fault system (Clark,
1964).
Foothills
18
Metamorphi sm
Rocks of the western metamorphic belt are of apparent low grade, and original structures and textures
commonly are preserved.
Determination of grade is
difficult in most of the region because most rocks are
fine-grained and commonly contain relict,
minerals.
pre-metamorphic
Regional metamorphism of predominantly
green-
schist facies is reported throughout the region, but
rocks of the a l m a n d i n e - a m p h i b o l i t e
prehnite-pumpellyite
facies (Baird, 1 9 6 2 ) ,
facies (McMath, 1 9 6 6 ) , and epidote-
amphibolite facies (Hietanen, 1973) occur locally.
Con-
tact metamorphism of albite-epidote hornfels facies and
hornblende hornfels facies is common in zones adjacent
to plutons (Best, 1963; Redmond, 1966; Russell
and
Cebull, 1974).
Interpretations of the Geologic
History
During Paleozoic time, the Sierra Nevada region was
on the western continental margin of North America, and
C a l a v e r a s , Shoo Fly, and upper Paleozoic
pyroclastic
rocks were deposited along this margin in predominantly
eugeosynclinal
environments.
The Late Devonian to
Middle Pennsylvanian Antler orogeny may have affected
some of these rocks.
Later, the rocks were deformed
during the Late Permian to Early Triassic Sonoma
orogeny,
19
which is interpreted by Schweikert and Wright (1975) as
the partial closing of a marginal
basin.
A variety of i n t e r p r e t a t i o n s are proposed for the
Mesozoic history of this region.
According to Bateman
(1968) the region was occupied by a large downwarping
synclinorium in which sediments accumulated to great
thicknesses.
The Sierra Nevada batholith was formed by
melting of a portion of the sedimentary pile and subsequent intrusion of m a g m a s .
Kistler and others
(1971)
interpret the region as being a topographic high which
shed sediments into epicontinental seas on the east and
onto and/or across the continental margin on the west
during the M e s o z o i c .
Plutons were intruded into this
high as it drifted westward over an upper mantle heat
source similar to that at an oceanic ridge.
Schweikert
and Cowan (1975) suggest that early in Mesozoic time the
region was located on a continental margin occupied by an
Andean arc.
Later, two volcanic arcs collided with the
continental m a r g i n , and an arc-trench complex then developed farther to the west.
The batholith was formed by
plutonism related first to subduction associated with the
early Andean arc and then to subduction associated with
arc-trench complex to the west.
Minor d i s t u r b a n c e s occurred throughout Mesozoic time,
but in the Late Jurassic a severe d i s t u r b a n c e , the classical Nevadan o r o g e n y , took place.
These minor disturbances
20
are related by d i f f e r e n t workers to synclinorium
develop-
ment, pluton e m p l a c e m e n t s , and/or melange formation in
subduction z o n e s .
The Nevadan orogeny is viewed by
Bateman (1968) as the major folding event which deformed
Upper Jurassic rocks.
Schweikert and Cowan (1975) con-
sider it to be the product of arc-continental
margin
collision and suggest that the Foothills fault
system
formed at that time.
The Cenozoic history of the Sierra region bagan with
an interval of relative stability and erosion.
the region experienced episodes of volcanic
uplift, t i l t i n g , and faulting
Later,
activity,
(Bateman, 1 9 6 8 ) .
CHAPTER III
DESCRIPTION OF ROCK TYPES AND MESOSCOPIC
STRUCTURES
IN THE MERCED RIVER AREA
Four different rock types occur in the Merced River
area:
stone.
chert, pel tic-si 1iceous rock, slate, and limeThe rock type terminology used here is in mixed
sedimentary and m e t a m o r p h i c t e r m s ; it reflects the lowgrade c h a r a c t e r of the metamorphism that affected all
rocks hut failed to alter the original appearance of many.
Each rock type is described below in regard to its field
a p p e a r a n c e , m i n e r a l o g y , and m i c r o s c o p i c texture.
Because
the rocks are fine-grained and l o w - g r a d e , some mineral
identifications and textural interpretations are less
than d e f i n i t i v e .
The types of mesoscopic
structures
formed during deformation are affected by rock type;
t h u s , specific rock types along the Merced River are
characterized by certain mesoscopic s t r u c t u r e s , although
some structures occur in more than one rock type.
How-
ever, structural geometries are expressed similarly in all
lithologies.
The cross-sectional traverse in the Merced
River area is divided into six subareas as shown in
Figure 3 to display the structural geometries and to
document possible changes in them over lateral d i s t a n c e s .
21
22
L i t h o s t r a t i g r a p h i c R e l a t i o n s h i p s Within
the Rock Succession
L i t h o s t r a t i g r a p h i c r e l a t i o n s h i p s within the rock
succession along the Merced River are summarized as
follows:
slates on the west side of the area
(downstream)
grade into p e l i t i c - s i l i c e o u s rocks w h i c h , in turn, are
i n c r e a s i n g l y i n t e r l a y e r e d with cherts on the east (ups t r e a m ) ; minor amounts of limestone occur as elongate
bodies within the interlayered p e l i t i c - s i l i c e o u s
and c h e r t s .
rocks
The gradational change from pelitic-siliceous
rock to slate is c h a r a c t e r i z e d by an increase in the
amount of s i l t - s i z e detrital q u a r t z , the disappearance of
s e d i m e n t a r y l a y e r i n g , and the development of a conspicuous c l e a v a g e , which probably reflects an increase in mica
content.
The interlayering of p e l i t i c - s i l i c e o u s rock and
chert is c h a r a c t e r i z e d by repeated t r a n s i t i o n s , through
intervals of a few feet to a few tens of feet, in which
massive p e l i t i c - s i l i c e o u s rock of moderate to high mica
content grades into d i s t i n c t l y layered chert.
In t r a n s i -
tion i n t e r v a l s , the massive rock gives way to rock displaying a planar feature which typically parallels nearby
layering.
In t u r n , such rock grades laterally into a
layered rock which does not have the highly siliceous
a p p e a r a n c e of the adjacent layered chert.
Limestones
within the i n t e r l a y e r e d cherts and pelitic-si 1iceous rocks
23
are p o r t i o n s of e l o n g a t e d
bodies mapped by Clark
These bodies have a trend parallel
to the
predominant
strike of l a y e r i n g , but the nature of contacts
them and s u r r o u n d i n g
(1964).
between
rocks is u n c l e a r .
Cherts
Cherts crop out in subareas
I, II, III, and
IV.
They are c o n s p i c u o u s l y layered and commonly are c h a r a c terized by a l t e r n a t i n g light and dark-colored
layers or by p e l i t i c - c h e r t
layers.
siliceous
layers between highly
These thinly layered cherts commonly are
siliceous
folded.
They are c h a l c e d o n i c or finely g r a n u l o s e in appearance
and light- to d a r k - g r a y , commonly with w h i t e , r e d - b r o w n ,
and b r o w n - g r a y w e a t h e r e d
surfaces.
Mi n e r a l o g y
Estimated
Average %
% Range
Quartz
75%
60% - 85%
White mica
(muscovite and
serici te)
17%
10% - 3 0 %
Mi neral
Dark mica
(biotite and unid e n t i f i e d brown
micas)
5%
Opaques
3%
3% -
8%
5%
A c c e s s o r y m i n e r a l s : c h l o r i t e , calcite (a late
r e p l a c e m e n t of q u a r t z ) , s p h e n e , and t o u r m a l i n e .
N u m b e r of s a m p l e s :
4.
24
Textures
On a m i c r o s c o p i c s c a l e , quartz in the cherts consists mainly of small crystals with sub-polygonal
polygonal
to
s h a p e s , indistinct to straight b o u n d a r i e s , and
incomplete to undulose e x t i n c t i o n .
Layering is defined
by v a r i a t i o n s in the c o n c e n t r a t i o n of
"radiolarian
g h o s t s " and micas within the quartz (Fig. 6 ) .
The "radio-
larian g h o s t s , " which are most numerous in bands of
m o d e r a t e mica c o n c e n t r a t i o n , are circular to elliptical
areas which contain slightly larger quartz crystals and
are devoid of mica.
That such structures
represent
relict r a d i o l a r i a n skeletons is uncertain.
Some layers
in these rocks are composed mostly of m i c a , others contain
wery
small amounts of m i c a , and still others have moderate
amounts of mica concentrated along their b o u n d a r i e s .
The
fine m i c a s , chiefly white but also dark (some identified
as b i o t i t e ) , are aligned parallel to layering (Fig. 6 ) ,
and this alignment is crenulated.
Veins of larger quartz
crystals (Fig. 6) occur parallel to layering, at a high
angle to l a y e r i n g , or randomly distributed in a ptygmatic
fashion.
Dark m i c a s , some identified as biotite, com-
monly are c o n c e n t r a t e d along these veins.
Such micas are
small, p o i k i l o b l a s t i c , and equidimensional .
Some white
micas d i s p l a y a random o r i e n t a t i o n ; chlorite is poikilob l a s t i c , occurs in f a n s , and a p p a r e n t l y replaces b i o t i t e .
25
CL
SL
It
c
II
Figure 6 . - - P h o t o m i c r o g r a p h of chert sample from Merced
River a r e a , showing comp&sitional layering ( C L ) ,
s^-surface (SL) of micas aligned parallel to layer
ing, " r a d i o l a r i a n g h o s t s " ( R G ) , and quartz veins
(QV).
Smallest scale unit = .042 mm.
Ordinary
light.
26
Mesoscopic
Structures
Folds in the layered cherts are the most
structures in these rocks (Fig. 7 ) .
conspicuous
Although such folds
crop out in most s u b a r e a s , they are best exposed in road
cuts and river banks near the Geological
area I (Fig. 3 ) .
Exhibit in sub-
Throughout the area, folds are commonly
chevron or s h a r p - c r e s t e d , open folds as is illustrated
by visual c o m p a r i s o n s between them and the idealized fold
shapes proposed by Hudleston (1973; Fig. 8 ) .
However,
some are r o u n d - c r e s t e d , open f o l d s , and a few are box
folds.
The number of round-crested folds may be greater
crested folds due to the ease with which their hinge
i
lines are m e a s u r e d .
3
than reported here because emphasis was placed on sharp-
C h a r a c t e r i s t i c s of folds in the
layered cherts include broken crests (Fig. 9 ) , bulbous
hinges (Fig. 1 0 ) , and limbs intruded into crests
10).
At places the folds are interformational
unfolded layers above and below (Fig. 1 0 ) .
(Fig.
with
Because the
folds are c o n s i d e r e d to be basically c y l i n d r i c a l , the
hinge lines represent fold axes.
Layers in the cherts are approximately k inch
(.8
cm) to 6 inches (15 cm) thick and at places exhibit
t h i c k e n i n g and thinning which resembles pinch-and-swel 1
structures
(Fig. 1 1 ) .
Individual layers are not as con-
19
27
Figure 7.--Typical stream-polished outcrop of folded
cherts along Merced River. Folds have a somewhat
chaotic a p p e a r a n c e . Hammer in front of clump of
grass in left central portion of photograph shows
seale;
28
•
:
)
—T"
B
1
2
3
D
—
1
1
!
i1
*
1
8
1
1
9
14
1
7
12
1
9
1
U9
1
1
i(
i
j
1
1
'' '
•
:
4
•
5
I4>
i
;
1
1
Figure 8.--Idealized fold shapes (after Hudleston, 1973)
to which folds in the Merced River area were compared (above) and tabulations of 66 such comparisons ( b e l o w ) .
SOI
29
fi«
!-3
19
Figure 9.--Chevron fold in cherts along the Merced River
Crest above and to the left of hammer is broken.
30
Figure 10.--Fold with bulbous hinge (below p e n ) and with
limb intruded into crest (to the right of p e n ) ;
cherts of Merced River area. This fold i l l u s t r a t e s
on a small scale the intraformational c h a r a c t e r of
the f o l d s ; layers above and to the right of the fold
are u n f o l d e d .
31
13
m
Si
19
Figure 11.--Apparent pinch-and-swel1
of Merced River area.
structures in cherts
32
t i n u o u s as initial
i m p r e s s i o n s of outcrops suggest.
They
commonly pinch out against a t h i c k e n i n g in an adjacent
layer or b i f u r c a t e into additional
layers.
F r a c t u r e c l e a v a g e is expressed as regular
fractures
that commonly are confined to one layer (Fig. 12) and as
l i g h t - c o l o r e d q u a r t z veins that d e l i n e a t e regular planar
features.
At places this c l e a v a g e is a regular set of
f r a c t u r e s in unfolded l a y e r s .
It fans across open folds
l o c a l l y , but g e n e r a l l y its geometric r e l a t i o n s h i p to the
folds is u n c l e a r .
M i c r o s c o p i c a l l y , the fracture
cleavage
is a p p r o x i m a t e l y parallel and a p p a r e n t l y related to
quartz veins which intersect layering at a high a n g l e .
L i n e a t i o n s in cherts occur as c r e n u l a t i o n s and
ges
"rid-
C r e n u l a t i o n s are fine to coarse linear " c r i n k l e s "
on p e l i t i c , layering s u r f a c e s , and " r i d g e s " are commonly
13
14
(fli
19
aw
l i g h t - c o l o r e d , raised traces on layering s u r f a c e s .
Only
a small number of " r i d g e s " were m e a s u r e d ; they plot parallel to c r e n u l a t i o n s on equal-area p l o t s , although on some
layers they are not precisely parallel to and are less
regular than the c r e n u l a t i o n s .
These c r e n u l a t i o n s are the
m i c r o s c o p i c c r e n u l a t i o n s of the s^-surface defined by
micas a l i g n e d parallel to l a y e r i n g , and "ridges" appear
to be i n t e r s e c t i o n s of quartz veins and layering
J o i n t s in cherts are subhorizontal
surfaces.
and nearly
33
ill
(rji
Figure 12.--Fracture cleavage in cherts, Merced River
area. Pen is parallel to layers; cleavage is
approximately perpendicular to layers.
34
p e r p e n d i c u l a r to fold a x e s .
Hence, good exposures of
fold profiles commonly are found on joint surfaces..
Pelitic-Si 1iceous Rocks
Rocks in the area which are not clearly chert, slate,
or limestone are considered here as pelitic-siliceous
rocks.
Such rocks crop out in subareas I, II, III, IV,
and V.
In many outcrops layering is not evident or
appears as f r a c t u r e s , c l e a v a g e s , or irregular ridges produced partly by w e a t h e r i n g , and in a few outcrops layering is chaotically disrupted.
These rocks commonly are
black to dark-gray to red-brown and less commonly
b l u e - g r a y , and white-gray.
gray,
Some are composed of elongate
light-colored clasts which "float" in a darker-colored
Si
matrix
(Fig. 1 3 ) .
These clasts range in length from
1/16
(9
inch (2mm) to 5 inches ( 1 3 c m ) .
Exposed rock surfaces
'C
SOI
•<
have p h y l l i t i c , q u a r t z o s e , or earthy appearances, and
many are stained or streaked.
Mineralogy
Estimated
Average %
% Range
Quartz
40%
15% - 70%
White mica
(muscovite, seric i t e , and possibly some minor
talc)
40%
10% - 8 0 %
Mi neral
35
II*
!Si
ijii
Ml
Figure 1 3 . - - E l o n g a t e clasts in p e l i t i c - s i l i c e o u s rocks
along Merced River. The pen is a p p r o x i m a t e l y
parallel to long axes of c l a s t s .
36
(Continued)
Opaques
Dark mica
(biotite and u n i dentified brown
mi cas)
15%
2% - 4 0 %
5%
2% -
8%
Accessory minerals: calcite, chlorite, tourmaline,
and s p h e n e .
(In one sample calcite clasts account
for 6 0 % of the rock, but calcite mainly is a late
r e p l a c e m e n t of q u a r t z . )
Number of s a m p l e s : 10.
Textures
The p e l i t i c - s i l i c e o u s rocks have a greater variation
of quartz and mica content and display less homogeneity
of t e x t u r e than do the c h e r t s .
In g e n e r a l , the quartz is
c h a r a c t e r i z e d by small crystals with polygonal
r
shapes,
(gi
i«i
indistinct or sutured b o u n d a r i e s , undulose or incomplete
19
'C
e x t i n c t i o n , and possibly some lattice orientation.
"Radiolarian g h o s t s " are visible in a few samples.
White
and dark micas are finely crystalline and commonly
aligned in an s^-surface which parallels
compositional
layers rich in quartz and mica (Fig. 1 4 ) .
Some samples,
mainly those with elongate c l a s t s , show no layering, but
have an s^-surface defined by an anastomosing alignment of
micas.
M e s o s c o p i c a l l y , this ^ - s u r f a c e does not appear to
have an o r i e n t a t i o n parallel to that of layering in other
samples and, t h e r e f o r e , is considered as a separate
37
;
19
Figure 1 4 . - - P h o t o m i c r o g r a p h of sample of pelitic-si 1iceous r o c k , Merced River area. Shown are c o m p o s i tional l a y e r i n g ( C L ) , the ^ - s u r f a c e of micas aligned
parallel to layering ( S L ) , and quartz veins (QV)
o r i e n t e d at high angle to l a y e r i n g . Smallest scale
unit = .042 mm. Crossed n i c o l s .
38
structure.
T h u s , an s_-surface parallel
to layering
an s^-surface o b l i q u e to l a y e r i n g are defined
in the
pel itic-sil iceous r o c k s , a l t h o u g h both are not
t o g e t h e r in any s a m p l e .
s u r f a c e parallel
layering
In many samples e i t h e r the i -
Veins of larger quartz
to l a y e r i n g , at a high angle to
(Fig. 1 4 ) , or r a n d o m l y in a ptygmatic
are a s s o c i a t e d with l o c a l i z e d
s^-surface parallel
compositionally
observed
to l a y e r i n g or the s^-surface o b l i q u e to
is c r e n u l a t e d .
occur parallel
and
of c a l c i t e or dark m i c a .
Some veins
The e l o n g a t e clasts are
lenses m a i n l y of q u a r t z , but also
A few clasts a p p e a r to be pul-
led a p a r t , and some d i s p l a y internal
o b l i q u e to external
layering
folds or d i s r u p t i o n s of the
to l a y e r i n g .
defined
fashion.
crystals
s^-surfaces.
s_-surfaces which
F i n a l l y , most
s i l i c e o u s r o c k s also contain r a n d o m l y oriented
are
m
peliticwhite
19
99
m i c a s , fans of dark mica
(commonly
i d e n t i f i a b l e as bio-
t i t e ) , and s p r a y s of c h l o r i t e a p p a r e n t l y r e p l a c i n g
Mesoscopic
biotite.
Structures
Some p o o r l y d e f i n e d
f o l d s , which r e s e m b l e those in
the c h e r t s , o c c u r in the pelitic-si 1iceous r o c k s .
s i n g l e , very
A
p o o r l y d e f i n e d fold in subarea V is the fold
in c l o s e s t g e o g r a p h i c a l
p r o x i m i t y to the slates of sub-
area VI w h i c h d i s p l a y no f o l d s .
Layering
in these rocks c o m m o n l y
is more subtle in
39
o u t c r o p than in the c h e r t s .
M i c r o s c o p i c a l l y , it is
c o m p o s i t i o n a l , a product of varying mica c o n c e n t r a t i o n s .
" U n i d e n t i f i e d " s.-planes in the pel itic-sil iceous
rocks are p e n e t r a t i v e planar features that resemble
f r a c t u r e s , poorly developed c l e a v a g e , or wery
layering.
ill-defined
In t h i n - s e c t i o n , some of these features are
the i - s u r f a c e parallel to l a y e r i n g , and others the s_surface o b l i q u e to l a y e r i n g .
L i n e a t i o n s in the p e l i t i c - s i l i c e o u s rocks are crenulations and long axes of elongate c l a s t s .
The crenula-
tions are fine to coarse linear " c r i n k l e s " on pelitic
surfaces or fine ribs and f u r r o w s .
They occur on layers
or " u n i d e n t i f i e d " s^-planes where they are highlighted at
places by parallel color s t r e a k s .
9I»
•14)1
The clasts are elongate
parallel to c r e n u l a t i o n s in samples in which they occur
\iV
:?ai
together.
M e a s u r e m e n t s of the orientation of their
long axes are few because the clasts are not commonly
exposed in the two necessary planes of view.
However, the
few m e a s u r e m e n t s made also indicate an orientation
to that of c r e n u l a t i o n s .
These mesoscopic
similar
crenulations
a p p a r e n t l y are the same as the m i c r o s c o p i c c r e n u l a t i o n s ,
and the c l a s t s are the c o m p o s i t i o n a l l y defined lenses
described
previously.
Joints in the p e l i t i c - s i l i c e o u s rocks are nearly
40
horizontal
and not d i s t i n g u i s h a b l e from those in c h e r t s .
Slates
S l a t e s crop out in subareas V and V I .
These rocks
are c o n s p i c u o u s l y c l e a v e d , d i s p l a y no l a y e r i n g , and
g e n e r a l l y are s l i g h t l y p h y l l i t i c .
They are
commonly
d a r k - g r a y and less commonly gray, l i g h t - g r a y , or b l u e gray and are w e a t h e r e d or stained b r o w n , r e d - b r o w n , and
cream.
Mi n e r a l o g y
Estimated
Average %
% Range
W h i t e mica
(muscovite, seric i t e , and p o s sibly t a l c )
55%
30% - 70%
Quartz
20%
15% - 3 5 %
Opaques
15%
5% - 3 0 %
Dark mica
8%
1% - 2 0 %
Chlori te
2%
0% - 10%
Mi neral
Accessory minerals
and s p h e n e .
Number of s a m p l e s :
feldspar, tourmaline, epidote.
5.
Textures
M i c r o s c o p i c a l l y , the slates d i s p l a y a prominent is u r f a c e w h i c h is defined by small aligned w h i t e and dark
m i c a s and e l o n g a t e s i l t - s i z e q u a r t z g r a i n s .
This
surface
41
is c r e n u l a t e d
l o c a l l y , (Fig. 15) and
compositionally
d e f i n e d lenses of b i o t i t e , c h l o r i t e , q u a r t z , or white
mica are e l o n g a t e parallel to it.
Some lenses
apparently
d e f l e c t the s^-surface, d i s p l a y a separate internal
s^-sur-
face of aligned m i c a s , and/or contain c o n c e n t r a t i o n s of
b i o t i t e , some of which appear to be smeared into the
p r o m i n e n t s_-surface.
Large, equidimensional
quartz
c r y s t a l s with u n d u l o s e e x t i n c t i o n occur in veins that
parallel the p r o m i n e n t s^-surface or are randomly d i s t r i buted in a p t y g m a t i c f a s h i o n .
Commonly associated
these q u a r t z v e i n s , but also scattered throughout
r o c k , are l a r g e , p o i k i l o b l a s t i c , randomly oriented
tite c r y s t a l s .
with
the
bio-
C h l o r i t e , some of which occurs in fanning
a g g r e g a t e s , a p p e a r s to be both a r e p l a c e m e n t of this
b i o t i t e and new mineral
Mesoscopic
growth.
Structures
Slaty c l e a v a g e is the p r e d o m i n a n t m e s o s c o p i c
ture in the s l a t e s .
struc-
In t h i n - s e c t i o n it is the prominent
s^-surface.
L i n e a t i o n s in the slates are fine c r e n u l a t i o n s on
cleavage surfaces.
M i c r o s c o p i c a l l y , they appear as crenu-
l a t i o n s of the p r o m i n e n t ^ - s u r f a c e .
A d d i t i o n a l l y , in one
sample small c o m p o s i t i o n a l l y defined lenses have their
long axes parallel
to the c r e n u l a t i o n s .
42
PS
PS
Figure 15.--Photomicrograph of a sample of silty-slate
from Merced River area ,' i 11 ustrati ng local crenulations (CR) of the prominent ^-surface ( P S ) .
Smallest scale unit = .042 mm. Crossed nicols.
43
Attitudes of joints vary more within the slates than
in other rock t y p e s ; and joints chiefly are oriented
perpendicular to cleavage.
Limestones
Limestones crop out in subareas II and III where
they are thinly layered and in some places contain
layers of pelitic material.
inter-
These dull to finely granu-
lose rocks are chiefly dark-gray but locally black or
white.
At some places they contain veins of black
calcite c r y s t a l s .
Mineralogy
Mi neral
Estimated
Average %
% Range
Calcite
77%
60% - 90%
Quartz
10%
1% - 2 0 %
Opaques
6%
5% -
7%
White mica
(muscovite and
serici te)
4%
1% -
6%
Biotite
(phlogopitic)
3%
1% -
5%
Accessory m i n e r a l s :
Number of samples:
epidote
2.
Textures
Layering in the limestones is expressed by thin
44
bands of mica in a calcite m a t r i x or by c o n c e n t r a t i o n s of
c a l c i t e , c a l c i t e plus q u a r t z , and mica plus q u a r t z .
Small white mica and biotite crystals are aligned parallel to l a y e r i n g .
Most calcite crystals are small
and
have indistinct b o u n d a r i e s and undulose e x t i n c t i o n ; generally they are equidimensional
but in a few places are
elongate parallel to the mica a l i g n m e n t .
Oriented at a
large angle to layering are subparallel to parallel
of l a r g e r , i n t e r l o c k i n g calcite c r y s t a l s .
are e q u i d i m e n s i o n a l
veins
These crystals
or slightly elongate parallel to the
length of the vein and display both complete and undulose
extinction.
layering.
Veins of quartz are parallel or oblique to
Quartz crystals in these veins are larger than
most of those quartz crystals interspersed with the small
calcite c r y s t a l s .
The former have polygonal
undulose e x t i n c t i o n .
shapes and
Although much mica is aligned
paral-
lel to l a y e r i n g , some is randomly oriented, and some of
the biotite is equidimensional
Mesoscopic
and
poikiloblastic.
Structures
S t r u c t u r e s found in the limestones are not distinq u i s h a b l e in a p p e a r a n c e from those in the c h e r t s .
How-
e v e r , no folds resembling those in the cherts are
o b s e r v e d , although a few minor ptygmatic folds occur in
zones of jumbled and disrupted layering.
defined c o m p o s i t i o n a l l y
Layering is
by varying c o n c e n t r a t i o n s of
45
c a l c i t e , m i c a , and q u a r t z .
The fracture cleavage paral-
lels and appears related to veins of large calcite
crystals.
The lineations are c r e n u l a t i o n s and "ridges."
C r e n u l a t i o n s of layering surfaces have no apparent m i c r o scopic e x p r e s s i o n , w h e r e a s "ridges" appear to be the
i n t e r s e c t i o n s of calcite veins and layering.
Joints are
also i n d i s t i n g u i s h a b l e from those in the cherts.
Subareas
In o r d e r to document geometric relationships of the
mesoscopic structures of the Merced River area, the
cross-sectional
(Fig. 3 ) .
traverse is divided into six subareas
The primary division is based on lithologic
differences.
Subarea VI is composed entirely of slate;
subarea V e n c o m p a s s e s the gradation from slate to peliticsiliceous rock; and subareas I through IV are composed
primarily of cherts and pelitic-siliceous rocks.
Sec-
ondly, subareas I, II, III, and IV are delineated on the
basis of the o r i e n t a t i o n of the traverse with respect to
the general n o r t h - s o u t h strike of layering; subareas I and
III are parallel to s t r i k e , subarea II is p e r p e n d i c u l a r ,
and subarea IV is o b l i q u e .
G e o m e t r i c R e l a t i o n s h i p s of
Mesoscopic
Structures
O r i e n t a t i o n s of structural
features are summarized
on the e q u a l - a r e a p r o j e c t i o n s shown in Figure 16.
46
Figure 1 6 . - - L o w e r - h e m i s p h e r e , equal-area p r o j e c t i o n s ,
for Merced River area, of l i n e a t i o n s , fold a x e s ,
(hinge l i n e s ) , and poles to axial p l a n e s , cleava g e , " u n i d e n t i f i e d " s^-planes, l a y e r i n g , and
joints for each subarea (Fig. 3 ) . Contours are
0%, 5%, 1 0 % , e t c . , of points per 1% area, except
for the plots of fold axes in subarea I and
c l e a v a g e in subareas IV and V I , which have contours of 0%, 1 0 % , 2 0 % , e t c . , per 1% area.
47
SUBAREA
LINEATIONS
FOLD
AXIAL
AXES
PLANES
(hinge lines)
CLEAVAGE
(subcireas i , II, III, IV '
fracture deovoge,
suboreo VI - slaty cleovoge)
4 8 PTS.
48 PTS.
33 PTS.
97 PTS.
13 PTS.
6 PTS.
3 2 PTS.
52 PTS.
10 PTS.
5 PTS-
22 PTSL
66 PTS.
UNIDENTIFIED
S-PLANES
(iKrt distinguishable os
clemiroge or layering in
oujiUcrop)
LITHOLOGIC
L A Y E R I N G.
73 PTS.
JOINTS
41 PTS.
II
13 PTS.
83 PTS.
46 PTS.
III
43 PTS.
29 PTS.
95 PTS.
58 PTS.
I PT.
44 PTS.
IV
98 PTS.
14 PTS.
5 PTS.
2 4 PTS.
56 PTS.
5 0 PTS.
35 PTS.
VI
75 PTS.
86
PTSL
90 PTS.
SUBAR
II
III
IV
VI
is
48
Lineations occur in all rock types and all subareas.
They
plunge steeply to the southeast in subareas I, II, and
III; to the south-southeast in subarea IV; to the south,
southwest, and west in subarea V; and to the west and
southwest in subarea VI.
The successive lineation maxima
appear to fall on a great circle.
In subareas I through
V, folds characterize the cherts and also occur in peliticsiliceous rocks.
They plunge steeply to the southeast in
subareas I and II; to the southeast and south in subarea
III; to the southeast, south, and southwest in subarea
IV; and to the west in subarea V.
Fold axes have orien-
tations similar to those of lineations and their maxima
also appear to plot successively along the same great
circle.
However, no folds are observed in subarea VI so
direct comparison with lineations is impossible there.
Axial planes of folds in subareas I, II, III, and IV
mainly strike almost east-west and dip steeply south.
The
poles to axial planes appear to be slightly dispersed
along a great circle, about a 3 parallel to fold axes.
Slaty cleavage in the slates of subarea VI, fracture
cleavage in the cherts of subareas I through IV, and the
"unidentified" s-plane in the pelitic-siliceous rocks of
subarea V, vary about a nearly east-west strike and dip
steeply south.
ff
The "unidentified" i-plane in the pelitic-
49
siliceous rocks probably r e p r e s e n t s both layering and
c l e a v a g e s u r f a c e s , which may be i n d i s t i n g u i s h a b l e
both are c h a r a c t e r i z e d
by mica a l i g n m e n t .
because
T h u s , on the
basis of similar o r i e n t a t i o n s , the " u n i d e n t i f i e d " s^-plane
is considered to be cleavage in subarea V, cleavage and
layering in subarea IV, and layering in subarea II.
The
c h e r t s , p e l i t i c - s i l i c e o u s r o c k s , and limestones are
layered.
Layering in subareas I, II, III, and IV, the
single layer measured in subarea V, and the
"unidentified"
£-plane in subarea II vary about a north-south strike and
dip steeply to the east.
Poles to layering form a dis-
c o n t i n u o u s great circle girdle about a 3 which
the m e a s u r e d fold axes and l i n e a t i o n s .
parallels
Joints in subareas
I through IV dip shallowly westward and are nearly perpendicular to fold axes and l i n e a t i o n s , whereas in subareas
V and VI they are more diversely oriented and appear to
be p e r p e n d i c u l a r to c l e a v a g e .
TEXAS TECH LIBRARt
CHAPTER IV
STRUCTURAL
INTERPRETATION OF THE
MERCED RIVER AREA
Structural
i n t e r p r e t a t i o n s in the Merced River area
involve q u e s t i o n s about the relationships of mesoscopic
structures in the various rock types and the deformational
and m e t a m o r p h i c chronology of the area.
Structural
Relationships
The following designations are assigned to structures of the Merced River area which are described in the
preceding c h a p t e r :
S Q = lithologic
layering
F, = folds of layering
S
= s_-surface defined by micas aligned parallel to
layering
Sp = c l e a v a g e , including both fracture and slaty
cleavage
1- = l i n e a t i o n s , including c r e n u l a t i o n s , "ridges,"
and elongate c l a s t s .
Figure 17 summarizes the data in Figure 16 and
illustrates
synoptic r e l a t i o n s h i p s of these s t r u c t u r e s .
S Q , o r i g i n a l l y sedimentary layering, is present in
c h e r t s , l i m e s t o n e s , and some pelitic-si 1iceous rocks.
also includes relict layering observed on a microscopic
50
It
51
v-^f.ly^,
Figure 1 7 . - - S y n o p t i c l o w e r - h e m i s p h e r e , equal-area plots
of fold a x e s , l i n e a t i o n s , l a y e r i n g , c l e a v a g e , and
axial planes of Merced River area.
1
1*
0
axes:
S u p e r i m p o s e d 10% c o n t o u r s of fold axes
of subareas I, II, I I I , and IV (from
Fig. 16; plots for s u b a r e a s II, III,
and IV not c o n t o u r e d in Fig. 1 6 ) . 3 1
and a great circle (dashed l i n e )
through the m a x i m a also are shown.
S u p e r i m p o s e d 1 0 % c o n t o u r s of l i n e a t i o n s of
each subarea (from Fig. 1 6 ) . 3i and a great
c i r c l e (dashed l i n e ) through the maxima also
are i l l u s t r a t e d .
S u p e r i m p o s e d 5% contours of poles to layering
of subareas I, II, III, and IV (from Fig. 1 6 ) .
Also shown is a great circle (dashed l i n e )
d i s t r i b u t i o n of the poles around 3-..
Superimposed 1 0 % contours of poles to
c l e a v a g e of subareas I, II, III, IV, and
VI and to " u n i d e n t i f i e d " s_-planes of subarea V
(from Fig. 1 6 ) . 3^ and synoptic S 2 , which
r e p r e s e n t s maximum overlap of the 1 0 % c o n t o u r s ,
also are indicated.
F, axial
planes:
Contoured plot of poles to axial
planes of subareas I, I I , I I I ,
and IV (from Fig. 1 6 ) ; c o n t o u r s
0%, 5%, and 10% per 1% area.
Synoptic S2 and p o s s i b l e great
circle (dashed l i n e ) d i s t r i b u t i o n
of these poles around 3i are also
shown.
52
SUBAREA
I
jlllj] SUBAREA
II
SUBAREA ill
SUBAREA IV
SUBAREA
V
SUBAREA VI
AXIAL PLANES
53
scale as compositional
pelitic-siliceous
lenses in slates and other
rocks.
In the cherts and pelitic-
siliceous rocks it is deformed by f o l d s , here labelled
F^.
The great circle d i s t r i b u t i o n of poles to S Q in sub-
areas I through
IV defines 3^, which is approximately
parallel to F^ axes (Fig. 1 7 ) .
Poles to F^ axial
are weakly dispersed around 3^ (Fig. 1 7 ) .
F^ f o l d s , which
in the field appear to be f l e x u r e - s l i p , are
tional and c o n s e q u e n t l y may be minor folds
with l a r g e - s c a l e s t r u c t u r e s .
delineated
planes
intraformaassociated
No large-scale folds are
in the study area, but the limited extent of
S Q pole c o n c e n t r a t i o n s along the great circle about 3,
(Fig. 17) suggests that such f o l d s , if present, are open
and r o u n d - c r e s t e d or that only a limb of a large-scale
fold is p r e s e n t .
S-, the alignment of fine micas parallel to S Q in
c h e r t s , l i m e s t o n e s , and some pelitic-siliceous
a p p a r e n t l y is of m e t a m o r p h i c origin.
rocks,
In slates and other
p e l i t i c - s i l i c e o u s rocks it probably is preserved as the
internal s^-surfaces within compositional
41).
lenses (see page
S, may have formed in response to interlayer slip
during early F- f l e x u r e - s l i p
folding.
Sp is the slaty c l e a v a g e , the "unidentified" s^-plane
in subarea V, and the fracture c l e a v a g e .
Because
these
s t r u c t u r e s have similar o r i e n t a t i o n s , they are considered
54
related.
Synoptic S2 strikes w e s t - n o r t h w e s t and dips
steeply south.
This a t t i t u d e is determined from the
maximum overlap of 10 percent contours from Figure 16
(Fig. 1 7 ) .
The specific expression of Sp is a function
of l i t h o l o g y .
In slates S^ is a slaty c l e a v a g e ; in some
p e l i t i c - s i l i c e o u s rocks it is a poorly expressed cleavage
("unidentified" s^-plane); in cherts and limestones it is
a fracture c l e a v a g e .
S2 is interpreted as being axial
plane c l e a v a g e to F^ folds because S^ and F^ axial planes
have similar o r i e n t a t i o n s (Fig. 1 7 ) .
L i n e a t i o n s are labelled 1^, although they are of
various types which include c r e n u l a t i o n s , elongate c l a s t s ,
and " r i d g e s . "
Sj and Sp.
Most of the lineations are crenulations of
If the crenulations represent intersections of
a third plane with S, and S^, two sets of lineations
might be e x p e c t e d , and double maxima should appear on
lineation p l o t s .
H o w e v e r , double maxima do not occur,
except in subarea VI where lineations are all crenulations
of Sp.
T h u s , 1, are not considered as intersections of
S, and Sp with a third plane.
Lineations and F, axes
have similar o r i e n t a t i o n s and are dispersed together along
a great circle (Fig. 1 7 ) .
This geometry for 1^ and F^
axes suggests that 1, c r e n u l a t i o n s are microfolds of S^
and Sp and that they are related to F^ folding.
Designa-
tion of " r i d g e s " and e l o n g a t e clasts also as 1^ appears
55
acceptable because they, too, are related to F, folding.
1
^
"Ridges" are intersections of S^ and S Q , and elongate
clasts appear to have formed as S Q was disrupted and
pulled apart by S^ d e v e l o p m e n t .
Because the formation of
"ridges" and clasts is related to S^, and because they are
parallel to F^ a x e s , these lineations are interpreted as
being g e n e t i c a l l y related to F^ folds.
The temporal
relationships between s t r u c t u r e s , as
implied in the foregoing d i s c u s s i o n , are summarized in
Figure 18.
Il
^
Fi
S2
^«
s,
fc
SEDIMENTARY
(Ms?) M ,
Ms
Di
i^?--^.
Figure 18.--Schematic outline of deformational chronology
of Merced River area. Metamorphic events (Mi and M ^ )
and deformational events (D? and D i ) are discussed in
the following s e c t i o n s .
Metamorphic
Events
A l o w - g r a d e , synkinematic metamorphic event, M^,
is indicated by the alignment of secondary biotite and
56
other micas in S^ and S^.
Because the micas are
s m a l l , i d e n t i f i c a t i o n of biotite and the certain
recog-
nition of micas as s y n k i n e m a t i c are subject to question.
H o w e v e r , the evidence supports the conclusion that M
is
a g r e e n s c h i s t facies m e t a m o r p h i c event, possible of
biotite zone g r a d e .
A period of minor static m e t a m o r p h i s m , M , is represented by r a n d o m l y oriented white mica, biotite, and
chlorite.
zone g r a d e .
At its onset M^ appears to have been of biotite
Later, retrogression to chlorite zone c o n d i -
tions is documented by the replacement of static biotite
by static c h l o r i t e .
Because these static minerals are
undeformed by S 2 , M^ must post-date S^ and F^.
the time r e l a t i o n s h i p between M
However,
and 1- is unclear.
No
static m i n e r a l s that are unquestionably deformed in conjunction with Ij c r e n u l a t i o n s are observed, and no static
m i n e r a l s that are superimposed on the crenulations are
noted.
Deformational
Chronology
A c h r o n o l o g y which accounts for most of the structural and m e t a m o r p h i c relationships in the Merced River
area as the products of one deformational
(D.) and meta-
morphic (M-) event is outlined in Figure 18.
D, is a
d e f o r m a t i o n a l event c h a r a c t e r i z e d by F, folding and
57
associated with low-grade m e t a m o r p h i s m , M,.
S, devel-
oped parallel to S Q during early flexure folding of the
rock s u c c e s s i o n .
As folding continued in c h e r t s , lime-
s t o n e s , and some p e l i t i c - s i l i c e o u s rocks of subareas I,
II, III, and IV, S2 developed as a fracture cleavage
a p p r o x i m a t e l y parallel to F^ axial planes, whereas in
slates and some pelitic-siliceous rocks of subareas
IV,
V, and V I , S2 developed as a slaty cleavage which obliterated S Q , S , , and F, folds.
K
formed late in D, as a
crenulation or microfolding of S-. and Sp around 3^.
M
If
is interpreted as occurring after 1, development, it
may represent either an extension of M, beyond D^ or a
later, separate static event.
If it occurred before 1,
f o r m a t i o n , it is possibly a minor interlude in D^ - Mtime during which recrystal1ization proceeded more
rapidly than movement within the rock succession
(Fig.
18).
The steep plunge of F, a x e s , the weak distribution
of Fj axial planes around &^,
axes and K
and the distribution of F^
along a great circle require some explanation
in the context of this interpretation.
First, the steep
plunge of F, axes might result from tilting of the rock
succession b e f o r e , d u r i n g , or after D^.
Such tilting
could have occurred after D^ and probably would not have
58
affected some areas in the western Sierra Nevada.
In
those areas folds like F. would be in a near horizontal
position.
H o w e v e r , no such folds are reported.
An
a l t e r n a t i v e would involve tilting of the entire Sierra
Nevada, but this suggestion seems implausible.
F, folds
could have d e v e l o p e d s y n c h r o n o u s l y with tilting during
D j , although the m e c h a n i s m for such tilting in conjunction with folding is not understood.
Finally, tilting
could have transpired during an earlier d e f o r m a t i o n , D,.
Steep Fj folds would then be a product of the folding of
a steeply dipping rock succession.
However, evidence in
support of D , is lacking.
121 di 11
21 :4
V
S e c o n d , the great circle distribution of poles to
a
V u
-J H - J ' l
F, axial planes around ^^ may be the product of these
iiu
axial planes having a fanning geometry about 3i-
11 n
Such a
« '«
geometry may indicate that F, folds are minor folds
•<
associated with large-scale folds.
Third, the d i s t r i b u t i o n of F^ axes and K
along a
great c i r c l e , which corresponds closely in orientation
with synoptic S2 (Fig. 1 7 ) , is interpreted as a dispersal
of fold axes and lineations in the cleavage ( S 2 ) .
The
dispersal is probably a result of inhomogeneity in the
movement pattern during D^.
Such an inhomogeneity might
be a b o u n d a r y c o n d i t i o n between a body of interlayered
chert and p e l i t i c - s i l i c e o u s rock deforming chiefly by
•^
mm
•<
•<
59
f l e x u r e - s l i p and a body of slate deforming mainly by
shear.
Indeed, the greatest dispersal of 1. occurs in
subarea V (Fig. 1 6 ) , the subarea in which the transition
from p e l i t i c - s i l i c e o u s rocks to slates occurs.
The most
consistent o r i e n t a t i o n of fold axes and lineations are in
subareas I and II (Fig. 1 8 ) , the subareas farthest
from this lithologic
removed
transition.
Although the preceeding discussion appears to satisfactorily explain the steep plunge of F^ a x e s , the distribution of Fj axial p l a n e s , and the distribution of F^ axes
and 1^, an alternate interpretation for each of these
geometries is shown in Figure 19.
Interpretation A sug-
gests that D^ was an isoclinal folding event
by low-grade metamorphism
(M,) (Fig. 1 9 ) .
accompanied
By this inter-
pretation S Q was folded about horizontal axes into isoclinal folds with subvertical axial planes, and S^
developed as an axial plane cleavage to such folds.
Steeply plunging F, folds would result from the folding
of a subvertical
rock succession during D^.
However,
d o c u m e n t a t i o n of this interpretation is lacking; horizontal or subhorizontal
folds are not observed in the
area, and according to Clark (1964) sedimentary tops in
the area are c o n s i s t e n t l y to the east.
Interpretation B explains the great circle distribution of F, axial planes around 3^ as a result of later
60
A
F?
Si
*»
»
SSDIMKNTARY
•
M?
.>
Ml
^ Ms
mm*
'1
B
f-
—OR -
-*
•-•
SEDIMENTARY
Ml
^ M
—
-
O R -
—
4^i
Di
c
h
'^>
SEDIMENTARY
OR
"^
>?J^
i^i
,.
Figure 19.--Schematic outline of alternative (compared
with Fig. 18) deformational chronologies (A, B, and
C) for Merced River area. Interpretation A accounts
for the steep plunge of li and Fi a x e s ; interpretation B for the great circle distribution of Fi axial
p l a n e s ; and interpretation C for the great circle
distribution of Ij and Fi axes.
61
coaxial folding (Fig. 1 9 ) .
During this later folding,
Fj axial planes were folded around axes which have
o r i e n t a t i o n s similar to that of 3,; 1, may have formed at
this time.
H o w e v e r , later coaxial folding is unlikely
because S2 is unfolded.
F u r t h e r m o r e , folded F, axial
planes are not observed in the field.
Interpretation C utilizes a later folding event to
explain the great circle distribution of F- and 1, (Fig.
19).
This folding is considered to be flexure folding
(rather than shear f o l d i n g ) because no post F , - K
planes are observed in the area.
"shear"
Because F^ axes and 1,
are distributed along a great circle or a large small
c i r c l e , the axes of folds formed during this postulated
later event should plunge shallowly to the north-northeast.
F u r t h e r m o r e , if this interpretation is correct,
S Q should be folded about a shallowly
plunging 3.
north-northeast
However, such shallowly plunging folds are
absent, and S Q is unaffected by such folding.
Therefore,
a l t e r n a t i v e C is not favored.
The deformational
chronology discussed first and
outlined in Figure 18 is preferred because it is simpler
and more complete than any of the alternative interpretations.
A l t e r n a t i v e s B and C conflict with some of the
data of this study.
A l t e r n a t i v e A is an attractive
e x p l a n a t i o n for D ^ , but it is poorly documented.
CHAPTER V
DESCRIPTION OF ROCK TYPES AND MESOSCOPIC
STRUCTURES
IN THE LITTLE BALD MOUNTAIN AREA
Cross-sectional
traverses in the Little Bald Moun-
tain area are divided into three subareas as shown in
Figure 4.
Homogeneity with respect to the strike of
layering is the basic criteria used to delineate the subareas (Fig. 2 0 ) .
Layering strikes chiefly northeast to
east-northeast in subarea I, northeast and west-northwest in subarea II, and northeast in subarea III.
Sub-
areas I and II are separated where layering first has a
consistent west-northwest strike.
The boundary between
subareas II and III is marked by a gap in the traverse
due to lack of outcrop.
The subarea divisions
generally
correspond with the three rock types of the area:
chert
predominates in subarea I, pelitic-siliceous rock in
subarea II, and clastic rock in subarea III.
The pelitic-
siliceous rocks appear to be interlayered with cherts in
part of subarea II, but the lithostratigraphic
relation-
ship between clastic rocks and pelitic-siliceous rocks is
unclear.
As in the Merced River area, a mixed
sedimentary
and metamorphic terminology is used here to describe
these low-grade metamorphic rocks.
Rocks in the Little
Bald Mountain area are less well exposed and more highly
62
63
Ill
il
LITHOLOGIC
LAYERING
144 PTS.
CLEAVAGE
(luboreo i - froeture
claovogt)
(suboreo III crenulotion cicovoge)
(suboreo ii - both)
136 PTS.
LINEATIONS
119 PTS.
FOLDS
•
FOLD AXES
•
AXIAL
PLANES
Figure 2 0 . - - L o w e r - h e m i s p h e r e , equal-area projections, for
Little Bald Mountain area, of poles to layering, poles
to c l e a v a g e , l i n e a t i o n s , fold axes, and poles to axial
planes for each subarea (Fig. 4 ) . Contours are 0%,
4%, 8 % , 16%, etc., of points per 1% area. Subareas
are indicated by Roman numerals.
64
weathered than those in the Merced River area.
Although,
in part, the same rock type designations are used, rocks
classified as a specific type at Little Bald Mountain are
not identical to rocks of the same designation along the
Merced River.
Subarea I:
Cherts
In subarea I rocks are predominantly layered cherts
with minor amounts of slate.
The cherts are white, tan,
dark-gray, red, gray, red-brown, and blue-gray.
Mineralogy
Mineral
Estimated
Average %
% Range
Quartz
95%
90% - 9 7 %
Opaques
3%
1% -
7%
White mica
(s e r i c i t e )
2%
1% -
5%
Accessory minerals
Number of samples:
brown mica and chlorite.
9.
Textures
Microscopic examination of the cherts from subarea I
shows that the quartz in a few samples is nearly microcrystalline but mainly occurs as small, equidimensional
crystals with indistinct boundaries and incomplete to
undulose extinction.
Layers are defined compositionally
by concentrations of m i c a , o p a q u e s , and quartz.
In some
65
t h i n - s e c t i o n s boundaries between layers are accentuated
by c o n c e n t r a t i o n s of "radiolarian ghosts," which appear
as circular to elliptical areas that are devoid of mica
and contain larger, more distinct quartz crystals.
Whether or not they represent relict radiolarian skeletons is unclear.
Micas mainly are sericite, but some are
brown mica of uncertain type.
Their alignment defines an
s^-surface which parallels layering or intersects it at a
small a n g l e ; hereafter this ^-surface is referred to as
the major s-surface.
Subparallel veins of opaques and/or
quartz intersect layering at a high angle.
Quartz crys-
tals in these veins are larger and more distinct than
most of the q u a r t z .
Locally, mica is aligned parallel to
the v e i n s , and in a few samples mica is aligned in a
minor ^ - s u r f a c e which intersects layering and the major
^ - s u r f a c e at a high angle.
Additionally, some micas are
randomly o r i e n t e d , and small amounts of chlorite occur in
fanning a g g r e g a t e s .
Mesoscopic
Structures
Mesoscopic structures in subarea I include layering,
fracture c l e a v a g e , l i n e a t i o n s , and folds.
Individual
layers are chiefly h inch (1.3 cm) to 6 inches (15 cm)
thick.
Some have irregular thickenings and thinnings
which in cross-section resemble pinch-and-swel1
structures
66
and which on layering surfaces appear as circular to
elliptical
"lumps" that range in diameter from approxi-
mately H inch (1.3 cm) to 1 foot (30 cm) (Fig. 2 1 ) .
At
some localities these "lumps" are elongate parallel to
lineations.
F r a c t u r e s , thin layers of pelitic material,
or red-brown coatings on layering surfaces commonly
accentuate the layering.
In general, layers strike
northeast to east-northeast and tend to dip steeply to
the north; poles to layering are distributed in a girdle
through the center of the equal-area plot shown in
Figure 20.
Fracture cleavage (Fig. 2 2 ) , which at some places
offsets layering and at others is refracted by layering
appears in some outcrops as two orthogonal sets of fractures (Fig. 2 1 ) .
However, the fractures are considered
to be a single cleavage because on an equal-area plot
(Fig. 20) the two maxima of poles to cleavage are
closely spaced.
This cleavage has vertical to subverti-
cal dips and variable north-northwest strikes (Fig. 2 0 ) .
M i c r o s c o p i c a l l y , it appears parallel and related to the
veins and the minor ^ - s u r f a c e .
Lineations are fine linear "crinkles" or crenulations that are expressed most clearly on pelitic layering
surfaces and much less clearly on cherty layering surfaces.
They have a wide range of orientations but
;o.a«rx-r-(ur,,-.'
67
F i g u r e 2 1 . - - " L u m p s " in chert l a y e r s . Little Bald Mountain
a r e a . A large "lump" is located above hammer and
small ones below hammer. A p p r o x i m a t e l y orthogonal
f r a c t u r e s in the cherts are s h o w n ; hammer handle
p a r a l l e l s the d o m i n a n t f r a c t u r e set, and hammer head
p a r a l l e l s w e a k e r set.
F i g u r e 2 2 . - - F r a c t u r e c l e a v a g e in c h e r t s at Little Bald
Mountain.
68
d o m i n a n t l y plunge steeply to the northeast
(Fig. 2 0 ) .
M i c r o s c o p i c a l l y , they apparently are crenulations of the
major s^-surface, but in some samples they may be the product of the intersection at a small angle of the major
1-surface and layering or of the intersection at a large
angle of the minor s^-surface and layering.
Folds of layering and also the major s^-surface are
divided into two g r o u p s , those which plunge
shallowly
and those which plunge steeply (not specifically delineated in Fig. 2 0 ) .
The former, of which only ten were
m e a s u r e d , are round- to sharp-crested, measure a few
inches to several feet in ^ - w a v e l e n g t h , and trend in
diverse directions
(Figs. 20 and 2 3 ) .
The latter, of
which only three were measured, are sharp- to roundc r e s t e d , measure a few inches to a few feet in ^ - w a v e length, and plunge steeply to the south or west
20).
(Fig.
Only two axial planes were measured, one for a
shallowly plunging fold and the other for a steeply
plunging fold.
The former strikes east-west and dips
steeply south, and the Tatter strikes northeast and dips
steeply east (Fig. 2 0 ) .
The single steeply plunging
fold, examined m i c r o s c o p i c a l l y , has fractures and an
incipient alignment of mica and opaque material
to its axial
plane.
parallel
69
Figure 2 3 . - - S h a l l o w l y plunging fold in cherts of Little
Bald M o u n t a i n area. Hammer on right limb of fold
shows s c a l e .
70
Slate
Interlayers
Minor amounts of black to dark-gray slate are
interlayered with the c h e r t s .
The single sample of
these slates examined in thin-section contains 70 percent
o p a q u e s , 20 percent white mica, 10 percent q u a r t z , and a
minor amount of brown mica and ferrian zoisite ( ? ) .
M i c r o s c o p i c a l l y , slate interlayers display a distinct
p e n e t r a t i v e _s-surface of aligned mica and opaque material
which is parallel to subparallel to layering.
surface is strongly crenulated
This s^-
(Fig. 2 4 ) , the crenulations
defining a w e l l - e x p r e s s e d lineation on layering surfaces.
These lineations parallel the other lineations of subarea
I.
In these slaty rocks veins consist of large quartz
c r y s t a l s , occur in a jumbled, almost ptygmatic
fashion,
and appear to be deformed in conjunction with the crenulation of the s^-surface.
Randomly oriented white and
brown micas and a small amount of ferrian zoisite
(?)
also occur in these quartz veins.
Subarea II:
Pelitic-Si1iceous
Rocks
Most rocks in subarea II are gradational
in composi-
tion and t e x t u r e between cherts and fine-grained
clastic
rocks and here are labelled pelitic-siliceous rocks; in
parts of this subarea they are interlayered with c h e r t s .
Such rocks are light- to dark-gray and weather to brown,
71
Figure 2 4 . - - P h o t o m i c r o g r a p h of slate interlayer in cherts
of Little Bald Mountain area. Sample displays
c r e n u l a t i o n s (CR) of the ^ - s u r f a c e (SL) defined by
o p a q u e s and micas aligned parallel to layering.
S m a l l e s t scale unit = .042 mm. Ordinary light.
72
red-brown, and tan.
They are more highly crystalline
than rocks in other subareas, and some are spotted with
dark-gray mineral
growths.
Mineralogy
Mineral
Estimated
Average %
% Range
Quartz
57%
25% - 90%
Muscovite
20%
5% - 35%
10%
0% - 25%
Opaques
8%
1% - 15%
Biotite
5%
0% - 10%
Staurolite
(?)
Accessory minerals: plagioclase, chlorite, anortho
clase, paragonite ( ? ) , garnet, and tourmaline.
Number of samples: 5.
Textures
In thin-section, layering in the pelitic-siliceous
rocks is defined by concentrations of mica, quartz,
opaque matter, tuffaceous material, and
ghosts."
"radiolarian
Most quartz is finely crystalline; individual
grains have indistinct to straight boundaries and
slightly undulose to incomplete extinction.
The few
layers of tuffaceous material are composed chiefly of
opaque m i n e r a l s , some mica, and scattered euhedral
grains
of a n o r t h o c l a s e , partially altered to paragonite ( ? ) .
Parallel or subparallel to layering is a s^-surface of
73
a n a s t o m o s i n g trains of opaque minerals and aligned musc o v i t e , b i o t i t e , and elongate quartz (Fig. 2 5 ) .
This
s_-surface is deformed by an incipient crenulation cleava g e , which is defined by fractures outlined by opaques,
c r e n u l a t i o n s of the ^ - s u r f a c e , and a weak alignment of
muscovite and/or biotite parallel to the cleavage.
In
some t h i n - s e c t i o n s patches of incipient, poikiloblastic
staurolite (?) envelop q u a r t z , muscovite, and biotite
inclusions (Fig. 2 5 ) ; some inclusions are aligned in the
s^-surface and/or are deflected into the incipient cleavage.
In hand specimen the staurolite (?) patches occur
as dark-gray spots and tend to be concentrated and
elongated along cleavage surfaces.
Two rock samples also
contain poikiloblastic plagioclase that has quartz and
biotite i n c l u s i o n s , and other samples contain fanning
aggregates of chlorite.
Large poikiloblastic
biotite
and m u s c o v i t e also occur oriented randomly, parallel to
the s-surface, or parallel to the cleavage (Fig. 2 5 ) .
Mesoscopic
Structures
L a y e r i n g , c l e a v a g e , lineations, and folds comprise
the m e s o s c o p i c structures of subarea II.
Except for folds,
these structures are difficult to identify in many outc r o p s ; hence,--the degree of confidence in data for this
subarea is less than that for subarea I.
At places,
74
.CL
.SL
CL
SL'
Figure 2 5 . - - P h o t o m i c r o g r a p h of sample of p e l i t i c - s i l i c e o u s
r o c k . Little Bald Mountain area, showing compositional layering ( C L ) , ^ - s u r f a c e (SL) of o p a q u e s ,
m i c a s , and quartz aligned parallel to l a y e r i n g ,
static micas ( M ) , and patches of incipient staurol i t e ( ? ) ( S ) . Orientation of cleavage ( C ) , defined
e l s e w h e r e in the t h i n - s e c t i o n , is also indicated.
Smallest scale unit = .042 mm. Crossed n i c o l s .
75
layering is like that in subarea I, but in others it
may be represented by color banding or an
1-plane.
"unidentified"
This "unidentified" £-plane is an expression
of the s^-surface parallel to layering, but in outcrop it
may not be visibly associated with compositional
tions.
varia-
Layering varies nearly 180^ in strike, but it has
two predominant o r i e n t a t i o n s , a northeast strike with
steep southeast dip and a west-northwest strike with
steep north dip (Fig. 2 0 ) .
Cleavage appears in places as fracture c l e a v a g e , at
others as crenulation c l e a v a g e , and at still others as
an "unidentified" s^-plane; microscopically, this cleavage
is the incipient crenulation cleavage.
dominant o r i e n t a t i o n s :
It has three
northwest strike with steep
northeast d i p , north-south strike with vertical to subvertical d i p , and northeast strike with vertical to subvertical dip (Fig. 2 0 ) .
Lineations plunge steeply to the southeast
(Fig. 2 0 ) .
They occur as linear "crinkles" or fine
discontinuous
ribs and furrows on layering surfaces.
Thin-section
examination suggests that they are crenulations of the
^ - s u r f a c e parallel to layering or the intersections of
cleavage with
layering.
Folds of subarea II plunge steeply to the south and
have axial planes which strike north-northwest to north-
76
east and dip steeply eastward (Fig. 2 0 ) . These folds
are mainly chevron folds, but some are round-crested.
In one, mineral alignment and small offsets in layering
parallel the axial plane (Fig. 2 6 ) .
Subarea III:
Clastic Rocks
Clastic rocks in subarea III are low-grade metamorphic equivalents of silty shales or argillaceous siltstones.
They are gray, brown-gray, and blue-gray;
weather to tan, red, and red-brown; and have granular to
phyllitic appearances.
Mineralogy
M'
1
Estimated
Ml neral
-a
sr
"^ "^'^^'
Average %
Mica
65%
(mainly muscovite,
but may include
sericite, chlorite, and brown
mica)
Quartz
25%
10%
Opaques
«/ oannp
* "^ange
^^
25% - 85%
5% - 65%
2% - 15%
Accessory minerals: chlorite, biotite, feldspar,
and tourmaline.
Number of samples: 5.
Textures
Microscopically, rocks in subarea III show a compositional layering defined by concentrations of mica, quartz,
or opaque material.
Parallel or subparallel to this
77
F i g u r e 2 6 . - - S t e e p l y p l u n g i n g fold in subarea II, Little
Bald M o u n t a i n a r e a . Small offsets in layering
parallel the axial p l a n e .
78
layering is a ^ - s u r f a c e defined by aligned micas and
e l o n g a t e q u a r t z grains (Fig. 2 7 ) .
The quartz grains are
c h i e f l y s i l t - s i z e and occur as both unit crystal and
polycrystalline grains.
In quartz-rich l a y e r s , the i-
surface is not as well defined as in mica-rich layers.
An i n c i p i e n t l y to highly developed crenulation
cleavage
is a s s o c i a t e d with crenulations of this ^ - s u r f a c e
(Fig. 2 7 ) .
Some mica and quartz aligned in the s^-surface
are deflected into the crenulation c l e a v a g e , which is
commonly outlined by opaque m a t e r i a l .
Some samples dis-
play additional microtextural d e t a i l s .
In one sample,
the compositional
layering is a collection of composi-
t i o n a l l y defined lenses.
In a n o t h e r , poorly defined
m o r t a r zones of quartz parallel both the s^-surface and
the c r e n u l a t i o n c l e a v a g e .
Two samples show a few quartz
or c h l o r i t e crystals that are bent into the crenulation
c l e a v a g e and which contain inclusions aligned with the
s^-surface.
F i n a l l y , about half of the samples contain
some randomly oriented m i c a .
Mesoscopic
Structures
M e s o s c o p i c s t r u c t u r e s in subarea III include layering, a foliation parallel to layering, c l e a v a g e , lineat i o n s , and f o l d s .
In this s u b a r e a , layering and cleavage
are difficult to identify in many o u t c r o p s ; h e n c e , the
79
cc
A
,CL
SL
CL
SL
F i g u r e 2 7 . - - P h o t o m i c r o g r a p h of sample of clastic rock from
L i t t l e Bald M o u n t a i n a r e a , i l l u s t r a t i n g compositional
l a y e r i n g ( C L ) , s^-surface (SL) of micas and quartz
a l i g n e d parallel to l a y e r i n g , and c r e n u l a t i o n cleavage ( C C ) . S m a l l e s t scale unit = .042 mm.
Crossed
nicols.
80
degree of confidence in the data for this subarea also is
less than that in subarea I.
Layering is expressed
primarily as color banding which parallels a phyllitic
to slaty foliation.
The foliation commonly occurs where
no layering is visible and is apparently the ^-surface
m i c r o s c o p i c a l l y defined as being parallel to layering.
The dominant attitude of layering is northeast
and vertical to steep northwest dip.
strike
This attitude was
determined from measurements of the attitudes of both
layering and the foliation parallel to layering
(Fig.
20).
C l e a v a g e , where strongly developed, is a crenulation
c l e a v a g e ; elsewhere it is identified as a penetrative
£-plane which intersects layering.
Where crenulation
cleavage is well developed, the foliation parallel to
layering is more highly developed than in other places.
This cleavage and the microscopically described
crenula-
tion cleavage are the same structure and have a northwest strike and steep southwest dip (Fig. 2 0 ) .
Lineations are fine linear "crinkles" or narrow
raised traces on layering or cleavage surfaces.
plunge steeply to the southwest (Fig. 2 0 ) .
They
Where
cleavage is well d e v e l o p e d , lineations are the product
of intersections of cleavage with layering and the s^surface parallel to layering; elsewhere they are crenulations of the s - s u r f a c e .
81
The two folds, noted in this area, are of the "kink"
variety, and both plunge steeply to the south or southeast (Fig. 2 0 ) .
CHAPTER VI
STRUCTURAL INTERPRETATION OF THE
LITTLE BALD MOUNTAIN AREA
Interpretations of the Little Bald Mountain area
are directed toward conclusions about the relationships
of mesoscopic structures and the deformational and metamorphic chronology of the area.
Unequivocal, structural
interpretations of this area cannot be formulated from
data collected exclusively for this study, because the
data, especially with respect to folds, are limited in
amount and areal extent.
However, work by Redmond
(1966)
in an area which encompasses Little Bald Mountain and a
larger area to the southwest and by Morahan (in progress)
in an area immediately to the north (Fig. 28) is utilized
to document
interpretations.
Structural
Relationships
For the Little Bald Mountain area, the following
symbols are assigned to structures described in the
preceding chapter:
S Q = lithologic
layering
F- = shallowly plunging folds of layering
only in subarea I)
(present
Fp = steeply plunging folds of layering (present in
all subareas)
82
83
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84
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i-
85
Sj = s.-surface defined by aligned micas parallel
layering
to
S2 = c l e a v a g e , including both fracture and crenulation cleavage
Ij = l i n e a t i o n s , including crenulations and intersections of cleavage with layering and with the
1-surface parallel to layering
Figure 29 is a synoptic diagram illustrating the orientation of these structures; it summarizes data presented in
Figure 20.
S Q is deformed into F- and F^ folds; synoptically it
strikes northeast and dips steeply to the south (Fig. 2 9 ) .
A shallowly northeast plunging pi (Fig. 29) is defined by
a great circle fit of poles to S Q in subarea I (great
circle not shown in Fig. 2 0 ) ; this great circle through
the center of the plot is difficult to determine
because
poles to S Q are dispersed along the edge of the net (Fig.
20).
This dispersion along the edge of equal-area
plots
of poles to S Q from all subareas (Fig. 20) defines a
nearly vertical 33 (Fig. 2 9 ) .
F^ folds, which appear in
the field to be flexure-slip folds, are associated
here
with 6j because they also plunge shallowly (Fig. 2 9 ) .
F, axes have diverse trends (Fig. 2 9 ) ; apparently they
are folded about a subvertical axis, possibly ^2'
^^^^
folding (Fg) is a refolding of S Q and probably resulted in
the S Q dispersal which makes the great circle fit for e^
difficult to determine.
Fg axes are subvertical
and
86
apparently related 3^ (Fig. 2 9 ) .
These f^ ^^l^s appear
to be formed mainly by flexure-slip, but indications of
movement parallel to the axial planes of some suggest
that shear folding also was involved (see page 76).
Sj is the metamorphic alignment of micas parallel or
subparallel to S Q .
F2 folds.
It is folded along with S Q in F^ and
Where S2 is present as a crenulation cleavage,
Sj is crenulated and, therefore, in part, pre-dates Sp.
Probably, Sj is a product of at least one of the following:
(1) flexure-slip along S Q during F^ folding, (2)
flexure-slip along S Q during early F2 folding, (3) slip
along a pre-existing s^-surface during the development of
S2 as a crenulation cleavage (Hobbs and others, 1976, p.
219).
Because minor static mineral growth (see page 78).
appears to post-date S, and pre-date S^ in two samples,
a time interval appears to have separated S. and Sp
development.
In the following paragraph, S2 development
is interpreted as being related to F2 folding.
Conse-
quently, a time interval appears to separate S, development and Fp folding, and S, tentatively is viewed as
having formed during F- folding.
S, is considered also
to have been enhanced by interlayer slip during S2
d e v e l o p m e n t , because pronounced expression of S^ is
associated with a strong development of S^ (see page 80).
Fracture cleavage and crenulation cleavage are both
87
designated S2 because fracture cleavage in subarea I and
c r e n u l a t i o n cleavage in subarea III have similar attitudes (Fig. 2 0 ) .
Although the cleavage in subarea II is
d i v e r s e l y oriented, it also is considered as S2 because
it h a s , in part, an attitude similar to that of the
fracture and crenulation cleavages (Fig. 2 0 ) .
Synopti-
c a l l y , S2 strikes north-northwest and dips subvertically
(Fig. 2 9 ) .
$2 development post-dates F^ folding, because
S2 transects F^ folds (Fig. 2 9 ) .
An interpretation of
S2 as axial plane cleavage to Fp folds is permitted by
the sparse data for F2 axial planes, but such an interpretation is uncertain.
L i n e a t i o n s , 1,, are chiefly steeply plunging.
They
probably initially developed as slip along S, lead to
crenulations.
Subsequently, in clastic rocks and some
p e l i t i c - s i l i c e o u s rocks the crenulations evolved into Sp
crenulation c l e a v a g e ; in such rocks 1, is the intersection of Sp with S Q - S,.
In cherts of subarea I crenu-
lations of S, failed to develop into crenulation cleavage.
These interpretations are supported by the fact
that in subareas II and III, which contain peliticsiliceous and clastic r o c k s , respectively, Ij maxima are
close to the intersection of synoptic Sg and S Q , whereas
in subarea I the 1, maximum lies on synoptic S Q but not
on synoptic S^ (Fig. 2 9 ) .
r
88
The preceding interpretations of temporal
relation-
ships of the mesoscopic structures are summarized in the
schematic chronology outlined in Figure 3 2 .
11
F|
Fi
s.
^
s,
*"
»
SEDIMENTARY
M,
. •
H
M2
M3
D2
t>l
Figure 32.--Schematic outline of deformational chronology
of Little Bald Mountain area. Metamorphic events
(Ml, M 2 , and M 3 ) and deformational events, (Dj and
D2) are discussed in the following sections.
Metamorphic
Events
Events of greenschist facies metamorphism, M^ and
Mp, appear to be synchronous with the development of S^
and S 2 , respectively.
Some micas aligned in Sj and
others aligned in S^ appear to be biotite, but because
the crystals are small this identification is somewhat
uncertain.
However, thin-section observations warrant a
conclusion that M^ and Mg are synkinematic events, possibly of biotite-zone grade.
The minor static metamor-
phism which occurred between S^ and S^ development may be
89
a separate event, but probably indicates that M,
r e c r y s t a l l i z a t i o n outlasted S^ formation.
Samples from subarea II, the subarea in close
proximity to batholithic rocks (Fig. 4 ) , display an
additional static metamorphic event, M^.
Here, static
growth of s t a u r o l i t e ( ? ) , plagioclase, and some m u s c o v i t e ,
b i o t i t e , and chlorite post-dates development of Sp.
This
mineral assemblage indicates that M^ is of at least the
al b i t e - e p i d o t e hornfels facies and possibly attained the
hornblende hornfels facies.
Recognition of M^ only in
rocks near the batholith suggests that M^ is related to
emplacement of plutons.
Thus, M^ is interpreted as a
contact metamorphic event which was caused by the intrusion of a portion of the Sierra Nevada batholith.
Related Work in Adjacent Areas
From a study area which includes the Little Bald
Mountain area (Fig. 2 8 ) , Redmond (1966) reports layering,
c l e a v a g e , f o l d s , and lineations which here are correlated
with S Q , S 2 , F 2 , and I j , respectively.
The structural
data reported by Redmond for subarea III of his area
(summarized in Fig. 30) are representative of most of his
study a r e a , which is centered about the Middle Fork of the
American River and extends from the Sierra Nevada batholith to the Melones fault zone.
In Redmond's subarea III,
90
which overlaps the Little Bald Mountain area, layering
and cleavage strike slightly more northerly than S Q and
S 2 . r e s p e c t i v e l y (Figs. 29 and 3 0 ) , and some layering is
transposed into the cleavage.
Folds and lineations are
steeply plunging like F2 and 1^, and Redmond
interprets
them as being dispersed in the cleavage plane (Figs. 29
and 3 0 ) .
He notes that the cherts at Little Bald
Mountain (subarea I of this study) are "anomalous" in
that they dip shallowly.
However, he shows little data
and does not discuss possible interpretations of the
shallow d i p s .
Morahan (in p r o g r e s s ) , who mapped an area north of
Little Bald Mountain (Fig. 2 8 ) , reports structures
related to two periods of deformation.
A portion of the
Paleozoic rock succession in Morahan's area is continuous
with the rock succession of Little Bald Mountain.
On the
mesoscopic and microscopic scales the first deformation,
proposed by Morahan, is poorly defined but may be expressed by a metamorphic alignment of micas parallel to layering.
However, it is well recorded by an angular uncon-
formity between tilted Mesozoic rocks and more intensely
deformed Paleozoic rocks.
The second deformation is
expressed by the tilted disposition of Mesozoic strata
and by cleavage and steeply plunging folds and lineations
in Paleozoic rocks.
Data from the Paleozoic rock succes-
91
sion are summarized
in Figure 31.
According to Morahan
the second deformation is characterized by folding and
transposition of layering into cleavage which parallels
the axial planes of northwestwardly plunging mesoscopic
folds (Fig. 3 1 ) .
This cleavage is correlated with Sp
because it has a similar orientation (Figs. 29 and 31)
and is locally a crenulation cleavage like Sp of subarea
III.
Although the orientations of steeply plunging
folds and lineations in Morahan's study area differ from
those of F2 and 1^, they probably are correlative (Figs.
29 and 3 1 ) .
In Redmond's area, fold axes and lineations
are dispersed in the cleavage; the difference in attitudes
of fold axes and lineations between Morahan's area and
Little Bald Mountain may be a product of a similar dispersal in Sp on a more regional
scale.
Deformational- Chronology
The mesoscopic structural features of the Little
Bald Mountain area are interpreted as products of two
deformational
and metamorphic events followed by an
interval of contact metamorphism (Fig. 3 2 ) .
The deforma-
tional events correspond to the two events proposed by
Morahan (personal c o m m u n i c a t i o n ) .
Fj and S. developed during the first
event, D,.
deformational
This event was accompanied by greenschist
92
f a d e s metamorphism, M^, which may have continued briefly
after D^ movement stopped.
S^ possibly formed in
response to slip along S Q during F^ flexure-slip folding.
Because S Q has an approximately constant attitude in much
of the area, the deformational geometry of D, must be
such that S Q had a single predominant orientation after
Fj folding.
This condition is met by assuming isoclinal
or intraformational folding.
If F^ folds are isoclinal,
they represent originally northeast trending horizontal
folds with near vertical axial planes.
If F, folds are
intraformational, they represent an originally limited
number of folds in S Q , which may have formed as S Q was
tilted to a subvertical position.
Folds that form in
such a manner might be expected to have axes parallel to
the axis of tilting, and 3, may represent this axis.
Observations during this study are insufficient to delineate between these two interpretations.
Thus, D^ is
viewed as a deformational event characterized by either
isoclinal folding or tilting.
Sp, Fp, and 1- define a second deformational
event.
Dp, which also was accompanied by greenschist facies
metamorphism, Mp.
of Sp.
Dp is characterized by the development
Both Morahan (in progress) and Redmond
(1966)
report that transposition of S Q into S2 is a prominent
feature of Sg development.
However, transposition of S Q
93
into S2 is uncommon in the Little Bald Mountain area.
This absence of transposition may be the result of:
(1)
orientation of $2 at a large angle to S Q , thereby hindering transposition and/or (2) the predominance of chert,
a competent rock, making transposition difficult.
Hence,
in the Little Bald Mountain area Sp appears to be superimposed on S Q and F^ as a crenulation or fracture cleavage.
The relationship of S2 development and F2 folding
is u n c l e a r .
Possibly, F2 folds and 1^ crenulations
developed early in D2.
As crenulations evolved
crenulation c l e a v a g e , $2 developed as an axial
into
plane
cleavage.
M^ is a contact metamorphic event associated with
the intrusion of plutons of the Sierra Nevada batholith.
It is considered of probable Late Jurassic age because
radiometric age dates for a portion of the batholith
nearest the Little Bald Mountain area are of that age
(Evernden and Kistler, 1970, locations 58, 59, and 6 0 ) .
M- principally affects rocks in subarea II, which is the
subarea showing greatest variation in S Q and S2 orientations (Fig. 2 0 ) .
This variation suggests that pluton
emplacement caused some minor disruption of S Q and S2.
However, if such disruption occurred, it is probably local
since Redmond
(1966) reports that batholith
emplacement
had little effect on the structures of most of the area.
CHAPTER VII
COMPARISON OF THE MERCED RIVER AND LITTLE BALD
MOUNTAIN AREAS:
THEIR POSSIBLE
TECTONIC RELATIONSHIP
Comparison of the Merced River and Little Bald
Mountain areas indicates several similarities.
Both are
similar with respect to lithologic successions and mesoscopic structures.
Furthermore, comparison of the pro-
posed deformational chronologies for each area reveals a
close correspondence.
The latter tentatively is inter-
preted as indicating that the same series of deformational
events occurred in both areas.
If such an interpretation
is correct, this common deformational history is regional
in extent and, hence, is part of the generalized geologic
history of the western Sierra Nevada.
In delineating
this history below, observations from both areas are
utilized in evaluating the importance of penecontemporaneous deformation as a possible first deformational
event;
this subject will be considered first.
Penecontemporaneous
Deformation
Mesoscopic structures of possible penecontemporaneous origin include folds, disrupted layering, pinch-andswell structures, "lumps," bifurcated layers, and pinch-
94
95
outs.
Particularly in the Merced River area, the intra-
formational character of the folds and their chaotic
appearance at some localities suggest such an origin.
H o w e v e r , the involvement of a metamorphic s.-surface
(S^)
in Fj folds (Merced River) and in F^ and F2 folds (Little
Bald M o u n t a i n ) , the consistent orientation of F. axes
(Merced R i v e r ) and F2 axes (Little Bald M o u n t a i n ) , and
the presence of a metamorphic cleavage (S2) parallel to
Fj axial planes (Merced River) and possibly F2 axial
planes (Little Bald M o u n t a i n ) indicate that the folds
developed in consolidated rock which deformed under lowgrade metamorphic conditions.
Disrupted or jumbled zones of layering, which are
chiefly in the pelitic-siliceous rocks of the Merced
River area, have the appearance of zones of sedimentary
slumping and mixing.
However, examination of samples
from such areas reveals that the metamorphic alignment of
micas parallel to layering (S,) is also disrupted.
Thus,
the disruption must post-date at least some metamorphism.
Pinch-and-swel1
structures and the "lumps" are
considered together because they appear to be the same
feature seen in cross-sectional and surface views, respectively.
At some places the "lumps" are round, whereas
at others they are elongate parallel to lineations.
These
o b s e r v a t i o n s suggest that these features initially were
96
round and later were deformed.
This apparently round
shape suggests that these structures are neither penecontemporaneous nor tectonic in origin.
For this reason,
and because these features are observed only in cherts,
they tentatively are interpreted as sedimentary structures related to layered chert deposition.
The bifurcated chert layers also are unlike any commonly reported tectonic or penecontemporaneous structures.
Hence, they too are considered as possible sedimentary
structures associated with layered chert deposition.
Lastly, pinch-outs of layers resemble penecontemporaneous deformational structures called wedges (Whitten,
1966, p. 175-178).
Such features are formed in uncon-
solidated sediments where gravity-induced slip causes a
layer to break and slide over another part of the same
layer.
Interpreting these pinch-outs as wedges appears
to be satisfactory, although an interpretation of their
being primary sedimentary structures in layered chert
cannot be dismissed.
Of the structural features observed during this
study, the pinch-outs are the only ones for which an
interpretation of penecontemporaneous deformation appears
warranted, although some of the other structures may
have sedimentary origins.
In particular, none of the
conspicuous folds of either study area appear to be
I
97
penecontemporaneous.
Hence, penecontemporaneous defor-
mation is only of negligible importance in both the
Merced River and Little Bald Mountain areas.
Comparison of Rock Successions
The rock successions are similar in both the Merced
River and Little Bald Mountain areas in that chert is
predominant to the east, fine-grained clastic rock is
prevalent to the west, and pelitic-siliceous rock occurs
in the middle.
In both areas the cherts (possibly radio-
larian cherts) are layered, but the amount of pelitic
material is greater in those along the Merced River than
in those near Little Bald Mountain.
In the Merced River
area pelitic material commonly occurs in thin, peliticchert layers that, together with the siliceous layers,
are referred to as cherts, whereas in the Little Bald
Mountain area infrequent layers of pelitic material are
slate interlayers within the chert.
In both areas the
pelitic-siliceous rocks are transitional between the
cherts and clastic rocks.
However, near Little Bald
Mountain they are less extensively exposed, less commonly
interlayered with cherts, and more highly metamorphosed
( M J than in the Merced River area; along the Merced River
such rocks have a greater variation in field appearance
and commonly are interlayered with the cherts.
In both
98
areas the fine-grained clastic rocks are of similar composition.
At Little Bald Mountain they are low-grade
metamorphic equivalents of silty-shales or argillaceous
siltstones and are coarser-grained than along the Merced
River where they are chiefly si 1ty-slates.
Minor amounts
of limestone are present in the Merced River area, but
limestone is absent in the Little Bald Mountain area,
although it is reported from the surrounding area
(Red-
mond, 1 9 6 6 ) .
Comparison of Mesoscopic
Structures
The same types of mesoscopic structures are present
in both areas.
An alignment of metamorphic micas (S,)
parallels layering ( S Q ) , except in the slates pf the
Merced River area where S Q and S, were obliterated by
Sp development.
sions.
Cleavage (Sp) has a variety of expres-
It is a fracture cleavage in cherts of both areas,
whereas in clastic rocks it is slaty cleavage in the
Merced River area and crenulation cleavage in the Little
Bald Mountain area.
crenulations.
Lineations (Ij) are predominantly
However, in the Merced River area 1^ also
consists of elongate clasts and "ridges;" in the Little
Bald Mountain area 1, includes intersections of S2 and
SQ.
Steeply plunging folds occur in both areas (F^ in the
Merced River area and F2 in the Little Bald Mountain area)
Additionally, in the Little Bald Mountain area a set of
99
shallowly plunging folds (F^) is present.
Although structures of both locations are similar,
their orientations differ.
Along the Merced River S Q has
a general north-south strike, and S2 has a west-northwest strike.
In the Little Bald Mountain area S Q strikes
northeast and S2 strikes north-northwest.
F, and K
in
the Merced River area plunge chiefly to the southeast and
are strongly dispersed in S2.
F2 and 1^ in the Little
Bald Mountain area plunge chiefly to the south, and 1.
are moderately dispersed in S Q ; however, in the surrounding area both are dispersed in S2 (Redmond, 1 9 6 6 ) .
Correlation of Deformational
Chronologies
The validity of attempting to correlate the deformational chronologies of the Merced River area and Little
Bald Mountain area (Fig. 33) is questionable.
The areas
are over 100 miles (175 km) apart, and the structural
orientations in each area differ.
However, both are in
the same geologic province, the western Sierra Nevada,
contain similar rock successions within the same formation, the Calaveras Formation, have similar types of
mesoscopic s t r u c t u r e s , and display similar deformational
chronologies.
T h u s , based primarily on the correlati.on
of the two deformational chronologies, which is summarized in Figure 3 3 , two regional deformational
tentatively are postulated.
events
100
MERCED RIVER AREA
(M.7) M 1
•l?-*^^
LITTLE BALD MOUNTAIN AREA
Ji
Ml
M2
Di
D2
FIRST
REGIONAL
SEDIMENTARY
M3
SECOND
REGIONAL
lOEFORAAATIONAM
DEFOR^yv^lONAL
EVENT
EVENT
Figure 33.--Tentative correlation of the deformational
chronologies of the Merced River and Little Bald
Mountain areas (Figs. 18 and 3 2 ) .
101
D^ in the Merced River area and D^ in the Little
Bald Mountain area appear to correspond and together
define an event of apparent regional extent which hereafter is referred to as the first deformational
event.
During this event, which is characterized either by
tilting or by isoclinal folding, layering attained
dips in both areas.
folds formed.
steep
In the Little Bald Mountain area F.
S^ also may have developed during this
first deformation; some minor evidence in the Little Bald
Mountain area suggests such an interpretation, and data
along the Merced River are permissive of it.
However, an
alternate interpretation, that of S, forming during the
second deformational
event, also is acceptable in terms
of the overall deformational
history because the first
event is not defined solely by S-.
D- in the Merced River area and Dp in the Little
Bald Mountain area are correlated; hereafter they are
referred to collectively as the second
event.
deformational
This regional event is one of fold (F,, Merced
River area, and Fp, Little Bald Mountain area) and
cleavage (Sp) development.
In the Merced River area
evidence for Sp being axial plane to the folds is strong,
whereas in the Little Bald Mountain area such a relationship is uncertain.
In both areas variations in rock type
similarly affect the structures formed during the second
102
deformation.
In clastic rocks slaty or crenulation
c l e a v a g e d e v e l o p e d , whereas cherts are characterized by
folds and fracture cleavage.
M3 (Little Bald M o u n t a i n ) and M
(Merced River) are
considered to be unrelated because M^ is a localized
contact event associated with batholithic
whereas M
regional
intrusion,
may be a static episode associated with M,
^
1
metamorphism.
In this study the preceding regional
deformational
history is proposed on the basis of a correlation of
similar deformational
c h r o n o l o g i e s , whereas some such
tectonic correlations are based on similarity of structural trend.
Structural trends in the Little Bald Moun-
tain and Merced River areas are different.
This differ-
ence may indicate that the similarities of the two a r e a s ,
including that of their deformational c h r o n o l o g i e s , is
only c o i n c i d e n t a l , or it may be explained as a result of
regional
inhomogeneities during both deformational
events.
D i s t i n g u i s h i n g between these two possibilities is beyond
the scope of this study, because it would require tracing
structural o r i e n t a t i o n s from one area to the other, or
p o s s i b l y , precisely dating each event in both areas.
H o w e v e r , the implications of the first and second deformational e v e n t s ' being regional in extent warrant
discussion.
further
103
Regional
Interpretation
If the two deformational events are regional in
extent, they may represent the Sonoma and Nevadan orogenies.
In the Little Bald Mountain area the D. and Dp
occurred no earlier than Carboniferous-Permian time,
because that is the apparent age of the Calaveras Formation, and no later than Late Jurassic time because that
is when plutons associated with M^ were intruded and
cooled.
In this time interval two orogenies are believed
to have occurred in the western Sierra Nevada.
The
Sonoma orogeny, a Permian-Triassic event, and the Nevadan
orogeny, a Late Jurassic event, are here tentatively
correlated with the first and second deformational events,
respectively.
In the western Cordillera the Sonoma orog-
eny is characterized by northeast stratigraphic and
structural trends, and in the western Sierra Nevada structures characteristic of the Nevadan orogeny generally have
northwest to north-northwest trends.
In the Little Bald
Mountain area the northeast strike of S Q and the northnorthwest strike of S2 appear at least to be in accord
with the structural trends of the Sonoma and Nevadan
orogenies, respectively.
In the Merced River area the deformations occurred
not earlier than Carboniferous-Permian time, but a later
time limit is not indicated by the data of this study.
104
Structural trends of D^ ( S Q strikes north-south) and D^
(S2 strikes w e s t - n o r t h w e s t ) are uncharacteristic of
either of these orogenies.
Rather, the possible associa
tion of D^ and D^ with the Sonoma and Nevadan orogenies,
r e s p e c t i v e l y , is based mainly on the correlation of
deformational
chronologies between the two study areas.
If this correlation proves untenable, the relationship
of D<p and D. in the Merced River area to other deformations in the western Sierra Nevada is unclear.
However,
the tentative correlation of the first and second deformational events to the Sonoma and Nevadan orogenies,
r e s p e c t i v e l y , is presently permissible for both the
Merced River and Little Bald Mountain areas.
CHAPTER VIII
SUMMARY OF CONCLUSIONS
The conclusions of this study are summarized as
follows:
(1) Penecontemporaneous deformation is insignificant in
the deformational history of both the Merced River and
Little Bald Mountain areas.
(2) Slaty or crenulation cleavage in fine-grained clastic
rocks and fracture cleavage in cherts is geometrically,
and probably genetically, related to the steeply plunging
folds in the cherts.
This relationship is well documented
in the Merced River area but is less certain in the Little
Bald Mountain area.
(3) Two deformations apparently occurred in both areas.
The first is poorly documented, particularly in the
Merced River area, but may be an event characterized by
tilting or isoclinal folding.
The second deformation is
defined by prominent structures in both areas and is
characterized by folding and cleavage development.
(4) The proposed first and second deformational
events
are possible correlatives of the Permian-Triassic Sonoma
orogeny and the Late Jurassic Nevadan orogeny,
respectively.
105
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