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Home , Clarence Dutton, Markagunt Plateau, Marysvale Volcanic Field

Pyroclastic of the Johnson Valley Reservoir

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 (andesine). Other phenocryst types are: clinopyroxene, titanomagnetite, (sometimes altered to iddingsite), ilmenite and apatite. Chemical data relates the J.V.R. andesite to other nearby alkali-rich volcanic rocks.

Keywords , , 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 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 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, flows and pyroclastic deposits are exposed in the southern , northern Markagunt Plateau, and southern Sevier Plateau

(Anderson et al., 1975). Moreover, the Fish Lake volcanics are the easternmost extrusive 2

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, and Loa. The Fish Lake Plateau (green) lies on the east- Basin and Range ern margin of the High La Sal Plateaus and the north- Mtns. 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 Colorado Plateau modified from 39° figures obtained Richfield Salina from Lehi F. Hintz’s Geologic History of Utah, the introductory Wasatch Plateau figure of Pavant Range Callaghan 1939 Fish Lake Plateau and data from the 38° Sevier Plateau Thousand Utah Geological Lake Mtn Survey official website (online).

Tushar Mtns

Awapa Plateau Beaver Loa 37° 114° 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

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 (including the Mount Belknap Rhyolite) and . 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 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 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 unit and a 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 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 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 , the largest of which is occupied by Fish Lake proper. A second set of NNW-SSE trending lineaments suggests another 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 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

S

e

v

e n

M i le

C r e e 38 37.5' N k

Fish Lake x JVR Hightop

F r e m

o n t e k a L h is F Mytoge R i v Mountains e r

38 30' N 0 5 N kilometers 111 45' W 111 37.5' W 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 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 Marvine Peak, 11610 ft N

200 ft

7 6 ? 5 5 4 3 2 breccia 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

GEOLOGIC MAP OF THE FISH LAKE PLATEAU, UTAH Figure 4. Geologic Map of the Fish Lake Plateau. Mapped is Tlc Geologic Contacts the Fish Lake USGS 7.5’ quadrangle and portions of the Loa and Tjvr Tjvr Stratigraphic, volcanic Forsyth Reservoir quadrangles. Authors Christopher M. Bailey, A Qs or erosional Caroline Webber and 2005 NSF Fish Lake REU. Tlc Tpc 4 5 , Fault not mapped Tlc Geologic Units Tjvr (dashed beneath Tlc Tjvr Tjvr Qs Surficial Deposits- includes Fremont River 2 1 surficial deposits) terraces, glacial till and outwash, debris flow fans, To Tlc lacustrine and alluvial deposits, and selected Tlc colluvium. Tjvr Tlc Tb Olivine Basalt

Tsc Sandstone and Conglomerate- poorly to Tlc Qs moderately cemented, well-bedded sedimentary Tlc rocks in the Fremont River valley Tpc Tlc Tjvr Tlc To Osiris Dacite- phenocryst-rich, biotite and Qs plagioclase bearing dacite Tjvr Qs Qs B Tlc Tlc Tjvr Tpc Pelican Canyon Andesite- phenocryst-poor, Tsc Tf spherulite-rich, variably vesiculated andesite Qs Tlc Tlc Tlc Lake Creek Andesite- phenocryst-poor, Qs glass-rich andesite Tlc Tjvr Tjvr Tjvr Johnson Valley Reservoir Andesite- Tf phenocryst-rich, plagioclase and pyroxene bearing andesite A’ Qs Tf Flagstaff Formation- siltstone, sandstone, and 3 limestone with a few layers containing chert and Qs quartzite clasts (to 10 cm) Tlc Tjvr Tlc Tlc Tlc Tjvr Map Sample ID Locale UTM Coordinates Qs Tsc Qs Type Location JVR check dam N4273565, E445075 1 Tjvr To 2 GMR148 Frying Pan Hill N4273780, E442780 Tb Tlc To To 3 GMR077 Mytoge Scarp N4265920, E438890 Qs 0 3 Tlc 4 GMR122 Mt. Marvine N4280090, E444240 6 miles N To 5 GMR021 Mt. Terrill N4283210, E443770 0 5 Tjvr 6 VA088 Mytoge Slope N4260660, E439051 kilometers B’ 12

640 Figure 5. Stratigraphic column Osiris Dacite 620 valid for the Fish Lake USGS Pelican 7.5’ quadrangle. This figure 560 Canyon shows a universal thinning of the Andesite 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. Lake Creek Andesite is >220 m thick Andesite 310 throughout the quadrangle, but 290 300 thins away quickly at the SE margin of the plateau near Rabbit

Thickness (m) 220 Valley. The Osiris Dacite is of variable thickness (0-24 m) throughout the study area, J.V.R. Andesite suggesting it was deposited over subdued topography. The thick- ness of the Flagstaff Limestone is unknown. 0 0 Flagstaff Limestone Figure 6. Cross sections A-A’ and B-B’. Scale 1:1. NW SE

A Tpc A' Tlc 12,000 Tlc 12,000 Tlc 11,000 Tlc Tlc 11,000 10,000 Tlc Tsc Tjvr 10,000 Tjvr Tjvr 9000 9000 Tjvr Tjvr Tjvr 8000 Tf Tjvr 8000 Tf Tf Tf Tf Tf 7000 7000 Elevation (ft.) ? ? ? 6000 6000 B B' 12,000 12,000 11,000 Tlc Tlc Fish Lake Tlc Tlc Tjvr 11,000 10,000 Tlc To 10,000 Tlc 9000 Tjvr Tjvr 9000 8000 Tjvr Tjvr 8000 Tf 7000 7000

Elevation (ft.) ? ? ? 6000 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.

Type Sample 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

12 Johnson Valley Reservoir Andesite

Lake Creek Andesite Trachyte 10 Basaltic Andesite of Mattox 1991 Latite of Mattox 1991 Trachyandesite

Lava Flows of Riley , 8 Nelson 1989 Basaltic Lava Flows of Deer Spring Draw, Nelson 1989 Trachyandesite Dacite 6 Alk

4 Basalt Figure 7. IUGS plot for Basaltic Andesite extrusive volcanics. Fish Lake volcanics are J.V.R. Andesite Andesite and Lake Creek Andesite of Webber. Basaltic andesite and 2 latite of Mattox, lava flows of Riley Spring and Deer Spring Draw of Nelson also plotted. 15 45 50 55 60 65 70 SiO2 20 8 Al2O3 CaO 19 6

18 4 17

2 16

55 60 65 70 55 60 65 70 10 5 FeO MgO J.V.R. Andesite

8 4 Lake Creek Andesite Basaltic And. of Mattox Latite of Mattox 6 3 Lava of Riley Spring Lava of Deer Spring Draw 4 2 Figure 8 (cont. on following page). Harker variation 2 diagram for the units plotted 1 in Figure 7. 16

55 60 65 70 55 60 65 70 1.4 8 Figure 8, cont. TiO2 K2O 1.2 6 1

0.8 4 0.6

0.4 2

0.2

55 60 65 70 55 60 65 70 5 0.6 Na2O P2O5 4

0.4 3

2 0.2

1 17

55 60 65 70 55 60 65 70 Cpx Si Ti

SEM Image 200 µm

Al Fe Mg

Ca Na K 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 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 . 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

References Cited

Anderson, J. J., Rowley, P. D., Fleck, R. J., and Nairn, A. E. M., 1975, Cenozoic geology of southwestern High Plateaus of Utah. Callaghan, E., 1939, Volcanic sequence in the Marysvale region in southwest-central Utah: Transactions - American Geophysical Union, v. Part 3., p. 438-452. Cunningham, C. G., Steven, T. A., Rowley, P. D., Naeser, C. W., Mehnert, H. H., Hedge, C. E., and Ludwig, K. R., 1994, Evolution of volcanic rocks and associated ore deposits in the Marysvale volcanic field, Utah: Economic Geology and the Bulletin of the Society of Economic Geologists, v. 89, no. 8, p. 2003-2005. Dutton, C. E., 1880, Report on the geology of the high plateaus of Utah. Fisher, R. V., 1984, Pyroclastic Rocks: Berlin, Heidelberg, New York, Tokyo, Springer-Verlag, 472 p. Fleck, R. J., Anderson, J. J., and Rowley, P. D., 1975, Chronology of mid-Tertiary volcanism in High Plateaus region of Utah. Hardy, C. T., and Muessig, S. J., 1952, Glaciation and drainage changes in the Fish Lake Plateau, Utah: Geological Society of America Bulletin, v. 63, no. 11, p. 1109-1116. Hintz, L. F., Geologic History of Utah, Brigham Young University, 181 p. Mattox, S. R., 1991, Petrology, age, geochemistry, and correlation of the Tertiary volcanic rocks of the Awapa Plateau, Garfield, Piute, and Wayne counties, Utah. -, 2001, Geologic map of the Moroni Peak Quadrangle, Wayne County, Utah. McGookey, D. P., 1960, Early Tertiary stratigraphy of part of central Utah: Bulletin of the American Association of Petroleum Geologists, v. 44, no. 5, p. 589-615. Nelson, S. T., 1989, Geologic map of the Geyser Peak Quadrangle, Wayne and Sevier counties, Utah. Steven, T. A., Rowley, P. D., Cunningham, C. G., and Anonymous, 1983, of the Marysvale volcanic field, west central Utah, in Symposium on Calderas and related volcanic rocks; the Krakatau centennial; a part of the American Geophysical Union, 1983 fall meeting. Webber, C. E., Bailey, Charles M., 2003, [Undergraduate thesis]: The College of William and Mary. Williams, P. L., and Hackman, R. J., 1971, Geology of the Salina Quadrangle, Utah.