Pyroclastic Geology of the Johnson Valley Reservoir Andesite

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 8 Geologic Mapping 8 Petrography 8 Whole Rock Geochemistry 8 S.E.M. 9 Results 9 Stratigraphic Column of the Fish Lake USGS 7.5’ Quadrangle 9 Geologic Map 10 Petrography 10 Whole Rock Geochemistry 13 S.E.M. 13 Discussion 19 Emplacement Mechanism 19 Comparisons with nearby Alkaline rocks 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

Needles Range

Fish Lake Plateau

Sev

ier

Pla

teau

Aw

apa

Pla

teau

ThousandLake Mtn

Loa

Salina

Beaver

Richfield

Tush

ar M

tns

Was

atch

Pla

teau

Pav

ant R

ange

Bas

in a

nd R

ange

Col

orad

o P

late

au

Figure 1. Map of central and southern Utah includ-ing an outline of the High Plateaus (gray) and selected Late Eocene and Oligocene volcanic intru-sive (solid red) and extru-sive 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’ quad-rangles Salina, Rich-field, Beaver and Loa. The Fish Lake Plateau (green) lies on the east-ern margin of the High Plateaus and the north-eastern margin of the Marysvale field. To the west of Fish Lake and across Grass Valley is the northern Sevier Plateau. South of Fish Lake is the Awapa Plateau. Map is modified from figures obtained from Lehi F. Hintz’s Geologic History of Utah, the introductory figure of Callaghan 1939 and data from the Utah Geological Survey official website (online).

112° 110°114°

39°

38°

37°

2

Marysvale Volcanic Field

La SalMtns.

HenryMtns.

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

MytogeMountains

Fish

Lake

111 37.5' W

Fish LakeHightop

x Mt. Marvine

JVR

111 45' W

38 30' N

38 37.5' N

Seven

Mile

Creek

x

Fremo

nt

River

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.

N5

kilometers

0

5

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 graben-

bounding 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).

?

123

45

56

7

NS

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. Thebedrock 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.

Marvine Peak, 11610 ft

200 ft

breccia

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.

Tjvr

Tjvr

Tjvr

Tjvr

Tjvr

Tjvr

Tjvr

Tjvr

Tjvr

Tjvr

Tjvr

Tsc

Tsc

Qs

Qs

Qs

Qs

Qs

Qs

Qs

Qs

Tjvr

Tjvr

Tjvr

Tjvr

Qs

Qs

Qs

QsTlc

Tlc

Tlc

TlcTlc

TlcTlc

Tlc

Tlc

Tlc

Tlc

Tlc

Tlc

Tlc

TlcTf

Tf

Tlc

Tlc

Tlc

Tlc

Tlc

Tlc

Tlc

TbTo To

Tpc

Tpc

To

To

To

N

kilometers

miles

not m

appe

d

0 5

0 3

Sandstone and Conglomerate- poorly tomoderately cemented, well-bedded sedimentaryrocks in the Fremont River valley

Surficial Deposits- includes Fremont Riverterraces, glacial till and outwash, debris flow fans,lacustrine and alluvial deposits, and selectedcolluvium.

Osiris Dacite- phenocryst-rich, biotite andplagioclase bearing dacite

Olivine Basalt

Pelican Canyon Andesite- phenocryst-poor,spherulite-rich, variably vesiculated andesite

Lake Creek Andesite- phenocryst-poor,glass-rich andesite

Johnson Valley Reservoir Andesite-phenocryst-rich, plagioclase and pyroxenebearing andesite

Flagstaff Formation- siltstone, sandstone, andlimestone with a few layers containing chert andquartzite clasts (to 10 cm)

Geologic Units

Geologic Contacts

Fault

Stratigraphic, volcanicor erosional

(dashed beneathsurficial deposits)Qs

Tb

Tsc

To

Tpc

Tlc

Tjvr

Tf

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.

12

4 5

6

,

UTM CoordinatesLocaleMap Sample ID

123456

Type Location

GMR148GMR077GMR122GMR021VA088

JVR check damFrying Pan HillMytoge ScarpMt. MarvineMt. TerrillMytoge Slope

N4273565, E445075N4273780, E442780N4265920, E438890N4280090, E444240N4283210, E443770N4260660, E439051

3

GEOLOGIC MAP OF THE FISH LAKE PLATEAU, UTAH

11

A

A’

B

B’

6000700080009000

10,00011,00012,000

Ele

vati

on

(ft

.)

600070008000900010,00011,00012,000

6000700080009000

10,00011,00012,000

Ele

vati

on

(ft

.)

600070008000900010,00011,00012,000

Tjvr

Tjvr

TjvrTjvr

Tjvr

Tjvr

Tjvr TjvrTjvr

Tjvr

Tjvr

TjvrTf

? ? ?

? ? ?Tf

Tf Tf Tf Tf Tf

Tlc

Tlc Tlc TlcTlc

Tlc

Tlc

Tlc

TlcTlc

TlcTlc

Tpc

Tsc

Fish LakeTo

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 thick-ness of the Flagstaff Limestone is unknown.

Figure 6. Cross sections A-A’ and B-B’. Scale 1:1.

12

A A'

B B'

0 0

290

560

620640

220

300310

Th

ickn

ess

(m)

NW SE

J.V.R. Andesite

Lake CreekAndesite

PelicanCanyonAndesite

Osiris Dacite

Flagstaff Limestone

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

Type Location GMR 021 GMR 077 VA 088 GMR 122 GMR 148

SiO2 55.14 56.62 57.46 57.31 57.1 55.68 Al2O3 16.34 15.95 16.11 16.29 16.06 16.04 Fe2O3 8.49 8.16 8.11 8.06 7.94 8.71 CaO 6.11 5.83 5.77 5.76 5.54 6.33 MgO 3.69 4.04 3.60 3.43 3.45 4.21 Na2O 3.43 3.50 3.57 3.55 3.4 3.44 K2O 2.94 3.16 3.34 3.43 3.37 2.89

Cr2O3 <0.01 <0.01 <0.01 <0.01 0.01 <0.01 TiO2 1.00 1.01 1.02 1.04 0.99 1.04 MnO 0.14 0.12 0.12 0.13 0.12 0.13 P2O5 0.34 0.37 0.36 0.43 0.35 0.38 SrO 0.08 0.08 0.08 0.08 0.08 0.09 BaO 0.07 0.05 0.08 0.07 0.04 0.06 LOI 1.99 0.99 0.31 0.46 1.33 0.92 Total 99.77 99.89 99.93 100.05 99.78 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.

Feldspar Cpx Ol Glass Si 2.60 2.00 1.02 76.7 Al 1.36 0.08 -- 11 Cr -- -- -- -- Ti -- -- -- 2.22 Ni -- -- -- -- Fe 0.03 0.36 0.69 1.56 Mg -- 0.86 1.26 -- Mn -- -- -- -- Ca 0.46 0.66 -- -- K 0.05 -- -- 5.91 Na 0.49 -- -- 2.59

Total 5.00 3.96 2.98 An45 En71 Fo65

Dacite

Andesite

Basaltic AndesiteBasalt

BasalticTrachyandesite

Trachyandesite

Trachyte

45 50 55 60 65 70

2

4

6

8

10

12

SiO2

Alk

15

Lake Creek Andesite

Johnson Valley ReservoirAndesite

Basaltic Andesite of Mattox1991

Latite of Mattox 1991

Lava Flows of Riley Spring,Nelson 1989

Lava Flows of Deer SpringDraw, Nelson 1989

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

19

18

17

16

55 60 65 70

55 60 65 70 55 60 65 70

55 60 65 70

2

4

6

8

2

4

6

8

10

1

2

3

4

5

Al2O3 CaO

FeO MgOLake Creek AndesiteJ.V.R. Andesite

Basaltic And. of Mattox

Latite of MattoxLava of Riley SpringLava of Deer SpringDraw

16

Figure 8 (cont. on following page). Harker variation diagram for the units plotted in Figure 7.

1.4

1.2

1

0.6

0.8

0.4

0.2

55 60 65 70

55 60 65 70 55 60 65 70

55 60 65 70

2

4

6

8

1

2

3

4

5

0.2

0.4

0.6

TiO2

K2O

Na2O P

2O

5

Figure 8, cont.

17

SEM Image

Si

Fe

Na

Al

Ca

Ti

Mg

K

Cpx

200 µm

Figure 9. Element map of a small area of VA088 thin section. Light areas means high returns. Dark area means poor returns.

18

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).

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.

20

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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