GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 7 2020 © 2020 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to GIW@utahgeology.org. FALLOUT TUFFS IN THE EOCENE DUCHESNE RIVER FORMATION, NORTHEASTERN UTAH—AGES, COMPOSITIONS, AND LIKELY SOURCE Michael S. Jensen, Bart J. Kowallis, Eric H Christiansen, Casey Webb, Michael J. Dorais, Douglas A. Sprinkel, and Brian Jicha GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover East view of the prominent 5.5-m-thick tuff bed at the base of the Lapoint Member of the Duchesne River For- mation. The base of the light-colored tuff bed is used as the contact with the underlying Dry Gulch Creek Mem- ber of the Duchesne River Formation. This outcrop of the tuff is located about 13 km west of Vernal, Utah, just off Utah SR-121 at 40.416519 N. latitude and 109.746494 W. longitude. See figure 3 and appendix 1 in this article for details. i UGA Board 2020 President Leslie Heppler lheppler@utah.gov 801.538.5257 2020 President-Elect Riley Brinkerhoff riley.brinkerhoff@gmail.com 406.839.1375 2020 Program Chair Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 2020 Treasurer Greg Gavin greg@loughlinwater.com 801.538.4779 2020 Secretary Elliot Jagniecki ejagniecki@utah.gov 801.537.3370 2020 Past President Peter Nielsen peternielsen@utah.gov 801.537.3359 UGA Committees Environmental Affairs Craig Eaton eaton@ihi-env.com 801.633.9396 Geologic Road Sign Greg Gavin greg@loughlinwater.com 801.541.6258 Historian Paul Anderson paul@pbageo.com 801.364.6613 Membership Rick Ford rford@weber.edu 801.626.6942 Outreach Greg Nielsen gnielsen@weber.edu 801.626.6394 Public Education Zach Anderson zanderson@utah.gov 801.537.3300 Matt Affolter gfl247@yahoo.com Publications Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Publicity Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Social/Recreation Roger Bon rogerbon@xmission.com 801.942.0533 AAPG House of Delegates 2017–2020 Term Tom Chidsey tomchidsey@utah.gov 801.537.3364 State Mapping Advisory Committee UGA Representative Bill Loughlin bill@loughlinwater.com 435.649.4005 UGA Newsletter Newsletter Editor Bill Lund uga.newsletter@gmail.com 435.590.1338 UGA Website — www.utahgeology.org Webmaster Paul Inkenbrandt paulinkenbrandt@utah.gov 801.537.3361 Become a member of the UGA to help support the work of the Association and receive notices for monthly meetings, annual field conferences, and new publi- cations. 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Earthquake Safety Committee Chair Grant Willis gwillis@utah.gov 801.537.3355 Douglas A. Sprinkel Azteca Geosolutions 801.391.1977 GIW@utahgeology.org dsprinkel@gmail.com Bart J. Kowallis Brigham Young University 801.422.2467 bkowallis@gmail.com Steven Schamel GeoX Consulting, Inc. 801.583-1146 geox-slc@comcast.net Thomas C. Chidsey, Jr. Utah Geological Survey 801.537.3364 tomchidsey@utah.gov John R. Foster Utah Field House of Natural History State Park Museum 435.789.3799 eutretauranosuchus@ gmail.com Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 7 2020 1 ABSTRACT Thin fallout tuffs are common in the terrestrial deposits of the Eocene Duchesne River Formation on the flanks of the Uinta Mountains of eastern Utah. Their ages and compositions provide new insight into the tectonic events and magmatic history of the western Cordillera and provide important constraints on the Cenozoic land mammal chronology. Whole-rock compositions of the volcanic ash show that they underwent post-emplacement argillic alteration, typical of a wetland/floodplain depositional setting. However, immo- bile element ratios and abundances, such as Zr/Ti, La/Nb, and Y are typical of rhyolites formed in a subduc- tion-related setting. Glass shards preserved in one sample all had SiO2 values >75%, typical of high-silica rhyolite. Preserved phenocrysts in the ash beds include quartz, sanidine, plagioclase, and biotite with vari- able amounts of accessory zircon, apatite, titanite, and allanite. Biotite compositions have Fe/(Fe+Mg) ratios typical of calc-alkaline igneous rocks and clusters of chemical compositions suggest a genetic relationship to three or four separate eruptions. Sanidine compositions from five samples range from Or73 and Or79. Only one sample had preserved plagioclase with compositions ranging between An22 – An49. Allanite from the ash beds has lower total rare earth elements (REE) concentrations than allanite from other well-studied rhyo- lites. Titanite in one sample has lower concentrations of REE, Fe, and Al than expected of rhyolites and is probably detrital. Plagioclase and sanidine from two different tuff beds near the middle of the Duchesne River Formation yielded analytically indistinguishable 40Ar/39Ar 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, one of the few volcanically active regions of western North America at the time. These new radiometric ages, along with stratigraphic relations and previously published ages for tuffs in the Bishop Conglomerate (which unconformably overlies the Duchesne River Formation), constrain the timing of late Laramide uplift in the region from 39 to about 37 Ma and post-Laramide epeirogenic uplift from 34 Ma to 30 Ma. Finally, the ages also provide additional evidence that the Duchesnean North Amer- ican Land Mammal Age ended in the Eocene, which was originally named and defined from the Duchesne River Formation. Fallout Tuffs in the Eocene Duchesne River Formation, Northeastern Utah—Ages, Compositions, and Likely Source Michael S. Jensen1, Bart J. Kowallis1, Eric H Christiansen1, Casey Webb1, Michael J. Dorais1, Douglas A. Sprinkel2, and Brian Jicha3 1 Department of Geological Sciences, Brigham Young University, Provo, UT, 84602; wasabae@gmail.com 2 Azteca Geosolutions, Pleasant View, UT 84414; Utah Geological Survey, Salt Lake City, UT 84114; dsprinkel@gmail.com 3 University of Wisconsin, Department of Geoscience, Madison, WI, 53706; bjicha@geology.wisc.edu Citation for this article. Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B., 2020, Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source: Geology of the Intermountain West, v. 7, p. 1–27, 2 appendices, supplemental data, https://doi.org/10.31711/giw.v7.pp1–27. © 2020 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to GIW@utahgeology.org. 2 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 INTRODUCTION Preserved volcanic ash in sedimentary rock sec- tions can provide insights into the geologic history of a region through radioisotopic dating and geochemical analysis, leading to better interpretations of tectonic events and settings (Kowallis and others, 2001; Hildreth and Wilson, 2007; Smith and others, 2014; Christiansen and others, 2015; Hong and others, 2019). Radiomet- ric dating of tuffs also provides better age constraints on sedimentary rocks and fossils (Kowallis and others, 1991, 1998, 2001; Riggs and others, 2003; Smith and Carrol, 2015). Additionally, accurate dates of volcanic ash in sedimentary succession can be used as important markers for stratigraphic correlation (Huff, 2016). We have employed high-precision, single-crystal, 40Ar/39Ar laser-fusion dating along with whole-rock and mineral geochemical analyses on the fallout tuffs in the Duchesne River Formation located in the Uinta Basin of eastern Utah. Previous work on the Duchesne Riv- er Formation tuff beds has provided some information on their age (Andersen and Picard, 1974; Bryant and others, 1989; Sprinkel, 2007) but the dates were impre- cise and geochemical data was essentially nonexistent. In the overlying Bishop Conglomerate, which also con- tains altered fallout tuffs, previous work 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 al- tered middle Cenozoic tuffs, as well as inferences about the original composition (Kowallis and others, 2005). In this study, we attempt to answer the following ques- tions: 1. What are the ages of the fallout tuffs within the Duchesne River Formation? 2. What phenoclasts are preserved within these tuffs and what are their compositions? 3. How has alteration affected the chemistry of the Duchesne River Formation tuffs and can infer- ences be made about their original composition? 4. Where is/are the eruptive source or sources of the tuffs? 5. Do more precise ages help to better constrain the age of fauna within the Duchesne River Forma- tion, which serves as the type formation for the Duchesnean North American Land Mammal Age (NALMA)? 6. Do the ages obtained from the ash beds help con- strain periods of uplift of the Uinta Mountains? GEOLOGIC SETTING Laramide Orogeny The Laramide orogeny was characterized by base- ment block uplift along high-angle reverse faults appar- ently due to shallow subduction of the Farallon plate from 70-34 Ma (Dickinson and others, 1988; Liu and others, 2010; Jones and others, 2011; Fan and Carrapa, 2014; Yonkee and Weil, 2015). The east-west-trending Uinta Mountain range is one such basement block and has experienced multiple periods of uplift and erosion (Hamilton, 1978; Hansen, 1986; Dickinson and others, 1988) as recorded by the sediments in the adjacent Uin- ta Basin (figure 1), including the Green River, Uinta, and the Duchesne River Formations. Ponded basins, such as the Uinta Basin, were common during the Lar- amide and were often filled with fresh or saline lakes that acted as efficient sediment traps (Carroll and Bo- hacs, 1999; Tanavsuu-Milkeviciene and others, 2017). 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 have been used to date the timing of volcanic, tectonic, and sedimentary events (Bryant and others, 1989; Remy, 1992; Smith and others, 2003; Kowallis and others, 2005; Smith and Car- roll, 2015). During the early stages of the Laramide orogeny, volcanism was uncommon due to shallow subduction of the Farallon plate (DeCelles, 1994; Dickinson, 2004; Schellart and others, 2007). However, as the Farallon plate steepened, subduction-related volcanism resumed about 54 Ma beginning in present-day Montana and Idaho (Norman and Mertzman, 1991) and then migrat- ed south into the Nevada-Utah region around 40 Ma, finally reaching the southern Great Basin about 36 Ma (Lipman and others, 1972; Humphreys, 1995; Castor and others, 2000, 2003; Best and others, 2013a). Volca- 3 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 Tds Tdl Tdd Tdb DRF-B DRF-C through DRF-I DRF-J Tb K DRF-A 1 2 9 24 54 3 3 0 1 0 1 kilometers miles Tuff bed (Tb) Bishop Conglomerate Duchesne River Forma�on (Tds) Starr Flat Mbr (Tdl) Lapoint Mbr (Tdd) Dry Gulch Creek Mbr (Tdb) Brennan Basin Mbr (K) Cretaceous units undivided Tuff sample loca�ons Volcanic field Strike and dip symbol VNW - Vernal NW quadrangle B 40°30' 40°22'30'' 109°45' 109°37'30'' DRF-K Utah State Route SR-121 A Challis 51-45 Ma Absaroka 55-45 Ma NE Nevada 43-39 Ma Southern Rocky Mountains 36.5-30 Ma 100 mi Keetley 35-32 Ma Oregon Nevada Idaho W yo m in g Arizona New Mexico Utah Colorado California Tuscarora 40-39 Ma 114°116°118°120° 112° 110° 108° 36° 38° 40° 42° VNW Uinta Mountains 100 km Tin�c 35-30 Ma Central Nevada 36-18 Ma Indian Peak- Caliente 36-18 Ma Thomas-Keg-Desert Mtn 42-30 Ma Marysvale 30-22 Ma Figure 1. (A) Regional map of western United States with boundaries and ages of middle Cenozoic volcanic fields that formed as a result of slab rollback. (B) A simplified geologic map of the Vernal NW quadrangle showing locations of tuff samples, the principal geologic units, and contacts of the different members of the Duchesne River Formation. The Cretaceous units are undivided. 4 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 nic activity during this time period was so intense it has been called the middle Cenozoic ignimbrite flare-up. The Challis, Absaroka, northern Nevada-Utah, and the central and southern Great Basin volcanic fields doc- ument this time of intense volcanic activity (Brooks, 1995a; Chandler, 2006; Henry, 2008; Best and others, 2016). These repeated volcanic episodes have been pro- posed as a contributor to the global cooling at the Eo- cene-Oligocene boundary (Zanazzi and others, 2007; Jicha and others, 2009). Silicic volcanic rocks from this time period typically range from dacite to rhyolite in composition and are commonly preserved as fallout tuffs, ash-flow tuffs, flow breccias, and silicic domes (Lipman and others, 1972; Brooks and others, 1995c). Like other distal fallout tuffs, the Duchesne River For- mation tuffs were likely sourced from large plinian or coignimbrite eruptions (Blaylock, 1998; Kowallis and others, 2005; Chandler, 2006; Christiansen and others, 2015). Duchesne River Formation Stratigraphy Sedimentary rocks, which record the uplift and ero- sion history of the Uinta Mountains and contain fall- out tuffs that can be used to study and date that record, are well exposed on the southern flanks of the Uinta Mountains in the Vernal NW quadrangle just west of Vernal, Utah (figure 1). Along with Cretaceous units, the predominant formation within the quadrangle is the Duchesne River Formation where all four mem- bers (in ascending stratigraphic order: Brennan Basin, Dry Gulch Creek, Lapoint, and Starr Flat) are exposed (figure 2). Tuffaceous beds were not found in the bas- al Brennan Basin Member and the capping Starr Flat Member in the study area; however, tuff beds have been reported within these members elsewhere in the Uin- ta Basin region (Sprinkel, 2007, 2018a). The Dry Gulch Creek and Lapoint Members contain numerous tuffa- ceous beds. These members are predominantly com- posed of siltstone and mudstone that intertongue with fine- to medium-grained sandstone and conglomerate with both members becoming progressively coarser towards the uplifted Uinta Mountain front (Warner, 1966; Andersen and Picard, 1972, 1974; Sato and Chan, 2015; Webb, 2017). Tuff beds in the Dry Gulch Creek and Lapoint Members have been mostly altered to clay minerals (Andersen and Picard, 1974). They are light to medium gray and stand out against the moderate-red to reddish-brown siltstone and mudstone (figure 3). Sev- eral tuff beds are laterally extensive throughout much of the quadrangle and the lowermost tuff bed is used as the contact between the Lapoint and Dry Gulch Creek Members (Webb, 2017) (figure 3a). The oldest and youngest members of the Duchesne River Formation, the basal Brennan Basin Member and the capping Starr Flat Member, respectively, are predominantly pebble to boulder conglomerates with minor sandstone and siltstone, indicating they either were deposited in closer proximity to the uplifting Uin- ta Mountain front to the north or were deposited when streams flowing off the uplift were transporting coarse material farther out into the basin as a result of increased gradient (Sato and Chan, 2015). The Starr Flat Member is capped by the Gilbert Peak erosion surface, a wide- spread Oligocene surface of erosion and non-deposition found on both the south and north flanks of the Uinta uplift (Hansen, 1986; Sprinkel, 2007; Webb, 2017). The Gilbert Peak erosion surface is considered to mark the end of the Laramide orogeny in the Uinta Basin region (Hansen, 1986; Aslan and others, 2017). The stratigra- phy and distribution of the Duchesne River Formation members within the study area are reported in greater detail in Webb (2017). In addition to the tuff beds, the Duchesne River Formation also contains key mammal fossils and has been used as the type section of the Duchesnean NALMA (Wood and others, 1941; Clark and others, 1967; Prothero, 1995). The Duchesnean NALMA 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 middle Eocene (Emry, 1981; Rassmussen and others, 1999; Alroy, 2000). SAMPLING AND ANALYTICAL METHODS Stratigraphic relations and structural evolution of the area were established by geological mapping (Webb, 2017). During the course of the mapping, four samples were collected from three tuff beds within the upper and 5 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 DRF-K (25 cm thick) DRF-B (1 m thick) DRF-C (30 cm thick) DRF-A (50 cm thick), 39.47± 0.16 Ma DRF-D, E, F, G (5.5 m thick) DRF- H, I (25 cm thick), 39.36 ± 0.15 Ma DRF-J (27 cm thick) La te E oc en e D uc he sn e R iv er La po in t D ry G ul ch C re ek Ep oc h Fo rm at io n M em be r Stratigraphy A B (approximately 30 m) (approximately 25 m) (approximately 35 m) Formation Symbol Thickness meters not to scale Lithology Notes Bishop Conglomerate Starr Flat Member Lapoint Member Dry Gulch Creek Member Brennan Basin Member Tb Tds Tdl Tdd Tdb 315 102 - 243 0 - 111 0 - 150 437 - 564 Eo ce ne Unconsolidated deposits Q less than 50Quaternary D uc he sn e R iv er F or m at io n Gilbert Peak Erosion Surface Massive conglomerate Interbedded conglomerate/ sandstone and siltstone Prominent ash beds throughout Duchesneodus uintensis fossils Distinct sandstone tongues of Tds Fan conglomerates grade southwestward to sandstone Diplacodon elatus fossils 40Ar/39Ar age 34.03 ± 0.04 Ma angular unconformity local angular unconformity Group 1 Group 2 Group 3 Group 4 Geochemical Groups 40Ar/39Ar age 39.47 ± 0.16 Ma 40Ar/39Ar age 39.36 ± 0.15 Ma Epoch Oligocene 40Ar/39Ar age 30.54 ± 0.22 Ma Moderate-red to redish-brown mudstone, siltstone, and sandstone Redish-brown to red-purple siltstone, sandstone, and minor pebble conglomerate Halfway Hollow Quarry Un0117 (Agriochoerus maximus) Carnegie Titanothere Quarry Un0012 (Duchesneodus uintensis) Stratigraphic interval of fossil mammal quarries Figure 2. (A) Stratigraphic column of Paleogene units in the Vernal NW quadrangle from Webb (2017). 40Ar/39Ar ages from the Bishop Conglomerate are from Kowallis and others (2005) and fossils are from Burger and Tacket (2014). Red box indi- cates area of stratigraphic column shown in part B. (B) Stratigraphic column of the Dry Gulch Creek and Lapoint Members of the Duchesne River Formation showing the relative locations and thicknesses of the tuff beds (gray). Note that samples DRF-D, DRF-E, and DRF-F are from the same 5.5-m-thick tuffaceous bed which serves as the contact between the Dry Gulch Creek and Lapoint Members. Samples DRF-H and DRF-I are also from the same tuffaceous bed but collected from different locations about 2 km apart. Geochemical groups based on the chemical composition of the samples within each tuff. Note that the tuffs are grouped stratigraphically. 6 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 middle Dry Gulch Creek Member and seven samples were collected from four tuff beds within the Lapoint Member and within the Vernal NW quadrangle (table 1). Roughly 5 to 8 kg of sample was collected at each site (site descriptions and photographs can be found in appendix 1). A split of each sample was processed to liberate the mineral phenoclasts, which were extract- ed by standard washing and heavy liquid techniques. Phenocrysts were mounted in epoxy and then polished for microprobe analysis at Brigham Young University (BYU). Table 2 summarizes the analytical parameters used for each mineral. A portion of each sample was also prepared for X-ray fluorescence (XRF) analyses of major and trace elements also at BYU. Two hundred sanidine grains from sample DRF-H and two hundred plagioclase grains from DRF-A were hand-picked from the clean concentrates at BYU. They were dated using single crystal 40Ar/39Ar laser fusion meth- ods at the University of Wisconsin-Madison WiscAr Geo- chronology Lab. The 40Ar/39Ar ages were calculated rel- ative to the FC-201 sanidine standard age of 28.201 Ma and a total 40K decay constant of 5.643 e-10/a (Kuiper and others, 2008). Methodology for laser fusion dating of single crystals of sanidine are outlined on the Wis- cAr Lab website (http://geochronology.geoscience.wisc. edu/analytical-approaches/). A summary of all the ages and the analytical parameters is given in appendix 2. 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 Lapoint Member Dry Gulch Creek Member A B Figure 3. (A) View to the east of the basal 5.5-m-thick tuff bed at the contact (underlined in yellow) between with Dry Gulch Creek and the Lapoint Members. The gray color of the ash stands out against the reddish-orange colors of the siltstone. Sam- ples DRF-D, DRF-E, DRF-F, and DRF-G were collected from this key layer which shows evidence of detrital mixing and has likely been thickened by post-sedimentary processes. (B) Collection site of sample DRF-C shown between the red lines is 18 m above the basal ash bed in A, which is covered by Quaternary unconsolidated gravel. 7 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 eruption age can be determined. This method is par- ticularly critical for tuff beds collected from sedimenta- ry sections where even slight reworking of the ash can contaminate it. Both samples contained detrital feldspar much older than the Duchesne River Formation. 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. ISOTOPIC AGES Several isotopic ages have previously been report- ed from the prominent tuff bed near the base of the Lapoint Member of the Duchesne River Formation (ta- ble 3). Fairly consistent K-Ar and 40Ar/39Ar biotite ages of approximately 41 ± 0.5 Ma from this bed have been reported by McDowell and others (1973), Andersen and Picard (1974), and Prothero and Swisher (1992) whereas fission track ages from the same bed in the Lapoint Member averaged 34 ± 1–3 Ma (Bryant and others, 1989). Additionally, a biotite K-Ar age of 37.6 ± 1.4 Ma and a zircon fission track age of 36.7 Ma ± 3.9 Ma were obtained from a tuff within the younger Starr Flat Member from the Neola NW quadrangle (table 3). The previous K-Ar and fission track ages from the Duchesne River Formation all have relatively large uncertainties. The biotite ages may also suffer from alteration, typical of biotite in this type of environment, giving erroneous ages (Smith and others, 2008). We chose to date feldspar because it is more resistant than biotite to alteration and Table 1. Characteristics of tuffs from the Duchesne River Formation. Table 2. Parameters for electron-microprobe analyses. Member Sample Latitude Longitude Phenocryst Assemblage Age (Ma) Temperature (°C) Rock Type (Zr/Ti -Nb/Y) Tectonic Settting (Nb-Y)Feld Bt Lapoint DRF-A 40.491478 -109.746676 Qz-Bt-Sa-Pl-Aln-gls-Zrn 39.47 +/- 0.16 630 711 Dacite Volcanic Arc Lapoint DRF-B 40.449856 -109.720892 Qz-Bt-Sa-Mc 704 Dacite Volcanic Arc Lapoint DRF-C 40.417679 -109.749658 Qz-Bt-Aln 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-Mc 650 Dacite Volcanic Arc Dry Gulch Creek DRF-H 40.418116 -109.747051 Qz-Bt-Sa-Mc-Aln 638 Trachyte Volcanic Arc Dry Gulch Creek DRF-I 40.418127 -109.747087 Qz-Bt-Sa-Mc-Aln-Ttn-Zrn 39.36 +/- 0.15 645 Trachyte Volcanic Arc Dry Gulch Creek DRF-J 40.409754 -109.699966 Qz-Bt-Sa 698 Rhyolite Volcanic Arc Dry Gulch Creek DRF-K 40.397333 -109.726184 Qz-Bt-Mc 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 are an average of multiple grains, calculated using the thermometer of Luhr and others (1984). Mineral abbreviations from Whitney and Evans (2010). Aln is allanite, Ap is apatite, Bt is biotite, gls is glass, Mc is microcline, Pl is plagioclase, Sa is sanidine, Ttn is titanite, Qz, is quartz, Zrn is zircon. Mineral Standard Analytical conditions for unknown Biotite Lemhi Biotite 20 nA current, 15kv acceleration voltage, 5µ beam size Sanidine orthoclase 20 nA current, 15kv acceleration voltage, 5µ beam size Plagioclase anorthosite 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 Note: Names of standards are from the list of standards used by the BYU Department of Geological Science. 8 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 typically provides more accurate ages (Kowallis and others, 1998, 2001, 2005). Sanidine extracted from our sample DRF-H gave a 40Ar/39Ar age of 39.36 ± 0.15 Ma and plagioclase from DRF-A gave an age of 39.47 ± 0.16 Ma (figure 4). DRF-H was taken from a tuff bed at the top of the Dry Gulch Creek Member, only 2 to 3 m from the contact with the Lapoint Member. DRF-A was taken from a tuff bed within the overlying Lapoint Member several ki- lometers northeast of DRF-H near where the Lapoint Member pinches out between the Starr Flat Member and Brennan Basin Member (figure 1). The ages are stratigraphically inverted but are not analytically distin- guishable from one another. Despite being very close in age, DRF-A and DRF-H are not from the same ash bed based on field mapping and chemical and mineralogical differences outlined below. This is significant because sample DRF-A represents a tuff bed that was mapped as part of the Starr Flat Member (Sprinkel, 2007). Our recent mapping at 1:24,000 scale (Webb, 2017) and the new age suggest that the tuff bed is from the upper part of the Lapoint Member where the Lapoint and Starr Flat Members intertongue. The analytical uncertainties for these new ages are much smaller than previous ones and constrain the boundary between the Lapoint and Dry Gulch Creek Members to about 39.4 Ma. These ages are significantly younger than the previously reported ~41 Ma ages from the same series of tuff beds (McDowell and others, 1973; Andersen and Picard, 1974; Prothero and Swisher, 1992). Table 3. Compilation of selected isotopic ages of the Duchesne River Formation. 35 40 45 50 55 DRF-H (sanidine) 39.36 0.15 Ma± n = 13 of 25 A Age (Ma) 35 40 45 50 55 DRF-A (plagioclase) 39.47 0.16 Ma± n = 11 of 16 B Figure 4. Rank order plots with probability density curves for feldspar ages from samples DRF-H (A) and DRF-A (B) from the Dry Gulch Creek and Lapoint Members of the Duchesne River Formation, respectively. Individual 40Ar/39Ar dates, as well as weighted mean ages, are shown with 2σ analytical un- certainties. Filled circles are the ages that were used in the age calculation. Reference/Report Sample ID Age ±2sd (Ma) Method Member 7.5' Quadrangle Latitude (N) Longitude (W) This Study DRF-H 39.36 ± 0.15 40Ar/39Ar, plagioclase Lapoint Vernal NW 40° 24'59.4" 109° 44'47.6" This Study DRF-A 39.47 ± 0.16 40Ar/39Ar, sanidine Dry Gulch Creek Vernal NW 40° 29'28.9" 109° 41'28.94" Bryant and others (1989) N-83-2 36.7 ± 3.9 Fission Track, zircon Starr Flat Neola 40° 28'43" 110° 05'54' Bryant and others (1989) ICP-1A 30.5 ± 1.4 Fission Track, zircon Starr Flat Ice Cave Peak 40° 35'49" 109° 59'52" Constenius and others (2011) KNC070109-1 38.90 ± 0.80 U-Pb, zircon Starr Flat Wolf Creek Summit 40° 23'240" 111° 00'908" Bryant and others (1989) VNW-1 36.9 ± 1.8 Fission Track, zircon Lapoint Vernal NW 40° 24'33" 109° 42'01" Bryant and others (1989) LA-1 35.2 ± 1.6 Fission Track, zircon Lapoint Lapoint 40° 24'52" 109° 45'43" Prothero and Swisher (1992) LP1 39.47 ± 0.17 40Ar/39Ar biotite Lapoint Vernal NW Not reported Not reported Bryant and others (1989) BLU-83-1 33.0 ± 3.4 Fission Track, zircon Dry Gulch Creek Bluebell 40° 19'41" 110° 09'17" Bryant and others (1989) BLU-83-2 34.5 ± 4.4 Fission Track, zircon Dry Gulch Creek Bluebell 40° 19'40" 110° 09'21" UGS Apatite to Zircon (2014) HC08122012-1 40.66 ± 1.88 U-Pb, zircon Brennan Basin Hancock Cove 40° 18'02.89" 110° 04'58.03" 9 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 MINERAL ASSEMBLAGES AND COMPOSITIONS In addition to providing ages, magmatic phenocrysts in tuff beds can provide information that can help in determining the composition of the source magma, in correlating ash beds across a broader area, and provide insights into the tectonic setting. Table 1 summarizes the mineral assemblage of each of the samples collected from the Duchesne River Formation. The presence of quartz, biotite, and sanidine and absence of pyroxene and hornblende is a common subduction-related rhy- olitic mineral assemblage and typical of other volcanic ash beds from the late Eocene of western North America (Lipman and others, 1972). Allanite was found in four samples. Titanite and apatite were the least abundant minerals and were only found in one sample each. Be- cause these ashes were deposited in a dominantly fluvial environment, there is an inherent risk of contamination by detrital grains and all samples contained sedimenta- ry grains of quartz, carbonate, and diagenetic clay. Quartz Quartz grains were present in all the Duchesne Riv- er Formation samples. Igneous quartz grains were iden- tified by their prismatic, euhedral shape, and presence of 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 the Duch- esne River Formation tuffs and are the main mafic phase present. 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 weath- ering than sanidine, quartz, and allanite. The biotite phenocrysts collected 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. Chloritic alteration is a concern because K can be replaced with H2O and Cl during diagenesis (Bisdom and others, 1982; Smith and others, 2008). These chemical changes lead to low ana- lytical totals and the resulting isotopic ages cannot be considered accurate (Smith and others, 2008). To avoid alteration, only black, euhedral grains were picked for electron microprobe analysis and from those, only anal- yses with totals (not including water) of 90% or above were considered. Al, Fe, and Mg in biotite are typically fairly immo- bile and may help in understanding the original compo- sition of the tuff beds (Christiansen and others, 2015). Molar Fe/(Fe+Mg) and total Al (atoms per formula unit – apfu) plot within or near the calc-alkaline rhyolite field (figure 5) and are similar to biotites from the subduc- tion-related Oligocene Fish Canyon Tuff and Jurassic tuffs from the Temple Cap, Carmel, and Morrison For- mations (Kowallis and others, 2001; Christiansen and others, 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. Samples DRF-D, DRF-F, and DRF-G are from different horizons within the same prominent ash bed at 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 Fe /(F e+ M g) Al total (apfu) Two-mica granites A-type Calc-alkaline Bishop Tuff Fish Canyon Tuff and Middle Jurassic ash beds Biotite Morrison Fm tuffs Group 1 Group 2 Group 3Group 4 DRF-A DRF-B Group 1 DRF-C DRF-D DRF-F DRF-G Group 2 DRF-H DRF-I Group 3 DRF-J DRF-K Group 4 Figure 5. Compositions of biotite from the tuffs of the Duch- esne River Formation compared to the Bishop Tuff, Fish Can- yon Tuff, Morrison Formation tuffs, and Middle Jurassic tuffs from the Carmel and Temple Cap Formations, which are all subduction-related pyroclastic deposits. Fields for different types of granite are from Christiansen and others (1986). The four clusters formed by the Duchense River Formation sam- ples are grouped stratigraphically. Compositions are presented in atoms per formula unit (apfu). 10 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 the same geographic location at the base of the Lapoint Member (figures 1 and 2, appendix 1) and form the high Al group. An enrichment of Al is sometimes the result of alteration as the more mobile elements like K and Na are removed and Al becomes relatively enriched (Christiansen and others, 2015). However, the grains with high Al also have K levels of at least 8 wt.% (Jensen, 2017), an indication that the biotite has undergone little alteration. Sericitic alteration could explain the elevated Al paired with normal levels of K but sericitic alteration occurs at higher temperatures and also decreases Mg below a 3/2 Mg/Fe ratio, which is not the case for the Duchesne River Formation tuffs (figure 6). Addition- ally, these samples have relatively high analytical totals (93 > wt.%) compared to other Duchesne River Forma- tion biotite phenocrysts. 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 Member and they all have the Fe/Mg ratios, Al values, and high analytical totals described above; this consistency would not be expected if the biotite was al- tered by secondary processes. Consequently, we consid- er these biotites to have magmatic compositions. The total Al vs Fe/(Mg+Fe) diagram also reveals distinct clusters of biotite from the Duchesne River Formation tuffs (figure 5). The clustering follows strati- graphic order with the oldest middle Dry Gulch Creek samples, DRF-K and DRF-J, plotting together. The up- per Dry Gulch Creek samples DRF-H and DRF-I were taken from the same ash bed at two different localities about 2 km apart and form an isolated cluster. These two groups have Fe/(Mg+Fe) ratios slightly higher than typical biotite in calc-alkaline rhyolite, which could be an indication of a more evolved and differentiated melt or that crystallization occurred at a lower ƒO2 than typ- ical calc-alkaline rhyolites. Also seen on figure 5, five biotite grains from DRF-D (yellow) have lower total Al and Fe/(Fe+Mg) ratios than the rest of DRF-D grains. These grains have analytical totals above 96 wt.% so their anomalous compositions could be an indication of a detrital component or of the secondary mixing of ash from two separate eruptions rather than alteration. DRF-D was collected at the top of the prominent, basal ash bed of the Lapoint Member (figure 2 and appendix 1) and could have experienced reworking in the floodplain depositional environment. DRF-C represents a tuff bed about 10 m above the basal Lapoint Member 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 To see if the groups selected on figure 5 are statisti- cally 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 7) shows that the samples in the total Al diagram cluster in the same way when TiO2, Al2O3, FeOt, MgO, Na2O, K2O, F, and Cl are all considered. These four clusters of biotite analyses are grouped stratigraphically which suggests that the composition of the magmatic source of the Duchesne River Formation tuffs became less evolved over time. For example, Fe drops from 1.8 to 1.0 apfu, while Mg increases from 1.0 to 1.7 apfu (figure 6) from the lower Dry Gulch Creek Member tuffs to the mid Lapoint Member tuffs. Biotite compositions are also useful in geother- mometry (Luhr and others, 1984). Titanium is a tem- perature-sensitive element in biotite and increases with increasing temperature (figure 8). Temperature calcula- tions from the biotite minerals from the Duchesne River Figure 6. 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 3/2 ratio eliminates sericite alteration as a cause for the high amounts of Al and K in these tuffs. The Mg/Fe ratio gradually increases over time. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 M g (a pf u) Fe (apfu) Group 1 Group 2 Group 4 Group 3 DRF-A DRF-B Group 1 DRF-C DRF-D DRF-F DRF-G Group 2 DRF-H DRF-I Group 3 DRF-J DRF-K Group 4 11 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 Formation tuffs range between 625 and 725°C. The low end of the range is suspect because typical biotite tem- peratures in fresh rhyolite are close to 700°C but samples DRF-C through DRF-I all plot at 650°C or lower. These low-temperature biotite minerals also show enrichment of Al but 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 9) plot in the oxidized, moderately contaminated (I-MC) field (Ague and Brimhall, 1988). The biotites have lower log(XF/XOH) than that in the Oligocene Fish Canyon Tuff and only overlap the lower log(XF/XOH) of biotite from the Late Jurassic Morrison Formation tuffs from eastern Utah (Christiansen and others, 2015). Based on lower Mg/Fe ratios, they are also more reduced than most of the Middle Jurassic ash beds of the Temple Cap and Carmel Formations collected in southwestern Utah (Kowallis and others, 2001). Nonetheless, they are sim- ilar to Sierran granites (Ague and Brimhall, 1988) and thus are similar to other subduction-related, typically oxidized silicic suites. Feldspar Alkali feldspar was the most common feldspar found in the Duchesne River Formation tuffs and was present in six samples whereas phenocrysts of plagioclase were found in only one sample (DRF-A from the middle part of the Lapoint Member). Plagioclase phenocrysts were commonly zoned and compositions ranged from An50 to An17 (figure 10). Since plagioclase has a low tolerance to weathering, it is unlikely to be a detrital component and is considered volcanic. Of the six samples with al- kali feldspar, only four contained volcanic sanidine with Or72-Or81. The other alkali feldspar grains in DRF-B, DRF-G, DRF-I, and DRF-K are detrital with Or values >Or89; in DRF-H these are likely the grains that give older Eocene to Jurassic ages and usually have higher K/Ca ratios. Using plagioclase and sanidine from DRF-A, a two-feldspar eruption temperature was calculated using DRF-A DRF-B DRF-C DRF-D(a) DRF-F DRF-G DRF-H DRF-I DRF-J DRF-K DRF-D(b) Group 4Group 1 Group 2 Group 3 Figure 7. 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. Note that DRF-D was separated into different clusters (a) and (b). This mixing is likely due to post-deposition detrital mixing. DRF-F, DRF-C, DRF-G, and DRF-D are somewhat mixed but become more distinctive when all elements are consid- ered, especially SiO2, FeOt, and MgO. Figure 8. Temperatures calculated from biotite compositions using the thermometer of Luhr and others (1984) which de- pends on the Ti/Fe ratio. The groups in this diagram are the same as in figure 5. 0.100 0.150 0.200 0.250 0.300 500 550 600 650 700 750 800 Ti (a pf u) T (°C) Luhr DRF-A DRF-B Group 1 DRF-C DRF-D DRF-F DRF-G Group 2 DRF-H DRF-I Group 3 DRF-J DRF-K Group 4 12 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 SolvCalc (Wen and Nakvasil, 1994) and the thermody- namic parameters of Elkins and Grove (1990). Average temperatures of 643°C, 630°C, and 608°C were calculat- ed at 5, 3, and 2 kilobars, respectively, using the average composition of 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 not like- ly to be reliable. Allanite Allanite, a common accessory mineral in silicic ig- neous rocks, is a complex, rare earth element-rich min- eral of the epidote group with a general formula of A2M- 3Si3O11(O, F)(OH), in which A is commonly Ca2+, Th4+, REE3+, or U4+, and M typically includes Al3+, Fe3+, or Fe2+ (Deer and others, 1997). Rare earth elements (RRE) are essential constituents in allanite through the coupled substitution of Ca2++ Fe3+ = REE3+ + Fe2+ (Giere and So- rensen, 2004). Euhedral grains of allanite were found in three different tuff beds in the Duchesne River For- mation, represented by four samples—DRF-A, DRF-C, DRF-H, and DRF-I. The grains are relatively unaltered with analytical totals between 98% and 100%. Duchesne River Formation allanite minerals have CaO concentrations ranging from 11 to 13 wt.%, which is several percent higher than in allanite from the Toba and Bishop Tuffs of Sumatra and California, respective- ly (figure 11). They also have about 0.65 apfu total light rare earth elements (LREE), which is about 0.3 apfu less than the allanites in the Toba or the highly-evolved Bishop Tuff as shown on figure 12. High Ca and low REE could be an indication that the melt in which the allanite formed was slightly depleted in REE and so Ca remained high 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, allanite from the Bish- op Tuff (Hildreth, 1979) has higher REE concentrations than allanite in the Toba Tuff but lower whole-rock REE abundances. Chesner and Ettlinger (1989) conclud- Lo g (X F/X O H ) Log (XMg/XFe) 0.0 -0.5 -1 -1.5 -2 -2.5 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 I-SCR I-SC I-MC I-WC Fish Canyon Tuff Middle Jurassic ash beds Morrison Formation tuffs DRF-A DRF-B Group 1 DRF-C DRF-D DRF-F DRF-G Group 2 DRF-H DRF-I Group 3 DRF-J DRF-K Group 4 Figure 9. Compositions of Duchesne River Formation biotite phenocrysts compared to the Fish Canyon Tuff, Morrison Formation tuffs of Christiansen and others (2015), and to ash beds in the Middle Jurassic of southern Utah (Kowallis and others, 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, strong- ly contaminated; I-MC—I-type, moderately contaminated; I-WC—I-type, weakly contaminated. These biotites are most like those in the moderately contaminated granites. Figure 10. Ternary diagram of feldspar compositions. DRF-A is the only sample with two feldspars. Sanidine phenocrysts from DRF-I and -J have relatively high Or levels. Potassium feldspars with >Or90 were interpreted to be detrital grains from plutonic rocks. These high Or grains are Late Jurassic to Early Cretaceous in age based on 40Ar/39Ar ages (Jensen, 2017). 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00Ab Or An DRF-A DRF-B DRF-H DRF-I DRF-J DRF-A plagioclase Detrital K feldspar 13 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 ed that physical parameters such as temperature exert a greater control on REE substitution in allanite than composition of the coexisting melt and suggested that higher temperatures correlate with higher REE abun- dances in allanite. The allanite-bearing tuffs from the Duchesne River Formation apparently had low erup- tive temperatures around 650 to 710°C based on bio- tite compositions. The lower REE content of allanite thus seems to agree with this notion. Low eruption temperatures could explain other chemical differences between the Duchesne River Formation allanite and other well-studied tuffs, if Ti in the octahedral site is a function of temperature as it is in biotite and horn- blende. Allanite in Duchesne River Formation tuffs has Ti concentrations between 0.0 to 0.06 apfu compared with 0.06 to 0.12 for allanite in the Toba Tuff and 0.16 and 0.19 for the Bishop Tuff. Petrik and others (1995) argued that the REE and Al content of allanite is a function of Fe3+/FeTotal and is thus an indicator of magmatic fO2. In this regard, allanite from Duchesne River Formation tuffs contains more Al than the other silicic tuffs used for comparison (>1.8 vs. 1.2–1.6 apfu). As a consequence, allanite has relatively low calculated Fe3+/FeTotal of 0.3–0.4 (figure 11b), even lower than in the Bishop Tuff (0.5–0.6), which crys- tallized near the QFM oxygen buffer (Hildreth, 1979). Thus, given the control of Fe3+ vs Al3+ by fO 2, it is possi- ble that the low REE contents are also related to relative- ly low fO 2. Moreover, Fe3+/FeTotal in allanite from DRF-H and DRF-I indicates more reducing conditions than to DRF-A and DRF-C. Based on Fe/(Fe+Mg) ratios (figure 5), biotite in these same samples show similar relation- ships and substantiates the control by oxygen fugacity. Figure 12. LREE-Y pattern for allanite in tuffs from the Duchesne River Formation compared to the average LREE abundances of allanite in the Bishop and Toba Tuffs; all are quite similar. The LREE-Y pattern for average titanite in sam- ple DRF-I is plotted for comparison. Allanites are strongly enriched in LREEs, including Y, compared to titanites. The compositional field for titanite in other Cenozoic tuffs of the Great Basin is shown for comparison. Figure 11. (A) Allanite phenocryst compositions from the Duchesne River Formation tuffs compared to allanite in the Bishop Tuff (California) and Toba Tuff (Sumatra), which are relatively more enriched in REEs and other A-site substitu- tions. (B) Duchesne River Formation allanites have lower in- ferred Fe3+/FeTotal ratios (0.3–0.4) than both the Bishop and Toba Tuffs, implying lower fO2 in the parent magmas. 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 0.60 0.80 1.00 1.20 1.40 1.60 1.80 R EE +Y +T h+ U +N a (p fu ) Ca (pfu) Bishop Tuff C&E Toba DRF-A DRF-C DRF-H DRF-I A B 0.00 0.25 0.50 0.75 1.00 1.0 1.5 2.0 2.5 3.0 R E E +Y +T h+ U ( ap fu ) Al (apfu) Bishop Tuff C&E Toba Fe3+/Fetot Tie lines DRF H DRF I DRF-A DRF-C Ferriallanite Allanite Epidote 0.20.4 0.6 0.8 Clinozoisite La Ce Nd Sm 100 1000 10,000 100,000 1,000,000 10,000,000 C ho nd rit e N or m al iz ed V al ue s Y DRF-I titaniteBishop Tuff Toba DRF-I DRF-A DRF-H DRF-C Allanite Titanite Representative Cenozoic titanite composition 14 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 Titanite Titanite was only found in one sample of one ash bed, DRF-I from the upper Dry Gulch Creek Member. It occurs as isolated, euhedral grains that lack clear ev- idence of a detrital origin but is also distinct from oth- er volcanic titanites. Titanite in DRF-I has an average composition of Ca0.99Ti0.91Si0.98O0.96F0.04 with only minor substitutions for Ca, Ti, and Si. Its major element com- position is similar to other volcanic titanites in the data- base of Kowallis and others (2019). However, Fe and Al are lower than most other volcanic titanites with average Fe+Al at 0.078 apfu compared with 0.11 apfu for titanite in other silicic tuffs (figure 13). It also has distinctively low Y and Mn and a high Nb/Y ratio, a characteristic shared by the whole-rock composition. These titanite grains have LREE + Y patterns similar to these other tuffs but with lower REE values and somewhat steeper slopes from La to Y (figure 12). The LREE/Y ratio of about 9.6 from titanite in DRF-I is higher than Ceno- zoic Great Basin tuffs, which typically have LREE/Y of about 3.6. The presence of titanite contradicts the evidence from biotite and allanite that the source magma was a reduced, low-temperature, rhyolitic magma, with low LREEs. Elevated Fe/(Mg+Fe) ratios in biotite and low calculated Fe3+/FeTotal ratios in allanite indicate fairly re- ducing conditions but this is unusual for titanite-bear- ing magmas that typically indicate oxidizing conditions. For example, Christiansen and others (2015) noted that the tuffs from the Morrison Formation containing ti- tanite were more oxidized than tuffs without titanite, as indicated by high Mg/Fe ratios in biotite. 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 0.1–0.43 for Duchesne River tuffs that lack titanite (figure 9). These contradic- tions of oxidizing vs. reducing conditions, along with the atypical compositions discussed above and shown on figure 13, and the fact that titanite was only found in DRF-I but not DRF-H (same tuff bed) probably indicate that the titanite grains are detrital rather than a mag- matic component of the tuff. 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 per- cent lower on average than is typical for apatite. F was very high, making up as much as 4.35 wt.% of the ap- atite grains, a strong indication of alteration (Deer and others, 1997). DRF-D has a strong detrital component and these apatite grains may be further evidence of de- trital mixing. WHOLE-ROCK COMPOSITION Whole-rock data was gathered using XRF tech- niques as well as microprobe analysis of glass shards from sample DRF-A. Whole-rock compositions pro- vide insights about the altered and original composition of the tuffs. Analytical totals from the whole-rock XRF analyses are good (>99.74 wt.%), but loss on ignition (LOI) ranges from 4.8 wt.% all the way up to 18 wt.% for DRF-C and DRF-J. However, most samples had LOI measurements less than 9% as shown in table 4. Sam- ples with high LOIs also show abnormally high CaO concentrations and the samples may have been con- taminated with detrital carbonate despite soaking the samples in an acid bath during preparation. Some glass shards managed to survive diagenesis 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.00 0.02 0.04 0.06 0.08 0.10 Fe a pf u Al pfu DRF-I titanite Volcanic Figure 13. Compositions of titanite from the Duchesne River Formation. DRF-I titanites are depleted in both Al and Fe compared to volcanic titanite from the Cenozoic tuffs com- piled by Kowallis and others (personal communication) and are probably detrital. 15 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 in sample DRF-A from the middle part of the Lapoint Member. DRF-A is the least altered of the all the Duch- esne River Formation tuffs based on SiO2 levels in the glass and immobile elements such as Nb, Zr, and Ti. The electron microprobe analyses indicate these shards are high-silica rhyolite with SiO2 >75% with low concen- trations of TiO2, Fe2O3, and MgO when normalized to 100% on a volatile free basis (see the supplemental data file). 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. However, by comparing immobile elements such as Nb, Zr, Ti, and REE, some information about the initial composition can be determined. Even though immobile element concentrations will typically be changed by alteration, they are similarly enriched or depleted so that ratios of Table 4. 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 52.7 68.84 73.86 70.11 65.58 67.37 66.66 54.16 73.62 TiO2 0.44 0.38 0.4 0.51 0.53 0.31 0.38 0.24 0.18 0.21 0.3 Al2O3 14.91 17.57 16.74 17.6 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 1.6 5.69 5.78 5.2 3.08 4.6 4.91 5.35 5.83 4.8 2.32 CaO 2.97 10.74 19.79 2.54 2.4 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 LOI 4.83 12.06 18.97 7.37 5.44 7.28 9.94 8.69 8.55 18.21 11.09 Analysis Total 99.99 99.74 100.3 99.93 99.97 99.92 100.01 99.91 99.94 99.94 100.16 Trace Elements 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 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 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 25 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 Note: Major oxides are in wt% and trace elements are reported in ppm. Tdl-Lapoint Member, Tdd-Dry Gulch Creek Member, LOI-loss on ignition. Normalized to 100% on a volatile free basis. 16 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 these elements can still be used to make general infer- ences about the original composition and volcanic set- ting (Kowallis and others, 2001; Christiansen and oth- ers, 2015). TiO2/Al2O3 ratios in altered ash beds from terrestrial settings can also be used to determine the extent of physical reworking and chemical alteration and TiO2/Al2O3 values <0.055 are indicative of primary ash composition (Hong and others, 2019). Despite ex- tensive argillic alteration seen in most of the Duchesne River Formation tuffs (discussed below) all the tuffs ap- pear to have maintained a TiO2/Al2O3 ratio <0.055. On the discrimination diagrams of Winchester and Floyd (1977), the Duchesne River Formation tuffs plot in the dacite and rhyolite fields based on Nb/Y vs. Zr/ Ti (figure 14a). Samples DRF-H and DRF-I have anon- ymously low Y resulting in high Nb/Y ratios so they plot in the trachyte field, but their Zr/Ti ratios are rhy- olitic. The distinctive low Y content could be the result of pre-eruptive fractionation of titanite and allanite which are both present in the tuff and have high parti- tion coefficients for Y. (As noted earlier, the titanite in this sample also has an anomalously high Nb/Y ratio.) When Zr/Ti is plotted against Ce ppm, all of the Duch- esne River samples plot in the rhyolite field (figure 14b) which agrees with the fresh glass analysis and the min- eral assemblages and compositions (e.g., sodic sanidine, quartz, low Ti biotite). Tectonic setting can also be inferred from immobile elements and careful use of some mobile trace elements. Figure 15 shows discrimination plots by Pearce (1984) that compare Nb, Y, and Rb concentrations. Both dia- grams show that the Duchesne River Formation tuffs have compositions characteristic of a volcanic arc set- ting, assuming the elements plotted have not been sig- nificantly changed by alteration. A volcanic arc setting also agrees with trace element patterns of the Duchesne River Formation tuffs (figure 16) which are typical of subduction-related calc-alkaline magmas including large negative Nb anomalies, strong positive Pb anoma- lies, and a generally decreasing trend from left to right. Average abundances of trace elements from volcanic arc tuffs that also experienced argillic alteration (described below) from the Jurassic Morrison Formation (Chris- tiansen and others, 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 and others, 2015). All the tuffs exhibit signs of argillic alter- ation to swelling clays as is typical of the alteration of volcanic glass. SiO2 values range from 68% in DRF-A all the way to 42% in DRF-C, all low for rhyolite magmas Figure 14. (A) Immobile element diagram modified from Winchester and Floyd (1977). Duchesne River Formation tuffs all plot in the rhyolite field. Ce has not been reported for the northeast Nevada volcanic field. (B) Most of the tuffs plot in the dacite and rhyolite field. DRF-H and DRF-I have anomalously low Nb/Y ratios. Unaltered ignimbrite compo- sitions from the northeast Nevada volcanic field (NENVF) are shown for comparison (Brooks, 1995a). 0.00 0.01 0.10 1.00 0.1 1.0 10.0 Zr /T i p pm Nb/Y Rhyolite Foidite Phonolite Trachyte Comendite Basalt Basaltic Andesite Trachyandesite Alkali Basalt Dacite Andesite NENVF 0.00 0.05 0.10 0.15 0.20 0.25 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 Zr /T i p pm Ce ppm DRF-A DRF-B DRF-C DRF-D DRF-E DRF-F DRF-G DRF-H DRF-I DRF-J DRF-K Comendite Rhyolite Dacite-Andesite Trachyandesite A B 17 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 which typically have SiO2 values in the low to mid 70s. Al2O3 tends to increase as SiO2 decreases in the altered Morrison Formation tuffs and a similar trend occurs in the Duchesne River Formation samples (figure 17). Ar- gillic alteration also causes an increase in MgO as SiO2 is removed, but the opposite relationship is seen in feld- spathically altered tuffs, which have relatively low MgO and high SiO2. Another important effect of the diagenesis of the tuffs was the deposition of carbonate minerals. The high CaO content (table 4) in the tuffs is as high as 20 wt.%; a normal rhyolite would have about 1 wt.%. Carbonate precipitation significantly diluted the less mobile ele- ments like Al2O3 and SiO2. The presence of argillic alteration of the tuffs, an in- dication of a freshwater setting (Christiansen and oth- ers, 2015), agrees with sedimentologic evidence on the depositional environment of the Duchesne River For- mation in a freshwater fluvial/wetland setting (Anders- en and Picard, 1972, 1974; Sato and Chan, 2015; Webb, 2017). Carbonates also formed and filled the pore spac- es during diagenesis. DISCUSSION Eruptive Source of the Duchesne River Formation Tuffs Age, inferred original composition, eruption type, Figure 15. Trace element discrimination diagrams from Pearce and others (1984). (A) Nb vs. Y. Duchesne River For- mation and northeast Nevada volcanic field (NENVF) tuffs plot in the volcanic arc field and are almost identical except for two anomalously low Y samples. (B) Rb vs. Y+Nb Duch- esne River Formation tuffs and NENVF tuffs plot in the vol- canic arc field. Rb is probably low in most samples because of secondary alteration. Figure 16. Trace element patterns normalized to primitive mantle (McDonough and Sun, 1994) for bentonitic tuffs from the Duchesne River Formation and the average argil- lically altered tuff from the Morrison Formation. The Duch- esne River tuffs and Morrison tuff average are similar and have general patterns typical of a continental subduction zone setting. The irregularity of some of the patterns is prob- ably the result of secondary alteration, e.g., Ba, Rb, La, Nd, and P are quite variable. 1 10 100 1 10 100 1000 N b (p pm ) Y (ppm) Within plate Volcanic arc Ocean ridge 1 10 100 1000 1 10 100 1000 R b (p pm ) Y + Nb (ppm) Syn - Collisional Within plate Volcanic arc Ocean ridge NENVF NENVF DRF-A DRF-B DRF-C DRF-D DRF-E DRF-F DRF-G DRF-H DRF-I DRF-J DRF-K A B 0.1 1.0 10.0 100.0 1000.0 R oc k/ P rim iti ve M an tle DRF tuffs Morrison Formation mean Ba Rb Th U K Nb La Pb Ce Sr Nd P Sm Zr Ti Y 18 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 and geographic location are the main criteria we have used to try to identify eruptive sources for the ash beds within the Duchesne River Formation. Any potential source would need to have erupted within a narrow age range around 39.4 Ma, have a high-silica, calc-alka- line 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 eventu- ally be deposited. Several large volcanic fields in western North Amer- ica (figure 1) might have been eruptive sources for the Duchesne River Formation tuff beds. These include the northeast Nevada (Brooks and others, 1995a, 1995b; 1995c; Rahl and others, 2002), Tuscarora (Henry and others, 1998; Smith and others, 2017), Absaroka (Chan- dler, 2006), Challis (Chandler, 2006), central Nevada (Best and others, 2009; 2013b), Indian Peak (Best and others, 2013c), and Robinson Mountain (Lund-Snee and others, 2016; Smith and others, 2017). These vol- canic fields either pre-date or post-date the Duchesne River Formation tuffs except for the northeast Nevada and Tuscarora fields. Ages of volcanic, volcaniclastic, and plutonic rocks of the Tuscarora volcanic field range from about 41 to 39 Ma (Coats and McKee, 1972; Berg- er and others, 1991; Henry and others, 1999; Castor and others, 2003; Henry, 2008). However, the only known large ash-flow tuff from this field, the tuff of Big Cot- tonwood Canyon, has a well-established age of 39.92 to 40.15 ± 0.10 Ma (Henry, 2008), recalculated for a Fish Canyon age of 28.201 Ma. Comparing these ages to our new ages on the Duchesne River Formation tuffs (40Ar/39Ar ages of 39.47 ± 0.16 Ma and 39.36 ± 0.15 Ma) shows that the tuff of Big Cottonwood Canyon is older. The Keetley Volcanics (Crittenden and others, 1973; Bromfield and others, 1977; Constenius and others, 2011), Tintic Mountain Volcanic Group (Moore, 1973; Keith and others, 1989), Marysvale volcanic field (Row- ley and others, 2002), and volcanic rocks of the Thomas caldera in north and central Utah are hundreds of kilo- meters closer to the Uinta Basin than those in Nevada but are several million years too young to be considered correlative as illustrated on figure 18 and summarized in table 5. Figure 18 also lists possible intrusive complexes that could be considered as the source for the Duchesne River Formation tuffs. However, the ages we have ob- tained on the tuffs do not match any of these regional intrusive complexes except the Bingham intrusive com- plex in northern Utah which was active from 39.18 to 37.2 Ma (Warnaars and others, 1978; Deino and Keith, 1997). The composition of the Bingham stock is more primitive and mafic and includes monzonite, quartz monzonite, and quartz latite with hornblende and py- 0.0 1.0 2.0 3.0 4.0 5.0 6.0 40 50 60 70 80 M gO (w t.% ) 1.0 6.0 11.0 16.0 21.0 26.0 31.0 36.0 40 45 50 55 60 65 70 75 80 85 90 Al 2O 3 ( w t.% ) SiO2 (wt.%) Morrison Argillic Morrison FeldspathicMorrison Zeolitic Morrison Feldspathic Morrison Zeolitic Morrison Argillic DRF Glass DRF Glass DRF-A DRF-B DRF-C DRF-D DRF-E DRF-F DRF-G DRF-H DRF-I DRF-J DRF-K Figure 17. Two element diagrams of the Duchesne River For- mation tuffs and fields for Morrison Formation tuffs showing the different effects of alteration type on major elements. The composition of the glass from DRF-A is plotted as represen- tative of the original unaltered composition of the tuffs. 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 the opposite relationship of argillic and zeo- litic samples which show little change in Al2O3 but significant additions of MgO—neither of which apply to the altered tuffs of the Duchesne River Formation. 19 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 Figure 18. Timeline illustrating the sequence of major regional magmatic and tectonic events in Utah and Nevada, as well as local events in the Uinta Basin area. The ages of the volcanic and plutonic events (orange and red lines) indicate that the northeast Nevada volcanic field is the most likely volcanic source of the Duchesne River Formation tuffs. References for the ages of volcanic fields and plutonic complexes are listed in table 5. U-Pb ages and K-Ar ages are included to show the large er- rors in the previously used ages. Other 40Ar/39Ar ages from various tuffs from the northeast Nevada volcanic field are plotted to demonstrate the volcanic activity of the northeastern Nevada region at this time. 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. D ee p C re ek fa ul t z on e Pe rio d Ep oc h M em be r Stra tig rap hy F or m at io n Pa le og en e O lig oc en e Eo ce ne B is ho p C on gl om er at e S ta rr F la t G ilb er t P ea k E ro si on S ur fa ce La po in t D ry G ul ch C re ek B re na n B as in D uc he sn e R iv er F or m at io n K/ArFission Track 40Ar/39Ar BC-1 LA-1 VNW-1 BC-2 U nc on fo rm ity N or th ea st N ev ad a P os t-L ar am id e ep iro ge ni c up lif t U in ta M ou nt ai n up lif t Tu sc ar or a DRF-A, H GHSS C en tra l N ev ad a M ar ys va le K ee tle y Ti nt ic M or on i/G ol de ns R an ch W hi tn ey an O re lla n C ha nd ro ni an D uc he sn ea n LM A Fo ld in g of D uc he sn e R iv er F or m at io n an gu la r u nc on fo rm ity B in gh am C an yo n Ib ap ah P ilo t P ea k C en tra l W as at ch R an ge M cG in ty 30 31 32 33 34 35 36 37 38 39 40 41 30 31 32 33 34 35 36 37 38 39 40 41 42 29 Selected Magmatic and Tectonic Events B in gh amR ob in so n M t Th om as /K eg /D ru m G ro us e C re ek G ro us e C re ek G ro us e C re ek E m ig ra nt P as s S ul ph ur S pr in gs R an ge H ar ris on P as s Major volcanic fields in Utah and Nevada Major plutonic complexes in Utah Syncline in Duchesne River Formation Uinta Mountain uplift Ages from tuffs within the northeast Nevada volcanic field Timing of Deep Creek fault zone Ages from tuffs within the Duchesne River Formation and Bishop Conglomerate EH CB PR Age Column Key Legacy and New Isotopic Ages (Ma) CMC TC CB 40Ar/39Ar ages from Smith and others, 2017 EH = Elko Hills CMC = Coal Mine Canyon CB = Copper Basin PR = Pinon Range TC = Taylor Canyon 40Ar/39Ar ages from Brooks and others, 1995 SS = Southern Snake Mountains GH = Gold Hill PR DRF-A DRF-H 40Ar/39Ar ages from Kowallis and others, 2005 BC = Bishop Conglomerate 1, 2 LA-1 and VNW-1 are fission track, zircon ages from Bryant and others, 1989 New sanidine and plagioclase 40Ar/39Ar ages from this work DRF = Duchesne River Formation A, H L oc al N on -D ep os iti on C en tra l W as at ch R an ge 20 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 roxene (Moore, 1973; Waite and others, 1997), minerals that are entirely absent in the Duchesne River Forma- tion tuffs. This suggests that the Bingham stock is an unlikely intrusive counterpart to the Duchesne River Formation tuffs. The volcanic rocks associated with the Bingham intrusion also have a more mafic composition except for some rhyolitic lava flows which erupted near the end of volcanic activity around 33 Ma. Another potential source is the volcanism in north- east Nevada. Coats (1964) mapped several tuffs in the Jarbidge quadrangle along the Nevada-Idaho border. The oldest of these, the Dead Horse Formation (Smith and others, 2017), is a composite unit composed of several ash-flow tuffs including rhyolitic tuffs with a phenocryst assemblage (quartz, biotite, sanidine, pla- gioclase, apatite, zircon, and allanite) similar to the Duchesne River Formation tuffs. The recalibrated ages of the tuffs in the Dead Horse Formation range from 47.61 ± 0.4 to 34.97 ± 0.25 Ma (Smith and others, 2017), overlapping the range of the Duchesne River Formation. Brooks and others (1995a, 1995b, 1995c) determined the compositions and ages of a number of ash-flow tuffs in the northeast Nevada volcanic field. 40Ar/39Ar ages from sanidine show that, of the multiple ash-flow tuffs in this volcanic field, several have ages with errors that overlap the 39.47 and 39.36 Ma ages reported in this paper for Duchesne River Formation tuffs. Based on age and compositional constraints, the most likely place to look for a source of the Duchesne River Formation tuffs is in the northeast Nevada volcanic field. The northeast Nevada field tuffs are silicic to inter- mediate composition, calc-alkaline tuffs, and have sim- ilar mineral assemblages including quartz, biotite, and feldspar. Whole-rock comparisons between the Duch- esne River Formation and northeastern Nevada tuffs also show similar abundances of major and trace ele- ments. For example, Nb/Y and Zr/Ti ratios of altered Duchesne River Formation tuffs are similar to those from the northeast Nevada volcanic field (Brooks and others, 1995a, 1995c) (figure 14a). A plot of Nb vs. Y Table 5. List of magmatic events, ages, and references used on figure 18. Plutonic Complex Age (Ma) Reference Central Wasatch Range 29.45, 32.2–35.38 John and others, 1997; Biek and others, 2005; Smyk and others, 2018 Sulphur Springs Range 31.5–36.9 Ryskamp and others, 2008 Grouse Creek 34.3, 36.3, 41.3 Egger, 2003; Strickland and others, 2011 Harrison Pass 36 Barnes and others, 2001 Bingham Canyon 37.0–38.6 Warnaars, 1978; Deino and Keith, 1997; Vogel and others, 2001 McGinty 37 Hintze and Kowallis, 2009 Pilot Peak 37.7–38.2 Wooden and others, 1999; Woodburne, 2004 Ibapah 39 Hintze and Kowallis, 2009 Volcanic Field Age (Ma) Reference Central Nevada 18–36 Best and others, 2013; Christiansen and others, 2015 Tintic 30.3–35 Keith and others, 1989 Marysvale 31–35 Rowley and others, 2002 Keetley 32–35 Constenius and others, 2011; Smyk and others, 2018 Bingham 34.2–39 Moore, 1973; Biek and others, 2005 Thomas/Keg/Drum 34.92–36.77 Shubat and Snee, 1992 Goldens Ranch/Moroni 35.9–39.9 Hintze and Kowallis, 2009 Emigrant Pass 36.4–38.2 Egger, 2003; Johnson, 2015 Robinson Mountain 37.5–38.5 Lund-Snee and others, 2015 NE Nevada 39–42.6 Brooks, 1995b; Smith and others, 2017 Tuscarora 39.8–40.5 Henry and others, 1995; Henry, 2008 Note: The Central Wasatch Range plutonic complex includes the Clayton Peak, Alta, Little Cottonwood, Flagstaff, Mayflower, Ontario, Glencoe, Valeo, Pine Creek, and Park Premier stocks. 21 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 (figure 15a) also shows that the Duchesne River For- mation tuffs have compositions similar to the north- east Nevada volcanic field. However, Rb (ppm) levels in Duchesne River Formation tuffs are lower than in the northeast Nevada volcanic field (figure 15b), probably due to leaching of Rb during alteration. The least altered tuff which retains glass shards (DRF-A) seems to have retained its initial Rb concentrations, however, and plots near the volcanic rocks from the northeast Neva- da volcanic field. Similar compositional data on the tuff beds in the nearby Dead Horse Formation of northern Nevada are lacking. Aside from correlating with other igneous units to determine the eruptive source of the tuffs, composition- al data can be used to make interpretations about the tectonic setting and eruptive conditions. The mineral assemblage, mineral compositions, eruption tempera- tures, and whole-rock compositions of the Duchesne River Formation tuffs indicate a rhyolite-dacite com- position and the immobile trace element plots provide evidence for a typical continental subduction-zone tec- tonic setting. There is no evidence of intraplate, A-type or extension-related magmatism in these tuffs. The rhy- olitic magmas from which these fall-out tuffs originated were likely generated as the subducting Farallon slab foundered and rolled back at the close of the Laramide orogeny (Best and others, 2016). Timing of Uinta Mountain Uplift The high-precision ages of the tuffs in the Duchesne River Formation also place important constraints on the tectonic history of the region. We propose that after a time of relative tectonic quiescence when the Brennan Basin, Dry Gulch Creek, and Lapoint Members were deposited, a renewed Laramide uplift began in the Uin- ta Mountains about 39 Ma, as illustrated on figure 18. The cessation 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 and others, 1989). This uplift event created a syncline in the Duchesne River Forma- tion, which dips to the south 1 to 9°, proximal to the Uinta Mountains to the north, and then dips to the north 1 to 3° farther south in the Uinta Basin (figure 1b). Uplift was followed by the development of an un- conformity called the Gilbert Peak erosion surface that formed over a period of approximately 4 million years during which uplift and deposition apparently slowed and erosion dominated (Webb, 2017). The Bishop Con- glomerate was deposited around 34 to 30 Ma (Kowallis and others, 2005) atop the Gilbert Peak erosion surface with some angular discordance between it and the un- derlying Duchesne River Formation members (figure 2). Hansen (1986), Dickinson and others (1988), Aslan and others (2017), and Sprinkel (2018a) all interpret- ed the Gilbert Peak erosion surface as the end of the Laramide in the Uinta Basin. Although evidence of post-Laramide but pre-extensional tectonic activity is seen throughout the Uinta Mountains, it is likely caused by epeirogenic regional uplift of the Uinta Mountains and nearby Rocky Mountains (Sprinkel, 2014, 2018a; Aslan and others, 2017). Hansen (1986) also considered changing climate conditions from warm and humid to cool and dry to explain the existence and composition of the Bishop Conglomerate. Duchesnean North American Land Mammal Age The Duchesne River Formation, near the town of Lapoint, Utah, is the type section of the Duchesnean NALMA (Emry, 1981), which extends from 42 to 38 Ma (Alroy, 2000). Some disagreement has arisen over the numerical age of the fauna that define the age, and whether the fauna is entirely late Eocene or part Eocene and part Oligocene (Emry, 1981; Prothero, 1995; Ras- mussen and others, 1999). The majority of fossils col- lected from the Duchesne River Formation come from the Brennan Basin and Lapoint Members (Rasmussen and others, 1999; Burger and Tacket, 2014). The Car- negie titanothere quarry (Un0012) is stratigraphically located in the lower Lapoint Member and the Halfway Hollow quarry (Un0117) is stratigraphically located in the uppermost Dry Gulch Creek Member (figure 2). However, the Halfway Hollow quarry was originally reported to be in the lower Lapoint Member (Emry, 1981). Workers 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 and im- precise radiometric dates have been the main reason for 22 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 the confusion about the age of the fauna. An 40Ar/39Ar age of 41.10 ± 0.32 Ma and a U-Pb zircon age of 40.66 ± 1.88 Ma were reported from the Brennan Basin Mem- ber (Sprinkel, 2018a). More importantly, two 40Ar/39Ar ages of 39.47 ± 0.16 and 39.36 ± 0.15 Ma from the Lapoint (sample DRF-A) and Dry Gulch Creek (sample DRF-H) Members are reported in this study. The 34.03 ± 0.04 Ma age of the tuff in the lower part of the over- lying non-Duchesnean Bishop Conglomerate (Kowallis and others, 2005) is conclusive evidence that the faunas found within the Duchesne River Formation are entire- ly late Eocene which, according to the official time scale of the International Commission on Stratigraphy, end- ed about 33.9 Ma. CONCLUSIONS The Duchesne River Formation is an important flu- vial-lacustrine deposit that records the uplift and devel- opment of the Uinta Mountains, a Laramide-age uplift in eastern Utah. Two key fallout tuff beds in the Duch- esne River Formation give 40Ar/39Ar ages on feldspars 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 part of that member, and the tuffs from the Dry Gulch Creek Member lie in the upper and middle parts. These new ages from the Duchesne River Formation constrain a period of Laramide uplift of the Uinta Mountains from about 39 to 37 Ma. The compositions of the phenocrysts in the thin (0.25–5.5 m) tuffs are typical of rhyolitic subduction-re- lated magmas. The mineral assemblage of quartz, bi- otite, feldspar, with accessory zircon, and allanite is typical for calc-alkaline magmas. However, calculated temperatures from biotite and feldspar along with rel- atively low Ti and REE concentrations in allanite sug- gest that the magmas were cooler and less oxidized than typical arc rhyolites. Biotite compositions suggest that the parent magmas evolved compositionally over time (figure 7). The tuffs fell into fresh-water environments and the glass altered to clay, but most of the phenoclasts survived. Whole-rock compositions of the tuffs are typ- ical of argillically altered rhyolite, indicated by an en- richment of Al and Mg, depletion of Si and K, stable levels of Nb, Y, Ti, Zr, and REE, and formation of smec- titic and illitic clays as a result of the diagenesis of the volcanic glass. The immobile trace element patterns are typical of rhyolites produced in a subduction zone. Based on age, composition, eruption style, and geo- graphic location, the northeast Nevada volcanic field is the most likely source of the Duchesne River Formation tuffs. Other potential sources are too old or too young or have different eruptive compositions. These tuffs are distal equivalents of slab rollback-related volcanism as it flared up in northern Nevada in the late Eocene. The faunas of the Duchesne River Formation, which define the Duchesnean NALMA, are about 39.4 Ma. This is younger than the previously published age of 41.0 Ma (Rasmussen and others, 1999). Based upon our new ages and ages reported earlier from the overly- ing Bishop Conglomerate, the Duchesnean NALMA is middle Eocene. ACKOWLEDGMENTS Funding for the project was provided by the De- partment of Geological Sciences and College of Physical and Mathematical Sciences at Brigham Young Universi- ty. Additional support was given by the Utah Geological Survey (UGS). Assistance with sample preparation and XRF analyses was provided by David Tomlinson (Rio Tinto Kennecott). Martha Hayden (UGS) provided us with quarry locations so we could compare the strati- graphic position of the quarry to our sample locations. We thank the following for their reviews and technical editing: Grant Willis, Stephanie Carney, and Michael Hylland (UGS) and Paul Murphey (San Diego Natural History Museum). REFERENCES Ague, J.J., and Brimhall, G.H., 1988, Regional variations in bulk chemistry, mineralogy, and the compositions of maf- ic and accessory minerals in the batholiths of California: Geological Society of America Bulletin, v. 100, p. 891–911, doi:10.1130/00167606. Alroy, J., 2000, New methods for quantifying macroevolutionary patterns and processes: Paleobiology, v. 26, no. 4, p. 707–733. Andersen, D.W., and Picard, M.D., 1972, Stratigraphy of the Duchesne River Formation (Eocene-Oligocene?), northern Uinta Basin, northeastern Utah: Utah Geological and Min- eral Survey Bulletin 97, 29 p. 23 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 Andersen, D.W., and Picard, M.D., 1974, Evolution of synorogen- ic clastic deposits in the intermontane Uinta Basin of Utah, in Dickinson, W.R., editor, Tectonics and sedimentation: So- ciety for Sedimentary Geology (SEPM) Special Publication 22, p. 167–189. Aslan, A., Boraas-Connors, M., Sprinkel, D.A., Karlstrom, K.E., Heizler, M., Lynds, R., and Becker, T.P., 2017, Cenozoic col- lapse of the eastern Uinta Mountains and drainage evolution of the Uinta Mountains region: Geosphere, v. 14, no. 1, p. 115–140. Barnes, C.G., Burton, B.R., Burling, T.C., Wright, J. E., and Karls- son, H.R., 2001, Petrology and geochemistry of the late Eo- cene Harrison Pass pluton, Ruby Mountain core complex, northeastern Nevada: Journal of Petrology, v. 42, p. 301–929. Best, M.G., Barr, D.L., Christiansen, E.H., Grommé, C.S., Dei- no, A.L., and Tingey, D.G., 2009, The Great Basin altiplano during the middle Cenozoic ignimbrite flareup—insights from volcanic rocks: International Geology Review, v. 51, p. 1–45. Best, M.G., Christiansen, E.H., and Grommé, S., 2013a, Intro- duction—the 36–18 Ma southern Great Basin, USA, ig- nimbrite province and flareup—swarms of subduction-relat- ed supervolcanoes: Geosphere, v. 9, p. 260–274, doi:10.1130/ GES00870.1. Best, M.G., Christiansen, E.H., and Grommé, S., 2013b, The 36–18 Central Nevada ignimbrite field and calderas, Great Basin, USA—multicyclic super-eruptions: Geosphere, v. 9, p. 1562–1636, doi:10.1130/GES00945.1. Best, M.G., Christiansen, E.H., and Grommé, S., 2013c, The 36–18 Indian Peak-Caliente ignimbrite field and calderas, southeastern Great basin, USA—multicyclic super-erup- tions: Geosphere, v. 9, p. 864–950, doi:10.1130/GES00902.1. Best, M.G., Christiansen, E.H., de Silva, S., and Lipman, P.W., 2016, Slab-rollback ignimbrite flare-ups in the southern Great Basin and other Cenozoic American arcs—a dis- tinct style of arc volcanism: Geosphere, v. 12, p. 1097–1135, doi:10.1130GES01285.1. Biek, R.F., Solomon, B.J., Keith, J.D., and Smith, T.W., 2005, Geo- logic 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., and Altemuller, H.J., 1982, Micromorphology of weathering biotite and its secondary products: Pedologie, v. 32, p. 225–251. Blaylock, G.W., 1998, Probable correlation of the Oligocene Whitney Ash beds of western Nebraska to ash-flow tuffs in Nevada and Utah: Provo, Utah, Brigham Young University, M.S. thesis, 45 p. Bromfield, C.S., Erickson, A.J., Jr., Haddadin, M.A., and Meh- nery, H.H., 1977, Potassium-argon ages of intrusion, extru- sion, and associated ore deposits, Park City mining district, Utah: Economic Geology, v. 72, p. 837–848. Brooks, W.E., Thorman, C.H., and Snee, L.W., 1995a, The 40Ar/39Ar ages and tectonic setting of the middle Eocene northeast Ne- vada volcanic field: Journal of Geophysical Research, v. 100, p. 10,403–10,416. Brooks, W.E., Thorman, C.H., and Snee, L.W., 1995b, Correction to the 40Ar/39Ar ages and tectonic setting of the middle Eo- cene Northeast Nevada volcanic field: Journal of Geophysical Research, v. 100, p. 15,545–15,548. Brooks, W.E., Thorman, W.E., Snee, L.W., Nutt, C.W., Potter, C.J., and Dubiel, R.F., 1995c, Summary of chemical analyses and 40Ar/39Ar-spectra data for Eocene volcanic rocks from the central part of the Northeast Nevada volcanic field: U.S. Geo- logical Survey Bulletin 1988-K, p. K1–K33. Bryant, B., Naeser, C.W., Marvin, R.F., and Mehnert, H.H., 1989, Upper Cretaceous and Paleogene sedimentary rocks and iso- topic ages of Paleogene tuffs, Uinta Basin, Utah: U.S. Geolog- ical Survey Bulletin 1787, p. J1–J22. Burger B.J., and Tacket, L., II., 2014, The stratigraphic importance of the brontothere (cf. Diplacodon elatus) in the Brennan Ba- sin Member of the Duchesne River Formation of Utah: Fossil Record, v. 17, p. 69–74. Carroll, A.R., and Bohacs, K.M., 1999, Stratigraphic classification of ancient lakes—balancing tectonic and climatic controls: Geology, v. 27, p. 99–102. Castor, S.B., Faulds, J.E., Fowland, S.M., and dePolo, C.M., 2000, Geologic map of the Frenchman Mountain quadrangle, Clark County, Nevada: Nevada Bureau of Mine and Geology Maps 127, 15 p., scale 1:24,000. Castor, S.B., Boden, D.R., Henry, C.D., Cline, J.S., Hofstra, A.H., McIntosh, W.C., Tosdal, R.M., and Wooden, J.P., 2003, The Tuscarora Au-Ag district—Eocene volcanic-hosted epither- mal deposits in the Carlin Gold region, Nevada: Economic Geology, v. 98, p. 339–366. Chandler, M.R., 2006, The provenance of Eocene tuff beds in the Fossil Butte Member of the Green River Formation of Wyo- ming—relation to the Absaroka and Challis volcanic fields: Provo, Utah, Brigham Young University, M.S. thesis, 89 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 ge- ology 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 volca- nism in the Brushy Basin Member of the Morrison Forma- tion—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). 24 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 Clark, J., Beerbower, J.R., and Kietzke, K.K., 1967, Oligocene sed- imentation, stratigraphy, paleoecology and paleoclimatolo- gy in the big Badlands of South Dakota: Fieldiana Geology Memoirs, v. 5, 158 p. Coats, R.R., 1964, Geology of the Jarbidge quadrangle, Neva- da-Idaho: U.S. Geological Survey Bulletin 1141-M, p. 24 Coats, R.R., and McKee, E.H., 1972, Ages of plutons and types of mineralization, northwestern Elko County, Nevada: U.S. Geological Survey Professional Paper 800-C, p. C165–C168. Constenius, K.N., Clark, D.L., King, J.K., and Ehler, J.B., 2011, Interim geologic map of the Provo 30' x 60' quadrangle, Salt Lake, Utah, and Wasatch Counties, Utah: Utah Geologi- cal Survey Open-File Report 586DM, 42 p., 1 plate, scale 1:62,500. Crittenden, M.D., Stuckless, J.S., and Kistler, R.W., 1973, Radio- metric dating of intrusive rocks in the Cottonwood area, Utah: Research Journal of the U.S. Geological Survey, v. 1, no. 2, p. 173–178. 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, v. 106, p. 32–56. Deer, W.A., Howie, R.A., and Zussman, J., 1997, Rock forming minerals—volume 1A, Orthosilicates (2nd edition): Bath, United Kingdom, The Geological Society, p. 663–669. Deino, A., and Keith, J.D., 1997, Ages of volcanic and intrusive rocks in the Bingham mining district, Utah, in John, D.A., and Ballantyne, G.H., editors, Geology and ore deposits of the Oquirrh and Wasatch Mountains, Utah: Society of Eco- nomic Geologists Guidebook Series, v. 29, p. 91–95, doi: https://doi.org/10.5382/GB.29. Dickinson, W.R., 2004, Evolution of the North American Cordil- lera: Annual Reviews of Earth and Planetary Science, v. 32, p. 13–45. Dickinson W.R., Klute, M.A., Hayes, M.J., Janecke, S.U., Lundin, E.R., McKittrick, M.A., and Olivares, M.D., 1988, Paleogeo- graphic and paleotectonic setting of Laramide sedimentary basins in the central Rocky Mountain region: Geological So- ciety of America Bulletin, v. 100, p. 1023–1039. Egger, A.E., Dumitri, T.A., Miller, E.L., Savage. C.F.I., and Wood- en, J.L., 2003, Timing and nature of Tertiary plutonism and extension in the Grouse Creek Mountains, Utah: Interna- tional Geology Review, v. 45, p. 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 Duch- esnean “Age:” Journal of Paleontology, v. 55, p. 563–570. Fan, M., and Carrapa, B., 2014, Late Cretaceous-early Eocene Laramide uplift, exhumation, and basin subsidence in Wyo- ming—crustal responses to flat slab subduction: Tectonics, v. 33, p. 509–529, doi:10.1002/2012TC003221. Giere, R., and Sorenson, S.S., 2004, Allanite and other REE-rich epidote-group minerals: Reviews in Minerology and Geo- chemistry, v. 56, p. 431–493. Hamilton, W., 1978, Mesozoic tectonics of the western U.S., in Howell, D.G., and McDougall, A.K., editors, Mesozoic pa- leogeography of the western United States: Los Angeles, Cal- ifornia, Pacific Section, Society of Economic Paleontologists and Mineralogists, p. 33–70. Hansen, W.R., 1986, Neogene tectonics and geomorphology of the eastern Uinta Mountains in Utah, Colorado, and Wyo- ming: U.S. Geological Survey Professional Paper 1356, 78 p. Henry, C.D., 2008, Ash-flow tuffs and paleovalleys in northeast- ern Nevada—implications for Eocene paleogeography and extension in the Sevier hinterland, northern Great Basin: Geosphere, v. 4, p. 1–35, doi: 10.1130/GES00122.1. Henry, C.D., Boden, D.R., and Castor, S.B., 1998, Geology and mineralization of the Eocene Tuscarora volcanic field, Elko County, Nevada, in Tosdale, R.M., editor, Contributions to the gold metallogeny of northern Nevada: U.S. Geological Survey Open-File Report 98-338-B, p. 279–290. Henry, C.D., Boden, D.R., and Castor, S.C., 1999, Geologic map of the Tuscarora quadrangle, Nevada: Nevada Bureau of Mines and Geology Map 116, 20 p., scale 1:24,000, Hildreth, E.W., 1979, The Bishop Tuff—evidence for the origin of compositional zonation in silicic magma chambers, in Chap- in, C.E., and Elston, W.E., editors, Ash-flow tuffs: Geological Society of America Special Paper 180, p. 43–75. Hildreth, E.W., and Wilson, C.J.N., 2007, Compositional zoning of the Bishop Tuff: Journal of Petrology, v. 48, p. 951–999, doi: 10.1093/petrology/cgm007. Hong, H., Alego, T.J., Fang, Q., Zhao, L., Ji, K., Yin, K., Wang, C., and Cheng, S., 2019, Facies dependence of the mineralogy and geochemistry of altered volcanic ash beds—an example from Permian-Triassic transition strata in southwestern Chi- na: Earth Science Reviews, v. 190, p. 58–88. Huff, W.D., 2016, K-bentonites—a review: American Mineralo- gist, v. 101, p. 43–70. Humphreys, E.D., 1995, Post-Laramide removal of the Farallon slab, western United States: Geology, v. 23, p. 987–990. Jensen, M.S., 2017, 40Ar/39Ar Ages, compositions, and like- ly source of the Eocene fallout tuffs in the Duchesne River Formation, northeastern Utah: Provo, Utah, Brigham Young University, M.S. thesis, 107 p. Jicha, B.R., and Brown, F.H., 2014, An age for the Korath Range, Ethiopia and the viability of 40Ar/39Ar dating of kaersutite in Late Pleistocene volcanics: Quaternary Geochronology, v. 21, p. 53–57. 25 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 Jicha, B.R., Scholl, D.W., and Rea, D.K., 2009, Circum-Pacif- ic flare-ups and global cooling near the Eocene-Oligo- cene boundary: Geology, v. 37, p. 303–306, doi: 10.1130/ G25392A.1. John, D.A., Turrin, B.D., and Miller, R.J., 1997, New K-Ar and 40Ar/39Ar ages of plutonism, hydrothermal alteration, and mineralization in the central Wasatch Mountains, Utah, in John, D.A., and Ballantyne, G.H., editors, Geology and ore deposits of the Oquirrh and Wasatch Mountains, Utah: Soci- ety of Economic Geologists Guidebook Series, v. 29, p. 47–57. Johnson, C.L., 2015, Petrology and geochemistry of the Emigrant Pass volcanics, Nevada—implications for a magmatic-hy- drothermal origin of the Carlin gold deposits: Corvallis, Or- egon State University, M.S. thesis, 73 p. Jones, C.H., Farmer, G.L., Sageman, B., and Zhong, S., 2011, Hy- drodynamic mechanism for the Laramide orogeny: Geo- sphere, v. 7, p. 183–201, doi: 10.1130/GES00575.1. Keith, J.D., Dallmeyer, R.D., and Kim, C.S., 1989, A re-evaluation of the volcanic history and mineral potential of the central East Tintic Mountains, Utah: Utah Geological and Miner- al Survey Open-File Report 166, 90 p., 2 plates [Eureka and Tintic Mountain quadrangles], scale 1:24,000. Kowallis, B.J., Christiansen, E.H., and Deino, A.L., 1991, Age of the Brushy Basin Member of the Morrison Formation, Col- orado 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., Turn- er, 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 Ev- erett, 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 pyro- clastic 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, BJ., Hunt, J.E., Sprinkel, D.A., May, S.B., Bradfield, T.D., and Brown, K.D., 2018, Geologic map of the Lake Mountain quadrangle, Uintah County, Utah: Utah Geological Survey Miscellaneous Publication 18-2DM, 13 p., 2 plates, scale 1:24,000. Kuiper, K.F., Deino, A., Hilgen, F.J., Krijfsman, W., Renne, P.R., and Wijbrans, J.R., 2008, Synchronizing rock clocks of earth history: Science, v. 320, p. 500–504, doi: 10.1126/sci- ence.1154339. Lee, J.Y., Marti, K., Severinghaus, J.P., Kawamura, K., Yoo, H.S., Lee, J.B., and Kim, J.S., 2006, A redetermination of the isoto- pic abundances of atmospheric Ar: Geochemica et Cosmo- chimica Acta, v. 70, p. 4507–4512. Lipman, P.W., Prostka, H.J., and Christiansen R.L., 1972, Ceno- zoic volcanism and plate-tectonic evolution of the western United States, I—early and middle Cenozoic: Philosophical Transactions Royal Society of London, v. 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, in Lucas, S.G., Zeigler, K.E., and Kondrash- ov, P.E., editors, Paleogene mammals: New Mexico Museum of Natural History and Science Bulletin 26, p. 97–101. Liu, L., Gurins, M., Seton, M., Saleeby, J., Muller, R.D., and Jack- son, J.M., 2010, The role of oceanic plateau subduction in the Laramide orogeny: Nature Geoscience, v. 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—min- eralogy 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., and Konstantinou, A., 2016, Cenozoic paleogeographic evolution of the Elko Basin and surrounding region, northeast Nevada: Geo- sphere, v. 12, p. 464–500, doi: 10.1130/GE801193.1. McDonough, W.F., and Sun, S.S., 1994, The composition of the earth: Chemical Geology Isotope Geoscience, v. 120, p. 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. Min, K., Mundil, R., Renne, P.R., and Ludwig, K.R., 2000, A test for systemic errors in 40Ar/39Ar geochronology through com- parison with U/Pb analysis of a 1.1-Ga rhyolite: Geochemica et Cosmochimica Acta, v. 64, no. 1, p. 73–98. Moore, W.J., 1973, Igneous rocks in the Bingham mining district, Utah: U.S. Geological Survey Professional Paper 629-B, 42 p. Norman, M.D., and Mertzman, S.A., 1991, Petrogenesis of Chal- lis volcanics from central and southwestern Idaho—trace element and Pb isotopic evidence: Journal of Geophysical Research, v. 96, p. 13,279–13,293, doi: 10.1029/91JB00285. Pearce, J., 1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrolo- gy, v. 25, p. 956–983. Petrik, I., Broska, I., Lipka, J., and Siman, P., 1995, Granitoid al- lanite-(Ce)—substitution relations, redox conditions, and REE distributions on an example of I-type granitoids, west- ern Carpathians, Slovakia: Geology Carpathica, v. 46, p. 79–94. 26 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 Prothero, D.R., 1995, Geochronology and magnetostratigraphy of Paleocene North American land mammal “ages”—an up- date: Society for Sedimentary Geology (SEPM) Special Pub- lication 54, p. 305–316, ISBN 1-565576-024-7. Prothero, D.R., and Swisher, C.C., III, 1992, Magnetostratigraphy and geochronology of the terrestrial Eocene-Oligocene tran- sition in North America, in Prothero, D.R., and Berggren, W.A., editors, Eocene-Oligocene climatic and biotic evolu- tion: New Jersey, Princeton University Press, p. 46–73. Rahl, J.M., McGrew, A.J., and Foland, K.A., 2002, Transition from contraction to extension in the northeastern Basin and Range—new evidence from the Copper Mountains, Nevada: Journal of Geology, v. 110, p. 179–194. Rasmussen, D.T., Hamblin, A.H., and Tabrum, A.R., 1999, The mammals of the Eocene Duchesne River Formation, in Gil- lette, D.D., editor, 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. Geo- logical Survey Bulletin 1787–BB, 79 p. Renne, P.R., Deino, A.L., Hilgen, F.J., Kuiper, K.F., Mark, D.F., Mitchell, W.S., III, Morgan, L.E., Mundil, R., and Smit, J., 2013, Time scales of critical events around Cretaceous-Pa- leogene boundary: Science, v. 339, p. 684–687, doi:10.1123/ science.1230492. Riggs, N.R., Ash, S.R., Barth, A.P., Gehrel, G.E., and Wooden, J.L., 2003, Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona—an example of dating a continental sandstone: Geological Society of America Bul- letin, v. 115, p. 1315–1323. Rowley, P.D., Cunningham, C.G., Steven, T.A., Workman, J.B., Anderson, J.J., and Theissen, K.M., 2002, Geologic map of the central Marysvale volcanic field, southwestern Utah: U.S. Geological Survey Geologic Investigations Series 1-2645-A, 1 plate, scale 1:62,500. Ryskamp, E.B., Abbott, J.T., Christiansen, E.H., Keith, J.D., Ver- voot, J.D., and 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, v. 4, p. 496– 519, doi: 10.1130/GES0013.1. Sato, T., and Chan, M.A., 2015, Source-to-sink fluvial systems for sandstone reservoir exploration—example from the bas- al Brennan Basin Member of Tertiary Duchesne River For- mation, northern Uinta Basin, Utah, in Vanden Berg, M.D., Ressetar, R., and Birgenheier, L.P., editors, Geology of Utah's Uinta Basin and Uinta Basin: Utah Geological Association Publication 44, p. 91–107. Schellart, W.P., Freeman, J., Stegman, D.R., Moresi, L., and May, D., 2007, Evolution and diversity of subduction zones con- trolled by slab width: Nature, v. 446, p. 308–311. Shubat, M.A., and Snee, L.W., 1992, High-precision 40Ar/39Ar geochronology, volcanic stratigraphy, and mineral deposits of Keg Mountain, west-central Utah: U.S. Geological Survey Bulletin 2012, p. G1–G16. Smith, M.E., Singer, B., and Carroll, A., 2003, 40Ar/39Ar geochro- nology of the Eocene Green River Formation, Wyoming: Geological Society of America Bulletin, v. 115, p. 549–565. Smith, M.E., Singer, B.S., Carroll, A.R., and Fournelle, J.H., 2008, Precise dating of biotite in distal volcanic ash—isolating sub- tle alteration using 40Ar/39Ar laser incremental heating and electron microprobe techniques: American Mineralogist, v. 93, p. 784–795, doi:10.2138/am.2008.2517784. Smith, M.E., Carroll, A.R., Jicha, B.R., Cassel, E.J., and Scott, J.J., 2014, Paleogeographic record of the Eocene Farallon slab rollback beneath western North America: Geology, v. 42, p. 1039–1042, doi: 10.1130/G36025.1. Smith, M.E., and Carroll, A.R., editors, 2015, Stratigraphy and pa- leolimnology of the Green River Formation, western USA— syntheses in limnology 1: Dordrecht, Springer Science+Busi- ness Media, 355 p., doi: 10.1007/978-94-017-9906-5_1. Smith, M.E., Cassel, E.J., Jicha, B.R., Singer, B.S., and Canada, A.S., 2017, Hinterland drainage and lake formation in re- sponse to middle Eocene Farallon slab removal, Nevada, USA: Earth and Planetary Science Letters, v. 479, p. 156–169, doi:10.1016/j.epsl.2017.09.023 Smyk, E., Hollings, P., Baker, M., Cooke, D.R., Thompson, J.A., Thompson, J.M., and Creaser, R., 2018, Geochemistry and geochronology of the intrusive rocks of the central Wasatch Mountains igneous belt, Utah, USA—implications for por- phyry mineralization: Utah Geological Association Publica- tion 47, p. 305–327. 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. Sprinkel, D.A., 2014, The Uinta Mountains—a tale of two geog- raphies and more: Utah Geological Survey, Survey Notes, v. 46, no. 3, p. 1–4. Sprinkel, D.A., 2018a, Interim geologic map of the Duchesne 30' x 60' quadrangle, Duchesne and Wasatch Counties, Utah: Utah Geological Survey Open-File Report 689, 38 p., 2 plates, scale 1:62,500. Sprinkel, D.A., 2018b, Mysteries of the Uinta Mountains: Utah Geological Survey, Survey Notes, v. 50, no. 3, p. 1–3. Stoener, R.W., Schaeffer, O.A., and Katcoff, S., 1965, Half-lives of argon-37, argon-39, and argon-42: Science, v. 148, p. 1325– 1328, doi:10.1123/science.148.3675.1325. 27 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 Strickland, A., Miller, E.L., and 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, v. 119, p. 185–206, doi: 10.1086/658294. Tanavsuu-Milkeviciene, K., Sarg, J.F., and Bartov, Y., 2017, Dep- ositional cycles and sequences in an organic-rich lake ba- sin—Eocene Green River Formation, Lake Uinta, Colorado and Utah, USA: Journal of Sedimentary Research, v. 87, p. 210–229, doi: http://dx.doi.org/10.2110/jsr.2017.11. Utah Geological Survey, and Apatite to Zircon Inc., 2014, U-Pb detrital zircon geochronology result for the Brennan Basin Member of the Duchesne River Formation, Duchesne 30' x 60' quadrangle, Duchesne and Wasatch Counties, Utah: Utah Geological Survey Open-File Report 635, 56 p. Vogel, T.A., Cambray, F.W., and Constenius, K.N., 2001, Origin, and emplacement of igneous rocks in central Wasatch Moun- tains, Utah: Rocky Mountain Geology, v. 36, p. 119–162. Waite, K.A., Keith, J.D., Christiansen, E.H., Whitney, J.A., Hat- tori, K., Tingey, D.G., and Hook, C.J., 1997, Petrogenesis of the volcanic and intrusive rocks associated with the Bing- ham porphyry Cu-Au-Mo deposit, Utah, in John, D.A., and Ballantyne, G.H., editors, Geology and ore deposits of the Oquirrh and Wasatch Mountains, Utah: Society of Economic Geologists Guidebook Series, v. 29, p. 69–90. Ward, J.H., 1963, Hierarchical grouping to optimize an objective function: Journal of the American Statistical Association, v. 58, p. 236–244. Warnaars, F.W., Smith, W.H., Bray, R.E., Lanier, G., and Shafiqul- lah, M., 1978, Geochronology of igneous intrusions and por- phyry 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. Webb, C.A., 2017, Geologic mapping of the Vernal NW quad- rangle, Uintah County, Utah, and stratigraphic relationships of the Duchesne River Formation and Bishop Conglomerate: Provo, Utah, Brigham Young University, M.S. thesis, 68 p., 2 plate, scale 1:24,000. Wen, S., and Nekvasil, H., 1994, SolvCalc—an interactive graph- ics program package for calculating the ternary feldspar solvus and for two-feldspar geothermometry: Comput- ers & Geosciences, v. 20, p. 1025–1040, doi:10.1016/0098- 3004(94)90039-6. Winchester, J.A., and Floyd, P.A., 1977, Geochemical discrimina- tion of different magma series and their differentiation prod- ucts using immobile elements: Chemical Geology, v. 20, p. 325–343, doi: 10.1013/009-2541(77)90057-2. Whitney, D.L., and Evans, B.W., 2010, Abbreviations for names of rock-forming minerals: American Mineralogist, v. 95, p. 184–187. 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 mam- mals of North America: New York, Columbia University Press, 391 p. Yonkee, W.A., and Weil, A.B., 2015, Tectonic evolution of the Sevier and Laramide belts within the North American Cor- dillera orogenic system: Earth-Science Reviews, v. 150, p. 531–593. Zanazzi A., Kohn, M.J., MacFadden B.J., and Terry, D.O., Jr., 2007, Large temperature drop across the Eocene-Oligocene transi- tion in central North America: Nature, v. 445, p. 639–642. 1–1 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 APPENDIX 1 Descriptions of Volcanic Ash Bed Samples DRF-A (latitude 40.491478, longitude -109.746676). Light grayish-tan tuffaceous sandstone, 1 to 2 m thick with visible biotite grains when examined with a hand lens. This ash is distinct from all the other ashes in the field and is a moderately sorted, subrounded, poorly lithified sandstone with little clay. Contains biotite, sanidine, plagioclase, allanite, glass shards, zircon, magnetite, and detrital quartz grains. This is the only ash bed with coexisting pla- gioclase and sanidine. Laterally continuous where exposed. 40Ar/39Ar age from plagioclase of 39.5 Ma. DRF-B (latitude 40.449856, longitude -109.720892). Light-gray clay matrix, 2 m thick with some visible biotite grains. Collected from middle Lapoint Member. Contains biotite, sanidine, and detrital grains including micro- cline. Laterally continuous where exposed. DRF-C (latitude 40.417679, longitude -109.749658). Light- to medi- um-dark-gray clay matrix, 0.4 to 1 m thick, with abundant visible biotite grains. Highest tuff collected from Bobcat Ridge. Contains biotite, allanite, and detrital grains. Laterally continuous where exposed. Pictured to the right. DRF-D (latitude 40.416545, longitude -109.746362). Medium- to dark-gray clay matrix, uppermost part of 5.5-m-thick tuff bed with some visible biotite grains. Collected from Bobcat Ridge. Poorly exposed due to erosion so lateral distribution is unknown. Contains biotite, apatite, zircon, and detrital grains. DRF-E (latitude 40.416564, longitude -109.746445). Light- to medium-gray clay matrix, upper-middle part of 5.5-m-thick tuff bed with some visible bio- tite grains. Laterally continuous and used as contact between Lapoint and Dry Gulch Creek Members. Contains biotite and detrital grains. DRF-F (latitude 40.416583, longitude -109.746469). Light- to medium-gray clay matrix, middle part of 5.5-m-thick tuff bed with some visible biotite grains. Later- ally continuous and used as contact between Lapoint and Dry Gulch Creek Mem- bers. Contains biotite and detrital grains. DRF-G (latitude 40.416519, longitude -109.746494). Light- to medium-gray clay ma- trix, lowermost part of 5.5-m-thick tuff bed with some visible biotite grains. Laterally con- tinuous and used as contact between Lapoint and Dry Gulch Creek Members. Contains bi- otite and detrital grains including microcline. Pictured to the right. DRF-H (latitude 40.418116, longitude -109.747051). Light- to medium-gray clay ma- trix, 25 cm thick with abundant visible biotite DRF-C ash DRF-G ash 1–2 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 APPENDIX 1 (CONTINUED) Descriptions of Volcanic Ash Bed Samples grains. Lowest tuff bed from Bobcat Ridge and directly beneath Lapoint and Dry Gulch Creek contact. Poorly exposed so lateral distribution is unknown. Sampled from same tuff bed as DRF-I. Contains biotite, sanidine, al- lanite, titanite, and detrital grains including microcline. DRF-I (latitude 40.418127, longitude -109.747087). Light- to medium-gray clay matrix, 25 cm thick with abundant visible biotite grains. Lowest tuff bed from Bobcat Ridge and directly below Lapoint and Dry Gulch Creek contact. Poor- ly exposed so lateral distribution is unknown. Sampled from same tuff bed as DRF-H. Contains biotite, sanidine, allanite, titanite, and detrital grains including microcline. 40Ar/39Ar age from sanidine of 39.36 ± 0.15 Ma. DRF-J (latitude 40.409754, longitude -109.699966). Light- to medium-gray clay matrix, 27 cm thick, and some visible biotite grains. Collected from the upper Dry Gulch Creek Member. Poorly exposed so lateral distribution is unknown. Contains biotite, sanidine, and detrital grains. Pictured to the right. DRF-K (latitude 40.397333, longitude -109.726184). Light- to medium-gray clay matrix, 25 cm thick with some biotite grains visible with a hand lens. Collected from the upper Dry Gulch Creek. Poorly exposed so lateral distribution is unknown. Contains biotite and detrital grains including microcline. Pictured below. DRF-J ash DRF-K ash 2–1 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 J- va lu e: 0. 00 78 19 9 ± 0. 00 00 05 5 (2 σ) In st ru m en t: M A P 21 5- 50 Sa m pl e: D R F- H St an da rd : Fi sh C an yo n sa ni di ne M at er ia l: sa ni di ne A ge (M a) : 28 .2 01 ± 0. 04 60 K ui pe r a nd o th er s ( 20 08 ) In cl ud ed in 40 A r ± 1 σ 4 0 39 A r ± \1 σ 3 9 38 A r ± 1 σ 3 8 37 A r ± 1 σ 3 7 36 A r ± 1 σ 3 6 % 40 A r* 40 A r* /39 A r K ± 2σ K /C a A ge ± 2σ w td . m ea n 0. 25 44 87 ± 0. 00 02 37 0. 08 75 18 ± 0. 00 01 52 0. 00 10 32 ± 0. 00 00 31 0. 00 03 39 ± 0. 00 02 76 0. 00 00 26 ± 0. 00 00 03 96 .8 9 2. 81 74 35 ± 0. 01 22 05 11 1. 17 1 39 .1 3 ± 0. 34  0. 11 41 78 ± 0. 00 02 04 0. 03 97 90 ± 0. 00 00 88 0. 00 04 46 ± 0. 00 00 19 0. 00 06 52 ± 0. 00 02 84 0. 00 00 05 ± 0. 00 00 03 98 .7 4 2. 83 33 94 ± 0. 02 58 37 26 .2 61 39 .3 5 ± 0. 71  0. 21 44 51 ± 0. 00 02 20 0. 07 50 33 ± 0. 00 01 39 0. 00 09 24 ± 0. 00 00 10 0. 00 07 43 ± 0. 00 02 80 0. 00 00 07 ± 0. 00 00 03 99 .0 1 2. 82 97 45 ± 0. 01 34 13 43 .4 17 39 .3 0 ± 0. 37  0. 09 50 63 ± 0. 00 02 19 0. 03 22 91 ± 0. 00 00 76 0. 00 03 52 ± 0. 00 00 20 0. 00 03 45 ± 0. 00 03 43 0. 00 00 17 ± 0. 00 00 03 94 .5 7 2. 78 41 28 ± 0. 02 89 60 40 .2 98 38 .6 7 ± 0. 80  0. 17 76 20 ± 0. 00 02 26 0. 06 24 24 ± 0. 00 01 15 0. 00 07 55 ± 0. 00 00 25 0. 00 05 62 ± 0. 00 02 95 0. 00 00 02 ± 0. 00 00 03 99 .5 9 2. 83 37 95 ± 0. 01 55 28 47 .7 38 39 .3 6 ± 0. 43  0. 72 27 95 ± 0. 00 03 70 0. 08 99 63 ± 0. 00 01 32 0. 00 10 42 ± 0. 00 00 20 0. 00 01 35 ± 0. 00 03 25 0. 00 00 13 ± 0. 00 00 03 99 .4 7 7. 99 15 50 ± 0. 01 62 65 28 6. 23 5 10 8. 89 ± 0. 43 0. 06 39 18 ± 0. 00 01 85 0. 01 99 79 ± 0. 00 00 64 0. 00 02 33 ± 0. 00 00 15 0. 00 00 50 ± 0. 00 03 28 0. 00 00 23 ± 0. 00 00 03 89 .1 4 2. 85 18 91 ± 0. 04 63 69 17 0. 88 7 39 .6 0 ± 1. 27  0. 28 86 60 ± 0. 00 02 59 0. 06 21 07 ± 0. 00 01 08 0. 00 07 10 ± 0. 00 00 30 0. 00 01 31 ± 0. 00 03 21 0. 00 00 29 ± 0. 00 00 03 96 .9 8 4. 50 73 15 ± 0. 01 76 78 20 3. 15 7 62 .2 1 ± 0. 48 0. 30 87 59 ± 0. 00 02 59 0. 03 16 51 ± 0. 00 00 73 0. 00 04 01 ± 0. 00 00 12 0. 00 03 06 ± 0. 00 02 47 0. 00 00 96 ± 0. 00 00 03 90 .6 9 8. 84 69 25 ± 0. 03 90 76 44 .4 74 12 0. 17 ± 1. 03 0. 13 75 80 ± 0. 00 02 03 0. 04 24 47 ± 0. 00 00 79 0. 00 05 09 ± 0. 00 00 14 0. 00 09 09 ± 0. 00 02 83 0. 00 00 36 ± 0. 00 00 03 92 .1 4 2. 98 64 86 ± 0. 02 30 81 20 .0 86 41 .4 5 ± 0. 63 0. 18 77 62 ± 0. 00 02 51 0. 06 52 40 ± 0. 00 01 01 0. 00 07 41 ± 0. 00 00 15 0. 00 04 36 ± 0. 00 03 24 0. 00 00 06 ± 0. 00 00 03 99 .0 8 2. 85 14 81 ± 0. 01 48 64 64 .4 12 39 .6 0 ± 0. 41  0. 08 98 66 ± 0. 00 02 03 0. 03 13 58 ± 0. 00 00 61 0. 00 03 80 ± 0. 00 00 18 0. 00 00 47 ± 0. 00 02 98 0. 00 00 00 ± 0. 00 00 03 99 .8 4 2. 86 10 80 ± 0. 03 12 99 28 5. 50 8 39 .7 3 ± 0. 86  0. 18 04 30 ± 0. 00 02 11 0. 04 87 43 ± 0. 00 00 94 0. 00 06 05 ± 0. 00 00 28 0. 00 00 43 ± 0. 00 03 98 0. 00 00 13 ± 0. 00 00 03 97 .8 3 3. 62 11 63 ± 0. 02 03 18 48 6. 11 5 50 .1 4 ± 0. 56 0. 74 22 68 ± 0. 00 03 77 0. 07 24 27 ± 0. 00 01 02 0. 00 08 79 ± 0. 00 00 27 0. 00 01 97 ± 0. 00 03 08 0. 00 00 12 ± 0. 00 00 03 99 .5 1 10 .1 98 21 2 ± 0. 02 00 30 15 8. 00 1 13 7. 84 ± 0. 52 0. 16 26 17 ± 0. 00 02 18 0. 05 69 21 ± 0. 00 01 02 0. 00 07 67 ± 0. 00 00 17 0. 00 02 88 ± 0. 00 03 13 0. 00 00 03 ± 0. 00 00 03 99 .4 1 2. 84 00 99 ± 0. 01 80 32 84 .8 74 39 .4 4 ± 0. 50  0. 07 29 44 ± 0. 00 02 19 0. 02 56 67 ± 0. 00 00 60 0. 00 03 09 ± 0. 00 00 23 0. 00 02 43 ± 0. 00 02 66 0. 00 00 06 ± 0. 00 00 03 97 .6 3 2. 77 45 01 ± 0. 03 82 66 45 .3 98 38 .5 4 ± 1. 05  0. 13 75 31 ± 0. 00 02 24 0. 04 79 40 ± 0. 00 00 95 0. 00 05 65 ± 0. 00 00 15 0. 00 00 57 ± 0. 00 03 15 0. 00 00 02 ± 0. 00 00 04 99 .5 9 2. 85 70 33 ± 0. 02 69 16 36 1. 56 1 39 .6 8 ± 0. 74  0. 14 47 88 ± 0. 00 02 19 0. 05 05 72 ± 0. 00 00 91 0. 00 05 87 ± 0. 00 00 15 0. 00 02 60 ± 0. 00 03 16 0. 00 00 01 ± 0. 00 00 03 99 .8 0 2. 85 72 33 ± 0. 02 02 01 83 .4 96 39 .6 8 ± 0. 56  0. 07 96 38 ± 0. 00 02 00 0. 02 79 15 ± 0. 00 00 65 0. 00 03 58 ± 0. 00 00 19 0. 00 03 56 ± 0. 00 02 52 0. 00 00 01 ± 0. 00 00 03 99 .7 9 2. 84 69 74 ± 0. 03 40 61 33 .6 91 39 .5 4 ± 0. 94  0. 90 46 83 ± 0. 00 05 34 0. 08 26 78 ± 0. 00 01 51 0. 00 10 34 ± 0. 00 00 20 0. 00 04 48 ± 0. 00 03 57 0. 00 00 16 ± 0. 00 00 03 99 .4 8 10 .8 85 44 0 ± 0. 02 38 03 79 .3 75 14 6. 77 ± 0. 62 0. 48 68 29 ± 0. 00 03 23 0. 04 23 81 ± 0. 00 00 82 0. 00 05 16 ± 0. 00 00 22 0. 00 01 06 ± 0. 00 02 57 0. 00 00 03 ± 0. 00 00 03 99 .8 3 11 .4 67 43 3 ± 0. 03 28 55 17 1. 63 8 15 4. 30 ± 0. 85 0. 79 73 24 ± 0. 00 04 64 0. 07 53 22 ± 0. 00 01 14 0. 00 09 14 ± 0. 00 00 21 0. 00 06 88 ± 0. 00 02 84 0. 00 00 47 ± 0. 00 00 04 98 .2 3 10 .3 98 01 8 ± 0. 02 18 81 47 .0 51 14 0. 44 ± 0. 57 0. 16 81 51 ± 0. 00 02 41 0. 05 22 16 ± 0. 00 00 90 0. 00 06 26 ± 0. 00 00 39 0. 00 02 19 ± 0. 00 03 08 0. 00 00 01 ± 0. 00 00 03 99 .9 0 3. 21 69 94 ± 0. 01 81 93 10 2. 74 2 44 .6 1 ± 0. 50 0. 30 32 05 ± 0. 00 03 06 0. 03 37 15 ± 0. 00 00 65 0. 00 04 00 ± 0. 00 00 22 0. 00 03 47 ± 0. 00 02 65 0. 00 00 12 ± 0. 00 00 03 98 .8 1 8. 88 66 06 ± 0. 03 26 98 41 .8 30 12 0. 69 ± 0. 86 0. 08 49 88 ± 0. 00 01 77 0. 01 96 70 ± 0. 00 00 53 0. 00 02 36 ± 0. 00 00 12 0. 00 01 18 ± 0. 00 03 60 0. 00 00 01 ± 0. 00 00 03 99 .5 3 4. 30 05 74 ± 0. 04 94 91 71 .7 25 59 .4 0 ± 1. 35 w ei gh te d m ea n ag e (1 3 of 2 5) : 39 .3 6 ± 0. 15 λ 4 0A r (0 .5 80 ± 0 .0 14 ) x 1 0-1 0 a -1 M in a nd o th er s ( 20 00 ) (40 A r/39 A r) K 0. 00 05 4 ± 0. 00 01 4 Ji ch a & B ro w n (2 01 4) 40 A r/36 A r 29 8. 56 ± 0. 31 Le e an d ot he rs (2 00 6) λ B - (4 .8 84 ± 0 .0 99 ) x 1 0-1 0 a -1 M in a nd o th er s ( 20 00 ) (38 A r/39 A r) K 0. 01 21 0 ± 0. 00 00 2 Ji ch a & B ro w n (2 01 4) 38 A r/36 A r 0. 18 85 ± 0. 00 03 Le e an d ot he rs (2 00 6) 39 A r (2 .5 8 ± 0. 03 ) x 1 0-3 a -1 St oe nn er a nd o th er s ( 19 65 ) (39 A r/37 A r) C a 0. 00 06 95 ± 0. 00 00 1 R en ne a nd o th er s ( 20 13 ) 37 A r (8 .2 3 ± 0. 04 2) x 1 0-4 h -1 St oe nn er a nd o th er s ( 19 65 ) (38 A r/37 A r) C a 0. 00 00 19 6 ± 0. 00 00 01 R en ne a nd o th er s ( 20 13 ) 36 C l (2 .3 03 ± 0 .0 46 ) x 1 0-6 a -1 (36 A r/37 A r) C a 0. 00 02 65 ± 0. 00 00 2 R en ne a nd o th er s ( 20 13 ) C om pl et e 40 A r/39 A r R es ul ts D ec ay c on st an ts In te rfe rin g is ot op e pr od uc tio n ra tio s A tm os ph er ic a rg on ra tio s APPENDIX 2 Summary of Ages and Analytical Parameters 2–2 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source Jensen, M.S., Kowallis, B.J., Christiansen, E.H, Webb, C., Dorais, M.J., Sprinkel, D.A., and Jicha, B. Geology of the Intermountain West 2020 Volume 7 APPENDIX 2 (CONTINUED) Summary of Ages and Analytical Parameters J- va lu e: 0. 00 78 19 9 ± 0. 00 00 05 5 (2 σ) In st ru m en t: M A P 21 5- 50 Sa m pl e: D R F- H St an da rd : Fi sh C an yo n sa ni di ne M at er ia l: sa ni di ne A ge (M a) : 28 .2 01 ± 0. 04 60 K ui pe r a nd o th er s ( 20 08 ) In cl ud ed in 40 A r ± 1 σ 4 0 39 A r ± \1 σ 3 9 38 A r ± 1 σ 3 8 37 A r ± 1 σ 3 7 36 A r ± 1 σ 3 6 % 40 A r* 40 A r* /39 A r K ± 2σ K /C a A ge ± 2σ w td . m ea n 0. 25 44 87 ± 0. 00 02 37 0. 08 75 18 ± 0. 00 01 52 0. 00 10 32 ± 0. 00 00 31 0. 00 03 39 ± 0. 00 02 76 0. 00 00 26 ± 0. 00 00 03 96 .8 9 2. 81 74 35 ± 0. 01 22 05 11 1. 17 1 39 .1 3 ± 0. 34  0. 11 41 78 ± 0. 00 02 04 0. 03 97 90 ± 0. 00 00 88 0. 00 04 46 ± 0. 00 00 19 0. 00 06 52 ± 0. 00 02 84 0. 00 00 05 ± 0. 00 00 03 98 .7 4 2. 83 33 94 ± 0. 02 58 37 26 .2 61 39 .3 5 ± 0. 71  0. 21 44 51 ± 0. 00 02 20 0. 07 50 33 ± 0. 00 01 39 0. 00 09 24 ± 0. 00 00 10 0. 00 07 43 ± 0. 00 02 80 0. 00 00 07 ± 0. 00 00 03 99 .0 1 2. 82 97 45 ± 0. 01 34 13 43 .4 17 39 .3 0 ± 0. 37  0. 09 50 63 ± 0. 00 02 19 0. 03 22 91 ± 0. 00 00 76 0. 00 03 52 ± 0. 00 00 20 0. 00 03 45 ± 0. 00 03 43 0. 00 00 17 ± 0. 00 00 03 94 .5 7 2. 78 41 28 ± 0. 02 89 60 40 .2 98 38 .6 7 ± 0. 80  0. 17 76 20 ± 0. 00 02 26 0. 06 24 24 ± 0. 00 01 15 0. 00 07 55 ± 0. 00 00 25 0. 00 05 62 ± 0. 00 02 95 0. 00 00 02 ± 0. 00 00 03 99 .5 9 2. 83 37 95 ± 0. 01 55 28 47 .7 38 39 .3 6 ± 0. 43  0. 72 27 95 ± 0. 00 03 70 0. 08 99 63 ± 0. 00 01 32 0. 00 10 42 ± 0. 00 00 20 0. 00 01 35 ± 0. 00 03 25 0. 00 00 13 ± 0. 00 00 03 99 .4 7 7. 99 15 50 ± 0. 01 62 65 28 6. 23 5 10 8. 89 ± 0. 43 0. 06 39 18 ± 0. 00 01 85 0. 01 99 79 ± 0. 00 00 64 0. 00 02 33 ± 0. 00 00 15 0. 00 00 50 ± 0. 00 03 28 0. 00 00 23 ± 0. 00 00 03 89 .1 4 2. 85 18 91 ± 0. 04 63 69 17 0. 88 7 39 .6 0 ± 1. 27  0. 28 86 60 ± 0. 00 02 59 0. 06 21 07 ± 0. 00 01 08 0. 00 07 10 ± 0. 00 00 30 0. 00 01 31 ± 0. 00 03 21 0. 00 00 29 ± 0. 00 00 03 96 .9 8 4. 50 73 15 ± 0. 01 76 78 20 3. 15 7 62 .2 1 ± 0. 48 0. 30 87 59 ± 0. 00 02 59 0. 03 16 51 ± 0. 00 00 73 0. 00 04 01 ± 0. 00 00 12 0. 00 03 06 ± 0. 00 02 47 0. 00 00 96 ± 0. 00 00 03 90 .6 9 8. 84 69 25 ± 0. 03 90 76 44 .4 74 12 0. 17 ± 1. 03 0. 13 75 80 ± 0. 00 02 03 0. 04 24 47 ± 0. 00 00 79 0. 00 05 09 ± 0. 00 00 14 0. 00 09 09 ± 0. 00 02 83 0. 00 00 36 ± 0. 00 00 03 92 .1 4 2. 98 64 86 ± 0. 02 30 81 20 .0 86 41 .4 5 ± 0. 63 0. 18 77 62 ± 0. 00 02 51 0. 06 52 40 ± 0. 00 01 01 0. 00 07 41 ± 0. 00 00 15 0. 00 04 36 ± 0. 00 03 24 0. 00 00 06 ± 0. 00 00 03 99 .0 8 2. 85 14 81 ± 0. 01 48 64 64 .4 12 39 .6 0 ± 0. 41  0. 08 98 66 ± 0. 00 02 03 0. 03 13 58 ± 0. 00 00 61 0. 00 03 80 ± 0. 00 00 18 0. 00 00 47 ± 0. 00 02 98 0. 00 00 00 ± 0. 00 00 03 99 .8 4 2. 86 10 80 ± 0. 03 12 99 28 5. 50 8 39 .7 3 ± 0. 86  0. 18 04 30 ± 0. 00 02 11 0. 04 87 43 ± 0. 00 00 94 0. 00 06 05 ± 0. 00 00 28 0. 00 00 43 ± 0. 00 03 98 0. 00 00 13 ± 0. 00 00 03 97 .8 3 3. 62 11 63 ± 0. 02 03 18 48 6. 11 5 50 .1 4 ± 0. 56 0. 74 22 68 ± 0. 00 03 77 0. 07 24 27 ± 0. 00 01 02 0. 00 08 79 ± 0. 00 00 27 0. 00 01 97 ± 0. 00 03 08 0. 00 00 12 ± 0. 00 00 03 99 .5 1 10 .1 98 21 2 ± 0. 02 00 30 15 8. 00 1 13 7. 84 ± 0. 52 0. 16 26 17 ± 0. 00 02 18 0. 05 69 21 ± 0. 00 01 02 0. 00 07 67 ± 0. 00 00 17 0. 00 02 88 ± 0. 00 03 13 0. 00 00 03 ± 0. 00 00 03 99 .4 1 2. 84 00 99 ± 0. 01 80 32 84 .8 74 39 .4 4 ± 0. 50  0. 07 29 44 ± 0. 00 02 19 0. 02 56 67 ± 0. 00 00 60 0. 00 03 09 ± 0. 00 00 23 0. 00 02 43 ± 0. 00 02 66 0. 00 00 06 ± 0. 00 00 03 97 .6 3 2. 77 45 01 ± 0. 03 82 66 45 .3 98 38 .5 4 ± 1. 05  0. 13 75 31 ± 0. 00 02 24 0. 04 79 40 ± 0. 00 00 95 0. 00 05 65 ± 0. 00 00 15 0. 00 00 57 ± 0. 00 03 15 0. 00 00 02 ± 0. 00 00 04 99 .5 9 2. 85 70 33 ± 0. 02 69 16 36 1. 56 1 39 .6 8 ± 0. 74  0. 14 47 88 ± 0. 00 02 19 0. 05 05 72 ± 0. 00 00 91 0. 00 05 87 ± 0. 00 00 15 0. 00 02 60 ± 0. 00 03 16 0. 00 00 01 ± 0. 00 00 03 99 .8 0 2. 85 72 33 ± 0. 02 02 01 83 .4 96 39 .6 8 ± 0. 56  0. 07 96 38 ± 0. 00 02 00 0. 02 79 15 ± 0. 00 00 65 0. 00 03 58 ± 0. 00 00 19 0. 00 03 56 ± 0. 00 02 52 0. 00 00 01 ± 0. 00 00 03 99 .7 9 2. 84 69 74 ± 0. 03 40 61 33 .6 91 39 .5 4 ± 0. 94  0. 90 46 83 ± 0. 00 05 34 0. 08 26 78 ± 0. 00 01 51 0. 00 10 34 ± 0. 00 00 20 0. 00 04 48 ± 0. 00 03 57 0. 00 00 16 ± 0. 00 00 03 99 .4 8 10 .8 85 44 0 ± 0. 02 38 03 79 .3 75 14 6. 77 ± 0. 62 0. 48 68 29 ± 0. 00 03 23 0. 04 23 81 ± 0. 00 00 82 0. 00 05 16 ± 0. 00 00 22 0. 00 01 06 ± 0. 00 02 57 0. 00 00 03 ± 0. 00 00 03 99 .8 3 11 .4 67 43 3 ± 0. 03 28 55 17 1. 63 8 15 4. 30 ± 0. 85 0. 79 73 24 ± 0. 00 04 64 0. 07 53 22 ± 0. 00 01 14 0. 00 09 14 ± 0. 00 00 21 0. 00 06 88 ± 0. 00 02 84 0. 00 00 47 ± 0. 00 00 04 98 .2 3 10 .3 98 01 8 ± 0. 02 18 81 47 .0 51 14 0. 44 ± 0. 57 0. 16 81 51 ± 0. 00 02 41 0. 05 22 16 ± 0. 00 00 90 0. 00 06 26 ± 0. 00 00 39 0. 00 02 19 ± 0. 00 03 08 0. 00 00 01 ± 0. 00 00 03 99 .9 0 3. 21 69 94 ± 0. 01 81 93 10 2. 74 2 44 .6 1 ± 0. 50 0. 30 32 05 ± 0. 00 03 06 0. 03 37 15 ± 0. 00 00 65 0. 00 04 00 ± 0. 00 00 22 0. 00 03 47 ± 0. 00 02 65 0. 00 00 12 ± 0. 00 00 03 98 .8 1 8. 88 66 06 ± 0. 03 26 98 41 .8 30 12 0. 69 ± 0. 86 0. 08 49 88 ± 0. 00 01 77 0. 01 96 70 ± 0. 00 00 53 0. 00 02 36 ± 0. 00 00 12 0. 00 01 18 ± 0. 00 03 60 0. 00 00 01 ± 0. 00 00 03 99 .5 3 4. 30 05 74 ± 0. 04 94 91 71 .7 25 59 .4 0 ± 1. 35 w ei gh te d m ea n ag e (1 3 of 2 5) : 39 .3 6 ± 0. 15 λ 4 0A r (0 .5 80 ± 0 .0 14 ) x 1 0-1 0 a -1 M in a nd o th er s ( 20 00 ) (40 A r/39 A r) K 0. 00 05 4 ± 0. 00 01 4 Ji ch a & B ro w n (2 01 4) 40 A r/36 A r 29 8. 56 ± 0. 31 Le e an d ot he rs (2 00 6) λ B - (4 .8 84 ± 0 .0 99 ) x 1 0-1 0 a -1 M in a nd o th er s ( 20 00 ) (38 A r/39 A r) K 0. 01 21 0 ± 0. 00 00 2 Ji ch a & B ro w n (2 01 4) 38 A r/36 A r 0. 18 85 ± 0. 00 03 Le e an d ot he rs (2 00 6) 39 A r (2 .5 8 ± 0. 03 ) x 1 0-3 a -1 St oe nn er a nd o th er s ( 19 65 ) (39 A r/37 A r) C a 0. 00 06 95 ± 0. 00 00 1 R en ne a nd o th er s ( 20 13 ) 37 A r (8 .2 3 ± 0. 04 2) x 1 0-4 h -1 St oe nn er a nd o th er s ( 19 65 ) (38 A r/37 A r) C a 0. 00 00 19 6 ± 0. 00 00 01 R en ne a nd o th er s ( 20 13 ) 36 C l (2 .3 03 ± 0 .0 46 ) x 1 0-6 a -1 (36 A r/37 A r) C a 0. 00 02 65 ± 0. 00 00 2 R en ne a nd o th er s ( 20 13 ) C om pl et e 40 A r/39 A r R es ul ts D ec ay c on st an ts In te rfe rin g is ot op e pr od uc tio n ra tio s A tm os ph er ic a rg on ra tio s Supplemental Data — Compositions of Duchesne River Formation Biotite Phenocrysts Page 1 Sample DRF-C SiO2 36.68 38.06 37.06 35.99 39.79 36.84 36.25 35.55 36.62 28.85 28.74 29.02 28.97 29.49 28.68 29.45 31.55 TiO2 3.39 3.10 3.46 3.53 3.21 3.34 3.40 3.49 3.33 3.40 3.60 3.42 3.45 3.59 3.52 3.43 3.23 Al2O3 13.40 12.62 13.76 13.61 11.99 13.87 14.23 13.63 12.62 14.46 14.58 14.37 14.45 14.76 14.37 14.23 16.38 FeO t 16.96 15.84 16.70 16.90 15.18 16.87 17.27 17.19 15.82 18.78 18.04 17.83 16.03 18.93 18.13 16.87 21.00 MnO 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.27 0.25 0.21 0.18 0.29 0.26 0.25 0.21 MgO 11.59 10.77 12.20 11.90 11.66 12.31 12.27 11.51 11.53 12.74 13.06 12.71 14.37 12.13 12.98 13.68 10.06 CaO 0.30 0.49 0.23 0.26 0.39 0.21 0.13 0.21 0.30 0.04 0.01 0.00 0.01 0.08 0.09 0.01 0.08 Na2O 0.21 0.34 0.37 0.32 0.21 0.34 0.33 0.34 0.25 0.47 0.52 0.47 0.52 0.51 0.46 0.43 0.36 K2O 7.72 7.32 7.95 7.92 7.47 8.13 8.35 8.19 7.56 8.49 8.52 8.45 8.23 8.29 8.42 8.31 9.04 BaO 0.87 1.06 0.85 0.92 1.08 0.85 0.83 0.75 F 0.37 0.40 0.50 0.48 0.33 0.34 0.42 0.43 0.35 0.37 0.28 0.31 0.34 0.37 0.30 0.28 0.41 Cl 0.09 0.09 0.09 0.09 0.13 0.09 0.10 0.07 0.05 0.14 0.10 0.11 0.11 0.15 0.15 0.13 0.09 H2O* 3.50 3.40 3.51 3.45 3.51 3.59 3.56 3.45 3.43 3.36 3.41 3.36 3.39 3.37 3.37 3.41 3.47 Total 94.04 92.24 95.60 94.23 93.70 95.75 96.11 93.86 91.71 92.05 92.04 90.96 90.80 92.85 91.42 91.15 96.42 DRF-A DRF-B Supplemental Data — Compositions of Duchesne River Formation Biotite Phenocrysts Page 2 Sample SiO2 TiO2 Al2O3 FeO t MnO MgO CaO Na2O K2O BaO F Cl H2O* Total 30.62 33.22 32.54 34.49 32.90 31.86 32.10 31.55 31.10 30.41 29.86 29.53 30.38 29.30 30.19 31.15 28.94 3.19 2.86 2.85 2.55 2.79 2.91 2.99 2.71 2.81 2.95 2.54 2.90 2.78 2.73 2.97 2.63 3.16 15.48 16.52 16.85 17.41 17.59 17.05 17.58 18.04 18.18 16.87 16.52 16.32 17.04 16.64 17.73 17.41 16.44 20.80 20.81 20.62 17.65 20.13 21.04 21.37 19.85 20.36 20.97 19.47 20.15 20.48 20.34 20.68 18.60 20.66 0.18 0.21 0.19 0.17 0.18 0.19 0.23 0.19 0.16 0.20 0.16 0.20 0.20 0.19 0.19 0.18 0.17 9.88 9.70 9.66 9.61 9.22 9.07 9.37 9.14 8.99 9.43 11.06 10.25 9.90 9.71 9.01 8.24 10.67 0.14 0.05 0.01 0.43 0.01 0.07 0.01 0.07 0.02 0.01 0.05 0.13 0.06 0.04 0.02 0.88 0.21 0.36 0.28 0.42 0.32 0.27 0.27 0.27 0.39 0.41 0.38 0.41 0.26 0.39 0.34 0.40 0.39 0.26 8.80 9.07 9.35 4.16 9.13 8.97 9.01 8.30 8.47 9.12 8.73 7.91 8.74 9.00 9.15 7.64 6.64 0.25 0.33 0.42 0.32 0.24 0.38 0.57 0.36 0.40 0.46 0.36 0.63 0.33 0.26 0.56 0.43 0.32 0.36 0.41 0.39 0.34 0.35 0.35 0.35 0.31 0.34 0.48 0.41 0.42 0.40 0.37 0.36 0.52 0.33 0.05 0.07 0.06 0.12 0.07 0.06 0.07 0.10 0.12 0.07 0.08 0.07 0.07 0.09 0.08 0.10 0.06 3.39 3.51 3.50 3.50 3.52 3.47 3.54 3.48 3.46 3.37 3.39 3.34 3.41 3.34 3.41 3.22 3.43 93.33 96.85 96.67 90.88 96.22 95.51 97.28 94.34 94.63 94.49 92.85 91.90 93.99 92.19 94.56 91.14 91.14 DRF-C Supplemental Data — Compositions of Duchesne River Formation Biotite Phenocrysts Page 3 Sample SiO2 TiO2 Al2O3 FeO t MnO MgO CaO Na2O K2O BaO F Cl H2O* Total DRF-F 28.53 29.48 28.95 29.62 28.32 35.79 36.16 36.28 34.87 35.50 36.61 35.33 35.26 36.28 35.56 31.03 3.21 3.12 2.97 3.00 2.91 3.04 3.15 3.38 2.93 2.94 4.32 3.05 3.19 4.47 2.95 3.10 16.79 16.05 15.55 16.19 15.92 16.04 15.86 15.44 17.02 17.40 13.67 15.94 16.29 14.11 16.89 15.97 20.88 19.91 14.15 14.23 20.63 15.93 16.18 15.89 19.58 19.69 17.32 20.00 18.15 18.17 19.56 18.94 0.23 0.17 0.13 0.07 0.21 0.01 0.00 0.01 0.01 0.00 0.03 0.01 0.00 0.02 0.02 0.13 10.12 10.81 15.78 16.60 9.83 13.39 13.38 13.34 8.99 9.23 12.64 10.00 9.97 11.73 9.19 10.61 0.12 0.30 0.05 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.00 0.27 0.29 0.88 0.71 0.30 0.50 0.49 0.50 0.32 0.26 0.48 0.30 0.36 0.44 0.31 0.33 7.68 6.69 9.12 9.18 9.16 9.11 9.08 8.97 9.29 9.26 8.65 9.27 9.09 8.85 9.27 8.85 0.42 0.29 0.18 0.06 0.33 0.60 0.29 0.37 1.75 1.92 0.27 0.44 0.41 0.49 0.37 0.42 0.44 0.37 0.46 0.39 0.37 0.39 0.07 0.03 0.19 0.06 0.05 0.05 0.05 0.06 0.05 0.04 0.13 0.05 0.05 0.15 0.04 0.06 3.42 3.40 2.76 2.83 3.34 3.64 3.68 3.62 3.53 3.57 3.58 3.58 3.49 3.59 3.57 3.39 91.90 90.74 91.68 93.66 91.15 97.75 98.24 97.75 96.80 98.11 97.65 97.73 96.10 98.01 97.57 93.21 DRF-C DRF-D Supplemental Data — Compositions of Duchesne River Formation Biotite Phenocrysts Page 4 Sample SiO2 TiO2 Al2O3 FeO t MnO MgO CaO Na2O K2O BaO F Cl H2O* Total 32.30 35.43 32.28 35.09 32.39 32.71 35.51 35.61 35.18 35.60 35.61 35.88 35.10 35.16 31.80 31.55 30.68 2.23 2.56 2.52 2.70 2.67 2.74 2.70 2.63 2.78 2.73 2.58 2.62 2.58 2.74 3.10 2.97 3.05 17.92 17.23 18.45 17.22 16.74 15.86 17.12 17.46 16.96 17.27 17.37 17.04 17.67 17.64 15.28 15.68 14.92 19.06 20.08 20.03 20.26 20.04 19.62 18.43 19.15 19.23 18.98 18.29 18.66 20.39 20.00 24.54 22.70 24.41 0.19 0.16 0.19 0.18 0.17 0.15 0.00 0.00 0.00 0.01 0.00 0.02 0.21 0.17 0.22 0.17 0.20 9.48 9.84 8.46 8.99 9.42 9.13 10.02 10.42 10.53 10.59 9.63 9.47 9.84 10.33 8.31 9.26 8.14 0.03 0.00 0.00 0.00 0.04 0.07 0.02 0.00 0.03 0.03 0.07 0.00 0.00 0.02 0.03 0.02 0.13 0.24 0.31 0.30 0.31 0.37 0.40 0.35 0.32 0.33 0.32 0.40 0.32 0.34 0.30 0.35 0.41 0.42 8.61 9.31 9.46 9.20 9.01 8.56 9.20 9.30 9.19 9.36 8.92 9.16 8.74 9.13 8.67 8.56 8.14 0.11 0.58 0.40 0.33 0.15 0.34 0.57 0.30 0.62 0.53 0.32 0.28 0.33 0.35 0.44 0.44 0.56 0.41 0.30 0.42 0.38 0.44 0.41 0.47 0.42 0.25 0.22 0.27 0.16 0.05 0.02 0.07 0.07 0.10 0.04 0.04 0.05 0.04 0.06 0.05 0.03 0.06 0.11 0.09 0.00 3.48 3.65 3.48 3.54 3.42 3.29 3.57 3.69 3.60 3.65 3.53 3.54 3.61 3.65 3.50 3.52 3.44 93.94 99.38 95.79 98.11 94.72 93.26 97.19 98.79 98.12 98.80 96.70 96.99 99.33 99.73 96.64 95.56 93.99 DRF-GDRF-F DRF-H Supplemental Data — Compositions of Duchesne River Formation Biotite Phenocrysts Page 5 Sample SiO2 TiO2 Al2O3 FeO t MnO MgO CaO Na2O K2O BaO F Cl H2O* Total 30.72 29.96 30.86 31.32 32.16 32.08 31.80 34.42 34.04 34.53 33.99 34.13 34.15 35.43 35.35 34.90 3.16 2.82 3.14 2.87 3.13 3.09 3.48 2.97 2.92 3.10 3.04 3.15 3.11 3.14 3.09 3.06 15.40 15.23 15.68 15.89 15.25 15.45 15.88 15.40 14.88 15.28 14.41 15.68 15.13 15.39 15.10 15.23 24.97 24.71 25.18 24.11 24.40 23.60 24.59 23.43 22.43 22.15 21.43 23.86 23.45 22.90 23.66 22.28 0.25 0.24 0.26 0.23 0.20 0.18 0.19 0.02 0.01 0.01 0.01 0.00 0.00 0.01 0.00 0.00 8.16 8.10 8.28 8.68 8.63 8.80 8.49 8.07 8.04 8.06 7.81 7.81 7.72 8.55 8.35 8.08 0.00 0.10 0.03 0.02 0.01 0.04 0.02 0.00 0.04 0.07 0.00 0.01 0.01 0.01 0.00 0.00 0.36 0.38 0.37 0.37 0.37 0.35 0.35 0.46 0.46 0.40 0.37 0.38 0.39 0.38 0.42 0.35 8.90 8.67 8.91 8.77 8.51 8.17 8.95 8.75 8.21 8.68 8.29 8.89 8.87 8.83 8.79 8.81 0.50 0.54 0.52 0.50 0.58 0.52 0.69 0.23 0.23 0.27 0.31 0.30 0.29 0.24 0.28 0.21 0.28 0.26 0.24 0.28 0.23 0.24 0.24 0.09 0.09 0.10 0.07 0.07 0.09 0.09 0.10 0.12 0.08 0.13 0.08 0.08 0.06 0.08 0.11 3.49 3.42 3.50 3.49 3.52 3.51 3.56 3.54 3.49 3.50 3.39 3.57 3.50 3.63 3.62 3.53 96.10 94.36 96.96 96.49 96.98 96.02 98.19 97.30 94.74 96.00 92.99 97.68 96.54 98.44 98.59 96.45 DRF-IDRF-H Supplemental Data — Compositions of Duchesne River Formation Biotite Phenocrysts Page 6 Sample SiO2 TiO2 Al2O3 FeO t MnO MgO CaO Na2O K2O BaO F Cl H2O* Total 35.13 34.52 31.69 32.58 34.78 35.42 34.87 34.29 35.77 36.13 34.13 35.78 34.59 34.62 31.80 34.70 35.12 4.50 4.19 2.86 4.30 4.45 4.40 4.17 4.26 3.42 4.33 4.46 4.35 4.40 4.22 3.19 4.16 4.34 13.91 13.82 14.00 14.05 14.09 14.75 12.85 14.12 14.64 14.35 14.20 14.18 14.20 13.63 15.12 13.41 14.05 23.25 22.51 21.23 22.52 23.64 23.42 21.72 22.85 15.38 22.65 23.03 23.58 23.16 22.51 23.79 22.23 22.62 0.21 0.27 0.23 0.27 0.21 0.20 0.18 0.21 0.17 0.25 0.21 0.23 0.25 0.26 0.24 0.19 0.21 8.99 9.04 10.25 9.36 9.25 9.80 8.84 10.01 13.75 9.80 9.47 9.56 9.63 9.20 8.53 9.54 9.54 0.11 0.01 0.29 0.12 0.02 0.03 0.20 0.05 0.16 0.01 0.05 0.04 0.08 0.09 1.72 0.12 0.09 0.36 0.35 0.03 0.36 0.39 0.35 0.24 0.45 0.54 0.37 0.39 0.45 0.46 0.14 0.18 0.14 0.37 8.36 8.48 6.09 8.13 8.59 8.55 7.64 8.11 7.83 8.47 8.32 8.52 8.21 7.89 3.70 7.11 8.10 0.79 0.94 0.04 0.66 0.83 1.11 0.35 0.98 0.71 0.89 0.86 0.67 1.20 0.23 0.40 0.25 0.93 0.33 0.45 0.30 0.37 0.49 0.37 0.44 0.45 0.40 0.34 0.39 0.40 0.47 0.41 0.28 0.45 0.45 0.19 0.22 0.08 0.17 0.20 0.18 0.26 0.22 0.06 0.22 0.21 0.20 0.19 0.27 0.08 0.24 0.21 3.56 3.43 3.42 3.43 3.50 3.65 3.35 3.51 3.61 3.64 3.52 3.61 3.52 3.45 3.48 3.43 3.51 99.50 97.99 90.37 96.12 100.20 102.03 94.87 99.27 96.25 101.24 99.01 101.35 100.11 96.68 92.35 95.73 99.31 DRF-J DRF-K Biotite Sample DRF-A DRF-B DRF-C DRF-C DRF-C DRF-D DRF-F DRF-F DRF-G DRF-H DRF-H DRF-I DRF-J DRF-K SiO2 36.68 38.06 37.06 35.99 39.79 36.84 36.25 35.55 36.62 28.85 28.74 29.02 28.97 29.49 28.68 29.45 31.55 30.62 33.22 32.54 34.49 32.90 31.86 32.10 31.55 31.10 30.41 29.86 29.53 30.38 29.30 30.19 31.15 28.94 28.53 29.48 28.95 29.62 28.32 35.79 36.16 36.28 34.87 35.50 36.61 35.33 35.26 36.28 35.56 31.03 32.30 35.43 32.28 35.09 32.39 32.71 35.51 35.61 35.18 35.60 35.61 35.88 35.10 35.16 31.80 31.55 30.68 30.72 29.96 30.86 31.32 32.16 32.08 31.80 34.42 34.04 34.53 33.99 34.13 34.15 35.43 35.35 34.90 35.13 34.52 31.69 32.58 34.78 35.42 34.87 34.29 35.77 36.13 34.13 35.78 34.59 34.62 31.80 34.70 35.12 TiO2 3.39 3.10 3.46 3.53 3.21 3.34 3.40 3.49 3.33 3.40 3.60 3.42 3.45 3.59 3.52 3.43 3.23 3.19 2.86 2.85 2.55 2.79 2.91 2.99 2.71 2.81 2.95 2.54 2.90 2.78 2.73 2.97 2.63 3.16 3.21 3.12 2.97 3.00 2.91 3.04 3.15 3.38 2.93 2.94 4.32 3.05 3.19 4.47 2.95 3.10 2.23 2.56 2.52 2.70 2.67 2.74 2.70 2.63 2.78 2.73 2.58 2.62 2.58 2.74 3.10 2.97 3.05 3.16 2.82 3.14 2.87 3.13 3.09 3.48 2.97 2.92 3.10 3.04 3.15 3.11 3.14 3.09 3.06 4.50 4.19 2.86 4.30 4.45 4.40 4.17 4.26 3.42 4.33 4.46 4.35 4.40 4.22 3.19 4.16 4.34 Al2O3 13.40 12.62 13.76 13.61 11.99 13.87 14.23 13.63 12.62 14.46 14.58 14.37 14.45 14.76 14.37 14.23 16.38 15.48 16.52 16.85 17.41 17.59 17.05 17.58 18.04 18.18 16.87 16.52 16.32 17.04 16.64 17.73 17.41 16.44 16.79 16.05 15.55 16.19 15.92 16.04 15.86 15.44 17.02 17.40 13.67 15.94 16.29 14.11 16.89 15.97 17.92 17.23 18.45 17.22 16.74 15.86 17.12 17.46 16.96 17.27 17.37 17.04 17.67 17.64 15.28 15.68 14.92 15.40 15.23 15.68 15.89 15.25 15.45 15.88 15.40 14.88 15.28 14.41 15.68 15.13 15.39 15.10 15.23 13.91 13.82 14.00 14.05 14.09 14.75 12.85 14.12 14.64 14.35 14.20 14.18 14.20 13.63 15.12 13.41 14.05 FeO t 16.96 15.84 16.70 16.90 15.18 16.87 17.27 17.19 15.82 18.78 18.04 17.83 16.03 18.93 18.13 16.87 21.00 20.80 20.81 20.62 17.65 20.13 21.04 21.37 19.85 20.36 20.97 19.47 20.15 20.48 20.34 20.68 18.60 20.66 20.88 19.91 14.15 14.23 20.63 15.93 16.18 15.89 19.58 19.69 17.32 20.00 18.15 18.17 19.56 18.94 19.06 20.08 20.03 20.26 20.04 19.62 18.43 19.15 19.23 18.98 18.29 18.66 20.39 20.00 24.54 22.70 24.41 24.97 24.71 25.18 24.11 24.40 23.60 24.59 23.43 22.43 22.15 21.43 23.86 23.45 22.90 23.66 22.28 23.25 22.51 21.23 22.52 23.64 23.42 21.72 22.85 15.38 22.65 23.03 23.58 23.16 22.51 23.79 22.23 22.62 MnO 0.00 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.27 0.25 0.21 0.18 0.29 0.26 0.25 0.21 0.18 0.21 0.19 0.17 0.18 0.19 0.23 0.19 0.16 0.20 0.16 0.20 0.20 0.19 0.19 0.18 0.17 0.23 0.17 0.13 0.07 0.21 0.01 0.00 0.01 0.01 0.00 0.03 0.01 0.00 0.02 0.02 0.13 0.19 0.16 0.19 0.18 0.17 0.15 0.00 0.00 0.00 0.01 0.00 0.02 0.21 0.17 0.22 0.17 0.20 0.25 0.24 0.26 0.23 0.20 0.18 0.19 0.02 0.01 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.21 0.27 0.23 0.27 0.21 0.20 0.18 0.21 0.17 0.25 0.21 0.23 0.25 0.26 0.24 0.19 0.21 MgO 11.59 10.77 12.20 11.90 11.66 12.31 12.27 11.51 11.53 12.74 13.06 12.71 14.37 12.13 12.98 13.68 10.06 9.88 9.70 9.66 9.61 9.22 9.07 9.37 9.14 8.99 9.43 11.06 10.25 9.90 9.71 9.01 8.24 10.67 10.12 10.81 15.78 16.60 9.83 13.39 13.38 13.34 8.99 9.23 12.64 10.00 9.97 11.73 9.19 10.61 9.48 9.84 8.46 8.99 9.42 9.13 10.02 10.42 10.53 10.59 9.63 9.47 9.84 10.33 8.31 9.26 8.14 8.16 8.10 8.28 8.68 8.63 8.80 8.49 8.07 8.04 8.06 7.81 7.81 7.72 8.55 8.35 8.08 8.99 9.04 10.25 9.36 9.25 9.80 8.84 10.01 13.75 9.80 9.47 9.56 9.63 9.20 8.53 9.54 9.54 CaO 0.30 0.49 0.23 0.26 0.39 0.21 0.13 0.21 0.30 0.04 0.01 0.00 0.01 0.08 0.09 0.01 0.08 0.14 0.05 0.01 0.43 0.01 0.07 0.01 0.07 0.02 0.01 0.05 0.13 0.06 0.04 0.02 0.88 0.21 0.12 0.30 0.05 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.00 0.03 0.00 0.00 0.00 0.04 0.07 0.02 0.00 0.03 0.03 0.07 0.00 0.00 0.02 0.03 0.02 0.13 0.00 0.10 0.03 0.02 0.01 0.04 0.02 0.00 0.04 0.07 0.00 0.01 0.01 0.01 0.00 0.00 0.11 0.01 0.29 0.12 0.02 0.03 0.20 0.05 0.16 0.01 0.05 0.04 0.08 0.09 1.72 0.12 0.09 Na2O 0.21 0.34 0.37 0.32 0.21 0.34 0.33 0.34 0.25 0.47 0.52 0.47 0.52 0.51 0.46 0.43 0.36 0.36 0.28 0.42 0.32 0.27 0.27 0.27 0.39 0.41 0.38 0.41 0.26 0.39 0.34 0.40 0.39 0.26 0.27 0.29 0.88 0.71 0.30 0.50 0.49 0.50 0.32 0.26 0.48 0.30 0.36 0.44 0.31 0.33 0.24 0.31 0.30 0.31 0.37 0.40 0.35 0.32 0.33 0.32 0.40 0.32 0.34 0.30 0.35 0.41 0.42 0.36 0.38 0.37 0.37 0.37 0.35 0.35 0.46 0.46 0.40 0.37 0.38 0.39 0.38 0.42 0.35 0.36 0.35 0.03 0.36 0.39 0.35 0.24 0.45 0.54 0.37 0.39 0.45 0.46 0.14 0.18 0.14 0.37 K2O 7.72 7.32 7.95 7.92 7.47 8.13 8.35 8.19 7.56 8.49 8.52 8.45 8.23 8.29 8.42 8.31 9.04 8.80 9.07 9.35 4.16 9.13 8.97 9.01 8.30 8.47 9.12 8.73 7.91 8.74 9.00 9.15 7.64 6.64 7.68 6.69 9.12 9.18 9.16 9.11 9.08 8.97 9.29 9.26 8.65 9.27 9.09 8.85 9.27 8.85 8.61 9.31 9.46 9.20 9.01 8.56 9.20 9.30 9.19 9.36 8.92 9.16 8.74 9.13 8.67 8.56 8.14 8.90 8.67 8.91 8.77 8.51 8.17 8.95 8.75 8.21 8.68 8.29 8.89 8.87 8.83 8.79 8.81 8.36 8.48 6.09 8.13 8.59 8.55 7.64 8.11 7.83 8.47 8.32 8.52 8.21 7.89 3.70 7.11 8.10 BaO 0.87 1.06 0.85 0.92 1.08 0.85 0.83 0.75 0.25 0.33 0.42 0.32 0.24 0.38 0.57 0.36 0.40 0.46 0.36 0.63 0.33 0.26 0.56 0.43 0.32 0.42 0.29 0.18 0.06 0.33 0.60 0.11 0.58 0.40 0.33 0.15 0.34 0.57 0.30 0.62 0.53 0.32 0.50 0.54 0.52 0.50 0.58 0.52 0.69 0.79 0.94 0.04 0.66 0.83 1.11 0.35 0.98 0.71 0.89 0.86 0.67 1.20 0.23 0.40 0.25 0.93 F 0.37 0.40 0.50 0.48 0.33 0.34 0.42 0.43 0.35 0.37 0.28 0.31 0.34 0.37 0.30 0.28 0.41 0.36 0.41 0.39 0.34 0.35 0.35 0.35 0.31 0.34 0.48 0.41 0.42 0.40 0.37 0.36 0.52 0.33 0.29 0.37 1.75 1.92 0.27 0.44 0.41 0.49 0.37 0.42 0.44 0.37 0.46 0.39 0.37 0.39 0.28 0.33 0.35 0.44 0.44 0.56 0.41 0.30 0.42 0.38 0.44 0.41 0.47 0.42 0.25 0.22 0.27 0.23 0.23 0.27 0.31 0.30 0.29 0.24 0.28 0.21 0.28 0.26 0.24 0.28 0.23 0.24 0.24 0.33 0.45 0.30 0.37 0.49 0.37 0.44 0.45 0.40 0.34 0.39 0.40 0.47 0.41 0.28 0.45 0.45 Cl 0.09 0.09 0.09 0.09 0.13 0.09 0.10 0.07 0.05 0.14 0.10 0.11 0.11 0.15 0.15 0.13 0.09 0.05 0.07 0.06 0.12 0.07 0.06 0.07 0.10 0.12 0.07 0.08 0.07 0.07 0.09 0.08 0.10 0.06 0.07 0.03 0.19 0.06 0.05 0.05 0.05 0.06 0.05 0.04 0.13 0.05 0.05 0.15 0.04 0.06 0.16 0.05 0.02 0.07 0.07 0.10 0.04 0.04 0.05 0.04 0.06 0.05 0.03 0.06 0.11 0.09 0.00 0.09 0.09 0.10 0.07 0.07 0.09 0.09 0.10 0.12 0.08 0.13 0.08 0.08 0.06 0.08 0.11 0.19 0.22 0.08 0.17 0.20 0.18 0.26 0.22 0.06 0.22 0.21 0.20 0.19 0.27 0.08 0.24 0.21 H2O* 3.50 3.40 3.51 3.45 3.51 3.59 3.56 3.45 3.43 3.36 3.41 3.36 3.39 3.37 3.37 3.41 3.47 3.39 3.51 3.50 3.50 3.52 3.47 3.54 3.48 3.46 3.37 3.39 3.34 3.41 3.34 3.41 3.22 3.43 3.42 3.40 2.76 2.83 3.34 3.64 3.68 3.62 3.53 3.57 3.58 3.58 3.49 3.59 3.57 3.39 3.48 3.65 3.48 3.54 3.42 3.29 3.57 3.69 3.60 3.65 3.53 3.54 3.61 3.65 3.50 3.52 3.44 3.49 3.42 3.50 3.49 3.52 3.51 3.56 3.54 3.49 3.50 3.39 3.57 3.50 3.63 3.62 3.53 3.56 3.43 3.42 3.43 3.50 3.65 3.35 3.51 3.61 3.64 3.52 3.61 3.52 3.45 3.48 3.43 3.51 Total 94.04 92.24 95.60 94.23 93.70 95.75 96.11 93.86 91.71 92.05 92.04 90.96 90.80 92.85 91.42 91.15 96.42 93.33 96.85 96.67 90.88 96.22 95.51 97.28 94.34 94.63 94.49 92.85 91.90 93.99 92.19 94.56 91.14 91.14 91.90 90.74 91.68 93.66 91.15 97.75 98.24 97.75 96.80 98.11 97.65 97.73 96.10 98.01 97.57 93.21 93.94 99.38 95.79 98.11 94.72 93.26 97.19 98.79 98.12 98.80 96.70 96.99 99.33 99.73 96.64 95.56 93.99 96.10 94.36 96.96 96.49 96.98 96.02 98.19 97.30 94.74 96.00 92.99 97.68 96.54 98.44 98.59 96.45 99.50 97.99 90.37 96.12 100.20 102.03 94.87 99.27 96.25 101.24 99.01 101.35 100.11 96.68 92.35 95.73 99.31 Supplemental Data — Compositions of Duchesne River Formation Biotite Phenocrysts Page &P Sanidine Composition of Duchesne River Formation Sanidine Phenocrysts Sample DRF-A DRF-B DRF-H DRF-I DRF-J DRF-J SiO2 64.68 64.83 61.31 60.82 65.73 64.31 65.17 88.63 66.50 66.03 65.79 64.63 64.72 65.89 64.86 65.19 65.31 64.57 62.59 62.74 64.80 63.08 65.00 62.90 64.49 65.19 65.31 64.57 62.59 62.74 64.80 63.08 65.00 62.90 64.49 Al2O3 18.72 19.00 19.57 19.71 18.60 18.25 18.55 6.70 19.23 18.83 19.01 18.59 18.64 18.89 19.26 18.35 18.30 18.46 19.33 18.96 18.67 19.00 18.53 19.03 18.50 18.35 18.30 18.46 19.33 18.96 18.67 19.00 18.53 19.03 18.50 Fe2O3 0.02 0.01 0.05 0.05 0.10 0.07 0.00 0.13 0.02 0.01 0.02 0.04 0.04 0.02 0.06 0.05 0.04 0.03 0.03 0.05 0.03 0.06 0.00 0.04 0.01 0.05 0.04 0.03 0.03 0.05 0.03 0.06 0.00 0.04 0.01 CaO 0.14 0.11 0.13 0.15 0.11 0.09 0.10 0.18 0.12 0.11 0.09 0.11 0.11 0.09 0.10 0.07 0.12 0.15 0.19 0.17 0.10 0.19 0.15 0.15 0.15 0.07 0.12 0.15 0.19 0.17 0.10 0.19 0.15 0.15 0.15 BaO 3.17 3.08 0.65 0.47 0.96 0.10 0.06 0.06 1.35 1.40 1.29 1.52 1.53 1.05 0.80 0.88 3.16 3.34 0.56 3.52 0.76 3.21 0.81 1.05 0.80 0.88 3.16 3.34 0.56 3.52 0.76 3.21 0.81 Na2O 2.12 2.13 2.79 2.79 1.95 2.03 2.80 0.94 2.71 2.72 1.97 1.92 2.10 2.02 2.02 2.82 2.87 2.91 2.69 2.65 2.85 2.54 2.82 2.69 2.83 2.82 2.87 2.91 2.69 2.65 2.85 2.54 2.82 2.69 2.83 K2O 13.31 13.11 11.78 11.97 13.60 13.77 12.26 4.00 13.04 13.09 13.60 13.53 13.28 13.35 13.38 12.43 12.38 12.49 11.84 11.72 12.59 11.52 12.35 11.71 12.49 12.43 12.38 12.49 11.84 11.72 12.59 11.52 12.35 11.71 12.49 Total 98.98 99.19 98.81 98.57 100.73 98.99 99.84 100.69 101.66 100.84 101.82 100.21 100.17 101.77 101.20 99.96 99.82 99.49 99.83 99.63 99.59 99.90 99.61 99.72 99.27 99.96 99.82 99.49 99.83 99.63 99.59 99.90 99.61 99.72 99.27 Plagioclase Compositions of Duchesne River Formation Plagioclase Phenocrysts Sample DRF-A DRF-A DRF-A DRF-A DRF-A DRF-A DRF-A DRF-A DRF-A SiO2 60.53 61.41 61.01 62.69 62.91 59.70 59.69 55.78 58.17 60.89 61.06 62.13 62.32 58.65 58.87 61.95 62.08 58.62 58.87 59.40 60.34 57.55 59.18 60.55 62.22 60.14 60.70 62.27 61.78 61.76 60.25 59.77 60.60 61.00 61.73 62.05 58.06 58.65 57.74 61.38 60.53 60.47 60.82 60.18 58.76 62.22 59.39 60.79 59.95 58.98 61.59 62.24 63.82 64.04 64.93 64.09 60.30 63.28 61.18 59.48 59.24 60.78 61.53 60.32 62.05 61.79 60.80 62.27 62.83 57.31 58.99 57.74 58.95 58.76 59.19 58.94 57.79 58.63 59.02 61.50 Al2O3 26.06 25.12 24.84 24.50 24.84 25.25 25.27 27.47 26.56 24.50 24.37 24.65 24.48 26.24 26.16 24.43 24.51 25.99 26.51 25.89 25.62 27.45 25.96 25.66 24.50 25.50 24.67 24.66 24.78 24.66 25.67 26.26 25.48 25.64 24.48 24.38 27.20 26.77 26.92 26.12 26.52 25.49 25.06 25.20 25.68 23.95 25.69 25.80 24.19 25.77 24.52 24.00 22.68 23.10 21.96 22.32 25.85 24.10 24.81 26.19 26.22 25.19 25.03 24.86 24.36 24.52 24.89 23.60 23.91 26.12 27.02 26.31 26.99 27.08 26.48 27.26 27.35 25.40 25.97 24.83 Fe2O3 0.15 0.17 0.15 0.06 0.04 0.04 0.04 0.28 0.24 0.07 0.05 0.04 0.06 0.26 0.25 0.13 0.14 0.13 0.21 0.18 0.20 0.15 0.12 0.12 0.12 0.17 0.12 0.13 0.09 0.07 0.14 0.17 0.17 0.15 0.10 0.07 0.34 0.27 0.29 0.25 0.25 0.11 0.10 0.15 0.16 0.19 0.17 0.16 0.08 0.04 0.04 0.06 0.03 0.07 0.05 0.07 0.15 0.12 0.17 0.18 0.19 0.17 0.19 0.21 0.15 0.18 0.07 0.07 0.06 0.20 0.15 0.13 0.23 0.16 0.16 0.17 0.21 0.20 0.11 0.12 CaO 7.60 6.86 6.53 5.73 6.10 7.05 7.01 9.94 8.61 6.06 6.02 5.93 5.83 8.47 8.02 6.07 6.04 8.15 8.54 7.92 7.48 9.65 7.96 7.19 6.13 7.34 6.74 6.32 6.37 6.53 7.29 7.99 7.20 6.79 6.07 5.96 9.31 8.76 9.32 7.47 7.96 7.17 6.83 7.25 7.73 5.56 7.90 7.35 6.45 7.67 6.06 5.48 4.39 4.57 3.52 4.12 7.48 5.65 6.55 8.03 8.24 7.03 6.76 7.15 6.17 6.00 6.72 5.32 5.21 8.56 8.72 8.63 8.59 8.76 8.30 8.98 9.33 7.69 7.78 6.56 Na2O 6.91 7.13 7.30 7.74 7.46 6.98 7.03 5.45 6.26 7.58 7.67 7.72 7.70 6.35 6.82 7.55 7.50 6.59 6.32 6.68 6.94 5.91 6.68 6.99 7.55 6.81 7.35 7.48 7.58 7.46 6.93 6.68 6.94 6.91 7.40 7.65 5.77 5.76 5.63 6.41 6.05 7.02 7.30 7.06 6.65 7.73 6.92 6.89 7.71 6.94 7.65 7.97 8.39 8.11 8.72 8.65 6.80 7.70 7.21 6.70 6.32 6.52 7.08 7.12 7.47 7.55 7.39 8.14 8.03 6.62 6.30 6.32 6.31 6.29 6.36 6.04 6.02 6.97 6.83 7.28 K2O 0.50 0.54 0.64 0.78 0.68 0.55 0.56 0.31 0.40 0.62 0.59 0.68 0.73 0.40 0.45 0.64 0.72 0.42 0.40 0.45 0.49 0.35 0.44 0.53 0.67 0.50 0.57 0.62 0.64 0.58 0.52 0.45 0.55 0.53 0.68 0.67 0.62 0.65 0.59 0.52 0.46 0.50 0.51 0.47 0.48 0.77 0.48 0.48 0.62 0.50 0.66 0.74 0.98 1.01 1.25 1.03 0.48 0.71 0.59 0.44 0.42 0.54 0.56 0.54 0.61 0.67 0.55 0.69 0.74 0.42 0.39 0.41 0.42 0.42 0.42 0.37 0.36 0.47 0.49 0.60 Total 101.75 101.23 100.47 101.50 102.03 99.57 99.59 99.23 100.23 99.72 99.75 101.15 101.13 100.36 100.58 100.77 100.99 99.89 100.86 100.52 101.07 101.06 100.35 101.04 101.18 100.47 100.14 101.48 101.24 101.06 100.80 101.32 100.94 101.02 100.46 100.77 101.30 100.85 100.48 102.14 101.77 100.75 100.60 100.31 99.46 100.41 100.56 101.46 98.99 99.89 100.51 100.47 100.29 100.90 100.44 100.28 101.06 101.56 100.50 101.02 100.63 100.24 101.14 100.20 100.80 100.71 100.42 100.09 100.78 99.22 101.55 99.53 101.49 101.45 100.91 101.76 101.05 99.36 100.19 100.89 Allanite Compositions of Duchesne River Formation Allanite Phenocrysts Sample DRF-I DRF-I DRF-I DRF-A DRF-A DRF-H DRF-H DRF-H DRF-C DRF-C SiO2 31.81 31.93 30.89 31.75 31.99 31.34 31.79 31.77 32.76 31.30 30.88 31.80 32.36 30.69 30.79 31.07 31.40 30.96 31.59 31.84 31.02 31.48 30.01 31.57 30.56 31.82 31.71 30.93 31.59 30.69 31.25 30.58 30.58 31.18 30.77 31.37 30.49 30.33 31.24 31.66 31.84 32.56 30.87 31.32 31.46 31.56 31.27 31.87 31.80 31.78 31.23 31.38 30.91 31.39 31.66 30.60 31.31 31.18 30.85 30.68 30.72 31.05 31.27 30.71 30.88 31.16 TiO2 0.34 0.41 0.32 0.29 0.30 0.28 0.33 0.32 0.29 0.43 0.26 0.31 0.51 0.40 0.28 0.30 0.28 0.38 0.38 0.29 0.52 0.56 0.69 0.47 0.69 0.39 0.43 0.55 0.37 0.61 0.77 0.59 0.44 0.61 0.32 0.56 0.32 0.35 0.50 0.42 0.55 0.34 0.19 0.54 0.58 0.32 0.33 0.56 0.32 0.46 0.24 0.31 0.22 0.32 0.52 0.41 0.40 0.46 0.41 0.67 0.40 0.20 0.48 0.29 0.37 0.49 Al2O3 17.40 17.34 16.64 17.44 17.70 17.28 17.75 17.15 17.14 16.52 16.39 16.60 17.39 16.21 16.52 16.95 17.04 17.19 16.94 17.27 15.34 15.94 16.01 15.73 15.79 16.99 15.74 15.60 15.77 15.48 15.69 15.60 15.44 15.46 15.87 15.53 15.77 15.56 16.16 17.39 16.57 17.81 16.68 16.61 16.89 17.19 16.91 17.34 17.02 17.41 16.63 17.07 16.57 17.02 16.79 15.65 15.83 16.11 15.93 16.16 15.87 16.17 15.92 15.57 15.86 15.98 FeO 12.27 12.56 12.41 12.40 12.32 12.31 12.39 12.15 12.36 11.90 12.29 12.38 12.15 12.51 12.18 12.21 12.10 12.57 12.37 12.17 13.03 12.60 12.28 13.10 12.41 12.57 12.72 12.55 12.88 13.01 11.55 12.55 12.91 11.89 12.88 12.68 12.64 12.82 12.60 12.36 12.26 12.07 12.53 12.38 12.45 12.27 12.15 12.04 12.42 11.91 12.58 12.52 12.64 12.33 12.46 12.82 12.66 12.50 12.60 12.13 12.59 12.65 12.72 12.70 12.46 12.89 MnO 0.34 0.20 0.34 0.49 0.27 0.37 0.27 0.31 0.46 0.40 0.36 0.35 0.25 0.35 0.27 0.27 0.19 0.19 0.42 0.32 0.17 0.14 0.15 0.19 0.19 0.34 0.33 0.27 0.29 0.20 0.10 0.14 0.23 0.06 0.41 0.21 0.28 0.29 0.24 0.22 0.19 0.24 0.43 0.22 0.24 0.26 0.37 0.16 0.39 0.14 0.50 0.17 0.41 0.32 0.19 0.31 0.22 0.26 0.36 0.18 0.35 0.52 0.21 0.48 0.32 0.29 MgO 0.58 0.62 0.60 0.51 0.49 0.57 0.57 0.58 0.60 0.59 0.62 0.57 0.68 0.56 0.60 0.57 0.57 0.56 0.66 0.62 0.94 0.90 1.05 0.95 1.24 0.55 0.94 1.09 1.00 1.01 1.68 1.01 0.89 1.72 0.99 1.02 0.91 0.95 1.05 0.73 0.80 0.72 0.55 0.75 0.69 0.67 0.72 0.80 0.70 0.70 0.58 0.55 0.57 0.70 0.67 0.96 0.97 1.05 1.01 1.13 1.02 0.83 0.96 1.00 0.95 0.87 CaO 12.73 12.26 12.20 12.45 12.77 11.80 11.82 12.92 12.04 11.84 12.38 12.50 12.57 12.13 12.42 12.69 12.47 11.57 12.76 12.73 13.46 13.50 13.43 13.12 12.80 12.66 11.79 12.21 11.99 12.62 12.24 13.40 12.81 12.11 11.40 13.26 11.83 11.24 13.37 12.94 12.50 14.48 11.76 12.27 12.94 12.75 12.19 14.30 12.23 13.16 11.53 11.71 11.46 12.47 12.98 12.16 12.95 13.33 12.58 13.38 12.35 11.18 13.48 11.23 12.41 13.57 U2O3 0.25 0.16 0.00 0.49 1.07 0.00 0.66 0.00 0.33 0.00 0.00 0.00 1.64 0.00 1.47 3.34 0.00 0.00 0.00 0.41 1.07 3.83 4.71 0.00 0.00 0.00 0.00 3.09 0.00 0.00 4.30 1.55 0.74 1.95 0.00 0.00 0.82 0.00 0.00 0.00 0.00 0.00 1.15 0.00 1.88 0.99 1.96 3.44 0.00 2.54 0.00 1.31 2.20 0.00 0.00 0.00 0.90 0.00 1.55 0.00 0.00 2.03 0.00 1.63 0.00 0.00 ThO2 1.77 1.59 1.53 1.80 1.92 1.51 1.20 1.66 1.49 1.35 1.86 1.79 1.62 1.46 1.42 1.12 1.76 1.73 1.71 1.34 1.34 0.67 0.62 1.31 0.67 1.70 1.16 0.74 0.88 0.54 0.86 0.90 1.43 0.59 0.82 1.10 1.22 0.89 0.82 1.27 1.39 0.97 1.66 1.33 1.46 1.51 1.07 1.38 1.08 0.87 1.50 0.67 1.69 1.39 1.47 1.11 1.17 0.91 0.89 0.96 0.88 1.26 1.01 1.07 0.51 1.07 Y2O3 0.15 0.11 0.13 0.13 0.06 0.12 0.06 0.09 0.10 0.09 0.11 0.13 0.08 0.16 0.19 0.16 0.12 0.04 0.07 0.13 0.16 0.11 0.12 0.18 0.05 0.10 0.11 0.11 0.08 0.18 0.03 0.11 0.10 0.03 0.12 0.14 0.19 0.17 0.09 0.05 0.09 0.12 0.18 0.14 0.12 0.08 0.19 0.14 0.21 0.09 0.18 0.06 0.20 0.08 0.09 0.15 0.15 0.15 0.17 0.16 0.09 0.15 0.15 0.19 0.11 0.17 La2O3 5.65 6.26 6.11 5.63 5.73 5.89 6.14 5.68 5.83 5.84 5.44 5.64 5.92 5.76 5.85 5.99 5.85 6.21 5.63 5.50 4.82 5.26 5.92 5.27 6.27 5.68 6.36 6.63 6.33 5.93 7.01 5.39 5.28 7.57 5.95 5.44 5.88 6.09 5.11 5.64 6.58 4.79 5.66 6.53 5.85 5.55 6.15 4.95 6.08 5.64 5.85 6.99 6.15 5.76 5.90 6.00 5.15 5.40 5.34 5.64 5.46 5.78 5.06 5.88 6.24 4.51 Ce2O3 9.75 10.85 10.75 9.98 9.59 10.69 10.67 9.55 10.59 10.04 10.52 9.87 9.84 10.59 10.26 10.13 10.13 10.47 10.22 10.27 9.74 10.09 9.90 10.28 11.08 10.29 11.07 11.23 11.27 10.78 11.39 9.86 10.10 12.22 11.27 10.44 11.10 11.82 9.96 10.14 10.49 9.08 10.82 10.78 9.80 9.87 10.51 8.67 10.86 10.33 11.01 11.95 10.74 10.26 9.94 10.85 10.13 10.11 10.48 10.47 10.96 11.55 9.60 11.60 11.52 9.29 Nd2O3 3.13 3.25 3.28 3.10 3.03 3.38 2.93 3.30 3.51 3.31 3.39 3.36 2.83 3.17 3.57 3.42 3.12 2.90 3.26 3.24 3.64 3.48 2.85 3.34 3.45 3.20 3.92 3.53 3.61 3.36 2.90 3.37 3.89 2.87 3.93 3.60 3.81 3.82 3.75 3.17 3.04 3.12 3.67 3.12 2.80 3.00 3.31 2.63 3.31 2.94 3.63 2.99 3.72 3.36 3.10 3.64 3.78 3.52 3.91 3.27 3.77 3.97 3.71 3.99 3.55 3.67 Sm2O3 0.41 0.38 0.46 0.47 0.39 0.55 0.31 0.42 0.31 0.33 0.57 0.50 0.29 0.49 0.44 0.39 0.46 0.37 0.31 0.45 0.61 0.49 0.43 0.50 0.44 0.44 0.51 0.46 0.57 0.53 0.31 0.45 0.60 0.16 0.54 0.54 0.63 0.57 0.70 0.33 0.33 0.39 0.42 0.40 0.33 0.37 0.36 0.37 0.49 0.41 0.52 0.41 0.53 0.38 0.42 0.65 0.57 0.55 0.54 0.33 0.51 0.62 0.57 0.49 0.38 0.66 H2O+ 1.60 1.60 1.56 1.60 1.61 1.58 1.61 1.59 1.62 1.55 1.55 1.58 1.61 1.54 1.54 1.56 1.57 1.57 1.59 1.59 1.55 1.56 1.53 1.57 1.55 1.59 1.57 1.55 1.57 1.55 1.56 1.54 1.53 1.56 1.56 1.56 1.53 1.53 1.57 1.60 1.58 1.63 1.56 1.57 1.58 1.59 1.57 1.60 1.59 1.59 1.57 1.58 1.56 1.58 1.58 1.54 1.56 1.57 1.55 1.55 1.55 1.56 1.56 1.54 1.55 1.56 Total 98.16 99.52 97.22 98.51 99.23 97.67 98.48 97.48 99.42 95.49 96.60 97.39 99.75 96.03 97.81 100.18 97.08 96.70 97.88 98.18 97.40 100.60 99.71 97.59 97.19 98.32 98.33 100.54 98.18 96.48 101.61 97.04 96.98 99.97 96.81 97.45 97.39 96.44 97.17 97.92 98.21 98.31 98.11 97.94 99.08 97.95 99.06 100.24 98.49 99.96 97.54 99.67 99.54 97.35 97.76 96.85 97.74 97.09 98.14 96.71 96.52 99.52 96.69 98.36 97.10 96.18 gg Titanite Compositions of Duchesne River Formation Titanite Phenocrysts Sample DRF-I DRF-I DRF-I DRF-I DRF-I DRF-I SiO2 28.64 29.14 28.92 29.25 29.61 28.68 28.21 29.06 29.39 27.88 29.12 29.11 28.69 29.04 28.02 28.18 28.98 28.13 28.72 28.00 27.07 28.28 27.90 26.89 27.49 28.18 28.08 27.62 28.47 28.31 27.82 27.89 28.97 27.37 27.33 27.48 28.55 28.66 28.19 28.17 28.22 29.03 29.18 27.45 28.25 28.49 27.25 28.44 28.30 27.87 29.15 Al2O3 0.70 1.56 0.82 0.73 0.63 0.92 0.79 0.85 0.91 0.86 0.77 0.65 0.83 0.75 0.97 0.84 0.86 0.78 0.62 0.77 0.86 0.83 0.89 0.65 0.61 0.67 0.86 0.92 0.84 0.92 0.75 0.80 0.64 0.86 0.80 0.81 1.01 0.97 0.69 0.65 0.67 0.95 1.06 0.98 0.81 1.03 0.64 0.86 0.71 0.74 0.68 TiO2 35.39 35.15 34.68 36.32 36.00 34.62 34.85 35.37 34.90 34.65 35.68 35.84 35.74 35.02 34.23 34.90 35.31 33.73 34.93 35.18 34.01 0.00 33.55 33.39 34.86 35.58 35.06 34.69 35.56 35.17 34.97 35.25 35.61 35.25 35.21 35.23 34.58 34.96 35.40 34.79 35.24 34.97 35.81 32.68 35.31 34.17 35.47 35.11 35.18 35.23 35.77 MgO 0.02 0.02 0.01 0.00 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.00 0.01 0.01 0.01 0.00 0.00 0.02 0.01 0.02 0.00 0.00 0.01 0.00 0.01 0.00 0.01 0.01 0.01 0.00 0.01 0.02 0.01 0.01 0.01 0.00 0.02 0.03 0.01 0.00 0.01 0.02 0.03 0.00 0.00 Fe2O3 1.37 0.94 1.41 1.15 1.39 1.59 1.67 1.46 1.53 1.40 1.41 1.28 1.34 1.49 1.68 1.42 1.54 1.82 1.19 1.48 1.72 1.33 1.80 4.54 1.43 1.32 1.55 1.73 1.45 1.50 1.27 1.44 1.25 1.67 1.46 1.54 1.67 1.56 1.26 1.34 1.32 1.56 1.16 2.03 1.52 1.87 1.29 1.50 1.69 1.41 1.32 K2O 26.77 27.94 26.31 27.66 27.58 26.54 27.06 27.34 27.51 27.15 27.03 27.32 27.25 26.31 26.05 26.58 26.87 26.27 27.02 26.45 25.79 29.40 25.89 25.67 26.00 26.76 26.09 26.54 27.36 27.36 26.32 26.74 27.05 27.21 26.87 27.06 26.54 26.83 26.79 26.88 26.81 27.22 27.73 24.66 27.03 26.29 26.58 27.22 27.27 27.15 27.27 SnO2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.01 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 V2O5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ta2O5 0.00 0.00 0.04 0.00 0.00 0.18 0.00 0.09 0.13 0.08 0.00 0.09 0.03 0.10 0.11 0.00 0.00 0.01 0.03 0.00 0.15 0.08 0.09 0.00 0.02 0.09 0.14 0.18 0.00 0.10 0.00 0.00 0.01 0.08 0.06 0.00 0.03 0.00 0.00 0.06 0.00 0.00 0.00 0.14 0.00 0.02 0.00 0.05 0.00 0.07 0.11 MnO 0.03 0.00 0.05 0.06 0.02 0.07 0.00 0.08 0.09 0.10 0.07 0.07 0.06 0.08 0.03 0.06 0.06 0.07 0.06 0.05 0.01 0.07 0.01 0.05 0.03 0.07 0.06 0.05 0.10 0.10 0.05 0.11 0.08 0.05 0.07 0.06 0.07 0.06 0.07 0.10 0.08 0.10 0.00 0.00 0.07 0.06 0.05 0.09 0.01 0.07 0.07 Nb2O5 0.98 0.24 0.87 0.23 0.20 0.62 0.13 0.46 0.49 0.65 0.64 0.80 0.53 0.68 0.51 0.48 0.45 0.59 0.65 0.57 0.00 0.58 1.32 0.68 0.91 0.80 0.56 0.53 0.54 0.30 0.77 0.44 0.61 0.40 0.45 0.59 0.54 0.52 0.87 0.55 0.66 0.65 0.75 1.30 0.51 0.47 0.72 0.77 0.20 0.87 0.41 Y2O3 0.12 0.05 0.13 0.10 0.07 0.15 0.17 0.21 0.11 0.29 0.12 0.11 0.14 0.30 0.30 0.11 0.13 0.29 0.12 0.14 0.30 0.18 0.45 0.08 0.14 0.11 0.60 0.34 0.15 0.08 0.14 0.19 0.16 0.11 0.18 0.09 0.25 0.38 0.09 0.11 0.11 0.20 0.06 0.70 0.11 0.26 0.12 0.08 0.03 0.13 0.12 La2O3 0.38 0.24 0.64 0.17 0.22 0.39 0.47 0.23 0.18 0.16 0.40 0.29 0.28 0.31 0.52 0.29 0.37 0.40 0.12 0.36 0.43 0.13 0.46 0.41 0.72 0.36 0.28 0.20 0.29 0.15 0.54 0.22 0.18 0.24 0.43 0.23 0.39 0.20 0.25 0.41 0.43 0.08 0.14 0.44 0.20 0.53 0.31 0.28 0.50 0.26 0.29 Pr2O3 1.21 0.61 1.57 0.51 0.56 1.15 1.22 0.61 0.82 0.64 1.15 0.96 1.11 1.22 1.54 0.99 1.03 1.50 0.70 1.19 1.53 0.55 1.67 1.00 1.49 1.02 1.25 0.82 0.79 0.85 1.49 0.68 0.79 0.92 1.18 0.73 1.54 0.94 1.04 1.14 1.19 0.44 0.51 1.93 0.95 1.68 1.20 0.74 0.90 0.85 0.87 Nd2O3 0.50 0.21 0.56 0.12 0.21 0.40 0.43 0.31 0.25 0.45 0.41 0.32 0.49 0.61 0.76 0.39 0.41 0.64 0.29 0.38 0.70 0.26 0.86 0.46 0.53 0.33 0.87 0.44 0.34 0.25 0.54 0.35 0.38 0.23 0.31 0.27 0.68 0.49 0.32 0.24 0.40 0.22 0.26 1.25 0.44 0.74 0.40 0.22 0.30 0.31 0.28 Sm2O3 0.13 0.08 0.09 0.03 0.15 0.11 0.09 0.07 0.13 0.01 0.10 0.17 0.10 0.20 0.10 0.13 0.01 0.17 0.07 0.05 0.06 0.04 0.00 0.07 0.00 0.11 0.03 0.15 0.06 0.10 0.11 0.03 0.05 0.00 0.31 0.13 0.13 0.12 0.12 0.02 0.05 0.06 F 0.26 0.69 0.25 0.32 0.26 0.31 0.55 0.38 0.41 0.28 0.34 0.26 0.35 0.27 0.26 0.41 0.31 0.33 0.28 0.31 0.32 0.34 0.24 0.30 0.19 0.26 0.22 0.36 0.40 0.46 0.25 0.39 0.28 0.45 0.35 0.43 0.35 0.30 0.27 0.28 0.24 0.28 0.57 0.17 0.43 0.37 0.26 0.41 0.37 0.27 0.34 O=F 0.11 0.29 0.11 0.13 0.11 0.13 0.23 0.16 0.17 0.12 0.14 0.11 0.15 0.11 0.11 0.17 0.13 0.14 0.12 0.13 0.14 0.14 0.10 0.13 0.08 0.11 0.09 0.15 0.17 0.19 0.11 0.16 0.12 0.19 0.15 0.18 0.15 0.13 0.11 0.12 0.10 0.12 0.24 0.07 0.18 0.16 0.11 0.17 0.16 0.12 0.14 Total 96.37 96.52 96.19 96.50 96.64 95.57 95.43 96.36 96.60 94.60 97.08 97.08 96.71 96.21 94.99 94.56 96.27 94.55 94.63 94.85 92.94 61.98 95.23 94.10 94.46 95.47 95.70 94.35 96.17 95.42 94.85 94.32 95.97 94.65 94.66 94.36 96.20 95.81 95.23 94.72 95.32 95.63 97.03 94.00 95.58 95.95 94.31 95.72 95.35 95.17 96.60 Apatite Compositions of Apatite Phenocrsysts from the Duchesne River Formation Sample DRF-D SiO2 0.16 0.01 4.71 2.27 3.99 0.02 0.66 3.72 2.16 1.66 Fe2O3 0.17 0.08 0.34 0.39 0.18 0.04 0.05 0.14 0.22 0.03 MnO 0.02 0.00 0.00 0.01 0.02 0.08 0.02 0.00 0.02 0.01 CaO 54.03 51.85 49.29 51.66 50.69 53.86 54.13 51.29 51.94 53.62 Na2O 0.59 1.29 0.49 0.62 0.42 0.62 0.40 0.52 0.58 0.39 La2O3 0.13 0.00 0.08 0.00 0.08 0.05 0.07 0.17 0.07 0.04 Ce2O3 0.23 0.00 0.00 0.10 0.20 0.07 0.11 0.01 0.00 0.02 P2O5 36.75 35.56 34.97 36.23 35.44 37.17 37.60 35.71 36.45 37.59 F 4.20 3.93 4.03 4.17 3.85 4.21 4.36 3.91 4.17 4.39 Cl 0.02 0.03 0.02 0.02 0.03 0.01 0.01 0.01 0.00 0.00 Sum 95.94 92.65 88.87 92.79 90.71 95.99 96.69 91.62 93.21 96.05 -O=F,Cl 1.77 1.66 1.70 1.76 1.63 1.78 1.84 1.65 1.76 1.85 SubTotal 94.17 90.99 87.17 91.03 89.08 94.21 94.85 89.97 91.45 94.20 H2O* -0.30 -0.22 -0.26 -0.30 -0.16 -0.30 -0.36 -0.17 -0.29 -0.36 Total 93.87 90.77 86.91 90.73 88.92 93.91 94.49 89.80 91.16 93.84 Glass Shard Table 10: Compositions of Glass Shards from DRF-A of the Duchesne River Formation *Normalized Average SiO2 76.55 68.82 69.39 68.29 68.67 69.39 69.59 70.68 69.67 68.74 68.33 70.11 68.56 70.03 68.70 66.64 70.56 TiO2 0.08 0.15 0.08 0.02 0.07 0.14 0.08 0.03 0.06 0.11 0.04 0.05 0.09 0.07 0.04 0.05 0.03 Al2O3 13.82 12.43 11.91 11.87 12.44 12.57 11.61 13.29 12.78 12.91 12.93 12.70 12.50 12.26 12.49 12.78 12.25 FeO 0.78 0.78 0.81 0.69 0.54 0.88 0.59 0.49 0.78 0.64 0.62 0.63 0.55 0.58 0.59 0.69 0.34 MnO 0.05 0.04 0.03 0.05 0.11 0.00 0.00 0.00 0.05 0.07 0.03 0.01 0.05 0.10 0.07 0.10 0.08 MgO 0.12 0.16 0.10 0.08 0.09 0.18 0.08 0.04 0.11 0.16 0.11 0.10 0.11 0.07 0.10 0.10 0.09 Na2O 2.90 2.65 2.73 2.83 2.69 2.38 2.54 2.70 2.86 2.64 2.51 2.64 1.99 2.84 2.69 2.39 2.89 CaO 0.66 0.58 0.41 0.45 0.48 0.79 0.37 0.81 0.71 0.85 0.87 0.71 0.48 0.45 0.43 0.83 0.41 K2O 5.11 4.52 4.92 4.86 4.70 4.18 5.01 5.04 4.24 4.19 4.23 4.41 4.78 4.58 4.75 4.49 4.97 Total 100.07 90.13 90.37 89.14 89.77 90.50 89.86 93.07 91.25 90.30 89.68 91.35 89.13 90.97 89.86 88.08 91.62 XRF DRF X-TAY FLUORESCENCE ANALYSES OF DUCHESNE RIVER FORMATION TUFFS Sample DRF-A DRF-B DRF-C DRF-D DRF-E DRF-F DRF-G DRF-H DRF-I DRF-J DRF-K 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 Normalized to 100% on a volatile-free basis 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 LOI 4.83 12.06 18.97 7.37 5.44 7.28 9.94 8.69 8.55 18.21 11.09 Anal Total 99.99 99.74 100.30 99.93 99.97 99.92 100.01 99.91 99.94 99.94 100.16 Sc 8.1 7 6 6 9 5 8 4 3 1 9 V 36.8 53 48 45 55 39 46 14 12 21 38 Cr 21.6 10 22 22 40 18 20 9 4 11 27 56 25.5 6.2 21.8 13.3 4.2 Ni 8.7 10 17 11 15 12 17 17 12 9 12 52 16 4 27 16 7 Cu 10 10 13 13 16 7 9 10 6 8 17 43 7 3 21 13 2 Zn 59.5 60 74 58 55 64 82 60 59 47 44 64 30 7 23 14 4 Ga 18.4 15 20 18 16 19 21 19 19 14 9 83 43 8 19 11 3 Rb 126.4 39 39 42 81 45 44 41 23 34 52 44 24 6 21 9 3 Sr 352.2 217 225 276 205 222 255 239 268 252 127 52 25 5 24 11 5 Y 18.3 17 21 21 41 18 16 6 5 30 16 23 15 5 33 17 2 Zr 174.4 135 140 150 153 135 122 103 88 104 182 27 16 5 35 16 2 Nb 12.9 12 13 14 12 9 10 18 18 11 7 47 4 2 24 12 5 Ba 750.3 421 179 184 280 432 363 68 48 193 189 40 9 2 12 9 3 La 28.4 22 22 33 44 23 27 12 11 26 25 Ce 56 52 43 64 83 44 52 23 27 47 40 Nd 25.5 16 7 30 43 24 25 15 16 4 9 Sm 6.2 4 3 7 8 6 5 5 5 2 2 Pb 21.8 27 21 23 19 21 24 33 35 24 12 Th 13.3 16 13 14 11 9 11 17 16 12 9 U 4.2 7 2 4 3 3 5 2 2 5 3 XRF NENVF X-TAY FLUORESCENCE ANALYSES OF NORTHEAST NEVADA VOCLANIC FIELD TUFFS sample 88T 56 90 B6 90 B11 88T 55 90 B3 90 B7A 90 B7B 90 B12 90 B13 90 B5 90 B19A 90 B19B 90 B24A 90 B24B 91 T10 90 B9A SiO2 72.84 69.01 69.59 78.85 76.30 78.11 77.30 80.00 76.36 77.89 78.01 80.42 77.75 73.64 71.63 71.78 TiO2 0.38 0.46 0.40 0.10 0.10 0.09 0.09 0.11 0.10 0.16 0.11 0.08 0.13 0.11 0.31 0.33 Al2O3 14.49 16.95 16.13 11.73 13.02 11.84 11.72 10.81 12.76 11.93 12.11 10.38 11.40 14.32 14.59 14.56 Fe2O3 1.82 1.84 2.66 0.45 0.95 0.76 1.79 1.06 1.38 1.22 0.69 0.35 1.15 2.23 2.77 2.74 MnO 0.02 0.02 0.02 0.02 0.04 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.04 0.03 0.03 MgO 0.54 0.67 0.62 0.12 0.30 0.25 0.22 0.19 0.23 0.54 0.17 0.10 0.23 0.85 0.91 0.62 CaO 2.34 2.81 2.63 0.77 0.90 0.89 0.80 0.88 0.91 1.46 0.99 0.12 0.76 1.94 3.06 3.11 Na2O 3.24 3.89 3.58 2.49 3.13 2.57 2.46 2.28 2.92 1.52 2.69 1.23 1.68 2.43 3.53 3.54 K2O 4.22 4.22 4.23 5.42 5.21 5.40 5.55 4.61 5.27 5.20 5.15 7.25 6.73 4.36 3.05 3.13 P2O5 0.11 0.14 0.14 0.05 0.05 0.07 0.05 0.05 0.05 0.06 0.05 0.05 0.14 0.09 0.13 0.16 LOI 0.96 1.48 2.41 0.67 1.25 1.01 1.65 1.96 1.65 4.69 1.69 1.27 1.74 6.90 1.35 0.99 Anal Total 98.98 99.44 99.12 99.59 99.54 98.95 98.93 99.09 99.61 99.44 99.11 99.51 99.10 99.79 99.36 99.20 Rb 144 157 152 201 203 251 154 205 115 177 183 200 146 110 111 Sr 598 573 109 163 154 130 138 143 480 176 79 153 246 450 441 Y 20 24 18 22 25 23 21 22 18 18 14 21 16 17 13 Zr 276 252 99 141 120 114 112 128 149 143 121 146 110 133 136 Nb 15 12 12 15 14 11 12 12 14 17 11 11 13 9 9 Ba 2110 2550 605 1043 915 711 967 773 1941 1146 895 1499 1658 1468 1535 XRF MOR X-TAY FLUORESCENCE ANALYSES OF MORRISON FORMATION TUFFS Sample MC-9 NH-18 NH-52 BD-5 BD-8 NOR-10 NOR-11 SiO2 76.66 83.94 87.66 82.21 81.59 TiO2 0.22 0.27 0.25 0.24 0.30 Al2O3 9.11 9.60 6.17 9.77 9.78 Fe2O3 3.96 1.15 2.20 1.82 2.42 MnO 0.18 0.02 0.04 0.05 0.05 MgO 1.93 0.42 0.66 0.71 0.90 CaO 5.07 1.20 1.21 1.19 0.40 Na2O 1.87 2.08 0.78 2.73 2.68 K2O 0.93 1.26 0.87 1.24 1.85 P2O5 0.07 0.06 0.16 0.04 0.03 LOI 6.31 2.25 2.21 2.63 2.43 Total 106.31 102.25 102.22 102.63 102.43 Sc 4 2 8 4 9 17 14 V 44 34 39 23 39 87 86 Cr 4 20 21 3 5 49 53 Ni na na na na na 11 13 Cu 1 14 12 9 11 21 14 Zn 35 25 39 29 40 69 51 Ga 9 13 7 11 11 32 22 Rb 28 57 45 54 65 234 206 Sr 437 158 78 178 143 292 204 Y 31 21 22 18 18 51 53 Zr 211 113 160 146 145 320 286 Nb 9 13 7 11 9 32 23 Ba 599 432 169 214 1405 126 156 La 15 23 24 14 12 67 61 Ce na na na na na 132 119 Nd 31 21 18 29 23 53 50 Sm na na na na na 10.6 9.5 Pb na na na na na 39 24 Th 3 10 6 7 7 29 22 U na na na na na 4 3