Pyroclastic Geology of the Johnson Valley Reservoir Andesite
Grant Rozier
Senior Integrative Exercise
March 10, 2006
Submitted in partial fulfillment of the requirements for a Bachelor of Arts degree from
Carleton College, Northfield, Minnesota
Pyroclastic Geology of the Johnson Valley Reservoir Andesite
Grant Rozier
Senior Integrative Exercise
March 10, 2006
Advisors:
Charles M. Bailey, William and Mary
Cameron Davidson, Carleton College
Abstract
The oldest volcanic unit exposed on the Fish Lake Plateau is the porphyritic,
phenocryst-rich andesite of the Johnson Valley Reservoir. This unit is a homogeneous,
250-m-thick accumulation of densely welded, vesicle-poor pyroclastic flow deposits,
unconformably overlying the Paleocene Flagstaff Limestone, and thinning to the
southeast. The bulk composition is an intermediate alkali-rich andesite. The dominant
phenocryst is plagioclase (andesine). Other phenocryst types are: clinopyroxene,
titanomagnetite, olivine (sometimes altered to iddingsite), ilmenite and apatite. Chemical
data relates the J.V.R. andesite to other nearby alkali-rich volcanic rocks.
Keywords
Utah, Oligocene, Plateaus, Pyroclastic flows, Calc-alkalic
Table of Contents:
Abstract
Introduction
1
Literature Review
3
Physiography of the Study Area
6
Methods
Geologic Mapping
Petrography
Whole Rock Geochemistry
S.E.M.
8
Results
Stratigraphic Column of the Fish Lake USGS 7.5’ Quadrangle
Geologic Map
Petrography
Whole Rock Geochemistry
S.E.M.
Discussion
Emplacement Mechanism
Comparisons with nearby Alkaline rocks
8
8
8
9
9
9
10
10
13
13
19
19
22
Conclusion
24
Acknowledgements
24
References Cited
25
1
Introduction
The High Plateaus of Utah are located in a zone of transition, southeast to
northwest, from the stable craton of the Colorado Plateau and the extensional tectonic
setting of the Basin and Range (Fig. 1). The structure and development of the Basin and
Range-Colorado Plateau Transition Zone is a contemporary topic in tectonic and
structural geology. The southern High Plateaus record intense Middle and Late Tertiary
volcanism in the Marysvale Volcanic field that both predates and evolves with the uplift
of the Colorado Plateau, the High Plateaus and Basin and Range faulting(Hintz). The
lithospheric structure and topography of the developing High Plateaus is better
understood through study of the stratigraphy and chemistry of the intrusive and extrusive
volcanics of the High Plateaus.
The Fish Lake Plateau, with peak elevation over 11610 feet, lies on the eastern
margin of the High Plateaus region of central and southern Utah (fig 1). Fish Lake itself
is an alpine lake 5 miles long and 1 mile wide at nearly 9000 feet elevation. It is the
headwaters of the Fremont River. The bedrock of the plateau is Oligocene and early
Miocene extrusive volcanic rocks (Williams and Hackman, 1971).
The Fish Lake volcanics are associated with the Marysvale volcanic pile in central
Utah. The Fish Lake, Awapa and Aquarius Plateaus lie on the eastern margin of the
volcanic pile centered near Marysvale, Utah (Fig. 1). There, the volcanic pile reaches a
maximum thickness of more than 3000 m (Anderson et al., 1975). The thickest
sequences of volcanic breccias, lava flows and pyroclastic deposits are exposed in the
southern Tushar Mountains, northern Markagunt Plateau, and southern Sevier Plateau
(Anderson et al., 1975). Moreover, the Fish Lake volcanics are the easternmost extrusive
2
au
late
Awa
p
aP
Tush
a
37°
114°
Was
atch
u
latea
ier P
Sev
r Mt
ns
Pa
van
tR
an
ge
Plat
eau
Co
lor
ad
oP
lat
ea
u
Ba
sin
an
dR
an
ge
Figure 1. Map of central
and southern Utah including an outline of the High
Plateaus
(gray)
and
selected Late Eocene and
Oligocene volcanic intrusive (solid red) and extrusive volcanics (red). It is
seen that the Marysvale
and the southern High
Plateaus are isomorphic.
The central Marysvale
field is divided by the
USGS 30’X60’ quadrangles Salina, Richfield, Beaver and Loa.
The Fish Lake Plateau
(green) lies on the eastern margin of the High
La Sal
Plateaus and the northMtns.
eastern margin of the
Henry
Marysvale field. To the
Mtns.
west of Fish Lake and
across Grass Valley is
the northern Sevier
Marysvale
Plateau. South of Fish
Volcanic Field
Lake is the Awapa
Needles Range
Plateau.
Map is
modified
from
39°
figures
obtained
Richfield Salina
from
Lehi
F.
Hintz’s Geologic
History of Utah,
the
introductory
figure
of
Callaghan
1939
Fish Lake Plateau
and data from the 38°
Thousand
Utah Geological
Lake Mtn
Survey
official
website (online).
Beaver Loa
112°
110°
3
volcanics in the east-west trending Marysvale-Pioche zone (Nelson, 1989).
In the well-studied central and southern Marysvale volcanic field, three periods of
igneous activity are distinguished: 34 to 22 Ma, 22-14 Ma and 9 to 5 Ma (Cunningham et
al., 1994). During the first period, the vast Mount Dutton Formation and Bullion Canyon
Volcanics (first described by Eugene Callaghan (Callaghan, 1939)) were erupted in the
southern and central Marysvale field, respectively, under a convergent tectonic regime.
In the second period, 22-14 Ma, the igneous activity suddenly transformed to a bimodal
alkali rhyolite (including the Mount Belknap Rhyolite) and basalt. This change is
attributed to a shift in tectonic regime from compression to initial extension (Cunningham
et al., 1994). Finally, a much more recent period of effusive basaltic volcanism 9 to 5 Ma
is associated with continued crustal extension.
Literature Review
Clarence Dutton, riding through the High Plateaus on horseback, first described
the physiography and structure of the Fish Lake Plateau, south central Utah (Dutton,
1880). He recognized the volcanic bedrock as "a great aggregate thickness of trachytes,
alternating with augitic andesites and some dolerites" (Dutton, 1880). In 1952, C.T.
Hardy and S. Meussig described the Fish Lake stratigraphy as part of their study of Fish
Lake Plateau glaciation. The Fish Lake volcanics were correlated with the Middle
Tertiary volcanics of Callaghan. They noted a lack of volcaniclastics that are commonly
observed westward. They described the oldest flows as dark grey hornblende trachyte
beneath massive red and light-grey trachytes (Hardy and Muessig, 1952). They observe
minimum thicknesses of volcanic material of 250-450 feet at Mt. Terrill and 1200-1800
4
feet at Mt. Marvine, and suspected that the volcanics thinned and dipped southeast.
Shortly afterwards in 1953, Donald P. McGookey, working independently, extended the
Bullion Canyon Volcanics of Callaghan (1939) north into the Fish Lake Plateau. He
identified andesites and latites based on mineralogy, and supposed an Oligocene age
(McGookey, 1960). However, he admitted, "No attempt was made by the writer to map
systematically the succession of flows that is exposed in the northern part of the Fish
Lake Plateau. Such a study would be a major project in itself."
Much later, Williams and Hackman mapped a Basaltic Andesite unit and a Latite
unit within their geologic map of the Salina quadrangle (Williams and Hackman, 1971).
Within the Fish Lake Plateau, they map an undifferentiated unit of basaltic andesite and
latite below the Tuff of Osiris, hereafter called the Osiris Dacite within my study area.
Most recently, Webber and Bailey (2003) produced a 1:24,000 geologic map of
the Fish Lake 7.5' USGS quadrangle as part of an undergraduate thesis. Therein, they
distinguish four bedrock units within the undifferentiated unit of Williams and Hackman.
From stratigraphically low to high, they are: Johnson Valley Reservoir Andesite (J.V.R.),
Frying Pan Hill Andesite, Lake Creek Andesite and Pelican Canyon Andesite (Webber,
2003).
The stratigraphically lowest unit is the unofficially named the Johnson Valley
Reservoir Andesite for its type location at a check dam below the Johnson Valley
Reservoir (38 36’32” N, 111 37’51” W) (Webber, 2003). It is the subject of this study.
The Osiris Dacite that caps the Fish Lake volcanics is an important marker bed
present throughout the Marysvale field. It was erupted from the Monroe Peak Caldera in
the Central Sevier Plateau, the largest caldera in the Marysvale volcanic field, as a simple
5
Figure 2. Digital Elevation Model of the study area, including Mt. Marvine, the Fish Lake
Hightop and the Mytoge Mountains. The essential elements of the structure of the Fish Lake
Plateau are discernable. Overall, the surface of plateau dips SE from its peak elevation at the
Fish Lake Hightop (11633 ft). This gentle slope is interrupted by a series of pronounced
NE-SW trending grabens, the largest of which is occupied by Fish Lake proper. A second set of
NNW-SSE trending lineaments suggests another graben series parallel to the Fremont River,
and this is confirmed by bedrock mapping. An ENE-WSW south of Fish Lake is the expression
of a large-displacement normal fault bounding the plateau. A parallel lineament to the north,
representing a fault or fracture of small displacement, cuts the Fish Lake hightop, Crater Lakes,
Mytoge Mountains and bisects Pelican Canyon. The northern plateau is marked by glacial
features, including the Sevenmile cirques above Sevenmile valley and the prominent Pelican
Canyon moraine intruding into Fish Lake.
x Mt. Marvine
Seven
M
e
il
Cre
ek
38 37.5' N
Fish Lake x
Hightop
JVR
sh
La
ke
nt
mo
Fre
Mytoge
Mountains
er
Riv
Fi
38 30' N
0
5
kilometers
111 45' W
111 37.5' W
N
6
cooling unit less than 60 meters thick (Steven et al., 1983). The densely welded tuff
contains primarily plagioclase, sanidine, biotite and pyroxene and covers greater than
4000 km2, mostly to the North and east (Steven et al., 1983). The five published isotopic
ages are 20.3±0.5 Ma (from Damon 1986b, page 42), 22.1±0.4 Ma, 22.3±0.4, 22.4±.4 and
22.8±.4 (Fleck et al., 1975).
Physiography of the Study Area
A Digital Elevation Model of the Fish Lake Plateau and explanation is presented
in Figure 2.
J.V.R. andesite crops out over much of the Fish Lake Plateau, both within and outside of
the Fish Lake quadrangle. J.V.R. outcrop is common in the hanging walls of the grabenbounding normal faults, particularly at the Mytoge mountain scarp, Crater Lakes and Cedarless
Flat grabens. It crops out in the walls of Pelican canyon and Doctor Canyon.
There are important exposures of J.V.R. andesite outside of the Fish Lake quadrangle.
The series of Splatter, Pole and Ivie canyons have J.V.R. andesite exposed in their lower
drainages. J.V.R. andesite is exposed on both sides of the Fremont River valley. Mt. Terrill has
a thick base of J.V.R. andesite, which is exposed in several continuous vertical buttresses by a
150-foot dip-normal cliff on the east side. The single most important exposure of J.V.R. andesite
is at Mt. Marvine, where the 200-foot-thick laterally continuous arête contains at least seven
distinct J.V.R. Andesite flows (Fig. 3).
Figure 3. Panoramic view of the east side of the Mt. Marvine Arete (UTM 4,280,000N, 444300E). The Mt. Marvine Peak, elevation
11610 feet, rises 200 feet above the talus slope below. North from the peak, the arete extends 200 meters. Southward from the peak, the
arete continues for several hundred more meters before it is lost. Overall, half a kilometer of laterally continuous bedrock is exposed. The
bedrock is entirely J.V.R. Andesite. At least seven separate deposits are distinguished and labelled below. Columnar jointing is observed
at the margins of several deposits (particularly deposit six). Voluminous breccias are not observed between deposits, and there are no
observed paleosols.
S
N
Marvine Peak, 11610 ft
200 ft
7
?
6
5
5
breccia
4
3
2
1
7
8
Methods
Geologic Mapping
The Fish Lake USGS 7.5' quadrangle and the areas immediately to the south and east
were mapped for bedrock and surficial deposits during a four-week field period June 26-July 23.
Mapping was done on foot with a handheld GPS unit. The primary objective was complete
coverage of the Fish Lake quadrangle. Approximately 400 bedrock stations were collected
within the Fish Lake quadrangle, in addition to the approximately 120 data points previously
mapped by Webber and Bailey in 2003. Bedrock data was also collected in the following areas
outside of the Fish Lake quadrangle: Mt. Terrill, Mt. Marvine, Spatter, Pole and Ivie Canyons,
Cedarless Flat, Elias and Briggs Hollow, Row of Pines Bench, Second Ledges, the Ledges,
Deadman's Hollow and the Fremont River. These stations plot in the Loa, Forsyth, Mt. Terrill
and Boobe Hole Reservoir 7.5' USGS quadrangles.
Petrography
Six J.V.R. andesite bedrock stations throughout the study area were sampled for
petrography. These stations are labeled on Figure 5. Six thin sections were produced
from these samples, each with a microprobe-quality polish. Petrography was done with
an optical microscope, with the purpose of describing mineralogy and textures.
Whole Rock Geochemistry
The same six bedrock stations mentioned above were sampled for the purpose of whole
rock geochemical analysis (Fig. 5). These samples are unweathered, lack vesicles and exhibit
9
little or no secondary mineralization or alteration. They were sent to ALS Chemex, where they
were crushed and analyzed for major oxides using XRF (ALS Chemex website).
S.E.M.
Phenocryst and matrix chemistries of VA088 were obtained with a Scanning
Electron Microscope (S.E.M.). After petrography, VA088 was coated with carbon and
analyzed with an S.E.M. equipped with an electron backscatter detector and INCA
software.
Results
Stratigraphic Column of the Fish Lake USGS 7.5' Quadrangle
One outcome of fieldwork is a general stratigraphic column for the thick volcanic
package of the Fish Lake quadrangle (Fig. 5). The Fish Lake volcanics sit atop an
unconformity, below which is the well-known Flagstaff Formation, a cherty freshwater
limestone that was deposited over much of Easter Utah during the Paleocene. The basal
unit of the 500-meter-thick Tertiary volcanic rocks is the J.V.R. Andesite. The rest of the
volcanic package, moving stratigraphically upward, is Lake Creek Andesite and Pelican
Canyon Andesite. The Frying Pan Hill Andesite of Webber and Bailey is absent because
it was determined in the field not to be a mappable unit. Bedrock stations previously
assigned to F.P.H. Andesite are now assigned to the Lake Creek unit. Above the Pelican
Canyon is another unconformity. Capping the Fish Lake volcanics is the Osiris Dacite,
which forms a thin, resistant cap of varying thickness (10-24 meters) to the Fish Lake
volcanic package. The dark, glassy basal vitrophyre that is sometimes present beneath
10
the Osiris Tuff (Anderson et al., 1975) is not observed beneath the Osiris Dacite on the
Fish Lake Hightop.
Geologic Map
The primary outcome of fieldwork is a geologic map of the Fish Lake quadrangle
extending into the western portion of the Forsyth Reservoir quadrangle and the northern
part of the Loa quadrangle (Fig. 4). The map includes stratigraphic contacts and faults
constrained by offset strata. They are assumed to be high-angle (>60 degree) normal
faults.
Two subparallel cross-sections, A-A' and B-B', approximately normal to the Fish
Lake graben, extend interpretations beneath the surface (Fig. 6).
Petrography
J.V.R. phenocryst percent ranges from 40% to 50%. 25% is plagioclase, mostly
between 1 and 3 mm, but ranging up to 5 mm. Euhedral to subhedral plagioclase laths
exhibit some combination of simple and polysynthetic twinning. Sieve texture is
common. 10% is euhedral or rounded, subhedral clinopyroxene, 1 to 2 mm, poor or one
directional cleavage. The remainder is composed of opaques, olivine, orthopyroxene and
iddingsite from olivine. The matrix is generally plagioclase crystallite-rich but also can
contain pyroxene crystallites, areas of glass and opaques.
Figure 10 illustrates some common petrographic textures.
11
Figure 4. Geologic Map of the Fish Lake Plateau. Mapped is
the Fish Lake USGS 7.5’ quadrangle and portions of the Loa and
Forsyth Reservoir quadrangles. Authors Christopher M. Bailey,
Caroline Webber and 2005 NSF Fish Lake REU.
GEOLOGIC MAP OF THE FISH LAKE PLATEAU, UTAH
Tjvr
A
d
e
pp
a
m
ot
Tjvr
Geologic Contacts
Stratigraphic, volcanic
or erosional
Qs
Tlc
4,5
Tpc
n
Geologic Units
Tlc
Tlc
Tlc
Qs Surficial Deposits- includes Fremont River
terraces, glacial till and outwash, debris flow fans,
lacustrine and alluvial deposits, and selected
colluvium.
Tjvr
To
2
Tlc
(dashed beneath
surficial deposits)
1
Tjvr
Tlc
Tjvr
Tlc
Tb Olivine Basalt
Tsc Sandstone and Conglomerate- poorly to
moderately cemented, well-bedded sedimentary
rocks in the Fremont River valley
To Osiris Dacite- phenocryst-rich,
plagioclase bearing dacite
biotite
and
Tpc Pelican Canyon Andesite- phenocryst-poor,
spherulite-rich, variably vesiculated andesite
Tlc
Tjvr
Fault
Tjvr
Qs
Tlc
Tpc
Tlc
Tlc
Qs
B
Qs
Tjvr
Qs
Tlc
Tjvr
Qs
Tlc
Tsc Tf
Tlc
Tlc
Creek AndesiteTlc Lake
glass-rich andesite
phenocryst-poor,
Qs
Tjvr
Tlc
Tjvr
Tjvr Johnson
Valley
Reservoir
Andesitephenocryst-rich, plagioclase and pyroxene
bearing andesite
Qs
1
2
3
4
5
6
UTM Coordinates
N4273565, E445075
N4273780, E442780
N4265920, E438890
N4280090, E444240
N4283210, E443770
N4260660, E439051
3
Tlc
Tlc
Tjvr
Locale
Type Location JVR check dam
GMR148
Frying Pan Hill
GMR077
Mytoge Scarp
GMR122
Mt. Marvine
GMR021
Mt. Terrill
VA088
Mytoge Slope
A’
Qs
Tf Flagstaff Formation- siltstone, sandstone, and
limestone with a few layers containing chert and
quartzite clasts (to 10 cm)
Map Sample ID
Tf
Tlc
Tjvr
Tlc
Qs
Qs
Tjvr
Tb
0
0
To
To
miles
kilometers
6
3
5
N
To
Tsc
Tlc
Tlc
Qs
To
B’
Tjvr
12
640
620
Osiris Dacite
Pelican
Canyon
Andesite
Thickness (m)
560
Lake Creek
Andesite
310
300
290
220
J.V.R. Andesite
0
0
Flagstaff Limestone
NW
Elevation (ft.)
A
12,000
11,000
10,000
9000
8000
7000
6000
Tpc
SE
Figure 5. Stratigraphic column
valid for the Fish Lake USGS
7.5’ quadrangle. This figure
shows a universal thinning of the
Fish Lake volcanics south and
east of the Fish Lake Hightop,
with the exception of the Osiris
Dacite. The Pelican Canyon
Andesite thins rapidly away from
the Fish Lake Hightop. J.V.R.
Andesite is >220 m thick
throughout the quadrangle, but
thins away quickly at the SE
margin of the plateau near Rabbit
Valley. The Osiris Dacite is of
variable thickness (0-24 m)
throughout the study area,
suggesting it was deposited over
subdued topography. The thickness of the Flagstaff Limestone is
unknown.
Figure 6. Cross sections A-A’
and B-B’. Scale 1:1.
A'
Tlc
Tlc
Tjvr
Tf
Tf
?
Tjvr
Tf
?
Tsc
Tlc
Tjvr
Tjvr
Tlc
Tjvr
Tjvr
Tf
Tjvr
Tf
Tf
?
Elevation (ft.)
B
12,000
11,000
10,000
9000
8000
7000
6000
12,000
11,000
10,000
9000
8000
7000
6000
Tlc
Tlc
B'
Tlc
Fish Lake
Tlc
Tjvr
Tf
?
Tlc
?
Tjvr
Tlc
Tjvr
Tjvr
Tlc
Tjvr
?
To
12,000
11,000
10,000
Tlc
9000
8000
7000
6000
13
Whole Rock Geochemistry
XRF returns weight percentages for the following major oxides: Si02, Al2O3,
Fe2O3, CaO, MgO, Na2O, K2O, Cr2O3, TiO2, MnO, P2O5, SrO, BaO, and loss on
ignition (LOI). Values normalized to 100% non-volatiles are reported in Table 1.
Each sample is classified according to the IUGS classification system for
extrusive volcanics of Le Bas and others, 1986 (Figure 8). SiO2 values range between
56.24%Wt. and 58.00%Wt. Total alkalis range between 6.39%Wt. and 7.01%Wt. Under
the Le Bas 1986 classification scheme, the J.V.R. Andesite samples plot as trachyandesite
and basaltic trachyandesite.
Results are also plotted on a Harker variation diagram in Figure 9.
S.E.M.
The S.E.M. returns weight percent for the major rock-forming cations Na, Mg, Al,
Si, K, Ca, Ti, Mn, Fe, P and Ni for each major phenocryst type (Feldspar, Pyroxene,
Opaque, Olivine) and matrix glass. Representative phenocryst chemistries normalized to
atomic proportions are given in Table 2. The feldspars are andesine with some
labradorite. The pyroxenes are augite. The opaques are titanomagnetite. The glass is
sanidine and titanomagnetite.
An element map of a small area of sample VA088 is presented in Figure 9. The
fact that the K does not have a mineral phase has implications for future radiometric
dating of the J.V.R. Andesite.
14
Table 1. Results from chemical analysis of the Johnson Valley Reservoir Andesite.
Sample
SiO2
Al2O3
Fe2O3
CaO
MgO
Na2O
K2O
Cr2O3
TiO2
MnO
P2O5
SrO
BaO
LOI
Total
Type
Location
55.14
16.34
8.49
6.11
3.69
3.43
2.94
<0.01
1.00
0.14
0.34
0.08
0.07
1.99
99.77
GMR 021
56.62
15.95
8.16
5.83
4.04
3.50
3.16
<0.01
1.01
0.12
0.37
0.08
0.05
0.99
99.89
GMR 077
57.46
16.11
8.11
5.77
3.60
3.57
3.34
<0.01
1.02
0.12
0.36
0.08
0.08
0.31
99.93
VA 088
57.31
16.29
8.06
5.76
3.43
3.55
3.43
<0.01
1.04
0.13
0.43
0.08
0.07
0.46
100.05
GMR 122
57.1
16.06
7.94
5.54
3.45
3.4
3.37
0.01
0.99
0.12
0.35
0.08
0.04
1.33
99.78
GMR 148
55.68
16.04
8.71
6.33
4.21
3.44
2.89
<0.01
1.04
0.13
0.38
0.09
0.06
0.92
99.92
Table 2. Representative phenocryst and glass chemistries from sample VA088.
Phenocrysts in mole fractions, feldspar and olivine normalized to four oxygen,
clinopyroxene to six. Glass in weight percent.
Si
Al
Cr
Ti
Ni
Fe
Mg
Mn
Ca
K
Na
Total
Feldspar
2.60
1.36
---0.03
--0.46
0.05
0.49
5.00
An45
Cpx
2.00
0.08
---0.36
0.86
-0.66
--3.96
En71
Ol
1.02
----0.69
1.26
----2.98
Fo65
Glass
76.7
11
-2.22
-1.56
---5.91
2.59
12
Johnson Valley Reservoir
Andesite
Trachyte
Lake Creek Andesite
10
Basaltic Andesite of Mattox
1991
Trachyandesite
Latite of Mattox 1991
Lava Flows of Riley Spring,
Nelson 1989
Basaltic
Lava Flows of Deer Spring
Draw, Nelson 1989 Trachyandesite
Alk
8
Dacite
6
4
Basalt
Basaltic Andesite
Andesite
2
50
55
SiO2
60
65
70
15
45
Figure 7. IUGS plot for
extrusive volcanics. Fish Lake
volcanics are J.V.R. Andesite
and Lake Creek Andesite of
Webber. Basaltic andesite and
latite of Mattox, lava flows of
Riley Spring and Deer Spring
Draw of Nelson also plotted.
20
Al2O3
19
8
CaO
6
18
4
17
2
16
55
10
60
65
70
FeO
55
5
MgO
6
3
4
2
Figure 8 (cont. on following
page). Harker variation
diagram for the units plotted
in Figure 7.
65
70
J.V.R. Andesite
Lake Creek Andesite
4
8
2
60
Basaltic And. of Mattox
Latite of Mattox
Lava of Riley Spring
Lava of Deer Spring
Draw
1
16
55
60
65
70
55
60
65
70
1.4
Figure 8, cont.
TiO2
1.2
8
K 2O
6
1
0.8
4
0.6
0.4
2
0.2
55
5
60
65
70
Na2O
55
0.6
60
65
70
P2O5
4
0.4
3
2
0.2
1
17
55
60
65
70
55
60
65
70
Si
Ti
Al
Fe
Mg
Ca
Na
K
Cpx
SEM Image
200 µm
18
Figure 9. Element map of a small area of VA088 thin section. Light areas means high returns. Dark area means poor returns.
19
Discussion
Emplacement Mechanism
Historically, the volcanics of Fish Lake have been called "lava flows"(Dutton,
1880; Hardy and Muessig, 1952; McGookey, 1960; Williams and Hackman, 1971).
Following the proposal of Webber and Bailey, I find that the petrographic textures and
structural observations made at Mt. Marvine qualify the J.V.R. Andesite as an
accumulation of densely welded pyroclastic flow deposits. The primary criterion is the
abundance of phenocrysts in the J.V.R. (up to 50%). This is not unusual since pyroclastic
flows are observed to have crystal abundances up to 50% (Fisher, 1984). However, the
observed crystal abundances would impart a crippling viscosity to a lava flow. Secondly,
broken phenocrysts, a common petrographic texture in J.V.R. Andesite (Fig. 10), are
commonly observed in pyroclastic flows (Fisher, 1984).
The structure pattern at Mt. Marvine (Fig. 3) suggests a rapid emplacement of
thin, mostly flat-lying, laterally continuous pyroclastic flows. Detailed mapping of the
entire Mt. Marvine outcrop is required, but as a first approximation there appears to be a
lack of well-developed erosional surfaces (breccia, paleosols). Also, no individual
cooling units are distinguished. It appears that the entire J.V.R. Andesite was emplaced
within a single eruptive epoch and relatively few eruptions ("eruptive epoch" and
"eruption" as defined by Fisher pg 347-8). Therefore, the J.V.R. Andesite is comprised
of a small number of thick, densely welded pyroclastic flow units (defined by Fisher pg.
349).
20
A
B
C
D
E
F
Figure 10. Photomicrographs of J.V.R. Andesite exhibiting some common petrologic textures.
Scale bars 1 mm. (A) (FL58) and (B) (FL64) show chipped laths of plagioclase under XPL. (C)
(FL63) and (D) (FL162) (XPL) show a glomerophyric cluster of plagioclase and pyroxene,
respectively. (E) (FL38) (XPL) records a preferred orientation of abundant plagioclase laths.
(F) (FL64) (PPL) shows sieve-textured plagioclase phenocrysts with pyroxene inclusions.
21
The J.V.R. Andesite must have defined a broad "volcanic plane," with a volume
of 100 km^3. I speculate that a collapsing caldera seems capable of extruding such a
volume of hot material over such a broad area in a short time. Caldera collapse is
responsible for the emplacement of thick volcanic planes elsewhere in the Marysvale
field, such as the 200 m of dense rocks SW of the Three Creeks Caldera in the northern
Tushar Mountains (Steven et al., 1983). The pressing question that follows this
interpretation is: where is the source caldera?
The Monroe Peak Caldera 30 km to the west is the nearest recognized caldera to
the Fish Lake volcanics but it is not a potential source. To have erupted both the J.V.R.
Andesite and later the far-reaching Osiris Tuff would require strong caldera resurgence.
This is unlikely, for we see that Steven et. al. states "no caldera in the Marysvale field
shows significant resurgence following subsidence." (Steven et al., 1983).
Previous authors have suggested source areas for the Fish Lake volcanics. Dutton
identified volcanic source areas for the Fish Lake volcanics in the eastern Sevier Plateau
and within the Fish Lake Plateau at Mt. Hilgard (Dutton, 1880). McGookey and Hardy
and Muessig echo Dutton while leaning toward the Sevier Plateau as the primary source
(Hardy and Muessig, 1952; McGookey, 1960). The pyroclastic flow method of
emplacement is somewhat incompatible with a source in the Sevier Plateau. If the J.V.R.
were a product of a built-up stratovolcano sending pyroclastic flows down onto a
topographic low, we would also expect to see alluvial deposits of material shed from the
stratovolcano's slopes. This is the case in the Awapa Plateau, where Mattox (1991)
observes a great thickness of volcanic mudflow breccia, conglomerate and sandstone (his
22
"Volcanic Rocks of Langdon Mountain") below and interlayered with extrusive Basaltic
Andesite and Latite (Mattox, 1991, 2001). However, no volcaniclastic deposits of any
kind are observed in the Northern Fish Lake Plateau.
These ideas will certainly change as we resolve our understanding of the
stratigraphy of the Fish Lake volcanics. It is crucial that we better understand the
structure at Mt. Marvine and the stratigraphic relations at the plateau margins if we are to
place the Fish Lake volcanics in a regional context.
Comparisons with nearby Alkaline rocks
J.V.R. Andesite shares similarities with the Basaltic Andesite of Williams and
Hackman and later Mattox (1991, 2001) and the lava flows of Riley Spring of Nelson
(1989). With the Basaltic Andesite, J.V.R. Andesite shares the constituent minerals,
particularly sieve-textured, embayed plagioclase, clinopyroxene and lesser olivine
(Mattox, 1991, 2001). Their groundmasses, mostly plagioclase microlites with smaller
amounts of Fe-Ti oxides and glass, are also similar.
The J.V.R. and lava flows of Riley Spring share stratigraphic position above the
Flagstaff Limestone (Nelson, 1989). It has fairly abundant phenocrysts (35%) of
dominantly plagioclase, either large, resorbed andesine or small, euhedral labradorite.
Augite, olivine (idd.) and Fe-Ti oxides are secondary phenocrysts.
All three units contain glomeroporphyritic clusters of plagioclase, clinopyroxene
or both commonly mixed with olivine and oxides (Mattox, 1991, 2001; Nelson, 1989),
(Fig. 10). This texture is interpreted to be acquired pre-eruption, as opposed to acquired
during transport (Nelson, 1989).
23
The whole rock chemical data facilitates comparison of Fish Lake volcanics with
other nearby volcanics. In addition to the geochemical data from the J.V.R. Andesite and
the Lake Creek trachyte, Figures 7 and 8 plot geochemical data from the basaltic andesite
of Mattox, the latite of Mattox, the lava flows of Riley Spring (Nelson 1989) and the lava
flows of Deer Spring Draw (Nelson 1989). These three pairs of units are very similar
chemically. The group of J.V.R. Andesite, B.A. of Mattox and lava flows of Riley
Spring plots between 55-60% SiO2 and 6-9% alkalis. The group of Lake Creek
Andesite, Latite of Mattox and lava flows of Deer Spring Draw plots between 64-68%
SiO2 and 9-12% alkalis. Relative to the other pairs, the Fish Lake volcanics are slightly
lower in alkalis and slightly higher in Fe and Mg (Fig. 8).
Presented with the chemical data, it is natural to assume a genetic relationship
between the trachyandesites and their counterpart trachytes. However, radiometric ages
and stratigraphic relations complicate this interpretation. The basal lava flow of Riley
Spring is 23.85±1.1 Ma; one lava flow of Deer Spring Draw, although chemically more
evolved, is 26.3±1.1 Ma. Responding to this "time inversion," Nelson suggests that the
spatial relation between the two units may be coincidental rather than genetic (Nelson,
1989). On the Awapa Plateau, three samples of Basaltic Andesite are dated: AP99 is
25.2±1.6 Ma, AP26 from the far east of the plateau is 25.8±1.4 Ma and AP47 from the
top of the unit immediately beneath the Osiris Tuff, is 23.2±1.5 Ma (Mattox, 1991).
Sample AP19 of Latite is dated at 23.1+-1.0 (Mattox 1991). The Latite and Basaltic
Andesite are known to interlayer (Mattox, 1991). Overall, a direct temporal genetic
relation between the two groups is uncertain.
Lack of vesicles, elevated Fe and Mg content, homogeneity, thickness and
24
emplacement mechanism argue that the J.V.R. Andesite deserves to be distinguished
from the nearby trachyandesites of Nelson and Mattox. However, stratigraphic data at
the margins may well result in a blending of these units.
Conclusion
Extensive bedrock mapping has clarified the stratigraphy of the volcanics in the
Fish Lake quadrangle. Stratigraphy combined with petrography has resulted in the
designation of the J.V.R. Andesite, the basal unit of the Fish Lake volcanics, as a densely
welded pyroclastic flow deposit erupted during a single eruptive epoch during the late
Oligocene or early Miocene. Chemical data is useful for comparing with the nearby
Basaltic Andesite of Mattox (1991) and the lava flows of Riley Spring of Nelson (1989),
but recording stratigraphic relations between the three units is still necessary to draw any
conclusions.
Acknowledgements
I acknowledge Charles M. Bailey, Assistant Professor of Geology at the College
of William and Mary and Cameron Davidson, Tenured Professor of Geology at Carleton
College as my advisors on this project. I acknowledge the Bernstein Development
Foundation's Geology Endowment Fund for their financial support.
25
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