40Ar/39Ar Ages, Compositions, and Likely Source of the Eocene Fallout

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2017-11-01 40Ar/39Ar Ages, Compositions, and Likely Source of the Eocene Fallout Tuffs in the Duchesne River Formation, Northeastern Utah Michael Seth Jensen Brigham Young University

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This Thesis is brought to you for free and open access by BYU ScholarsArchive. It has been accepted for inclusion in All Theses and Dissertations by an authorized administrator of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. 40Ar/39Ar Ages, Compositions, and Likely Source of the Eocene Fallout Tuffs in the Duchesne River Formation, Northeastern Utah

Michael Seth Jensen

A thesis submitted to the faculty of Brigham Young University in partial fulfillment of the requirements for the degree of

Master of Science

Bart J. Kowallis, Chair Eric H. Christiansen Jeffrey D. Keith

Department of Geological Sciences

Brigham Young University

40Ar/39Ar Ages, Compositions, and Likely Source of the Eocene Fallout Tuffs in the Duchesne River Formation, Northeastern Utah

Michael Seth Jensen Department of Geological Sciences, BYU Master of Science

Thin fallout tuffs in the Duchesne River Formation in the Uinta Basin, Utah are evidence that volcanism was active in northern Nevada and Utah in the late Eocene. The Uinta Basin is a sedimentary basin that formed during the Laramide orogeny. Ponded lakes of various salinity filled and emptied and during the late Eocene the northern rim was dominated by a wetland/floodplain depositional setting. Most of the tuffs have rhyolitic mineral assemblages including quartz, biotite, sanidine, and allanite. Rhyolitic glass shards were also found in one of the ash beds. Biotite compositions have Fe/(Fe+Mg) ratios typical of calc-alkaline igneous rocks and clusters of biotite compositions suggest 3 or 4 volcanic events. Sanidine compositions from five samples grouped at Or73 and Or79. Only one sample had plagioclase with compositions ranging between An22 - An49. Some beds also contained accessory phases of titanite, apatite, and zircon. Whole rock compositions of the altered volcanic ash beds indicate these tuffs underwent post-emplacement argillic alteration, typical of a wetland/floodplain depositional setting. Immobile element ratios and abundances, such as Zr/Nb and Y are typical of a subduction zone tectonic setting and rhyolitic composition. 40Ar/39Ar ages constrain the timing of volcanism. One plagioclase and one sanidine separate from two different tuff beds yielded ages of 39.47 ± 0.16 Ma and 39.36± 0.15 Ma respectively. These dates, along with the compositional data seem to limit the eruptive source for these fallout tuffs to the northeast Nevada volcanic field. These new ages, along with previously published ages in the Bishop Conglomerate which unconformably overlies the Duchesne River Formation, constrain the timing of two uplift periods of the Uinta Mountains at 39 Ma and 34 Ma. Finally, the ages also date the fauna of the Duchesnean Land Mammal Age to be about 39.4 Ma as opposed to less precise earlier estimates that placed it between 42 and 33 Ma.

Keywords: fallout tuffs, 40Ar/39Ar ages, laramide orogeny, vernal northwest quadrangle, northeast Nevada volcanic field, Duchesnean Land Mammal Age ACKOWLEDGEMENTS

I would first like to acknowledge Bart J. Kowallis, the Chair of my graduate committee,

for all his help and time spent discussing ideas and interpreting data. Without his guidance this project would not have been a success. I also thank and acknowledge Eric H. Christiansen for sharing his expertise about the volcanic history of western North America and for providing

Excel spreadsheets to help analyze and interpret data about the mineral and whole rock compositions. I also acknowledge the assistance of Michael J. Dorais, who helped calibrate and repair the microprobe at BYU so that I could obtain reliable data from mineral grains that were

sometimes weathered.

Doug Sprinkel, Senior Geologist from the Utah Geological Survey, introduced me to the field area, provided background information, and was always available to discuss ideas.

Brian Jicha at the Geochronology lab of the University of Wisconsin-Madison performed the 40Ar/39Ar dating and provided probability-density plots of the age data.

I also acknowledge fellow graduate student Casey Webb, who assisted with the sample

preparation of the volcanic ash, helped with sample collection, and spent many hours with me in

the field.

Finally, I thank my wife, Madeline. She has supported and encouraged me throughout

this entire process and always pushed me to do my best work.

Funding for the project was provided by Brigham Young University. TABLE OF CONTENTS

TABLE OF CONTENTS ...... iv LIST OF FIGURES ...... v LIST OF TABLES ...... vi INTRODUCTION ...... 1 GEOLOGIC SETTING ...... 3 Laramide Orogeny ...... 3 Duchesne River Formation ...... 5 SAMPLING AND ANALYTICAL METHODS ...... 7 ISOTOPIC AGES ...... 9 MINERAL ASSEMBLAGES AND COMPOSITIONS ...... 10 Quartz ...... 11 Biotite ...... 11 Feldspar ...... 15 Allanite ...... 16 Titanite ...... 18 Glass Shards ...... 19 Apatite ...... 19 WHOLE ROCK COMPOSITION ...... 19 Immobile Elements ...... 19 Argillic Alteration...... 21 DISCUSSION ...... 22 Eruptive Source of the Duchesne River Formation Tuffs ...... 22 Timing of Uinta Mountain Uplift ...... 26 Duchesnean Land Mammal Age ...... 27 CONCLUSIONS...... 28 REFERENCES CITED ...... 30 APPENDIX 1 ...... 99 APPENDIX 2 ...... 103

LIST OF FIGURES

Figure 1: Regional map and simplified geolgioc map of Vernal northwest quadrangle...... 38 Figure 2: Stratigraphic column of mapping units...... 39 Figure 3: Startigraphic diagram of Dry Gulch Creek and Lapoint members...... 40 Figure 4: Field photographs of tuffs...... 41 Figure 5: Individual 40Ar/39Ar dates...... 42 Figure 6: Total Al vs Fe/(Fe+Mg)...... 43 Figure 7: Discrimination diagram of K and Al in biotite...... 44 Figure 8: Analytical totals for Duchesne River Formation biotite ...... 45 Figure 9: Constellation plot of biotite analyses...... 46 Figure 10: Scatterbox matrix of biotite analyses ...... 47 Figure 11: Discrimination diagram of Mg and Fe in biotite...... 48 Figure 12: Biotite eruption temperatures...... 49 Figure 13: Log(XMg/XFe) vs. log(XF/XOH)...... 50 Figure 14: Discrimination diagram of F and Cl in biotite...... 51 Figure 15: Ternary diagram of feldspar compositions...... 52 Figure 16: Discrimination diagram of Ba and K in sanidine...... 53 Figure 17: Allanite compostions...... 54 Figure 18: Titanite compositions...... 55 Figure 19: Light rare earth elements in allanite and titanite...... 56 Figure 20: Compositions of glass shards...... 57 Figure 21: Immobile element diagram...... 58 Figure 22: Trace element diagram...... 59 Figure 23: Normalized trace element concentrations...... 60 Figure 24: Two element alteration diagrams...... 61 Figure 25: Isotopic ages and geologic events timeline...... 63 Figure 26: NAVDAT dactie-rhyolite regional map ...... 64

LIST OF TABLES

Table 1: Characteristics of tuffs from the Duchesne River Formation ...... 66 Table 2: Parameters for electron-microprobe analyses ...... 67 Table 3: Selected history of isotopic Dating of the Duchesne River Formation ...... 67 Table 4: List of events, ages, and references used in Figure 25 ...... 68 Table 5: X-ray fluorescence analyses of altered tuffs from the Duchesne River Formation ...... 70 Table 6: Compositions of Duchesne River Formation Biotite Phenocrysts ...... 72 Table 7: Compositions of Duchesne River Formation Sanidine Phenocrysts ...... 81 Table 8: Compositions of Duchesne River Formation Plagioclase Phenocrysts ...... 83 Table 9: Compositions of Duchesne River Formation Allanite Phenocrysts ...... 86 Table 10: Compositions of Duchesne River Formation Titanite Phenocrysts ...... 92 Table 11: Compositions of Glass Shards from DRF-A of the Duchesne River Formation ...... 97 Table 12: Compositions of Apatite Phenocrysts from the Duchesne River Formation ...... 98

INTRODUCTION

Preserved volcanic ash in sedimentary rock sections can provide insights into the

geologic history of a region by giving radiometric ages and geochemical data on volcanic events

(Hildreth and Wilson, 2007; Bogaard and Schirnick, 1995; Dalrymple et al., 1965) leading to better interpretations of tectonic settings (Shervais, 1982; Huff et al., 1992 Blaylock, 1998; Huff,

2000; Chandler, 2005; Smith et al., 2014; Christiansen et al., 2015), better age constraints on sedimentary rocks and fossils (Riggs et al., 2003; Kowallis et al., 1998, 2001; Sageman et al.,

2006; Meyers et al., 2012; Smith and Carrol, 2015) lead to improved inferences about paleogeography and paleoclimate (Chamberlain et al., 2011; Feng et al., 2013; Mulch et al.,

2015), and be used as important stratigraphic markers for correlation purposes (Huff, 2016).

We employ some of these techniques on the fallout tuffs from the Duchesne River

Formation in the Uinta Basin of eastern Utah to interpret the age and origin of an enigmatic part of the western Cordillera of the United States. Some interpretations regarding the tuff beds within the Duchesne River Formation have been published by previous workers (Anderson and

Picard, 1974; Bryant et al., 1989; Sprinkel, 2007). Imprecise dating techniques and alteration of the volcanic ash has made understanding their origin and nature difficult. We have applied improved dating techniques and more detailed chemical analysis of these altered ash beds and their preserved phenocrysts to help better understand their origin and history. Previous work in the overlying Bishop Conglomerate, which also contains altered fallout tuffs, has shown that more accurate and precise 40Ar/39Ar dating techniques and detailed chemical compositions can

be used to constrain the timing, tectonic setting, and sources of altered middle Cenozoic tuffs

(Kowallis et al., 2005). In this study, we build on the work of previous investigators and report

new 40Ar/39Ar ages for the tuff beds, along with their major- and trace-element compositions, phenocryst assemblages, and phenocryst chemistry to answer the following questions:

1) What is the age of the fallout tuffs within the Duchesne River Formation?

2) What phenocrysts are preserved within the Duchesne River Formation tuffs and what

are their compositions?

3) What are the major and trace element compositions of the Duchesne River Formation

tuffs? How has their composition been affected by alteration?

4) Where are the eruptive sources of the tuffs? Do they come from one source area or

were multiple areas active at the same time?

5) What is the age of the fauna within the Duchesnean Land Mammal Age?

6) Do the tuffs help constrain periods of uplift of the Uinta Mountains?

The weathering and alteration of the tuffs in the Duchesne River Formation add an additional complication to answering some of these questions. Through diagenesis of the glassy, silicic ash, and removal of soluble elements, the altered volcanic ash has lost its original composition. However, despite diagenetic alteration of the ash, compositions of phenocrysts that

resisted weathering can be used to make inferences about the original composition (Lund-Snee et

al, 2015). Additionally, Summa and Verosub (1992), Winchester and Floyd (1997), and Zhou et

al. (2000) have shown that ratios of immobile trace elements including REEs in rhyolitic ash can

be used to infer original magma compositions. The preservation of mineral compositions and

immobile elements permits the use of altered volcanic ash in this study.

GEOLOGIC SETTING

Laramide Orogeny

The Laramide Orogeny was characterized by basement block uplift along high angle

reverse faults due to shallow subduction of the Farollon slab (Dickinson et al., 1988; Liu et al.,

2010; Jones, 2011; Fan, 2014; Yonkee and Weil, 2015). The EW-trending Uinta Mountain range

is one such basement block, and has experienced multiple periods of uplift and erosion

(Hamilton, 1978; Hansen, 1984; Dickinson, 1988). These cycles are recorded by the sediments in

the adjacent Uinta Basin (Figure 1), including the Green River Formation, Uinta Formation, and

the Duchesne River Formation. Ponded basins such as the Uinta Basin, were common during the

Laramide and the basins were often filled with fresh and salt water lakes (Carroll and Bohacs,

1999; Tanavsuuu-Milkeviciene et al., 2017) which acted as efficient sediment traps. The

Paleocene-Eocene sedimentary deposits in the Uinta Basin are roughly 5000 m thick near the

center of the basin and fluctuate from deep lacustrine to fluvial deposits (Hintze and Kowallis,

2009). The lakes and fluvial environments were also efficient traps for erupted volcanic material,

and fallout tuffs have been found in many of the formations throughout the strata within the

Uinta Basin and date the timing of tectonic and sedimentary processes (Remy, 1992; Smith et al.,

2003; Smith and Carroll, 2015).

As the Laramide came to an end in western North America around 40-45 Ma (Dickinson,

1988) and a cooler, dryer climate replaced the warm, wet climate of the Eocene (Jicha et al.,

2009), the lakes dried up and fluvial and alluvial processes dominated (Anderson and Picard,

1974). Even after the end of the Laramide orogeny in other parts of North America however, the

Uinta Mountains continued to experience uplift. The ages from the tuffs within the Duchesne

River Formation help constrain the timing of late Eocene uplift of the Uinta Mountains to shortly

before a period of tectonic quiescence and erosion and/or non-deposition indicated by the Gilbert

Peak Erosion Surface (Sprinkel, 2014). The overlying Bishop Conglomerate possibly records the

final phase of Laramide deformation in the Uinta Mountains (Kowallis, 2005).

The existence of fallout tuffs from the time of the Laramide Orogeny is noteworthy

because volcanism was not common in western North America during the orogeny due to

shallow subduction of the Farallon Plate (DeCelles, 1994; Dickinson, 2004; Schellart, et al.,

2010). This is analogous to the Andean volcanic belt of western South America, which is

punctuated with volcanic gaps due to shallow subduction of the Pacific Plate (Gutshcer et al.,

2000; Martinod et al., 2010). The tuffs within the Duchesne River Formation likely document the

transition from minimal volcanic activity in the northern Utah/Nevada region during the Eocene,

to widespread, intense volcanism in the Oligocene as a consequence of the southward migration

of slab rollback and the restoration of hot asthenosphere beneath the continental lithosphere

(Humphreys, 1995, Norman and Mertzman, 1991; and Suahyah and Rogers, 1991).

After the magmatic lull, volcanism resumed in present day Montana and Idaho about 54

Ma then migrated south into the Nevada-Utah region around 40 Ma and finally reached the southern Great Basin about 20 Ma (Lipman et al., 1972; Humphreys, 1995, Castor et al. 2000,

Castor et al., 2003, Best and Christiansen, 2013). Volcanic activity during this time period was so intense it has been called the middle Cenozoic ignimbrite flare-up and the Challis, Absaroka, northern Nevada-Utah, and the central and southern Great Basin volcanic fields document this time of intense volcanic activity (Brooks, 1995; Henry et al., 2003; Chandler, 2005; Best and

Christiansen, 2016). Silicic volcanic rocks from this time period typically range from dacite to rhyolite in composition and are commonly preserved as fall-out tuffs, ash-flow tuffs, flow breccias, and silicic domes (Lipman et al, 1972; Brooks et al, 1995). Like other fall out tuffs, the

Duchesne River Formation tuffs were likely sourced from one or more large plinian or

coignimbrite eruptions (Blaylock, 1998; Chandler, 2005; Kowallis et al, 2005; Chrisitansen et al,

2015).

Duchesne River Formation

The study area for this project is the Vernal Northwest quadrangle, which lies along the

south flank of the Uinta Mountains and is just west of Vernal, Utah (Figure 1). Along with

Cretaceous units, the predominant formation within the quadrangle is the Duchesne River

Formation and all four members, Brenan Basin, Dry Gulch Creek, Lapoint, and Starr Flat are

exposed (Figure 2). The Dry Gulch Creek and Lapoint members of the Duchesne River

Formation are well exposed and contain numerous tuffaceous beds. These two members are

predominantly siltstone and mudstone with intertonguing beds of fine to medium grained

sandstone and conglomerate. The oldest and youngest members, the Brenan Basin Member and

the Starr Flat Member, are coarse grained conglomerates indicating a close proximity to the

uplifting Uinta Mountain front. The Starr Flat Member is capped by the Gilbert Peak erosion

surface (Sprinkel, 2007; Webb, 2017). The fining and thinning of coarse-grained beds to the

south, along with paleo-current directions, indicate that the dominate flow direction was to the

south, away from the mountain front and to the central part of the Uinta Basin (Sato and Chan,

2015; Webb, 2017). The stratigraphy and distribution of these units within the quadrangle are

reported in greater detail in a companion study (Webb, 2017).

In this study, tuff samples were collected from beds in the Dry Gulch Creek Member and

the Lapoint Member (Figure 3). The Dry Gulch Creek and Lapoint members were deposited in a

wetland/fluvial setting (Anderson and Picard, 1974) and allowed for the preservation of fallout

ash deposits. Since deposition, the glassy matrices of the ash beds have been mostly altered to

5 clay minerals (Anderson and Picard, 1974). The tuff beds are light to medium grey and stand out against the red to reddish-brown siltstone (Figure 4). Several tuff beds are laterally extensive throughout much of the quadrangle and one bed is used as the contact between the Lapoint and

Dry Gulch Creek members (Webb, 2017). Other beds that were not as laterally continuous show signs of possible reworking by streams and contain detrital grains. These non-laterally continuous beds may have been deposited in paleo-lows such as streams and ponds and preserved only in small areas where they were contaminated by other mineral grains carried by the streams.

Although we found no tuffs within the Brenan Basin Member in the Vernal NW quadrangle, there are documented locations within the nearby Ice Cave Peak quadrangle (Figure

1) where tuffs are present (Sprinkel, 2007; Poduska, 2015). In contrast to the siltstone and sandstone of the Brennan Basin Member in the Ice Cave Peak quadrangle, much of the Brenan

Basin within the Vernal Northwest quadrangle is a limestone-clast conglomerate, likely deposited in an alluvial fan setting proximal to the uplifting Uinta Mountains. It is possible that tuffs were deposited during Brenan Basin time but were subsequently eroded in the high-energy environment of the alluvial fan. The youngest member of the Duchesne River Formation, the

Starr Flat Member, is likewise is a predominantly conglomeratic unit in the Vernal NW quadrangle and no volcanic material was found.

Additionally, the Duchesne River Formation contains key mammal fossils and is used as the type section of the Duchesnean Land Mammal Age (Wood et al., 1941, Clark et al., 1967).

The Duchesnean Land Mammal Age is used throughout North America and the fauna within that time period are used to better understand the evolution of animals and climate in North America

during the Late Eocene (Emry, 1981; Rassmussen et al., 1999). More accurate and precise

isotopic dating better constrains the absolute age of these important fauna.

SAMPLING AND ANALYTICAL METHODS

Samples of the tuffs within the Duchesne River Formation were collected in the fall of

2015 and summer of 2016. Seven samples were collected from the Lapoint Member and four were collected from the upper and middle Dry Gulch Creek Member (Table 1). Roughly 5-8 kilograms of tuffaceous material was collected at each site by digging into the slope to expose the tuff to collect material that was less contaminated from sediments above and below.

A split of each sample was processed to liberate the mineral phenocrysts. These splits

were crushed in a roller mill and then soaked and scrubbed in water to remove the clay. The

remaining material was then sieved, collecting anything between 60-120 microns. This fraction

was then ultrasonically washed in a bath of distilled water until all of the clay was removed. Two

samples were also washed in a bath of 10% hydrochloric acid to remove carbonate. Once

cleaned, the grains were separated using standard magnetic and heavy liquid techniques.

Residual mineral assemblages of each sample are shown in Table 1.

After the grains were sufficiently clean and separated, individual grains of biotite,

sanidine, plagioclase, allanite, titanite, apatite, glass, and zircon were selected by hand, cast in

epoxy, and polished for analysis using an electron microprobe. Electron microprobe analyses

were done using a Cameca SX-50 at Brigham Young University. A 15 kv acceleration voltage,

20 nA beam current, and 5 micron beam size were used to analyze biotite, feldspar, and allanite.

A 15 kv acceleration voltage, a beam current of 10 nA, and a beam size of 5 microns were used

for glass and apatite analyses to prevent migration of volatiles during analysis. A 15 kv

7 acceleration voltage, 30 nA beam current, and 10 micron beam size was used on titanite.

Standards and conditions used for all the minerals are summarized in Table 2.

In order to determine the major and trace element compositions of the tuffs, 100 grams of whole rock was taken from each sample and powdered in a tungsten-carbide shatter box for two minutes and then dried for 2 days in an oven at 100 °C. Glass discs and pressed powder pellets were made for major and trace element analysis using a Rigaku ZSX Primus II wavelength dispersive x-ray fluorescence spectrometer. Natural rock powders were employed as calibration standards. BHVO-2 and RGM-1 were used as monitors to assess data quality. For a complete list of elements analyzed, results, and uncertainties see Table 4. Half of the samples reported loss on ignition at 1000°C higher than 10 wt. %. This is mostly due to water trapped in the clay minerals.

Four samples also had abnormally high CaO concentrations (8-16 wt %), likely a result of leaching of marl cements from surrounding limestone conglomerate beds into the tuffs. The added calcite also increases the LOI as CO2 is released during ignition of the powder.

Two hundred sanidine grains from sample DRF-H and two hundred plagioclase grains from DRF-A were hand-picked and radiometrically dated by Dr. Brian Jicha at the University of

Wisconsin-Madison WiscAr Geochronology Lab. They were dated using single crystal 40Ar/39Ar laser fusion methods following the methods described in Kuiper, (2008). One of the benefits of the single crystal method is that anomalously old or young grains can be identified and removed from a weighted average and a more accurate eruption age can be determined. Both samples contained detrital feldspar much older than the Duchesne River Formation tuffs so these grains were not used to calculate ages. One of the drawbacks of single crystal analysis however is that because the crystals and argon signal sizes are small, the analytical uncertainties are higher than

they would be using the multi-crystal fusion method; consequently, the analytical errors are on the order of 100,000 years rather than 10,000 years.

Sanidine from DRF-H was dated using a MAP 215-50 mass spectrometer and a standard

Fish Canyon sanidine age of 28.201± 0.0460 Ma from Kuiper et al., (2008), and a J-value of

0.0078199 ± 0.0000055. Plagioclase from DRF-A was dated using Noblesse 5-collector mass

spectrometer and the same age standard and J-value. Different instruments were used because the

sanidine separate contained numerous detrital grains of Jurassic age (Brian Jicha, personal

communication). These old grains are probably microcline and were mistaken for sanidine when

hand-picked and sent in for dating. They are considered to be detrital. A summary of all the ages

and the analytical parameters is given in Appendix 2.

ISOTOPIC AGES

Several isotopic ages have previously been determined from the prominent tuff bed in the

Lapoint Member of the Duchesne River Formation (Table 3). A fairly consistent K-Ar and

40Ar/39Ar biotite age of 41 ± 0.5 Ma has been ascertained for tuff beds in the basal part of the

Lapoint Member from McDowell et al. (1973), Anderson and Picard, (1974), Prothero and

Swisher, (1992), and Kowallis and Sprinkel, (2007), whereas fission track ages from the same

bed in the Lapoint Member averaged 34± 1-3 Ma (Bryant et al., 1989). A biotite K-Ar age of

37.6 ± 1.4 Ma and a zircon fission track age of 37.6 Ma ± 1.4 Ma were obtained from a tuff within the younger Starr Flat Member by Bryant et al. (1989). The previous K-Ar and fission track ages from the Duchesne River Formation all have large errors and biotite may be altered with low K or excess Ar giving erroneous ages (Smith et al, 2008). Samples were collected from some of same ash beds from the Lapoint Member as previous workers in an attempt to procure new, more accurate and precise ages. We chose feldspar because it is more resistant than biotite

to alteration and typically provides more accurate ages. Kowallis et al. (2005) dated tuffs from

the overlying Bishop Conglomerate using sanidine and obtained 40Ar/39Ar ages of 30.5 ± 0.2 Ma

at the top and 34.0 ± 0.1 Ma near the bottom of the formation.

Sanidine extracted from DRF-H gave an 40Ar/39Ar age of 39.36 ± 0.15 Ma and

plagioclase from DRF-A gave an age of 39.47 ± 0.16 Ma (Figure 5). DRF-H was taken from a

tuff bed at the top of the Dry Gulch Creek Member, only 2-3 meters from the contact with the

Lapoint Member. DRF-A was taken from a tuff bed within the overlying Lapoint Member

several kilometers northeast of DRF-H where the Lapoint Member pinches out between the Starr

Flat Member and Brenan Basin Member (Figure 1). Though not as precise as the Bishop

Conglomerate ages, the analytical uncertainties for these new ages are much smaller than

previous ones and constrain the boundary between the Lapoint Member and Dry Gulch Creek

Member to about 39.4 Ma. The ~39.4 Ma ages are significantly younger than the previously

reported 41 Ma ages from the same series of tuff beds (Anderson and Picard, 1974; McDowell et

al., 1974; Prothero and Swisher, 1992; Kowallis and Sprinkel, 2007). DRF-A and DRF-H are not

from the same ash bed based on chemical and mineralogical differences and field mapping, but the analytical uncertainties for these ages overlap and are statistically indistinguishable, meaning

the ashes must be close stratigraphically. This is significant because sample DRF-A represents a

tuff bed that was initially believed to be part of the Starr Flat Member (Sprinkel, 2007) but our

recent mapping at 1:24,000 scale (Webb, 2017) and the new age suggest that the tuff bed is

from the lower part of the Lapoint Member.

MINERAL ASSEMBLAGES AND COMPOSITIONS

In addition to providing ages, magmatic phenocrysts can provide information about the composition of volcanic ash, its correlation with other outcrops, and insight into the pre-eruptive 10 conditions and tectonic setting. Table 1 summarizes the mineral assemblage of each of the samples collected from the Duchesne River Formation. The volcanic mineral phases include quartz, found in 11 samples, biotite in 11 samples, sanidine in five samples, allanite in four samples, and plagioclase and glass shards were found in one sample. The presence of quartz, biotite, and K-feldspar and absence of pyroxene and hornblende is a common subduction–related rhyolitic mineral assemblage and is typical of other volcanic ash beds from the Late Eocene of western North America (Lipman, 1972). Euhedral grains of zircon were found in three samples.

Titanite and apatite were the least abundant minerals and were only found in one sample each.

Because these ashes were deposited in a dominantly fluvial environment, there is an inherent risk of contamination by detrital grains and all of the samples contained sedimentary grains of quartzite, carbonate, and clay. Five samples also contained microcline much older than the ash beds and likely a detrital component derived from the Uinta Mountains.

Quartz

Quartz grains were present in all of the Duchesne River Formation samples. Igneous quartz grains were identified by their prismatic, euhedral shape, and presence of still glassy melt inclusions. Detrital quartz grains were rounded, frosted, lacked melt inclusions, and tended to be larger than the volcanic quartz grains.

Biotite

Biotite phenocrysts were present in all of the Duchesne River Formation tuffs and are the only mafic phase represented. In several samples, abundant biotite was visible in the field without the aid of a hand lens. In general, biotite is less resistant to the effects of weathering than sanidine, quartz, and allanite and the biotite grains showed signs of alteration with many grains

being rounded and light brown in color; however, no evidence of chloritic alteration was found

in the polished grain mounts. This damage is a concern because K can be replaced with H2O and

Cl during diagenesis (Bisdom et al, 1982, Smith et al, 2008). These chemical changes lead to low

analytical totals and the resulting isotopic ages cannot be considered accurate (Smith et al.,

2008). In order to avoid erroneous analyses, only black, euhedral grains were picked for electron

microprobe analysis and from those, only those with totals (+ water) of 90% or above were

considered. For a complete list of analytical results see Table 6.

Al, Fe, and Mg in biotite are typically fairly immobile and may help in understanding the

original composition of the tuff beds (Christiansen et al, 2015). Molar Fe/(Fe+Mg) and total Al

(apfu) plot within or around the calc-alkaline rhyolite field (Figure 6) and are similar to the

subduction–related Oligocene Fish Canyon Tuff and Jurassic tuffs from the Carmel, Temple Cap

and Morrison formations (Kowallis et al, 2001; Christiansen et al, 2015). Total Al plots between

1.2 and 1.5 apfu for biotite in seven of the samples, but biotite in DRF-C, DRF-D, DRF-F, and

DRF-G have total Al greater than 1.5 apfu (Figure 6). This enrichment of Al is typically considered a result of alteration as the more mobile elements like K and Na are removed and Al becomes relatively enriched (Zielinski, 1982; Christiansen et al, 2015). However, the samples with high Al also have K levels of at least 8 wt % (Figure 7), an indication that the biotite has undergone little alteration. Additionally, these samples have relatively high analytical totals compared to other Duchesne River Formation biotites (Figure 8); consequently we consider these to have magmatic compositions. DRF-D, DRF-F, and DRF-G, were collected from the top, middle, and bottom of the same thick ash bed at the same locality at the base of the Lapoint

Memeber (Figure 4), so it is not surprising that they all have similar Fe/Mg ratios and Al levels if they are unaltered.

The total Al vs Fe/(Mg+Fe) diagram (Fig. 6) also shows four distinct clusters of biotite

from the Duchesne River Formation tuffs. The clustering follows stratigraphic order with the

oldest middle Dry Gulch Creek samples, DRF-K and DRF-J, plotting together. The upper Dry

Gulch Creek samples DRF-H and DRF-I were taken from the same ash bed at two different localities and form an isolated cluster. These two groups have Fe/(Mg+Fe) ratios slightly higher than typical calc-alkaline rhyolites which could be an indication of a more evolved and differentiated melt or that crystallization occurred at a lower fO2 than typical calc-alkaline

2+ rhyolites. Low Fe /FeTotal ratios in allanite from DRF-I and DRF-H, discussed below, also suggest low fO2 conditions, indicating that the elevated Fe/(Mg+Fe) ratios are likely the result of

low oxygen fugacity rather than a highly evolved melt.

Samples DRF-D, DRF-F, and DRF-G are from different horizons within the same prominent ash bed at the same location (Appendix 1) and form the high Al group at the base of

the Lapoint Member. Five biotite grains from DRF-D have lower total Al and Fe/(Fe+Mg) ratios and are represented by DRF-D(b) in Figure 9. These grains have good analytical totals so their anomalous compositions could be an indication of a detrital component rather than alteration.

DRF-D was collected at the top of the prominent ash bed (Appendix 1) and could have experienced reworking in the floodplain depositional environment. DRF-C represents a tuff bed about 10 meters above the prominent ash bed and also lies in the high Al group. Finally, the middle Lapoint samples, DRF-B and DRF-A form a loose cluster with similar Fe/(Fe+Mg) ratios within the range of typical calc-alkaline rhyolites. Allanites from DRF-A have slightly higher

2+ calculated Fe /FeTotal ratios compared to DRF-H and DRF-I and could explain the lower

Fe/(Fe+Mg) ratios.

In order to statistically verify that the groups plotted in the total Al vs Fe/(Fe+Mg) diagram are significant, a hierarchical cluster analysis was run using all the elements analyzed in biotite according to Ward’s minimum variance method (Ward, 1963) using the JMP software. A

constellation plot (Figure 9) and scatter plot matrix (Figure10) show that the same samples

grouped together in the total Al diagram are also grouped together when TiO2, Al2O3, FeOt,

MgO, Na2O, K2O, F, and Cl are all considered. These four clusters of biotite compositions are

grouped stratigraphically which suggests that the composition of the magmatic source of the

Duchesne River Formation tuffs changed over time. For example, Fe drops from 1.8 to 1.0 apfu,

while Mg increases from 1.0 to 1.7 apfu (Figure 11) from the lower Dry Gulch Creek Member

tuffs to the mid Lapoint Member tuffs.

Biotite compositions are also useful in geothermometry (Luhr et al., 1984). Titanium is a

temperature sensitive element in biotite and increases with increasing temperature (Figure 12).

Temperature calculations from the biotites from the Duchesne River Formation tuffs range between 625-725°C (Table 1). The low end of the range is suspect because typical biotite temperatures in rhyolite are close to 700°C but samples DRF-C through DRF-I all plot at 650°C

or lower. These low temperature biotites also show enrichment of Al and maintain K abundances

greater than 0.9 apfu. Speculatively, the low temperatures, and the high Al and K content could

be the result of metasediments being incorporated into the magma prior to eruption.

Comparison of log(XMg/XFe) and log(XF/XOH) from the Duchesne River Formation

biotites (Figure 13) plot in the oxidized, moderately contaminated (I-MC) field determined by

Ague and Brimhall, (1988). The biotites have lower F/OH ratios on average than the Fish

Canyon Tuff and most of the Morrison Formation tuffs and are more reduced than other middle

Jurassic ash beds. Christiansen et al (2015) noted the tuffs from the Morrison Formation

containing titanite were more oxidized than tuffs without titanite. The opposite relationship is

seen in the Duchesne River Formation biotites associated with titanite, which have log(XMg/XFe)

ratios around -0.2 as opposed to 0.3-0.4 from the Morrison Formation.

F and Cl ratios can be used to determine if the composition of the biotite is changed due

to post-depositional processes. Most of the Duchesne River Formation samples plot with the

four-to-one and two-to-one ratios (Figure 14), which is typical of calc-alkaline magmas,

implying that the biotites are reasonably well preserved.

Feldspar

Alkali feldspar was the most common feldspar found in the Duchesne River Formation

tuffs and was present in six samples while phenocrysts of plagioclase were found in only one

sample (DRF-A from the middle part of the Lapoint Member). Plagioclase grains are commonly

zoned and compositions ranged from An47 to An21 (Figure 15). Since plagioclase has a low

tolerance to weathering, it is unlikely to be a detrital component. Of the six samples with alkali

feldspar, only four contained volcanic sanidine with Or72-Or81. The other alkali feldspar grains in

DRF-B, DRF-G, DRF-I, and DRF-K appear to be detrital microcline with Or values >Or90 and

ages far older than the volcanic sanidine (Figure 15, Appendix 2). For a complete list of analytical results see Tables 7 and 8.

Feldspar compositions were not very useful in discriminating samples or for correlating one outcrop to another, or for comparison with a potential source volcanic field since sanidine and plagioclase were not abundant in any sample. From the analyses that were gathered, two

populations of sanidine around Or75 and Or79 are observed (Figure 15). DRF-J and -B have

lower Or sanidine, DRF-A and -H have higher Or sanidine and DRF-I has grains in both Or

groups. When Ba and K are compared, a different grouping pattern is seen (Figure 16). Given the limited number of analyses from DRF- H (5), DRF-I (8), DRF-B (2), and DRF-A (2) the pattern is likely a result of limited data. Additionally, the samples in each cluster are not related stratigraphically and are not grouped in the biotite or allanite composition diagrams.

Using plagioclase and sanidine from DRF-A, a two-feldspar eruption temperature was calculated using SolvCalc (Wen and Nakvasil, 1994) and the thermodynamic parameters of

Elkins and Grove (1990). Average temperatures of 642.9°C, 629.8°C, and 608.5°C were calculated at 5, 3, and 2 kilobars respectively using the five most sodic plagioclase grains ranging from An20 to An27 and one Or79 sanidine grain. The temperatures are low compared to

other volcanic rhyolite feldspars and since only one sanidine grain was used in the calculations

these temperatures are suspect.

Allanite

Euhedral grains of allanite were found in three beds, samples DRF-A, DRF-C, DRF-H,

and DRF-I, and are in relatively good condition compared to biotite and feldspar with analytical totals between 98% and 100%. Rare earth elements are essential constituents in allanite through

the coupled substitution of Ca2++ Fe3+ = REE3+ + Fe2+ (Giere and Sorensen, 2004). REE abundance typically decreases from core to rim in allanite from igneous systems (Gromet and

Silver, 1983) so our measurements were taken from the core of each grain for consistency.

Duchesne River Formation allanites have CaO concentrations ranging from 11-13 wt%, which is several percent higher than in allanite found in the Toba and Bishop Tuffs. They also have about

0.65 atoms per formula unit (apfu) REEs, which is about 0.3 apfu less than the highly-evolved

Bishop Tuff (Figure 17). For a complete list of analytical results see Table 9.

High Ca and low REEs could be an indication that the melt involved with allanite in these

Dushesne River Formation tuffs was slightly depleted in REEs and so Ca remained in the A site.

Chesner and Ettlinger (1989), however, found that REE abundance in allanite may not be a good

indicator of REE concentrations in the magmatic melt. For example, allanites from the Bishop

Tuff (Hildreth, 1977) have higher REE concentrations than allanite in the Toba Tuff but lower

whole rock REE abundances. Chesner and Ettlinger (1989) concluded that physical parameters

such as temperature exert a greater control on allanite fractionation and REE substitution than

composition of the coexisting melt. They noticed that higher temperatures correlate with higher

REE abundances in allanite. Temperatures calculated determined from the REE-rich, allanite-

bearing, Bishop Tuff range from 720-760°C (Hildreth, 1979; Hildreth and Wilson, 2007), and

temperatures determined from the Toba Tuff, which has slightly lower REEs in its allanite, range

from 710-765°C. The Duchesne River Formation allanite-bearing tuffs give temperatures around

650-710°C from biotite. These lower temperatures for the Duchesne River Formation tuffs could

explain the low REE content of the allanites.

The low eruption temperatures could explain other chemical difference between the

Duchesne River Formation allanites and those of other allanite bearing tuffs. Duchesne River

Formation tuffs have Ti concentrations between 0.025- 0.06 apfu compared with 0.06-0.12 for allanite in the Toba Tuff and 0.16 and 0.19 for the Bishop Tuff so a Ti substitution does not explain the low REEs in the Duchesne River Formation tuffs. Additionally, allanite from

Duchesne River Formation tuffs contains more Al, which substitutes for Fe in the M site, than the Bishop Tuff (1.8 vs. 1.2 apfu). Petrik et al. (1995) showed that the REE and Al content of

2+ allanite is a function of Fe /FeTotal. In this regard, the allanites from the Duchesne River

2+ Formation suggest lower Fe /FeTotal ratios of 0.3-0.4 (Figure 17b) even lower than in the Bishop

Tuff (0.5-0.6) which crystallized near the QFM oxygen buffer (Hildreth, 1979). DRF-H and

DRF-I allanites indicate reducing conditions while DRF-A and DRF-C allanites indicate more

oxidizing conditions. The biotites in these respective samples show the same reducing-oxidizing

relationship.

2+ Fe /FeTotal ratios of 0.3-0.4 are abnormal in mid-Cenozoic rocks but are observed in the

late Eocene (36 Ma) intrusive Harrison Pass pluton in the east Humboldt Mountains of

northeastern Nevada (Barnes et al, 2001). Northeastern Nevada is proposed as the source region

2+ for the Duchesne River Formation tuffs and the low Fe /FeTotal ratios could be a signature of

magmatic activity from that region though a more comprehensive examination of the volcanic

rocks in the region would be required to know for certain. Another mid-Cenozoic abnormality common to both the Duchesne River Formation and Harrison Pass pluton is the coexistence of magmatic accessory minerals titanite and allanite (Barnes et al., 2001).

Titanite

Titanite was only found in one tuff (DRF-I from the upper Dry Gulch Creek Member)

and has LREE + Y patterns similar to Cenozoic tuffs from the Great Basin (Figure 18). Other compositional comparisons of the Duchesne River Formation titanite to that of Great Basin tuffs reveal significant differences, however. Titanite in the Duchesne River Formation have significantly lower Fe+Al at 0.078 apfu compared with 0.11 apfu for other tuffs. Additionally, a

REEs/Y ratio of 9.6 from the titanite in DRF-I is higher than other Great Basin tuffs which typically have REEs/Y=3.6. Low levels of Y and REEs in titanite could indicate that the magma may have been REE poor, and that these constituents were depleted in the melt by fractionation of allanite, which is present in the mineral assemblage and is rich in LREE relative to titanite

(Figure 19). For a complete list of analytical results see Table 10.

Glass Shards

Despite the effects of weathering, some glass shards managed to survive diagenesis in

sample DRF-A from the middle part of the Lapoint Member. DRF-A is the least-altered of the all

the Duchesne River Formation tuffs. The electron microprobe analyses of these shards were

normalized to 100 % dry weight. All are high-silica rhyolite according to the IUGS classification diagram (Figure 20). For a complete list of analytical results see Table 11.

Apatite

Apatite was only found in one tuff, DRF-D, and proved to be significantly weathered despite a euhedral and prismatic appearance. The average analytical total was 93.56% and P2O5

and CaO were several weight percent lower on average than is typical for apatite. F was very

high, making up as much as 4.35 wt. % of the apatite grains, a strong indication of alteration

(Deer, Howie, and Zussman, 1996). DRF-D has a strong detrital component and these apatite

grains may be further evidence of detrital mixing. For a complete list of analytical results see

Table 12.

WHOLE ROCK COMPOSITION

Immobile Elements

Given the altered condition of the Duchesne River Formation tuffs and likely

contamination from detrital material, whole rock compositions do not represent the original

composition of the volcanic ash. Their total alkali and silica contents are plotted in Fig. 20 for

comparison with the glass shards in DRF-A. Authigenic clay minerals have replaced the volcanic glass and concentrations of both major and trace elements have been changed as a result.

Because of these changes, the initial composition of the ash is difficult to determine, but by

comparing immobile elements such as Nb, Zr, Ti, and REEs some information about the initial

composition can be found. Even though immobile element concentrations will typically be

changed by alteration, they should be similarly enriched or depleted so that ratios of these elements can still be used to make general inferences about the original composition and volcanic setting (Kowallis et al., 2001; Christiansen et al., 2015). The discrimination diagrams of

Winchester and Floyd (1977) in Figure 21 show that the Duchesne River Formation tuffs plot in the dacite and rhyolite fields based on Nb/Y vs Zr/Ti. Samples DRF-H and DRF-I have anonymously low Y resulting in a high Nb/Y ratio so they plot in the trachyte field, but their

Zr/Ti ratios are rhyolitic. The distinctive low Y content could be the result of titanite and allanite fractionation which are both present in the tuff. The Nb/Y and Zr/Ti ratios of the Duchesne River

Formation tuffs are similar to those from the northeast Nevada volcanic field, which was active at the time these tuffs were deposited (Brooks, 1995). When Zr/Ti is plotted against Ce ppm, all of the Duchesne River samples plot in the rhyolite field (Figure 21). Trace element data of Ce from the northeast Nevada volcanic field was not available for comparison.

Volcanic setting can also be inferred from immobile elements and careful use of mobile elements. Figure 22 shows discrimination plots by Pearce (1984) and compares Nb, Y, and Rb.

Both diagrams show that the Duchesne River Formation tuffs have compositions characteristic of a volcanic arc setting and are similar in composition to the northeast Nevada volcanic field. Rb

(ppm) levels in Duchesne River Formation tuffs are lower than the Northeast Nevada volcanic field, probably due to leaching of Rb during alteration. DRF-A seems to have retained initial Rb concentrations, however, and was the least altered of all the tuffs, containing glass shards and little diagenetic clay.

A volcanic arc setting indicated by Nb, Y, and Rb is in agreement with trace element

patterns from the Duchesne River Formation tuffs. Trace element patterns normalized to

primitive mantle (McDonough and Sun, 1995) show patterns typical of subduction related calc-

alkaline magmas including a large negative Nb anomaly, high Pb peak, and a generally

decreasing trend from left to right (Figure 23). Average abundances of trace elements from tuffs

which experienced argillic alteration from the Jurassic Morrison Formation (Christiansen et al.,

2015) are plotted as a comparison to the Duchesne River tuffs.

Argillic Alteration

The extensive secondary alteration exhibited by the Duchesne River Formation tuffs can provide insight into the depositional environment. Christiansen et al. (2015) found that altered tephra layers from the Jurassic Morrison Formation had three types of alteration depending on

water chemistry at the site of deposition. Argillic alteration caused a decrease in SiO2, an

increase in MgO, formation of smectitic and illitic clays, and was indicative of a

wetland/floodplain freshwater environment as described by Dunagan and Turner (2004).

Feldspathic alteration caused the precipitation of secondary feldspar in the form of albite, an

increase in SiO2, a decrease in Al2O3, TiO2, Fe2O3, and K2O, and were indicative of an alkaline

lacustrine system that evolved in a closed basin. Zeolitic alteration was identified by clinoptolite

and analcime as the main diagenetic minerals and underwent few compositional changes. This

type of alteration occurred on the fringes of evaporative lakes.

All of the Duchesne River Formation tuffs exhibit signs of argillic alteration. SiO2 values

range from 68% in DRF-A, all the way to 42% in DRF-C, which is low for rhyolite magmas,

which typically have SiO2 values in the low to mid 70’s. Al2O3 tends to increase as SiO2

decreases in the Morrison Formation tuffs and a similar trend is seen in the Duchesne River

Formation samples (Figure 24). Argillic alteration also causes an increase in MgO as SiO2 is removed but the opposite relationship is seen in feldspathic alteration which has relatively low

MgO and high SiO2. Additionally, the tuffs had to be washed extensively during mineral

separation to remove swelling clays which are a characteristic of mixed-layer smectite and illite.

The presence of argillic alteration of the tuffs agrees with sedimentological evidence on

the depositional environment of the Duchesne River Formation in a freshwater fluvial/wetland

setting (Webb et al., 2017; Sprinkel, 2008; Anderson and Picard, 1974; Anderson and Picard,

1972). These ashes were likely deposited on the overbank floodplain or in small ponds and

streams where water remained fresh and shallow.

DISCUSSION

Eruptive Source of the Duchesne River Formation Tuffs

Age, inferred original composition, eruption type, and geographic location are the main

criteria we have used to correlate the ash beds within the Duchesne River Formation with an

eruptive source. Any potential source would need to have erupted within a narrow range around

39.4 Ma, have a high silica, calc-alkaline composition, be capable of producing large plinian

eruptions, and lie to the west of the Uinta Basin where prevailing winds could transport the ash east to deposit it. Based on these constraints, the most likely source of the Duchesne River

Formation tuffs is the northeast Nevada volcanic field (Figure 25).

Previous workers attempted to correlate the tuffs of the Duchesne River Formation with an eruptive source. Anderson and Picard (1974) used rapid-scan X-ray fluorescence techniques developed by Jack and Carmichael (1969) to collect relative abundances of Rb, Zr, and Sr.

Anderson and Picard (1974) then compared these data with several known volcanic fields and

22 concluded that the Keetley volcanic field of central Utah (Fig. 1) was the likely source of the

Duchesne River Formation tephras. This conclusion is suspect however because Rb and Sr tend to behave as mobile elements during alteration and so abundances of these elements probably do not represent the actual composition of the magma.. Additionally, the ages used for correlation had large errors and based on the new ages from this study, the Duchesne River Formation tuffs are a few million years older than the Keetley volcanics (39.4 Ma vs 37.5 to 33.5 Ma; John et al,

1997). The only volcanic fields in western North America that appear to have been active at the time of the Duchesne River Formation tuffs are the northeast Nevada volcanic field and the

Tuscarora volcanic field (Figure 26). Ages of possible sources of the Duchesne River Formation tuffs are summarized in Figure 25.

A reconnaissance study of the northeast Nevada volcanic field was done by Brooks

(1995), including 40Ar/39Ar ages from sanidine. Multiple ash-flow tuffs have ages within error of the 39.47 and 39.36 Ma ages of the Duchesne River Formation tuffs (Figure 25). The Northeast

Nevada tuffs are silicic to intermediate-composition, calc-alkaline tuffs and have similar mineral assemblages including quartz, biotite, and feldspar. Whole rock comparisons between the

Duchesne River Formation and northeastern Nevada tuffs also show similar abundances of major and trace elements as shown in Fig. 21 and Fig. 22.

Just west of the large northeast Nevada volcanic field is the 2000 km2 Tuscarora volcanic field, located in north-central Nevada near the town of Tuscarora. The Big Cottonwood Canyon caldera in the central part of the field produced a large fallout ash which has been dated at 40.29±

0.14 Ma (Henry, 2008). The sanidine K/Ca ratio reported with this age was 68.9, higher than the

40.3 ratio of the dated tuffs from the Duchesne River Formation. The slightly older age and the higher K/Ca ratio seems to eliminate the Tuscarora as a potential source for the Duchesne River

Formation tuffs. Some younger eruptions in the Tuscarora field produced silicic lava domes and

flow breccias, but no large plinian or ignimbrite eruptions (Castor et al., 2003). It may be that the

Big Cottonwood Canyon tuff erupted at 40.29 Ma is present in the Duchesne River Formation,

but was not found in our study area.

The northeastern Nevada field is the only known volcanic field that was producing large

ash flow tuffs at the time of the Duchesne River Formation (Brooks, 1995). Other large volcanic fields in western North America such as the Absaroka (Chandler, 2005), Challis (Chandler,

2005), Tuscarora (Henry, 1995), Central Nevada (Best and Christiansen, 2008), Indian Peak

(Christiansen, 2015), Robertson Mountain (Lund Snee et al, 2015), Desert Mountain (Wooden et

al, 1999), either pre-date or post-date the Duchesne River Formation tuffs. The Keetley

(Crittenden, 1973; Bromfield and Erickson, 1977), Marysvale (Moore et al, 2008) and Tintic

(Moore et al, 2008) volcanics in north and central Utah are hundreds of kilometers closer to the

Uinta Basin than the northeastern Nevada field but are several million years too young to be considered correlative.

Fall out tuffs can also be correlated with their intrusive counterparts much in the same way the ashes can be correlated with extrusive rocks. Plutonism was occurring in the region as a consequence of slab rollback as shown in Figure 25. However, the timing of the Duchesne River

Formation tuffs does not match any intrusive complex except the Bingham intrusive complex in northern Utah which was active from 39.18-37.2 Ma (Warnaars et al., 1978; Deino and Keith,

1997) and some of the plutons within the Park City mining district (Jones et al, 1997). The composition of the Bingham stock however is more primitive and mafic and includes monzonite, quartz monzonite, and quartz latite with hornblende and pyroxene mafic phases present (Moore,

1973, Waite et al., 1997), both of which are entirely absent in the Duchesne River Formation

tuffs. The volcanic rocks associated with the Bingham intrusion also have a more mafic signature

except for some rhyolitic flows which were erupted near the end of volcanic activity around 33

Ma.

The plutons within the Park City mining district include the Little Cottonwood Stock,

Alta Stock, Pine Creek Stock, Valeo Stock, Mayflower Stock, Ontario Stock, Flagstaff Stock,

and the Park Premier Stock. Unless volcanic activity preceded the emplacement of these plutons,

the Park City mining district is likely not the source of the Duchesne River Formation tuffs since

most ages are too young (Jones, 1997). The ages for the stocks range from about 36-31 Ma with

a few anonymously high ages between 41 and 38 Ma (Vogel et al., 2001) taken from

hornblende, dated with the K/Ar method.

Aside from correlating with other igneous units, compositional data can be used to make interpretations about the tectonic setting and eruptive conditions. The mineral assemblage, mineral compositions, eruption temperatures, and whole rock compositions of the Duchesne

River Formation tuffs indicate a rhyolite-dacite composition typical of a subduction zone tectonic setting. There is no evidence of A-type or extension related magmatism in these tuffs.

The rhyolitic composition magmas from which these fall out tuffs originated were likely generated as the subduction angle of the Farallon slab began to steepen at the close of the

Laramide Orogeny. Since slab rollback and volcanism began to the north and moved south, the northeast Nevada volcanic field and the tuffs from the Duchesne River Formation likely represent some of the first evidences of slab-rollback related volcanism in the northern

Utah/Nevada region.

Correlating far-field volcanic units such as the Duchesne River Formation tuffs with an eruptive source area is especially difficult in the Basin and Range region due to extensive normal

25 faulting. Normal faults have dissected and displaced igneous rocks from their original locations and can be difficult to recognize in the field for their true identity (Best and Christiansen, 2015).

Additionally, normal faulting has exposed these igneous rocks to the elements and it is likely that many rocks deposited on the surface have been removed by erosion. While there is evidence to support the northeast Nevada volcanic field as the eruptive source of the Duchesne River

Formation, it is possible that the true source area has been permanently removed by geologic processes. For example the Flagstaff (39.7± 2 Ma) and Valeo (39.8 ±2.5) stocks from the park

City mining district, the Ibapah granite (39 ±0.5), and Grouse Creek (41.5-34 Ma) plutons (Jones et al., 1997) could have associated extrusive components that have eroded away. In addition to age data, mineral composition data from the northeast Nevada volcanic field could help to solidify this correlation, particularly allanite, titanite, and biotite.

Timing of Uinta Mountain Uplift

Conglomerates have often been interpreted to indicate uplift of mountain ranges

(DeCelles, 1994). The Starr Flat Member of the Duchesne River Formation likely indicates renewed uplift of the nearby Uinta Mountains and so the ages reported in this study can be used to date this period of uplift (Figure 25).

We propose that after a time of relative tectonic quiescence when the Brenan Basin, Dry

Gulch Creek, and Lapoint members were deposited, a period of uplift began in the Uinta

Mountains soon after 39.4 Ma. The duration of the uplift event is loosely constrained by a U-Pb zircon age of 37 ± 1.5 Ma from a tuff in the Starr Flat Member (Bryant et al., 1989). This uplift event created a shallow syncline in the Duchesne River Formation which dips to the south, proximal to the Uinta Mountains to the north, and then dips to the north further south in the

Uinta Basin. This uplift event is followed by a ~4 Ma angular unconformity called the Gilbert

Peak erosion surface in which uplift and deposition apparently slowed and erosion dominated.

Dickinson, (1988) interprets the Gilbert Peak erosion surface as the end of the Laramide in the

Uinta Basin but this and other studies (Kowallis et al., 2005; Sprinkel, 2007; Webb, 2017)

suggest that another Laramide deformation event followed this pause in tectonic activity. The

Bishop Conglomerate was deposited atop the Gilbert Peak erosion surface and may represent a final pulse of uplift or a rapid lowering of base level between about 34 to 30 Ma (Kowallis et al.,

2005) during the Oligocene.

Duchesnean Land Mammal Age

The Duchesne River Formation, near the town of Lapoint, Utah, is the type section of the

Duchesnean Land Mammal Age (Emry, 1981). Some disagreement has arisen over the numerical

age of the fauna which define the Age, and it has been unclear whether the fauna are entirely late

Eocene or part Eocene and part Oligocene (Emry, 1981, Rassmussen et al., 1999). The majority

of fossils have been collected from the Brenan Basin and Lapoint members (Rassmussen et al.,

1999, Burger and Tacket, 2014) and have relied on old isotopic ages taken from tuffs within

these members to constrain the age of the fauna (Lucas and Emry, 2004). These old radiometric

dates have been the main reason for the confusion about the exact age of the fauna. The two

40Ar/39Ar ages of 39.47 ± 0.16 and 39.36 ± 0.15 Ma reported in this study and the 34.03± 0.04

Ma age of the tuff in the lower part of the overlying non-Duchesnean Bishop Conglomerate are

strong evidence that the fauna found within the Duchesne River Formation are entirely Late

Eocene (Prothero, 1995).

CONCLUSIONS

Answers to the questions presented in the introduction regarding the age, mineral

assemblage and composition, the altered and original major and trace element compositions, the

source of the Duchesne River Formation tuffs, the tectonic setting, the age of the fauna within

the Duchesnean Land Mammal Age, and the timing of uplift of the Uinta Mountains can be

summarized as follows:

1) The two most prominent tuff beds give 40Ar/39Ar ages of 39.47± 0.16 Ma and 39.36 ±

0.15 Ma. The tuffaceous layers in the Lapoint Member are concentrated in the lower and middle

Lapoint, and the tuffs from the Dry Gulch Creek Member were taken from the upper and middle

Dry Gulch Creek. These new ages represent a reasonable age for all of the tuffs sampled within

Duchesne River Formation.

2) The compositions of the phenocrysts are typical of rhyolitic, calc-alkaline, subduction related magmatism. Quartz, biotite, feldspar, and allanite is a known mineral assemblage of calc- alkaline magmas. Calculated temperatures from biotite and feldspar and relatively low Ti and

REE’s in allanite suggest that the magma was cooler and less oxidized than typical rhyolite magmas. Clustering of biotite compositions suggest the parent magmas evolved compositionally over time. These ashes represent some of the first evidences of slab rollback-related volcanism in the Nevada/Utah region.

3) The whole rock altered compositions of the tuffs are typical of argillic alteration, indicated by an enrichment of Al and Mg, depletion of Si and K, stable levels of Nb, Y, Ti, Zr, and REEs, and formation of smectitic and illitic clays as a result of the diagenesis of the volcanic glass. The immobile element abundances are typical of rhyolites produced in a subduction zone.

4) Based on age, composition, eruption style, and geographic location, the northeast

Nevada volcanic field is the most likely source of the Duchesne River Formation tuffs. All other

known potential sources are a few millions of years too young or have an entirely different

composition. The tuffs within the Duchesne River Formation likely document the transition from

minimal volcanic activity in the northern Utah/Nevada region, to wide-spread volcanism due to the southward migration of slab-rollback.

5) The faunas of the Duchensne River Formation, which define the Duchesnean Land

Mammal Age, are about 39.4 million years old. This is younger than the previously published age of 41.0 Ma and are entirely Eocene in age.

6) The ages of the tuffs from the Duchesne River Formation and the overlying Bishop

Conglomerate constrain two periods of uplift of the Uinta Mountains at about 39 Ma and 34 Ma.

REFERENCES CITED Ague, J.J., and Brimhall, G.H., 1988, Regional variations in bulk chemistry, mineralogy, and the compositions of mafi c and accessory minerals in the batholiths of California: Geological Society of America Bulletin, v. 100, p. 891–911, doi:10.1130/00167606(1988).

Andersen, D.W., and Picard, M.D., 1972, Stratigraphy of the Duchesne River Formation (Eocene- Oligocene?), northern Uinta Basin, northeastern Utah: Utah Geological & Mineral Survey Bulletin 97, 29 p.

Andersen, D.W., and Picard, M.D., 1974, Evolution of synorogenic clastic deposits in the intermontane Uinta Basin of Utah, in Dickinson, W.R., ed., Tectonics and Sedimentation: SEPM Special Publication 22, p. 167-189.

Barnes C.G., Burton B.R., Burling, T.C., Wright, J. E., Karlsson, H.R., 2001, Petrology and geochemistry of the late Eocene Harrison Pass pluton, Ruby Mountain core complex, northeastern Nevada; Journal of Petrology, vol. 42, no. 5, pp. 301-929.

Best, M.G., Christiansen, E.H., and Gromme, S., 2013, Introduction: The 36–18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervolcanoes: Geosphere, v. 9, no. 2, p. 260–274, doi:10.1130/GES00870.1.

Best, M.G., Christiansen, E.H., and Gromme, S., 2013, The 36-18 central Nevada ignimbrite field and calderas, Great Basin, USA: Multicyclic super-eruptions: Geosphere, v. 9, no. 6, p. 1562-1636, doi:10.1130/GES00945.1.

Best, M.G., Christiansen, E.H., and Gromme, S., 2013, The 36-18 Indian Peak-Caliente ignimbrite field and calderas, southereastern Great basin, USA: mulitcyclic super-eruptions: Geosphere, v. 9, no. 4, p. 864-950, doi:10.1130/GES00902.1.

Biek, R.F., Solomon, B.J., Keith, J.D., and Smith, T.W., 2005, Geologic map of the Tickville Spring quadrangle, Salt Lake and Utah Counties, Utah: Utah Geological Survey Map 214, 2 plates, scale 1:24,000.

Bisdom, E.B.A., Stoops, G., Delvigne, J., Curmi P., Altemuller, H.J., 1982, Micromorphology of weathering biotite and its secondary products: Advisory panel on weathering phenomena and neoformations, vol. 32, no. 3, pp. 225-251, 1 table, 2 figures, 4 plates.

Blaylock, G.W., 1998, Probable correlation of the Oligocene Whitney Ash beds of western Nebraska to ash-flow tuffs in Nevada and Utah [M.A. thesis] Brigham Young University 50 p.

Bogaard, P., and Schirnick, C., 1995, 40Ar/39Ar laser probe ages of Bishop Tuff quartz phenocrysts substantiate long-lived silicic magma chamber at Long Valley, United States, GSA Bulletin, v. 23, no. 8, pp. 759-762, doi: 10.1130/0091-7613.

Bromfield, C.S., Erickson, A.J., Jr., Haddadin, M.A., Mehnery, H.H., 1977, Potassium-argon ages of intrusion, extrusion, and associated ore deposits, Park City mining district, Utah, Economic Geology, v. 72, pp. 837-848.

Brooks, W.E., Thorman, C.H., and Snee, L.W., 1995, The 40Ar/39Ar ages and tectonic setting of the middle Eocene northeast Nevada volcanic field: Journal of Geophysical Research, v. 100, no. B7, p. 10,403-10,416.

Brooks, W.E., Thorman, C.H., and Snee, L.W., 1995, Correction to “the 40Ar/39Ar ages and tectonic setting of the middle Eocene northeast Nevada volcanic field,” Journal of Geophysical Research, vol. 100, no. B8, pp. 15,545-12,548.

Bryant, B., Naeser, C.W., Marvin, R.F., and Mehnert, H.H., 1989, Upper Cretaceous and Paleogene sedimentary rocks and isotopic ages of Paleogene tuffs, Uinta Basin, Utah: U.S. Geological Survey Bulletin 1787, p. J1-J22.

Burger B.J., and Tacket II., 2014; The stratigraphic importance of the brontothere (cf. Diplacodon elatus) in the Brennan Basin Member of the Duchesne River Formation of Utah: Fossil Record, v. 17, p. 69-74.

Carroll, A. R., Bohacs, K. M., 1999, Stratigraphic classification of ancient lakes: Balancing tectonic and climatic controls; Geology, vol. 27, no. 2, pp. 99-102.

Castor, S.B., Faulds, J.E., Fowland, S.M., DePolo, C.M., 2000, geologic map of the Frenchman Mountain quadrangle, Clark County, Nevada: Nevada Bureau of Mine and Geology Maps 127, 1:24,000. 15 p.

Castor, S.B., Boden, D.R., Henry, C.D., Cline, J.S., Hofstra, A.H., McIntosh, W.C., Tosdal, R.M., Wooden, J.P., 2003, the Tuscarora Au-Ag district: Eocene volcanic-hosted epithermal deposits in the Carlin Gold region, Nevada: Economic Geology, vol. 98, pp. 339-366.

Chamberlain, C.P., Mix, H.T., Mulch, A., Hren, M.T., Kent-Corson, M.L., Davis, S.J., Horton, T.W., and Graham, S.A., 2012, The Cenozoic climatic and topographic evolution of the western North American Cordillera: American Journal of Science, v. 312, p. 213–262, doi: 10.2475/02.2012.05.

Chandler, M. R., 2006, The provenance of Eocene tuff beds in the Fossil Butte Member of the Green River Formation of Wyoming: relation to the Absaroka and Challis volcanic fields [M.A. thesis] Brigham Young University, Provo, Utah, 98 p.

Chesner, C.A., and Ettlinger, A.D., 1989, Composition of volcanic allanite from the Toba Tuffs, Sumatra, Indonesia: American Mineralogist, v. 74, p. 750-758.

Christiansen, E.H., Sheridan, M.F., and Burt, D.M., 1986, The Geology and Geochemistry of Cenozoic Topaz Rhyolites from the Western United States: Geological Society of America Special Paper 205, 82 p., doi:10.1130/SPE205-p1.

Christiansen, E.H., Kowallis, B.J., Dorais, M.J., Hart, G.L., Mills, C.N., Pickard, M., and Parks, E., 2015, The record of volcanism in the Brushy Basin Member of the Morrison Formation: Implications for the Late Jurassic of western North America: Geological Society of America Special Paper 513, p. 399–439, doi:10.1130/2015.2513(11).

Clark, J., Beerbower, J. R., and Kietzke, K. K., 1967, Oligocene sedimentation, stratigraphy, paleoecology and paleoclimatology in the Big Badlands of South Dakota: Fieldiana, Geology Memoirs, v. 5, p. 1-158.

Constenius, K.N., Esser, R.P., Layer, P.W., 2003, Extensional collapse of the Charleston-Nebo salient and its relationship to space-time variations in Cordilleran orogenic belt tectonism and continental stratigraphy: in Raynolds, R.G., and Flores, R.M., editors, Cenozoic Systems of the Rocky Mountain Region, Rocky Mountain SEPM, Denver, CO, p. 303-343.

Coogan, J.C., and Constenius, K.N., 2003, Structural continuity of the Sevier thrust belt across the Uinta arch: American Association of Petroleum Geologists field trip guidebook.

Dalrymple, G.B., Cox, A., Doell, R. R., 1965, Potassium-Argon age and paleomagnetism of the Bishop Tuff, California: GSA Bulletin, v. 76, pp. 665-674, doi: 10.1130/00136-7606

DeCelles, P.G., 1994, Late Cretaceous-Paleocene synorogenic sedimentation and kinematic history of the Sevier thrust belt, northeast Utah and southwest Wyoming. Geological Society of America Bulletin 106:32–56

Deer, W.A., Howie, R.A., and Zussman, J., 1997, Rock Forming Minerals: Volume 1A. Orthosilicates (2nd ed.): Bath, UK, The Geological Society, pp. 663-669.

Deino, A., and Keith, J. D., 1997, Ages of volcanic and intrusive rocks in the Bingham Mining District, Utah; in Geology and Ore deposits of the Oquirrh and Wasatch Mountains, Utah; Society of Economic Geologists Field Trip Guidebook, vol. 29, pp. 91-95.

Dickinson, W.R., 2004, Evolution of the North American Cordillera. Annual Reviews of Earth and Planetary Science, v. 32, pp. 13-45.

Dickinson W.R.., Klute, M. A., Hayes, M.J., Janecke, S. U., Lundin, E. R., McKittrick, M. A., Olivares M. D., 1988, Paleogeographic and paleotectonic setting of Laramide sedimentary basins in the central Rocky Mountain region: Geological Society of America Bulletin, v. 100, p. 1023-1039.

Dunagan, S.P., and Turner, C.E., 2004, Regional paleohydrologic and paleoclimatic settings of wetland lacustrine depositional systems in the Morrison Formation (Upper Jurassic), Western Interior, USA: Sedimentary Geology, v. 167, p. 269–296, doi:10.1016/j.sedgeo.2004.01.007.

Egger, A.E., Dumitri, T.A., Miller, E.L., Savage. C.F.I., Wooden, J.L., 2003, Timing and nature of Tertiary plutonism and extension in the Grouse Creek Mountains, Utah: International Geology Review, vol. 45, pp. 497-532.

Elkins, L. T., and Grove, T. L., 1990, Ternary feldspar experiments and thermodynamic models: American Mineralogist, v. 75, p. 544-559.

Emry, R. J., 1981, Additions to the mammalian fauna of the type Duchesnean, with comments on the status of the Duchesnean “Age”: Journal of Paleontology, v. 55, p. 563-570.

Feng, R., Poulsen, C.J., Werner, M., Chamberlain, C.P., Mix, H.T., and Mulch, A., 2013, Early Cenozoic evolution of topography, climate, and stable isotopes in precipitation in the North American Cordillera: American Journal of Science, v. 313, p. 613–648, doi: 10.2475/07.2013.01

Giere, R., and Sorenson, S.S., 2004, Allanite and other REE-rich epidote-group minerals: Reviews in Minerology and Geochemistry, vol. 56, pp. 431-493.

Gromet, L.P., and Silver, L.T., 1983, Rare earth distributions among minerals in a granodiorite and their petrogenetic implications: Geochim Cosmochim, vol. 74, pp. 925-939.

Gutscher, M.A., Malod, J., Rehault, J.P., 2002. Geodynamics of flat subduction: seismicity and tomographic constraints from the Andean margin: Tectonics vol. 19, p. 814-833.

Hamilton, W., 1978, Mesozoic tectonics of the western U.S.: in Howell, D.G., and McDougall, A.K., eds. Mesozoic paleogeography of the western United States., Society of Economic Paleontologists and Mineralogists, Pacific Section, Los Angeles, p. 33-70.

Hansen, W.R., 1986, Neogene tectonics and geomorphology of the eastern Uinta Mountains in Utah, Colorado, and Wyoming: U.S. Geological Survey Professional Paper 1356, 78 p.

Henry, C.D., 2008, Ash-flow tuffs and paleovalleys in northeastern Nevada: Implications for Eocene paleogeography and extension in the Sevier hinterland, northern Great Basin: geosphere, vol. 4, no. 1, pp. 1-35, doi: 10.1130/GES00122.1 13 figures; 3tables; 1 supplementary file.

Hildreth, W., and Wilson, C. J. N., 2007, Compositional zoning of the Bishop Tuff: Journal of Petrology, vol. 48, no. 5, pp. 951-999, doi: 10.1093/petrology/cgm007

Humphreys, E.D., 1995, Post-Laramide removal of the Farallon slab, western United States: Geology, v. 23, no. 11, p. 987-990.

Jack, R.N., and Carmichael, I.S., 1969, the chemical fingerprinting of acid volcanic rocks: California Division of Mines and Geology Special Report 100, pp. 17-32.

Jicha, B.R., Scholl, D.W., Rea, D.K., 2009, Circum-Pacific flare-ups and global cooling near the Eocene- Oligocene boundary: Geology, v. 37, no. 4, p. 303-306, doi: 10.1130/G25392A.1.

John, D. A., Turrin, B.D., Miller, R.J., 1997, New K-Ar and 40Ar/39Ar ages of plutonism, hydrothermal alteration, and mineralization in the central Wasatch Mountains, Utah; in Geology and Ore deposits of the Oquirrh and Wasatch Mountains, Utah; Society of Economic Geologists Field Trip Guidebook, vol. 29, pp. 47-57.

Johnson, C.L., 2015, Petrology and Geochemistry of the Emigrant Pass volcanics, Nevada; Implications for a magmatic-hydrothermal origin of the Carlin gold deposits [M.A. Thesis], Oregon State University, Corvallis, Oregon, p. 73

Jones, C.H., Farmer, G.L., Sageman, B., Zhong, S., 2011, Hydrodynamic mechanism for the Laramide orogeny: Geosphere, vil. 7, no. 1, pp. 183-201, doi: 10.1130/GES00575.1.

Kowallis, B.J., Christiansen, E.H., and Deino, A.L., 1991, Age of the Brushy Basin Member of the Morrison Formation, Colorado Plateau, western USA: Cretaceous Research, v. 12, p. 483–493, doi:10.1016/0195-6671(91)90003-U.

Kowallis, B.J., Christiansen, E.H., Deino, A.L., Peterson, F., Turner, C.E., Kunk, M.J., and Obradovich, J.D., 1998: The age of the Morrison Formation: Modern Geology, v. 22, p. 235–260.

Kowallis, B.J., Christiansen, E.H., Deino, A.L., Zhang, C., and Everett, B.H., 2001, The record of Middle Jurassic volcanism in the Carmel and Temple Cap Formations of southwestern Utah: Geological Society of America Bulletin, v. 113, p. 373–387.

Kowallis, B.J., Christiansen, E.H., Balls, E., Heizler, M.T., and Sprinkel, D.A., 2005, The Bishop Conglomerate ash beds, south flank of the Uinta Mountains, Utah: Are they pyroclastic fall beds from the Oligocene ignimbrites of western Utah and eastern Nevada?, in Dehler, C.M., Pederson, J.L., Sprinkel, D.A., and Kowallis, B.J., editors, Uinta Mountain geology: Utah Geological Association Publication 33, p. 131-145.

Kowallis and Sprinkel, 2007: 40Ar/39Ar geochronology results from the Duchesne River and Carmel Formations. New Mexico Geochronology Research Laboratory Internal Report # NMGRL-IR- 561.

Kuiper, K.F., Deino, A., Hilgen, F.J., Krijfsman, W., Renne, P.R., Wijbrans, J.R., 2008, Synchronizing rock clocks of earth history; Science, vol. 320 (5870), 500-504, DOI: 10.1126/science.1154339.

Lipman, P.W., Prostka, H.J., Christiansen R.L., 1972, Cenozoic volcanism and plate-tectonic evolution of the Western United States. I. Early and Middle Cenozoic: Philosophical Transactions Royal Society of London, A. 271, p. 217-248.

Lucas S.G., and Emry R.J., 2004, The Entolodont Brachyhyops (Mammilia Artiodactyla) from the Upper Eocene of Flagstaff Rim, Wyoming. Paleogene Mammals, New Mexico Museum of Natural History and Science Bulletin No. 26 p. 97-101.

Liu, L., Gurins, M., Seton, M., Saleeby, J., Muller, R.D., Jackson, J.M., 2010, The role of oceanic plateau subduction in the Laramide orogeny: Nature Geoscience, vol. 3, p. 353-357, doi:10.1038/NGE0829

Luhr, F.J., Carmichael, I.S.E., and Varekamp, J.C., 1984, The 1982 eruptions of El Chichon volcano, Chiapas, Mexico: Mineralogy and petrology of the anhydrite-bearing pumices: Journal of Volcanology and Geothermal Research, v. 23, p. 69–108, doi:10.1016/0377-0273(84)90057-X.

Lund-Snee, J.E., Miller, E.L., Hourigan, J.K., Konstantinou, A., 2015, Cenozoic paleogeographic evolution of the Elko Basin and surrounding region, northeast Nevada: Geosphere, vol. 12, no. 2, doi: 10.1130/GE801193.1.

Martinod, J., Husson, L., Roperch, P., Guillaume, B., Espurt, N., 2010, Horizontal subduction zones, convergence velocity and the building of the Andes: Earth and Planetary Science Letters, vol. 299, iss. 3-4, pp. 299-309.

Mauger, R.L., 1977, K-Ar ages of biotite from tuffs in Eocene rocks of the Green River, Washakie, and Uinta Basins, Utah, Wyoming, and Colorado: University of Wyoming Contributions to Geology, v. 15, p. 17-41.

McDonough, W.F., Sun, S.-s, 1994, The composition of the earth: Chemical Geology Isotope Geoscience, vol. 120, pp. 223-253.

McDowell, F. W., Wilson, J.A., and Clark, J., 1973, K-Ar dates for biotite from two paleontologically significant localities--Duchesne River Formation, Utah, and Chadron Formation, South Dakota: Isochron/West, v. 7, p. 11-12.

Meyers, S.R., Siewert, S.E., Singer, B.S., Sageman, B.S., Condon, D.J., Obradovich, J.D., Jicha, B.R., Sawyer, D.A., 2012, Intercalibration of radioisotopic and astrochronologic time scales for the Cenomanian-turonian boundary interval, Western Interioir basin, USA; Geology, vol. 40, no. 1, pp. 7-10, doi:10.1130/G32261.1.

Montellano, M., and Hidalgo, E.J., 2006, Mexican fossil mammals, who, where, and when? Studies on Mexican Paleontology, pp. 249-273.

Mulch, A., Chamberlain, C.P., Cosca, M.A., Teyssier, C., Methner, K., Hren, M.T., and Graham, S.A., 2015, Rapid change in high-elevation precipitation patterns of western North America during the Middle Eocene Climatic Optimum (MECO): American Journal of Science, v. 315, p. 317–336, doi: 10.2475/04.2015.02

Moore, W.J., 1973, Summary of radiometric ages of igneous rocks in the Oquirrh Mountains, north- central Utah: Economic geology vol. 68, pp. 97-101.

Moore, W.J., 1973, Igneous rocks in the Bingham Mining District, Utah; Geological Survey Professional Paper, v. 629-B.

Norman, M.D. and Mertzman, S.A. (1991). Petrogenesis of Challis volcanics from central and southwestern Idaho: Trace element and Pb isotopic evidence. Journal of Geophysical Research 96: doi: 10.1029/91JB00285. issn: 0148-0227.

Pearce, J., 1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrology, vol. 25, part 4, pp. 956-983.

Poduska, G.J., 2015, geologic mapping of Ice Cave Peak Qaudrangle, Uintah and Duchesne counties, Utah; with implications from mapping Laramide faults [M.A. thesis] Provo, Brigham Young University, 91 p. 21 figures, 1 plate, 1:24,000

Prothero, D.R., 1995, Geochronology and magnetostratigraphy of Paleocene North American land mammal “ages”: an update: Society for Sedimentary Geology, SEPM Special Publication No. 54, pp. 305-316, ISBN 1-565576-024-7.

Prothero, D.R., and Swisher, C.C., III, 1992, Magnetostratigraphy and geochronology of the terrestrial Eocene-Oligocene transition in North America, in Prothero, D.R., and Berggren, W.A., editors, Eocene-Oligocene climatic and biotic evolution: Princeton, New Jersey, Princeton University Press, p. 46-73.

Rasmussen, D.T., Hamblin, A.H., and Tabrum, A.R., 1999, The mammals of the Eocene Duchesne River Formation, in Gillette, D. D., ed., Vertebrate Paleontology in Utah: Utah Geological Survey Miscellaneous Publication 99-1, p. 421-428.

Remy, R. R., 1992, Stratigraphy of the Eocene part of the Green River Formation in the south-central part of the Uinta Basin, Utah: A multidisciplinary approach to research studies of sedimentary rocks and their constituents and the evolution of sedimentary basin, both ancient and modern; U.S. Geological Survey Bulletin 1787 Evolution of sedimentary basins-uinta and Picenace basins, ch. BB

Riggs, N.R., Ash, S.R., Barth, A.P., Gehrel, G.E., Wooden, J.L., 2003, Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone: GSA Bulletin, v. 115, no. 11, pp. 1315-1323.

Ryskamp, E.B., Abbott, J.T., Christiansen, E.H., Keith, J.D., Vervoot, J.D., Tingey, D.G., 2008, Age and petrogenesis of volcanic and intrusive rocks in the Sulphur Spring Range, central Nevada: Comparisons with ore associated Eocene magma systems in the Great Basin; Geosphere, vol. 4, no. 3, pp. 496-519, doi: 10.1130/GES0013.1.

Sageman, B. B., Meyers, S. R., Arthur, M. A., 2006, Orbital time scale and new C-isotope for Cenomanian-Turonian boundary stratotype; Geology, v. 34, no. 2, pp. 125-128, doi: 10.1130/G22074.1

Sato, T., and Chan, M.A., 2015, Source-to-sink fluvial systems for sandstone reservoir exploration: Example from the basal Brennan Basin Member of Tertiary Duchesne River Formation, northern Uinta Basin, Utah, in Vanden Berg, M.D., Ressetar, R., and Birgenheier, L.P., editors, The Uinta Basin: Utah Geological Association Publication 45, 17 p.

Schellart, W.P., Freeman, J., Stegman, D.R., Moresi, L., May, D., 2007, Evolution and diversity of subduction zones controlled by slab width: Nature, vol. 446, pp. 308-311.

Smith, M. E., Singer, B., and Carroll, A., 2003, 40Ar/39Ar geochronology of the Eocene

Green River Formation, Wyoming: Geological Society of America Bulletin, v. 115, no. 5, p. 549-565.

Smith, M. E., Singer, B.S., Carroll, A. R., Fournelle, J.H., 2008, Precise dating of biotite in distal volcanic ash: Isolating subtle alteration using 40Ar/39Ar laser incremental heating and electron microprobe techniques; American Mineralogist, vol. 93, pp.784-795, DOI:10.2138/am.2008.2517784. Smith, M. N., Carroll, A. R., Jicha, B. R., Cassel, E. J., Scott, J. J., 2014, Paleogeographic record of the Eocene Farallon slab rollback beneath western North America; Geology, v. 42, no. 12, pp. 1039- 1042, doi: 10.1130/G36025.1.

Smith, M. N. and Carrol, A. R., 2015, Stratigraphy and paleolimnology of the Green River Formation, western USA; Syntheses in Limnology 1, DOI 10.1007/978-94-017-9906-5_1.

Sprinkel, D.A., 2007, Interim geologic map of the Vernal 30’ x 60’ quadrangle, Uintah and Duchesne Counties, Utah, Moffat and Rio Blanco Counties, Colorado: Utah Geological Survey Open-File Report 506DM, compact disc, GIS data, 3 plates, scale 1: 100,000.

Strickland, A., Miller, E.L., Wooden, J. L., 2011, The timing of tertiary metamorphism and deformation in the Albion-Raft River-Grouse Creek metamorphic core complex, Utah and Idaho; The Journal of Geology, vol. 119, no. 2, pp. 185-206, DOI: 10.1086/658294.

Suayah, I. B., and Rogers, J. J. W., 1991, Petrology of the lower Tertiary Claron Formation in north central Oregon: The importance of magma mixing: Journal of Geophysical Research, v. 96, p. 13,357-13,371.

Summa, L.L., and Verosub, K.L., 1992, Trace element mobility during early diagenesis of volcanic ash: Applications to stratigraphic correlation: Quaternary International, vol. 13/14 p. 149-157.

Toole, W., 1951, Geology of the Keetley-Kamas volcanic area [M.A. thesis] University of Utah 38 p.

Tanavsuu-Milkeviciene, K., Sarg, J.F., Bartov, Y., Depositional cycles and sequences in an oragnaic-rich lake basin: Eocene Green River Formation, lake Uinta, Colorado and Utah, USA; Journal of Sedimentary research, vol. 87, pp. 210-229, doi: http://dx.doi.org/10.2110/jsr.2017.11.

Vogel, T.A., Cambray, F.W., Constenius, K.N., 2001, Origin, and emplacement of igneous rocks in central Wasatch Mountains, Utha: Rocky Mountain Geology, vol. 36, no. 2, pp. 119-162.

Waite, K.A., Keith, J.D., Christiansen, E.H., Whitney, J.A., Hattori, K., Tingey, D.G., Hook, C.J., 1997, Petrogenesis of the volcanic and intrusive rocks associated with the Bingham porphyry Cu-Au- Mo deposit, Utah; Society of Economic Geologists, Oquirhh Symposium, Special Volume.

Ward, J.H., 1963, Hierarchial grouping to optimize an objective function: Journal of the American Statistical Association, vol. 58, pp. 236-244.

Warnaars, F.W., Smith, W.H., Bray, R.E., Lanier, G., Shafiqullah, M., 1978, Geochronology of igneous intrusions and porphyry copper mineralization at Bingham, Utah; Economic geology, v. 73, no. 7, p. 1242-1249.

Warner, M.M., 1966, Sedimentational analysis of the Duchesne River Formation, Uinta Basin, Utah: Geological Society of America Bulletin, v. 77, p. 945-958.

Wen, S., and Nekvasil, H., 1994, SolvCalc: An interactive graphics program package for calculating the ternary feldspar solvus and for two- feldspar geothermometry: Computers & Geosciences, v. 20, pp. 1025–1040, doi:10.1016/0098-3004(94)90039-6.

Winchester, J.A., and Floyd, P.A., 1977, Geochemical discrimination of different magma series and their differentiation products using immobile elements: Chemical geology, v. 20, pp. 325-343, doi: 10.1013/009-2541(77)90057-2.

Wood, H. E., II, Chaney, R. W., Clark, J., Colbert, E. H., Jepsen, G. L., Reeside, J. B., Jr., and Stock, C., 1941, Nomenclature and correlation of the North American continental Tertiary: Geological Society of America Bulletin, v. 52, p. 1-48.

Woodburne, M.O., 2004, Late Cretaceous and Cenozoic mammals of North America: Columbia University Press, New York, p. 391

Yonkee and Weil, 2015, Tectonic evolution of the Sevier and Laramide belts within the North American Cordillera orogenic system: Earth-Science Reviews, v. 150, p. 531-593.

Zanazzi A., Kohn, M.J., MacFadden B.J., Terry Jr, D.O., 2007, Large temperature drop across the Eocene-Oligocene transition in central North America: Nature, v. 445, p. 639-642.

Zielinski, R.A., 1982, The mobility of uranium and other elements during alteration of rhyolite ash to montmorillonite: A case study in the Troublesome Formation, Colorado, U.S.A.: Chemical Geology, v. 35, p. 185–204, doi:10.1016/0009-2541(82)90001-8.

Zhou, Y., Bohor, B.F., Ren, Y., 2000, Trace element geochemistry of altered volcanic ash layers (tonsteins) in Late Permian cola-bearing formations of eastern Yunnan and western Guizhou Provinces, China: International Journal of Coal Geology, vol. 44, pp. 305-324.

FIGURES

Figure 1: (A) Regional map of western United States with boundaries and ages of Cenozoic volcanic fields which formed as a result of slab rollback. (B) is a simplified geologic map of the Vernal Northwest quadrangle showing locations of volcanic ash samples an and the principal geologic units and contacts of the different members of the Duchesne River Formation. The Cretaceous units undivided on the this map include the Mesaverde Formation, Mancos Shale, Frontier Formation, Mowry Shale, Dakota Formation, and Cedar Mountain Formation.

Figure 2: Stratigraphic column of Tertiary and Cretaceous units mapped in the Vernal Northwest quadrangle from Webb, (2017).

Figure 3: Stratigraphic diagram of the Dry Gulch Creek and Lapoint members of the Duchesne River Formation showing the relative locations and thicknesses of the tuff beds. Note that samples DRF-D, DRF-E, and DRF-F from the same 5.5 meter thick tuffaceous bed whi which serves as the contact between the Dry Gulch Creek and Lapoint. Samples DRF-G and DRF-H are also from the same tuffaceous bed but taken from different locations.

Figure 4: (A) Photograph of the prominent 5.5 m thick tuff bed with the contact between with Dry Gulch Creek Member and the Lapoint Member marked by the yellow line at base of the tephra layer. The grey color of the ash stands out against the red/orange colors of the siltstone. Samples DRF-D, DRF-E, DRF, and DRF-G were collected from this layer which is known as the “prominent ash bed.” This bed shows evidence of detrital mixing and has likely been thickened by post-sedimentary processes. (B) Ash bed DRF-C, shown between the red lines, is 18 meters above the prominent ash in A.

Figure 5: Rank order plots with probability density curves for feldspar ages from samples DRF- H and DRF-A from the Dry Gulch Creek and Lapoint members of the Duchesne River Formation, respectively. Individual 40Ar/39Ar dates, as well as weighted men ages, are shown with 2σ analytical uncertainties. Filled circles are the ages that were used in the age calculation. Plots were created by Brian Jicha at the University of Wisconsin-Madison WiscAr Geochronology Lab.

Figure 6: Compositions of biotite from the Duchesne River Formation compared to the Bishop Tuff, Fish Canyon Tuff, Morrison Formation tuffs, and Middle Jurassic tuffs from the Carmel and Temple Cap Formations, all subduction-related ashes. Fields for different types of granite are from Christiansen et al. (1986). Note the four clusters formed by the DRF samples.

Figure 7: Discrimination diagram of K and Al in biotite from the DRF. The high Al analyses also have high K, an indication the biotites are well preserved. Metasediments becoming incorporated into the pre-eruptive magma may explain the unusually high Al and K levels for lower Lapoint Member tuff beds (DRF-C, DRF-D, DRF-F, DRF-G).

Figure 8: Analytical totals for Duchesne River Formation biotites. Only analyses with totals >90% were used in this study.

Figure 9: Constellation plot showing how individual biotite analyses compare to each other on the basis of oxide wt. % of TiO2, Al2O3, FeOt, MgO, Na2O, K2O, F, and Cl. Colors correspond to groups of samples which are compositionally similar. Note that DRF-D has been divided into (a) and (b) subgroups. This mixing is likely due to detrital mixing post-deposition. DRF-F, DRF-C, DRF-G, and DRF-D are somewhat mixed but become more distinctive when all elements are considered, especially SiO2, FeOt, and MgO.

Figure 10: Scatter box matrix of the elements analyzed in Duchesne River Formation biotites. Colored clusters represent groups of samples with similar compositions for the given constituents. The clusters are made of the same samples in all the other biotite diagrams. The orange outlier analyses from DRF-C (Lapoint Member) is not included in any other diagram and given the high Na, F, and Mg are probably highly altered

Figure 11: Mg and Fe abundances for biotite phenocrysts from the tuffs of the Duchesne River Formation plot near or above a ratio of 3 to 2. A ratio of 3/2 eliminates sericite alteration as a cause for the high amounts of Al and K in many of samples.

Figure 12: Temperatures calculated from biotite compositions using the thermometer of Luhr et al, (1984) which depends on Ti/Fe ratio. Note that the same samples are grouped in this diagram as Figure 6.

Figure 13: Compositions of Duchesne River Formation biotites compared to the Fish Canyon Tuff, Morrison Formation tuffs (Christiansen et al., 2015), and to ash beds in the Middle Jurassic of southern Utah (Kowallis et al., 2001) in terms of log(XMg/XFe) vs. log(XF/XOH), where X is mole fraction. Granite fields are for Sierra Nevada granitoids from Ague and Brimhall (1988): I-SCR—I- type, strongly contaminated and reduced; I-SC—I-type, strongly contaminated; I-MC—I-type, moderately contaminated; I-WC—I-type, weakly contaminated.

Figure 14: The compositions of the majority of Duchesne River Formation biotite phenocrysts plot within typical F/Cl ratios of calc-alkaline rocks.

Figure 15: Ternary diagram of feldspar compositions. DRF-A is the only sample with two feldspars. Sanidines from the Duchesne River Formation have relatively high Or levels. Potassium feldspar with >Or90 were classified as detrital grains, possibly microcline. These high Or grains are late Jurassic to early Cretaceous in age.

Figure 16: Discrimination diagram of Ba and K in Duchesne River Formation sanidine grains. Grouping of samples is different than Figure 15 and the clusters likely do not indicate similarities between different tuffs.

Figure 17: (A) Allanite compositions from the Duchesne River Formation compared to the Bishop Tuff and Toba Tuff which are relatively more enriched in REEs and other A-site substitutes. (B) Duchesne River Formation allanites have lower Fe2+/FeTotal ratios (0.3-0.4) than both the Bishop and Toba tuffs, implying low fO2.

Figure 18: Compositions of titanite from the Duchesne River Formation. (A) Chondrite normalized chondrite values of LREEs and Y from 14 titanite grains from sample DRF-I. Tertiary ash field includes the Fish Canyon, Hiko, Leach Canyon, Racer Canyon, Toiyabe tuffs plotted for reference 9all Cenozoic tuffs from the western United States). DRF titanites are generally depleted in Ce, Nd, Sm, and Y compared to other Cenozoic volcanic titanites. (B) DRF-I titanites are depleted in both Al and Fe compared to other Cenozoic volcanic titanites.

Figure 19: LREEs plotted for Duchesne River Formation tuffs, average LREE abundances of the Bishop and Toba tuffs, and average LREE abundances in Duchesne River Formation titanites (DRF-I). Duchesne River Formation allanites are enriched in LREEs, including Y, compared to Duchesne River Formation titanites.

Figure 20: Glass shards from DRF-A plot within the rhyolite field according to the IUGS classification diagram from Le Bas et al., (1986).

Figure 21: A) Immobile element diagram modified from Winchester and Floyd (1977). Duchesne River Formation tuffs all plot in the rhyolite field. B) Duchesne River Formation tuffs plot in the dacite and bottom of the rhyolite field due to high Ti ppm. Unaltered fallout tuff compositions from the Northeast Nevada volcanic field shown for comparison (Brooks, 1995).

Figure 22: Trace element discrimination diagrams from Pearce et al. (1984). A) Rb ppm versus Y+Nb ppm. Duchesne River Formation tuffs and northeast Nevada tuffs plot in the volcanic arc field. B) Nb ppm versus Y ppm. Duchesne River Formation and northeast Nevada tuffs plot in the volcanic arc field and are almost identical except for two anomalous low Y samples.

Figure 23: Trace element concentrations normalized to primitive mantle (McDonough and Sun, 1995) for the Duchesne River Formation tuffs and Morrison Formation tuffs. Many of the samples from the Morrison Formation exhibit argillic alteration and were used to calculate average trace element abundances. The Duchesne River Formation tuffs and Morrison Formation tuff average are similar and have general patterns typical of a subduction zone setting.

Figure 24: Two element diagrams of the Duchesne River Formation tuffs and select Morrison Formation tuffs showing the different effects of alteration type on major elements. The Duchesne River Formation and Morrison Formation argillic samples show similar patterns of SiO2 depletion and MgO and Al2O3 enrichment. Morrison Formation feldspathic samples show an opposite relationship and zeolitic samples are neutral.

Figure 25: Timeline illustrating the timing of major, regional magmatic and tectonic events in Utah, Nevada as well as local events in the Uinta Basin area. The Age column arranges various dates by the method used to obtain the date. Note the large errors on the U/Pb, fission track, and K/Ar ages. Age 9, is a fission track age from Bryant et al., (1989) for the Lapoint Member and is grossly inaccurate and is partly responsible for the confusion about the exact age of the Duchesnean Land Mammal Age. Also note the ages of rhyolitic ashes from the northeast Nevada volcanic field shown by the orange triangles. These ages align well with the ages of the Duchesne River Formation tuffs. Only tuffs DRF-A and DRF-H are placed in absolute age location. The other undated tuffs are placed in their relative position and do not indicate an exact age. The ages of the volcanic and plutonic events (orange and red boxes) indicates that the northeast Nevada volcanic field is the most likely volcanic source of the Duchesne River Formation tuffs. Ages and references for the isotopic dates are as follows: 1) 30.54 +/- 0.22 Ma sanidine (Kowallis et al, 2005); 2) 34.03 +/- 0.04 sanidine (Kowallis et al., 2005); 3) 37 +/- 0.8 Ma zircon (Bryant, 1989); 4) 37.6 +/- 1.4 Ma zircon (Bryant, 1989); 5) 41.1 +/- 0.5 biotite (Kowallis and Sprinkel, 2007); 6) 41.5 +/- 0.5 Ma biotite (Kowallis and Sprinkel, 2007); 7) 39.36 +/- 0.15 sanidine (this report); 8) 39.47 +/- 0.16 plagioclase (this report); A 39.61 +/- 0.13; B 39.5 +/- 0.2; C 39.76 +/- 0.13; D 39.85 +/- 0.15; E 39.23 +/- 0.5, (Brooks, 1995). Time scale for Land Mammal Age from Prothero, (1995). Timing of the volcanic events were obtained from: Brooks, 1(995); Henry et al., (1995); Woodburne (2004); Constenius et al (2003); Moore et al (2008); Broomfield at al, (1977); Wooden et al (1999); Best and Christiansen (2013ab); Moore (1973); Johnson (2015); Shubat, (2011); Lund-Snee and Miller, (2016). The Keg Mountain event includes the Mount Laird Tuff, Keg Tuff, and Joy Tuff. Timing of the plutonic events were obtained from: Woodburne (2004); Constenius et al (2003); Egger et al (2003); William et al (2006); Wooden et al (1999); Vogel et al (2001); Bromfield et al (1977); Moore (1973); Biek et al (2006), Warnaars et al (1978), Strickland et al (2011), Ryskamp et al, (2008); Barnes et al, (2001); Deino and Keith, (1997), Jones et al, (1997). The plutonic events, other than the Bingham Canyon stock, Sulphur Springs Range. Granitic stocks of the Park City mining district include the Clayton Peak pluton, Little Cottonwood Stock, Alta Stock, Ontario Stock, and Park Premier Stock, Valeo Stock, Flagstaff Stock, and Pine Creek Stock. The Sulphur Springs Range includes the East Humboldt, and Ruby River Range granites. Folding of the Duchesne River Formation took place over a short span of time, and was likely coeval with the deposition of the Starr Flat Member. Renewed uplift events were constrained by the ages from DRF-A and DRF-H, the ages from the Bishop Conglomerate (Kowallis et al, 2005), and the occurrence of conglomerate units. The timing of the Deep Creek fault zone is modified from Poduska (2015).

Figure 26: Regional map of the western United States showing location of rhyoltic-dacitic volcanic rocks between 39.2-39.7 Ma from the Western North America Volcanic and Intrusive Rock Database (NAVDAT). Note that activity is focused in northeastern Nevada. The occurrences in Idaho and eastern Oregon are dated by regional correlations and have large uncertainties in their ages.

TABLES Table 1: Characteristics of tuffs from the Duchesne River Formation

Temperaeture Phenocrsyt (°C) Rock type Tect. Set. Member Sample Latitude Longitude Assemblage Age (Ma) Feld Bt (Zr/Ti -Nb/Y) (Nb/Y) Lower Lapoint Lapoint DRF-A 40.491478 -109.746676 Qz-Bt-Sa-Pl-Aln-Gls-(Zrn) 630 Dacite 39.47 +/- 0.16 711 Volcanic Arc Lapoint DRF-B 40.449856 -109.720892 Qz-Bt-Sa-(Mi) 704 Dacite Volcanic Arc Lapoint DRF-C 40.417679 -109.749658 Qz-Bt-Al 658 Dacite Volcanic Arc Lapoint DRF-D 40.416545 -109.746362 Qz-Bt-Ap-(Zrn) 697 Dacite Volcanic Arc Lapoint DRF-E 40.416564 -109.746445 Qz-Bt Dacite Volcanic Arc Lapoint DRF-F 40.416583 -109.746469 Qz-Bt 645 Dacite Volcanic Arc Lapoint DRF-G 40.416519 -109.746494 Qz-Bt-(Mi) 650 Dacite Volcanic Arc Dry Gulch DRF-H 40.491478 -109.746676 Qz-Bt-Sa-Aln-(Mi) Creek 638 Trachyte Volcanic Arc Upper Dry Dry Gulch Qz-Bt-Sa-Aln-Ttn-(Zrn- DRF-I 40.418127 -109.747087 Gulch Creek Creek Mi) 39.36 +/- 0.15 645 Trachyte Volcanic Arc Dry Gulch DRF-J 40.409754 -109.699966 Qz-Bt-Sa Creek 698 Rhyolite Volcanic Arc Dry Gulch DRF-K 40.397333 -109.726184 Qz-Bt-(Mi) Creek 696 Rhyolite Volcanic Arc

Note: Feldspar temperature calculated using the thermodynamic parameters of Elkins and Grove (1990) at a pressure of 5 kb. Biotite temperatures average of multiple grains, calculated using the thermometer of Luhr et al., (1984).

Table 2: Parameters for electron-microprobe analyses

Mineral Standard Analytical conditions for unknown Biotite Lemhi Bioite 20 nA current, 15kv acceleration voltage, 5µ beam size Sanidine orthoclase 20 nA current, 15kv acceleration voltage, 5µ beam size Plagioclase anorthothite 20 nA current, 15kv acceleration voltage, 5µ beam size Allanite none 20 nA current, 15kv acceleration voltage, 5µ beam size Titanite Sphene-T 30 nA current, 15kv acceleration voltage, 10µ beam size Apatite Apa-Durango 10 nA current, 15kv acceleration voltage, 5µ beam size Glass Rhyo-Gls 10 nA current, 15kv acceleration voltage, 5µ beam size

Table 3: Selected history of isotopic Dating of the Duchesne River Formation

Author Age Method Member Quad McDowell et al, (1974) 40.3 Ma K-Ar, biotite Lapoint Bryant (1989) 37.6 +/- 1.4 Ma K-Ar, biotite Starr Flat Neola Bryant, (1989) 36.9+/- 1.8 Ma Fission Track, zircon Lapoint VNW Bryant, (1989) 33.0 +/- 3.3 Ma Fission Track, zircon Dry Gulch Creek Bluebell Sprinkel (2007) 37 Ma U/Pb, zircon *Starr Flat VNW Kowallis (2007) 45.1 +/- 0.5 Ma K-Ar, biotite Dry Gulch Creek VNW Kowallis (2007) 41.1 +/- 0.5 Ma 40Ar/39Ar, biotite Lapoint Lapoint Kowallis (2007) 41.1 +/- 0.3 Ma 40Ar/39Ar, biotite Brennan Basin Lake Mtn This Study 39.36 +/- 0.15 40Ar/39Ar, plg Lapoint VNW This Study 39.47 +/- 0.16 40Ar/39Ar, sanidine Dry Gulch Creek VNW *Exact location uncertain.

Table 4: List of events, ages, and references used in Figure 25

Magmatic Event/Region Age (Ma) Reference northeast Nevada volcanic field 42.6-39 Ma Brooks, 1995 Grouse Creek 41.3±0.3, 36.3±0.2, 34.3±0.3 Strickland et al, 2011; Egger, 2003 Tuscarora 39.8-40.5 Ma Henry et al, 1995 Park City 35-41.1 Ma Bromfield and Erickson, 1977; Ibapah Granite 39 Ma Hintze and Kowallis, 2009 Bingham volcanics 34.2-39 Ma Moore, 1973; Biek, 2006 Bingham canyon stock 37.0-38.6 Ma Deino and Keith, 1997; Warnaars, 1978; Robinson Mt volcanics 37.5-38.5 Ma Lund-Snee et al, 2015 Emmigrant Pass volcanics 36.4-38.2 Ma Johnson, 2015; Egger, 2003 Goldhill/Pilot Peak 37.7-38.2 Ma Woodburne, 2004; Wooden et al, 1999 McGinty intrusion 37 Ma Hintze and Kowallis, 2009 Moroni/Goldens Ranch 35.9-39.9 Ma Hintze and Kowallis, 2009 Keg Mountain tuffs (keg, Mt Laird, Mt. Laird tuff 36.59±0.29; Keg tuff Joy) 36.77±0.12; Joy 34.92±0.16 Shubat, 2011 Marysvale volcanics 31-35 Ma Moore et al, 2008 central Nevada volcanics 18-36 Ma Best and Christiansen, 2008; Christiansen, 2015 Tintic volcanics 30.3-35 Ma Moore et al, 2008 keetely volcanics 32-35 Ma Crittenden, 1973; Bromfield and Erickson, 1977 Desert Mountain 35 Ma Wooden et al, 1999 Alta Stock 34.4 Ma Biek et al, 2006 Almo granite 34.4 Ma Hintze and Kowallis, 2009 new Duchesne River Formation Tuffs 39.4 Ma This study old Duchesne River Formation tuffs 41.5 Ma Kowallis and Sprinkel, 2007; McDowel, 1973 northeast Nevada volcanic field tuffs 42.6-39 Ma Brooks, 1995 Sulpher Springs Range 31.5-36.9 Ma Ryskamp et al, 2008 Harrison Pass 36 Ma Barnes et al, (2001)

Clayton Peak 36-35 John et al, 1997 Little Cottonwood 31-30 John et al, 1997 Ontario stock 36 John et al, 1997, Bromfield and Erickson, 1977 Park Premier 35-32 Ma John et al, 1997, Bromfield and Erickson, 1977 Pine Creek Stock 35-38.5 Ma John et al, 1997 Valeo Stock 34.6-40.3 Ma John et al, 1997 Flagstaff Stock 37.8-40.8 Ma John et al, 1997, Bromfield and Erickson, 1977 Mayflower Stock 32-40 Ma John et al, 1997, Bromfield and Erickson, 1977

Table 5: X-ray fluorescence analyses of altered tuffs from the Duchesne River Formation

Sample: DRF-A DRF-B DRF-C DRF-D DRF-E DRF-F DRF-G DRF-H DRF-I DRF-J DRF-K Member: Tdl Tdl Tdl Tdl Tdl Tdl Tdl Tdd Tdd Tdd Tdd Major Oxides SiO2 71.81 59.62 52.70 68.84 73.86 70.11 65.58 67.37 66.66 54.16 73.62 TiO2 0.44 0.38 0.40 0.51 0.53 0.31 0.38 0.24 0.18 0.21 0.30 Al2O3 14.91 17.57 16.74 17.60 13.29 15.41 16.45 21.29 21.34 14.64 7.55 Fe2O3 3.08 3.14 3.64 2.99 3.46 2.67 3.24 2.33 2.51 1.99 1.72 MnO 0.04 0.08 0.27 0.01 0.02 0.02 0.07 0.01 0.01 0.27 0.06 MgO 1.60 5.69 5.78 5.20 3.08 4.60 4.91 5.35 5.83 4.80 2.32 CaO 2.97 10.74 19.79 2.54 2.40 3.41 6.84 1.76 1.74 21.42 12.23 Na2O 2.06 1.71 0.09 0.91 0.66 1.47 0.91 0.61 1.08 1.29 0.17 K2O 2.98 0.93 0.45 1.25 2.39 1.88 1.51 0.99 0.61 1.14 1.81 P2O5 0.11 0.13 0.13 0.16 0.32 0.12 0.11 0.07 0.04 0.07 0.24 Analytical Total 99.99 99.74 100.30 99.93 99.97 99.92 100.01 99.91 99.94 99.94 100.16

LOI 4.83 12.06 18.97 7.37 5.44 7.28 9.94 8.69 8.55 18.21 11.09

Trace Elements Rb 126 39 39 42 81 45 44 41 23 34 52 Sr 352 217 225 276 205 222 255 239 268 252 127 Y 18 17 21 21 41 18 16 6 5 30 16 Zr 174 135 140 150 153 135 122 103 88 104 182 Nb 13 12 13 14 12 9 10 18 18 11 7 Sc 8 7 6 6 9 5 8 4 3 1 9 V 37 53 48 45 55 39 46 14 12 21 38 Cr 22 10 22 22 40 18 20 9 4 11 27 Ni 9 10 17 11 15 12 17 17 12 9 12

Cu 10 10 13 13 16 7 9 10 6 8 17 Zn 60 60 74 58 55 64 82 60 59 47 44 Ga 18 15 20 18 16 19 21 19 19 14 9 Ba 750 421 179 184 280 432 363 68 48 193 189 La 28 22 22 33 44 23 27 12 11 26 25 Ce 56 52 43 64 83 44 52 23 27 47 40 Nd 26 16 7 30 43 24 25 15 16 4 9 Sm 6 4 3 7 8 6 5 5 5 2 2 Pb 22 27 21 23 19 21 24 33 35 24 12 Th 13 16 13 14 11 9 11 17 16 12 9 U 4 7 2 4 3 3 5 2 2 5 3

Table 6: Compositions of Duchesne River Formation Biotite Phenocrysts

Sample DRF-A SiO2 36.68 38.05 37.05 35.99 39.79 36.83 36.24 35.54 36.62 TiO2 3.38 3.09 3.45 3.52 3.21 3.34 3.40 3.49 3.32 Al2O3 13.39 12.61 13.75 13.61 11.98 13.86 14.22 13.63 12.62 FeO t 16.964 15.84 16.70 16.90 15.17 16.86 17.27 17.18 15.82 MnO n.d. 0.01 n.d. n.d. n.d. n.d. 0.01 n.d. n.d. MgO 11.59 10.76 12.19 11.90 11.66 12.30 12.26 11.51 11.53 CaO 0.29 0.48 0.23 0.25 0.39 0.21 0.13 0.20 0.29 Na2O 0.21 0.33 0.36 0.32 0.21 0.34 0.32 0.34 0.24 K2O 7.71 7.31 7.95 7.91 7.46 8.12 8.34 8.18 7.56 BaO F 0.37 0.40 0.49 0.47 0.33 0.34 0.41 0.43 0.35 Cl 0.09 0.09 0.09 0.09 0.12 0.08 0.09 0.06 0.05 H2O* 3.5 3.4 3.51 3.45 3.51 3.59 3.56 3.45 3.43 Total 94.03 92.24 95.59 94.23 93.69 95.75 96.10 93.86 91.70