Research Paper T H E M E D I S S U E: C Revolution 2: Origin and Evolution of the Colorado River Syste m II

GEOSPHERE Cenozoic collapse of the eastern Uinta Mountains and drainage evolution of the Uinta Mountains region

G E O S P H E R E; v. 1 4, n o. 1 Andres Aslan 1 , Marisa Boraas-Connors 2 , D o u gl a s A. S pri n k el 3 , Tho mas P. Becker 4 , Ranie Lynds 5 , K arl E. K arl str o m 6 , a n d M att H eizl er 7 1 Depart ment of Physical and Environ mental Sciences, Colorado Mesa University, 1100 North Avenue, Grand Junction, Colorado 81501, U S A doi:10.1130/ G E S01523.1 2 Depart ment of Geosciences, Colorado State University, Natural Resources Building, Roo m 322, Fort Collins, Colorado 80523, U S A 3 Utah Geological Survey, 1594 W North Te mple, Salt Lake City, Utah 84114-6100, U S A 1 5 fi g ur e s; 1 t a bl e; 4 s u p pl e m e nt al fil e s 4 Exxon Mobil Exploration Co mpany, 22777 Spring woods Village Park way, Spring, Texas 77389, U S A 5 Wyo ming State Geological Survey, P. O. Box 1347, Lara mie, Wyo ming 82073, U S A 6 Depart ment of Earth and Planetary Sciences, University of Ne w Mexico, Redondo Drive NE, Albuquerque, Ne w Mexico 87131, U S A C O R RESP O N DE N CE: aaslan @colorado mesa .edu 7 Ne w Mexico Bureau of Geology and Mineral Resources, Ne w Mexico Tech, 801 Leroy Place, Socorro, Ne w Mexico 87801, U S A

CI T A TI O N: Aslan, A., Boraas- Connors, M., Sprinkel, D. A., Becker, T. P., Lynds, R., Karlstro m, K. E., and Heizler, M., 2018, Cenozoic collapse of the eastern ABSTRACT young (younger than 40 Ma) grains also support a volcanic ash-fall origin. Uinta Mountains and drainage evolution of the Uinta So me of the grains originated fro m the Southern Rocky Mountain volcanic Mountains region: Geosphere, v. 14, no. 1, p. 115– Coupled detrital sanidine and zircon data, co mbined with sedi mento - field, and were re worked into Bro wns Park For mation deposits. Ne w M DAs 140, doi:10.1130/ G E S01523.1. logical and stratigraphic observations, provide te mporal constraints on the of Bro wns Park For mation sedi ments that unconfor mably overlie Neoprotero - post-Lara mide paleogeographic and structural evolution of the eastern Uinta z oi c U M G r o c k s i n w e st er n m o st Br o w n s P ar k pr o vi d e e vi d e n c e f or a y o u n g er Science Editor: Ray mond M. Russo Associate Editor: Todd La Maskin Mountains region fro m the late Eocene to late Miocene (ca. 36–8 Ma). Maxi - (12–8 Ma) phase of extensional collapse of the eastern Uinta Mountains that mu m depositional ages ( M D As) calculated fro m detrital zircon U-Pb and was associated with 10–20 k m of north west ward-directed lengthening of the 4 0 3 9 Received 2 March 2017 detrital sa ni di ne Ar / Ar ages indicate that the most significant Paleogene Bro wns Park graben. These data are co mpatible with models for t wo stages Revision received 22 August 2017 fluvial syste m in the region, represented by the Bishop Conglo merate, e xi st e d of post-Lara mide epeirogenic uplift of the Uinta Mountains region, including Accepted 12 October 2017 fro m 36 to 27 Ma. The abundance of red sandstone and gray li mestone clasts, post–12 Ma tectonis m that set the stage for subsequent integration of the Published online 22 Nove mber 2017 paleocurrent directions, and the large nu mber of Grenville-age detrital zir - Green and Colorado Rivers after 8 Ma. cons suggest that the Uinta Mountain Group ( U M G) facies of the Bishop Conglo merate are tributaries that flo wed radially a way fro m the crest of the Uinta Mountains. To the north of the Uinta Mountains, these rivers joined a INTRODUCTION mainste m river in south western Wyo ming represented by the Bishop Con - glo merate Firehole Canyon (F C) facies. This facies consists of rounded cobble- The northern boundary of the Colorado Plateau physiographic province to pebble-sized quartzite clasts with minor quantities of volcanic rocks, has ( western U S A) is the Preca mbrian-cored east- west–trending Uinta Mountains west ward paleocurrent directions, and abundant young (younger than 40 Ma) uplift that for med in the Lara mide orogeny, ca. 70–50 Ma ( Hansen, 1984, 1986; detrital zirc o n a n d sa ni di ne grai ns. Detrital sa ni di ne a ge a n d ge oc he mical data Bradley, 1995) (Fig. 1). Like the Kaibab uplift and Grand Canyon to the south, suggest that these young detrital grains are tephra that originated fro m the this uplift is a key physiographic barrier for understanding Cenozoic drainage Basin and Range volcanic field, which was subsequently re worked into Bishop evolution of the Colorado Plateau. Po well (1876) found it enig matic that the O L D Conglo merate sedi ments. The more regional head waters of the mainste m Green River cut a deep gorge ( Canyon of Lodore) orthogonal to the Uinta G river could have been located east of the Uinta Mountains, or in the Challis M o u ntai ns u plift, a n d p ost ulate d t hat t he river c o urse pre date d t he u plift (a nte - and Absaroka volcanic fields and the Wind River Mountains located to the cedence). Sears (1924) postulated the opposite, that the uplift predated the north west of the region. The question of whether the F C facies of the Bishop Green River and that the river course was established at higher stratigraphic OPE N A C CESS Conglo merate represents part of an integrated river syste m that was a precur - levels s uc h t hat it mai ntai ne d t his pat h as t he river i ncise d resista nt r ocks i n s or to t he Platte River re mai ns u nres olve d. t he c ore of t he u plift (s u per p ositi o n). Si nce t he n, st u dies of t he i nte grati o n of Extensional collapse of the eastern Uinta Mountains was marked by the the Green River and Colorado River syste ms have entertained more co mplex cessation of Bishop Conglo merate fluvial deposition and the onset of Bro wns histories involving landscapes where modern river courses reflect linkage of Park For mation sedi mentation within the Bro wns Park graben beginning ca. internally drained basins and modification of paleoriver seg ments through a 25 Ma. Tuffaceous sandstone and siltstone and minor quantities of carbonate co mbination of piracy and do wn ward integration ( Black welder, 1934; Lucchitta, T hi s p a p er i s p u bli s h e d u n d er t h e t er m s of t h e accu mulated in a mosaic of fluvial and lacustrine environ ments representing 1 9 7 2; P e d er s o n a n d H a d d er, 2 0 0 5; A sl a n et al., 2 0 1 4; K arl str o m et al., 2 0 1 4; Ki m - C C- B Y- N C li c e n s e. an internally drained basin. Detrital sanidine age and geoche mical data for br o u g h et al., 2 0 1 5) c o u pl e d wit h C e n o z oi c u plift ( K arl str o m et al., 2 0 1 2).

© 2017 The Authors

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A bsaro k a V ol c a ni c Fi el d B elt

S weet water River and Thrust

Figure 1. Map sho wing generalized geol - G r e at Di vi d e B asi n ogy of western Wyo ming ( WY), north west - ern Colorado ( C O), and northeastern Utah n g F ol d ( UT). Red box outlines the general study

mi Roc k Fi g ur e 3 B area (Fig. 3 B). F m. —for mation. Inset map S pri ngs sho ws the study area within the western G r e e n Ri v e r U nite d States. W y o U plift Was ha ki e B asi n B asi n W Y Sand Wash Ui nt a Mo u ntai ns B asi n

Ui nt a B asi n

U T C O

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Reconstructing Cenozoic landscapes is co m monly ha mpered by i mpre - of Cenozoic landscape evolution (Lilligraven and Ostresh, 1988; S mith et al., cise chronologies that rely on techniques such as paleo magnetostratigraphy, 2 0 0 3, 2 0 0 8; M c Mill a n et al., 2 0 0 6), a n d t h e s e d etrit al d ati n g t e c h ni q u e s pr o vi d e bi o str ati gr a p hi c c orr el ati o n t o l a n d- m a m m al a g e s, a n d w h er e a v ail a bl e, r a di o - excellent opportunities to i mprove our understanding of paleogeography. metric dati n g of volca nic de p osits. T he a p plicati o n of detrital zirc o n U- P b ge o - The Cenozoic geologic history of south western Wyo ming, northeastern c hr o n ol o gy has pr ovi de d a n ot her t o ol t o c o nstrai n t he a ges of terrestrial clastic Utah, and north western Colorado (Fig. 1) provides the fra me work for studying d e p o sit s ( e. g., St e w art et al., 2 0 0 1; H o d g e s et al., 2 0 0 5; G e hr el s et al., 2 0 11; F a n the evolution of the upper Green River syste m beginning with Lakes Gosiute et al., 2015), and ne wly e merging techniques such as detrital sanidine 4 0 Ar/ 3 9 Ar and Uinta during the Eocene ( Bradley, 1936; Hansen, 1969a, 1969b, 1984, 1986; geochronology offer potentially even more precise means for establishing te m - S mith et al., 2008) (Fig. 2 A). As sho wn in Figure 2 B, Hansen (1986) suggested poral fra me works for interpreting the provenance of terrestrial sedi ments and that the Eocene lakes were replaced by an Oligocene east ward-flo wing river la n dsca pe evol uti o n ( Heref or d et al., 2016; Karlstr o m et al., 2017). Wit hi n t he syste m (ancestral Platte River) originating in the mountains of south western western United States, Lara mide inter montane basins are i mportant archives Wyo ming and northeastern Utah. In this model, subsequent Miocene collapse

A Eocene B Oli g o c e n e

MW R MW R T B W F

Lake WFT B G osi ut e Figure 2. Paleogeographic maps sho wing Hansen’s (1986) hypothesized evolution of Cenozoic fluvial syste ms in the Uinta Oli g o c e n e Di vi d e Mountains region. ( A) During the Eocene, U M U M internal drainage prevailed in the Rocky Mountain region and rivers drained into Lakes Gosiute and Uinta, located north and south of the Uinta Mountains, respec - L a k e Ui nt a 1 0 0 k m tively. U M — Uinta Mountains; WFTB — 1 0 0 k m Wyo ming fold and thrust belt; WR M — Wind River Mountains. (B) During the Oligocene, an east ward-flo wing ancestral C Mi o c e n e D Modern Platte River syste m flo wed through the Gree n River Basi n. ( C) D uri n g t he Mi oce ne, S n collapse of the eastern Uinta Mountains a C o nti n e nt al Di vi d e M R W k e R iv er for med the Bro wns Park graben (note that the scale differs fro m that of A, B, R ive r and D). ( D) Fro m the Miocene to the pres - at e r S w e e tw ent, drainage reorganization produced Gre e n Ri v er the modern river syste ms of the region.

WFT B Blue arro ws sho w flo w direction of rivers. G D B — Great Divide Basin. Br o w ns Par k B G D B e

a

graben r U M R i v e a k e R i r S n v e r tle it Mi o c e n e di vi d e L U M Y a m p a R iv er

4 0 k m 1 0 0 k m v e r lo ra d o R i C o

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of the eastern Uinta Mountains and the for mation of the Bro wns Park graben upper Eocene– Oligocene Bishop Conglo merate. In so me parts of north west - set the stage for drainage reorganization and capture of the ancestral east - ern Colorado, the erosion surface is overlain by the upper Oligocene– Miocene ward-flo wing Platte River (Fig. 2 C). Today the Green River flo ws south ward Bro wns Park For mation ( Hansen, 1986; Buffler, 2003). Prior studies sho w that across the Uinta Mountains through Lodore Canyon, and joins the Ya mpa the Bro wns Park For mation locally overlies the Bishop Conglo merate along the River in Echo Canyon (Fig. 2 D). Details of the transition fro m Eocene lakes eastern flank of the Uinta Mountains ( Hansen, 1986). to the modern Green River syste m, ho wever, re main poorly understood. In The upper Eocene– Oligocene Bishop Conglo merate was first described by particular, the ages of key Cenozoic deposits, the path ways of ancient rivers, Po well (1876) and studied in detail by Bradley (1936) and Hansen (1984, 1986). and the precise ti ming of the collapse of the eastern Uinta Mountains re main The unit consists predo minantly of fluvial sandy conglo merate, tuffaceous poorly constrained ( Hansen, 1986; Pederson and Hadder, 2005). conglo meratic sandstone, and lesser a mounts of tuffaceous mudrock that fill T he p ur p ose of t his pa per is t o i nte grate se di me nt ol o gic a n d strati gra p hic paleovalleys in northeastern Utah, south western Wyo ming, and north western observations, provenance infor mation, and age constraints provided by ne w Colorado. In general, the Bishop Conglo merate is unlithified but erosionally detrital zircon and sanidine data to i mprove our understanding of Cenozoic resista nt, a n d cr o ps o ut o n platea us a n d mesa t o ps. Strata are ty pically flat ly - landscape evolution in the Uinta Mountains region. Specifically, this study ing, tens of meters to a fe w hundred meters thick, and unconfor mably overlie (1) pr ovi des ne w c o nstrai nts o n t he ti mi n g of exte nsi o nal c olla pse of t he east - gently dipping Preca mbrian through early Paleogene rocks in both the Uinta ern Uinta Mountains, which set the stage for final integration of the Green and Green River Basins. The sedi ment source of the Bishop Conglo merate River across the Uinta Mountains, (2) exa mines the Cenozoic fluvial record of is interpreted to be pri marily fro m the Uinta Mountains, because the depos - the Uinta Mountains region and questions the interpretation of a late Eocene its thin and clast sizes decrease radially a way fro m the range front ( Bradley, to Oligocene east ward-flo wing ancestral Platte River syste m, and (3) high - 1936; Hansen, 1986). Co mpositionally, clasts are do minated by red sandstones lights the prevalence of far-transported Oligocene– Miocene ash-fall grains of the Neoproterozoic Uinta Mountain Group, and Paleozoic li mestones and and sedi ment re working in the Bishop Conglo merate and Bro wns Park For - sandstones ( Hansen, 1986). Bradley (1936) and Hansen (1986) interpreted the mation, which has i mportant i mplications for using detrital zircon and sani - Bishop Conglo merate to represent a ti me of alluvial sedi mentation associated dine data for interpreting the provenance and depositional ages of clastic with tectonic quiescence and cli matic change fro m hot and moist conditions sedi ments that have accu mulated during ti mes of extensive volcanic activity. in the Eocene to cooler and drier conditions during the Oligocene. The age of the Bishop Conglo merate is constrained by 4 0 Ar/ 3 9 Ar a ges (ca. 34–30 Ma) of v ol - GEOLOGIC SETTI NG canic tuffs located along the southern flank of the Uinta Mountains near V er n al, Utah ( Ko wallis et al., 2005). K- Ar ages of tuff of the Bishop Conglo merate lo - Late Cretaceous to early Cenozoic Lara mide tectonis m in south western cated near Vernal suggest a slightly younger mini mu m age (ca. 29–26 Ma) Wyo ming, northeastern Utah, and north western Colorado created uplifts in the ( Da mon, 1970; Hansen, 1986). The tuffs are thought to have been produced by study area, including the east- west–trending Uinta Mountains and the north- caldera-for ming eruptions within the Basin and Range extensional province in south–trending Rock Springs uplift (Fig. 1). North of the Uinta Mountains and c e ntr al Ut a h, i n cl u di n g t h e C ott o n w o o d c al d er a ( K o w alli s et al., 2 0 0 5). w e st of t h e R o c k S pri n g s u plift, E o c e n e b a si n fill, i n cl u di n g t h e Gr e e n Ri v er a n d The upper Oligocene– Miocene Bro wns Park For mation was also first de - Bridger For mations, accu mulated in the southern Green River Basin ( Bridger scribed by Po well (1876) and was studied in detail by Buffler (1967, 2003), Izett B a si n) u ntil c a. 4 7 M a ( M ur p h e y a n d E v a n off, 2 0 0 7; S mit h et al., 2 0 0 8) ( Fi g. 3 A). (1975), Hansen (1984, 1986), and Luft (1985) in north western Colorado and north - South of the Uinta Mountains, Eocene sedi mentary rocks represented by the eastern Utah. The Bro wns Park For mation contains tuffaceous sandstone and Green River, Uinta, and Duchesne River For mations, filled the Uinta Basin until mudrock with minor a mounts of li mestone that represent a mosaic of fluvial, at least ca. 40 Ma ( Mauger, 1977; Bryant et al., 1989; Prothero and S wisher, l a c u s tri n e, a n d e oli a n e n vir o n m e nt s (Iz ett, 1 9 7 5; L uft, 1 9 8 5; H a n s e n, 1 9 8 6; B uf fl er, 1992; Sprinkel, 2015). Eocene deposits ( Green River and Bridger For mations) 2003). Volcanic tuffs are locally abundant, and a conglo meratic facies underlies filled the Washakie and Sand Wash Basins to the east and southeast of the the fine-grained volcaniclastic deposits in the Sand Wash Basin near the Park R o c k S pri n g s u plift, r e s p e cti v el y, u ntil c a. 4 6 – 4 5 M a ( S mit h et al., 2 0 0 8). Range and Flat Tops of Colorado ( Kucera, 1962; Buffler, 2003). Although the eo - A regional unconfor mity separates Eocene and older Lara mide basin fill lian facies of the Bro wns Park For mation suggest that this unit covered a broad fro m younger units in Bro wns Park and the Green River and Uinta Basins (Fig. area ori gi nally (Izett, 1975), t he t hickest (t o 500 m t hick) a n d best preserve d de - 3 A). T his u nc o nf or mity is referre d t o as t he Gil bert Peak er osi o n s urface i n t he posits are in extensional grabens such as Bro wns Park and the Ya mpa River region ( Bradley, 1936), and the Green Mountain erosion surface in the Sand valley south of Stea mboat Springs in Colorado. Hansen (1986) docu mented that Was h Basi n ( B uffler, 1967). T his s urface bevels r ocks fr o m t he c ore of t he Ui nta the collapse of the eastern Uinta Mountains at the end of the deposition of the Mountains ( Neoproterozoic Uinta Mountain Group) to lo wer Paleogene units Bishop Conglo merate provided the acco m modation necessary to accu mulate in the surrounding Lara mide basins. In northeastern Utah, south western and preserve the Bro wns Park For mation. K- Ar ages of tuff fro m the Bro wns Wyo ming, and parts of north western Colorado, this surface is overlain by the Park For mati o n ra n ge fr o m ca. 25 t o 8 Ma (Izett, 1975; L uft, 1985; B uffler, 2003).

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Gr e e n Ri v er B asi n a n d A Bro wns Park Ui nt a B asi n

Mi o c e n e B r o w ns Par k F m .

Oli g o c e n e Bis h o p Conglo merate Bis h o p Conglo merate Fi g ure 3. ( A) Strati gra p hic c orrelati o n c hart of Eocene– Miocene units in the Green River and Uinta Basins and Bro wns Park Gil b e rt Pea k E r osi o n area ( modified fro m Hansen, 1984). Major Gil b e rt P e a k E r osi o n unconfor mities are sho wn by gray fill pat - Eocene Duchesne Rive r F m . tern. F m. —for mation. (B) Map sho wing distribution of Oligocene Bishop Conglo m - B ri d g e r F m . Ui nt a F m . erate and Miocene Bro wns Park For mation, a n d l ocati o ns of detrital zirc o n sa m ples. L o - G r e e n Ri v e r F m . G r e e n Ri v e r F m . cati o n of Fi g ure 4 is s h o w n. Detrital zirc o n sa mple abbreviations: GR AT — Green River Airport Terrace; A B — Antelope Butte; B C — Bitter Creek; C C — Crouse Canyon; EL K —Elk B Legend Springs; FC —Firehole Canyon; JEC —Jesse Rock Spri ng s U plift Mi o c e n e Bro w ns Par k F m. a n d e q ui v al e nt u nit s E wing Canyon; J W M —John Weller Mesa; W Y Oli g o c e n e Bis h o p C o n gl o me r at e LOD —Lodore Canyon; P W —Po wder Wash; S W —Sand Wash; SB —South Baxter; TF — G R A T Fi g ur e 4 D et rit al Z i r c o n S a m p l es Taylor Flat; UCC — Upper Crouse Canyon. Br o w n s P a rk For m ati o n A B Note that location Firehole Canyon (F C) Sa n d y F a ci es of t h e Bishop Conglo me r at e S B F C F a ci es of t h e Bishop Conglo me r at e represents t wo sa mples; one sa mple of F C U M G F a ci es of t h e Bis h o p C o n gl o me r at e the U M G ( Uinta Mountain Group) facies B C of the Bishop Conglo merate (sho wn as a red dot) and a second sa mple (not sho wn) representing the F C facies of the Bishop Conglo merate. Detrital sanidine sa mples J E C P W were also acquired at B C, EL K, and J W M. T F S a n d W a s h Sources: Esri —https:// w w w . e sri .c o m/; Ui nt a M o u nt ai n s C C US GS — U.S. Geological Survey; N OAA — V C B asi n U.S. National Oceanic and At mospheric U C C J W M L O D S W L S R Ad ministration. WY — Wyo ming; UT — Utah; C O —Colorado. U T E L K C O

D a t a R e p os it or y 1. D et a il e d Bi shop C o n gl o m er at e a n d Br o wns P ar k F o r ma o n d et ri t a l zi rc o n a n d s a ni di n e s a m pl e l o c a li t y i nf or m a o n .

L o c a o n U ni t/ F a ci es S a m p l e # L a tu d e L o n g it u d e S a m p l e S a m p l e S a m p l e T y p e El e v a o n El e v a o n ( ) ( m) Bi s h o p C o n g l o m e r a t e

Gr e e n R i v er , W Y T b c - U M G f a ci es G R A T - 8 2 7 1 1- 3 4 1. 46342 -1 0 9 . 4 8 8 9 9 G vl P it 7180 2 1 8 8 Fi r e h ol e M e s a , W Y T b c - F C f ac i e s F C- 102911 - 1 A 4 1. 31498 -1 0 9 . 3 6 2 0 5 O ut cr o p 7290 2 2 2 2 Fi r e h ol e M e s a , W Y T b c - U M G f a ci es F C- 8 2 7 1 1- 2 4 1. 51514 -1 0 9 . 3 6 2 8 1 O ut cr o p 7300 2 2 2 5 P o w d er W as h, C O T bc - U M G f ac i e s N R- 81611-1A - 1 C 4 0. 89909 -1 0 8 . 3 2 2 7 3 G vl P it 7290 2 2 2 2 S o ut h B ax t e r, W Y T bc - F C f a ci es S B - 7- 9 -1 2- 1 4 1. 36008 -1 0 9 . 1 1 8 7 O utc r o p 7 5 1 5 2 2 9 1 Bi e r C k M es a, W Y T bc - F C f a ci es B C - 7 1 1 1 2 - 1 41.31839 -1 0 9 . 2 2 8 3 2 O ut cr o p 7340 2 2 3 7 El k S p ri n g s, C O T bc - U M G f ac i e s T b p - 7 9 1 2- 1 4 0. 35814 -1 0 8 . 4 7 8 1 6 G vl P it 6560 1 9 9 9 S a n d W as h, C O T bc - U M G f ac i e s T b p - S W- 52913 40.63082 -1 0 8 . 3 7 7 9 8 O ut cr o p 5826 1 7 7 6 METHODS measure ments were li mited to the Firehole Canyon area where Bishop Con - A n t e lo p e B u e , W Y T b c - Sandy f ac i e s A N T -7 1 1 1 2- 1 4 1. 38363 -1 0 9 . 1 9 6 2 1 O ut cr o p 7706 2 3 4 9 U p p e r Cr o us e C y n, U T T b c - Sandy f ac i e s T bc - 3 2 6 1 4 - 1 40.77213 -1 0 9 . 1 1 0 4 4 O ut cr o p 6845 2 0 8 6

Br o w ns P a r k F m glo merate is locally ce mented. In many areas, the Bishop Conglo merate is un -

T a yl or F lat , U T T b p T b p - 3 2 9 1 4 - 1 40.87071 -1 0 9 . 1 9 5 7 6 O ut cr o p 6152 1 8 7 5 John W ell er M es a, C O T b p T b p- 7 9 1 2- 2 4 0. 64819 -1 0 8 . 5 5 5 9 9 O ut cr o p 6640 2 0 2 4 Field work consisted of describing and sa mpling selected outcrops of lithified. Paleocurrents were pri marily measured using i mbricated clasts within Lo d o r e C y n, C O T b p T b p - L O D - 5 2 913 40.73198 -1 0 8 . 8 7 3 7 9 O ut cr o p 5464 1 6 6 5 J e ss e E w i n g C y n, U T T b p T B P 1 2 - 3 4 0 . 9 1178 - 1 0 9 . 1 4377 O utc r o p 5 7 9 2 1 7 6 5 L o w e r Cr o us e C y n, U T T b p T B P 1 2- 4 4 0. 8 3 2 3 9 - 109.0881 1 O ut cr o p 5771 1 7 5 9 V e r m illi o n C k , C O T b p T B P 1 2- 5 4 0. 7 3 2 3 5 - 108.7629 7 O ut cr o p 5898 1 7 9 8 the Bishop Conglo merate and the Bro wns Park For mation in areas including conglo meratic units, although in a fe w instances measure ments were made Li le S n a k e Ri v e r, C O T b p T B P 1 2 - 6 4 0 . 6 1209 - 1 0 8 . 3 2736 O utc r o p 5 8 4 1 1 7 8 0 (1) the southern flank of the Rock Springs uplift and the adjacent Green River on foresets of trough cross-bedded sandstones. Measure ments were made at 1 Supple mental Ite m 1. Detailed Bishop Conglo merate Basin (Firehole Canyon, Aspen Mountain, Green River), (2) the Bro wns Park re - a total of 18 stations, and 5–8 measure ments were recorded at each station. and Bro wns Park For mation detrital zircon and sani - gion of northeastern Utah and north western Colorado, and (3) parts of the west - We collected 17 detrital zircon and 3 detrital sanidine sa mples fro m grav - di ne sa m ple l ocality i nf or mati o n. Please visit htt p:// d oi . or g / 1 0 .1130 / GES01523 . S 1 or t he f ull-text article ern Sand Wash Basin of north western Colorado (Elk Springs, Po wder Wash) elly, sandy, and tuffaceous beds of the Bishop Conglo merate and Bro wns Park o n w w w .gsapubs . or g t o vi e w S u p pl e m e nt al It e m 1. (Fig. 3 B). We also described clast types at selected localities. Paleocurrent For mation (Supple mental Ite m 1 1 ). Both zircon and K-feldspar grains were

GEOSPHERE | V ol u m e 1 4 | Nu mber 1 A sl a n et al. | C e n o z oi c c oll a p s e of t h e e a st er n Ui nt a M o u nt ai n s 11 9 Do wnloaded fro m https://pubs.geoscience world.org/gsa/geosphere/article-pdf/14/1/115/4034955/115.pdf by Univ Ne w Mexico user on 19 Dece mber 2018 Research Paper

D a t a R e p os it or y 2. Su m m a r y o f Bis h o p C o n g l o m e r a t e a n d B r o w n s P a r k F m. d et ri t a l zi r c o n U - P b d a t a . BI S H O P C O N GL O M E R A T E S A M P L E S se pa rated fro m bulk sa mples using standard methods. Detrital zircon ( DZ) of U-Pb ages of zircons collected fro m a single volcanic rock may sho w a dis - G R A T , G R A T -827113-3. U- P b ge ochronologi c a n al y s e s . Is ot o p e r a o s A p p ar e n t a g es ( M a ) A n al ys is U 2 0 6 P b U/ T h 2 0 6 P b * ± 207P b * ± 2 0 6 P b * ± e rr o r 206P b * ± 2 0 7 P b * ± 2 0 6 P b * ± B e st a g e ± U-Pb ages of ~100 detrital grains fro m each sa mple were deter mined by laser tri b uti o n wit h overla p pi n g u ncertai nties (ty pically 1 σ wit h L A-I C P- M S data sets) ( p p m) 2 0 4 P b 2 0 7 P b * ( %) 235 U* ( % ) 2 3 8 U ( %) c o rr. 238 U * ( Ma ) 2 3 5 U ( Ma ) 2 0 7 P b * ( Ma ) ( M a) ( M a)

G R A T -38 147 361 1 0 . 7 15.6445 44 . 8 0. 0 423 48. 7 0 . 0 048 19. 2 0 . 3 9 3 0. 9 5 . 9 4 2 . 1 2 0. 1 7 3 9 . 0 9 9 7. 4 3 0. 9 5 . 9 G R A T -48 80 192 3 0 . 8 9. 2 677 241 . 8 0. 0 769 243 . 1 0. 0 052 25. 2 0 . 1 0 3 3. 2 8 . 3 7 5 . 2 1 7 8. 1 1 7 6 4. 3 5 0 2. 6 3 3. 2 8 . 3 ablation– multicollector–inductively coupled plas ma– mass spectro metry (L A- such that the youngest age distribution in a sa mple may be a better esti mate G R A T -85 122 431 6 1 . 0 16.1563 85 . 5 0. 0 450 86. 4 0 . 0 053 12. 4 0 . 1 4 3 3. 9 4 . 2 4 4 . 7 3 7. 8 6 7 0 . 5 2 4 2 2. 7 3 3. 9 4 . 2 G R A T -10 247 42 0 0 . 5 12.9012 37 . 7 0. 0 566 38. 3 0 . 0 0 5 3 7. 0 0 . 1 8 3 4. 0 2 . 4 5 5 . 9 2 0. 8 1 1 3 4. 4 7 7 7. 7 3 4. 0 2 . 4 G R A T -11 105 300 3 0 . 9 - 4. 0119 706 . 0 - 0. 1823 706 . 2 0. 0 053 14. 0 0 . 0 2 3 4. 1 4 . 8 -204 . 4 NA NA NA 3 4 . 1 4. 8 G R A T -22 104 195 5 0 . 9 16.7710 62 . 2 0. 0 440 64. 2 0 . 0 054 16. 0 0 . 2 5 3 4. 4 5 . 5 4 3 . 7 2 7. 5 5 9 0 . 0 1 5 0 2. 3 3 4. 4 5 . 5 G R A T -40 155 340 7 1 . 0 16.8691 41 . 1 0. 0 439 41. 6 0 . 0 0 5 4 6. 2 0 . 1 5 3 4. 5 2 . 2 4 3 . 6 1 7. 8 5 7 7 . 4 9 3 1. 7 3 4. 5 2 . 2 M C-I CP- M S) at the Arizona Laser Chron Center (Tucson, Arizona). For individ - of t he a ge of a volca nic eve nt t ha n t he y o u n gest grai n. G R A T -63 157 399 2 1 . 1 11.7479 75 . 8 0. 0 633 76. 3 0 . 0 0 5 4 8. 4 0 . 1 1 3 4. 7 2 . 9 6 2 . 3 4 6. 1 1 3 1 8. 3 1 7 9 7. 5 3 4. 7 2 . 9 G R A T -25 137 314 7 0 . 9 17.4894 57 . 0 0. 0 426 57. 6 0 . 0 0 5 4 8. 5 0 . 1 5 3 4. 7 3 . 0 4 2 . 4 2 3. 9 4 9 8 . 4 1 3 6 8. 7 3 4. 7 3 . 0 G R A T -84 144 329 1 0 . 9 17.9868 46 . 3 0. 0 415 47. 1 0 . 0 0 5 4 8. 7 0 . 1 8 3 4. 8 3 . 0 4 1 . 3 1 9. 1 4 3 6 . 3 1 0 8 7. 5 3 4. 8 3 . 0 2 0 7 2 0 6 G R A T -90 88 237 7 0 . 7 6. 8 979 259 . 1 0. 1 087 259 . 3 0. 0 054 10. 0 0 . 0 4 3 4. 9 3 . 5 1 0 4. 7 2 6 3. 9 2 2 8 7. 4 1 6 3. 7 3 4. 9 3 . 5 u al zir c o n gr ai n s ol d er t h a n 9 0 0 M a, P b/ Pb ages are reported, whereas for As o utli ne d by Dicki ns o n a n d Ge hrels (2009), t here are at least five pri mary G R A T -47 186 358 0 0 . 5 18.9354 50 . 4 0. 0 397 50. 9 0 . 0 0 5 5 7. 0 0 . 1 4 3 5. 0 2 . 5 3 9 . 5 1 9. 7 3 2 0 . 7 1 2 2 0. 5 3 5. 0 2 . 5 G R A T -13 173 268 6 1 . 6 20.0384 56 . 5 0. 0 379 57. 0 0 . 0 0 5 5 7. 6 0 . 1 3 3 5. 4 2 . 7 3 7 . 8 2 1. 1 1 9 0 . 6 1 4 2 4. 6 3 5. 4 2 . 7 G R A T -14 123 348 8 0 . 9 30.8620 82 . 1 0. 0 249 83. 1 0 . 0 056 12. 8 0 . 1 5 3 5. 8 4 . 6 2 4 . 9 2 0. 5 - 9 2 8. 3 2 9 2 5. 6 3 5. 8 4 . 6 2 0 6 2 3 8 G R A T -37 624 11734 0. 4 21.4363 13 . 0 0. 0 361 13. 3 0 . 0 0 5 6 2. 7 0 . 2 1 3 6. 1 1 . 0 3 6 . 0 4. 7 3 1. 3 3 1 2. 5 3 6. 1 1 . 0 zircon grains younger than 900 Ma we report the P b/ U a ges. Zirc o n grai n methods that can be used as a basis for deter mining M D As: (1) the youngest G R A T -62 195 548 6 1 . 6 30.0064 116 . 8 0. 0 258 117 . 2 0. 0 0 5 6 9. 4 0 . 0 8 3 6. 1 3 . 4 2 5 . 9 3 0. 0 - 8 4 7. 0 0 . 0 3 6 . 1 3. 4 G R A T -46 135 233 3 0 . 8 29.9809 67 . 9 0. 0 259 69. 0 0 . 0 056 12. 6 0 . 1 8 3 6. 2 4 . 6 2 6 . 0 1 7. 7 - 8 4 4. 6 2 1 7 8. 8 3 6. 2 4 . 6 G R A T -19 259 424 9 1 . 3 37.9884 47 . 7 0. 0 205 48. 0 0 . 0 0 5 6 5. 6 0 . 1 2 3 6. 3 2 . 0 2 0 . 6 9. 8 N A N A 3 6. 3 2 . 0 G R A T -16 111 164 9 0 . 8 21.7228 51 . 6 0. 0 358 54. 1 0 . 0 056 16. 4 0 . 3 0 3 6. 3 5 . 9 3 5 . 7 1 9. 0 - 0. 5 1 3 2 5. 8 3 6. 3 5 . 9 G R A T -53 240 662 9 1 . 4 33.2395 54 . 0 0. 0 234 54. 2 0 . 0 0 5 6 5. 3 0 . 1 0 3 6. 3 1 . 9 2 3 . 5 1 2. 6 N A N A 3 6. 3 1 . 9 ages were filtered using a 20 % discordance filter and a 5 % reverse discordance single grain age within a distribution of detrital ages, (2) the youngest prob - G R A T -66 95 250 0 1 . 0 7. 2 712 148 . 6 0. 1 072 149 . 2 0. 0 057 13. 2 0 . 0 9 3 6. 3 4 . 8 1 0 3. 4 1 4 7. 7 2 1 9 6. 3 2 1 2. 6 3 6. 3 4 . 8 G R A T -72 130 306 2 0 . 8 20.3167 61 . 9 0. 0 384 62. 9 0 . 0 057 10. 8 0 . 1 7 3 6. 4 3 . 9 3 8 . 2 2 3. 6 1 5 8 . 4 1 6 0 3. 6 3 6. 4 3 . 9 G R A T -99 341 261 5 2 . 6 18.8904 19 . 1 0. 0 416 19. 4 0 . 0 0 5 7 3. 5 0 . 1 8 3 6. 6 1 . 3 4 1 . 4 7. 9 3 2 6 . 1 4 3 6. 2 3 6. 6 1 . 3 2 G R A T -86 218 492 5 1 . 4 26.8017 73 . 5 0. 0 293 73. 8 0 . 0 0 5 7 7. 0 0 . 0 9 3 6. 7 2 . 5 2 9 . 4 2 1. 4 - 5 3 4. 0 2 2 8 0. 9 3 6. 7 2 . 5 filter. Co mplete details are tabulated in Supple mental Ite m 2 . a bility pl ot ( Y P P) wit hi n a pr o ba bility de nsity f u ncti o n of a sa m ple of detrital G R A T -97 240 314 1 2 . 7 18.9827 30 . 3 0. 0 415 31. 2 0 . 0 0 5 7 7. 5 0 . 2 4 3 6. 7 2 . 7 4 1 . 3 1 2. 6 3 1 5 . 0 7 0 3. 4 3 6. 7 2 . 7 G R A T -28 114 260 3 1 . 0 20.3473 35 . 2 0. 0 389 39. 4 0 . 0 057 17. 7 0 . 4 5 3 6. 9 6 . 5 3 8 . 8 1 5. 0 1 5 4 . 9 8 4 8. 3 3 6. 9 6 . 5 G R A T -74 137 96 6 0 . 8 20.6993 53 . 9 0. 0 383 55. 5 0 . 0 058 13. 3 0 . 2 4 3 7. 0 4 . 9 3 8 . 2 2 0. 8 1 1 4 . 5 1 3 6 5. 2 3 7. 0 4 . 9 G R A T -41 135 208 8 1 . 0 7. 4 861 352 . 1 0. 1 061 352 . 2 0. 0 0 5 8 9. 8 0 . 0 3 3 7. 0 3 . 6 1 0 2. 4 3 5 7. 0 2 1 4 5. 5 6 0 2. 8 3 7. 0 3 . 6 G R A T -87 172 347 0 0 . 7 26.1810 41 . 0 0. 0 304 42. 3 0 . 0 058 10. 4 0 . 2 5 3 7. 1 3 . 8 3 0 . 4 1 2. 7 - 4 7 1. 6 1 1 2 5. 2 3 7. 1 3 . 8 Detrital sanidine ( D S) geochronology involved hand-picking clear K-feld - ages, (3) the weighted mean age of the youngest cluster of 2 or more grain G R A T -79 73 173 0 1 . 0 15.7119 142 . 2 0. 0 507 143 . 0 0. 0 058 14. 8 0 . 1 0 3 7. 1 5 . 5 5 0 . 2 7 0. 2 7 2 9 . 9 7 9 3. 3 3 7. 1 5 . 5 s par grai ns i n a n atte m pt t o c o nce ntrate sa ni di ne f or 4 0 Ar/ 3 9 Ar d ati n g of i n di vi d - a g e s wit h 1 σ overla p of u ncertai nty [ Y C 1 σ (2+)], (4) t he wei g hte d mea n a ge of 2 Supple mental Ite m 2. Su m mary of Bishop Conglo m - ual crystals (~45–65 grains per sa mple) via single-crystal laser fusion. Analy - the youngest cluster of 3 or more grain ages overlapping at 2 σ [ Y C 2 σ ( 3 +)], a n d erate and Bro wns Park For mation detrital zircon U-Pb d at a. Pl e as e visit htt p:// d oi . or g / 1 0 .1130 / GES01523 . S 2 ses were co mpleted at the Ne w Mexico Geochronology Research Laboratory (5) a M o nte Carl o a nalysis of t he u ncertai nty of t he y o u n gest s u bset of detrital or t he f ull-text article o n w w w .gsa p u bs . or g t o vie w ( N M G RL) using an A R G U S VI multicollector mass spectro meter, and individual a ges i n a n a ge distri b uti o n. T his a p pr oac h is i nc or p orate d i n t he Excel Is o pl ot S u p pl e m e nt al It e m 2. grains yielded precision of ~0.05 %–0.2 % (2 standard deviations) for 35–20 Ma add-in (Lud wig, 2012). This approach is si milar to the YPP deter mination, but with variation dependent upon grain size and radiogenic yield. Based on age t he act ual u ncertai nty ar o u n d t his y o u n gest peak is ex pl ore d differe ntly. I n t his D R 3. 4 0 Ar / 3 9 Ar data re p osit ory f or: and K/ Ca value, this method was very successful for Bro wns Park sa mple Tbp study, M D As of both zircon and sanidine sa mples were calculated as the mean Cenozoic collapse of the eastern Uinta Mountains and drainage 7912-2 and less so for the Bishop Conglo merate sa mple B C71112-1 that yielded standard weighted ages of the youngest group of three or more detrital grains evolu on of the Uinta Mountain regio n several microcline grains older than 500 Ma. Co mplete details are tabulated in fro m each sa mple [YC 2 σ (3+) of Dickinson and Gehrels, 2009] using Isoplot Andres Asla n 1 , Marisa Boraas- Connor s 2 , D o u g S pri n k el 3 , Tho mas P. Becker 4 , Ranie Lynds 5 , K arl E. K arlstr o m 6 , a n d M att H ei zl er 7 3 1 D e pt. of P h ys ical and Environ mental Sciences, Colorado Mesa University , Gr a n d J u n cti o n, C O Supple mental Ite m 3 . D S age spectra were co mpared to igni mbrite sanidine (L u d wi g, 2012) ( Ta ble 1). T he details of t hese calc ulati o ns are prese nte d i n S u p - 81501 2 D e pt. of G e os ci e n c es, C ol or a d o St at e U ni v ersit y , Ft. C olli ns, C O 8 0 5 2 3 data fro m the N M G RL database in an effort to deter mine source area. pl e m e nt al It e m 4 4 . 3 Ut a h G e ol o gi c al S ur v e y, S alt L a k e Ci t y , U T 84114-6100 4 E x x on Mobil Exploration Co mpan y, 22777 Spring woods Village Park wa y, S pri n g, T X 77389 5 W y o mi n g G e ol o gi c al S ur v e y , L ar a mi e, W Y 82072 Zircon and sanidine data sets were used to assess the maxi mu m depo - The mean square of weighted deviates ( MS W D) is co m monly used to as - 6 D e pt. of E art h a n d Pl a n et ar y S ci e n c es, U ni v ersit y of N e w M e xi c o, Al b u q u erque, N M 87131 7 Ne w Mexico Bureau of Geology and Mineral Resources, Ne w Mex ico Tech, Socorro, N M sitional ages ( M D As) of clastic deposits. Clastic deposits typically represent sess ho w well a cluster of ages may represent a single population ( Wendt and 87801 re working of material that was previously crystallized or for med. So meti mes Carl, 1991). In Isoplot (Lud wig, 2012), the weighted mean ( W M) is calculated 3 Supple mental Ite m 3. Su m mary of Bishop Conglo m - billi o ns of years se parate t he f or mati o n of a pare nt r ock a n d de p ositi o n of its fro m data for a series of grains by weighting each grain according to their erate and Bro wns Park For mation detrital sanidine clastic residuu m. As a result, radio metric ages of constituent minerals may variance with s mall uncertainties more heavily weighted than those with large 4 0 Ar/ 3 9 Ar data. Please visit htt p:// d oi . or g / 1 0 .1130 / GES01523 . S 3 or t h e f ull-t e xt arti cl e o n w w w .gsapubs not provide the best esti mate of depositional age, but the deposit can be no uncertainties. The M S W D provides a measure of the deviation of the data fro m . or g t o vi e w S u p pl e m e nt al It e m 3. older than the youngest constituent minerals. This gap bet ween the age of a a nor mal distribution with respect to analytical error. Ideally an M S W D of 1 deposit and the M D A given by the youngest crystallized minerals in a deposit indicates that data are scattered about a mean value in accordance with the

DR4. Maxi mu m deposi onal age calcula o n data re p osit ory f or: i s r ef err e d t o a s it s l a g ti m e ( e. g., C er v e n y et al., 1 9 8 8; T h o m a s et al., 2 0 0 4). T hi s a n al yti c al err or. A n M S W D t h at i s > 1 i n di c at e s t h at t h e a g e i s li k el y u nr eli a bl e, Cenozoic collapse of the eastern Uinta Mountains and drainage lag ti me can be mini mized, and the M D A may approxi mate actual depositional signifying a high degree of scatter not accounted for by the analytical uncer - evolu on of the Uinta Mountain region ages if fluvial sedi ments accu mulate during ti mes of se micontinuous felsic vol - tainty alone. Alternatively, an M S W D <1 suggests that the errors assigned to A n dres Asl a n 1 , Marisa Boraas-Connors 2 , Doug Sprinke l3 , Tho mas P. Becke r4 , R a ni e L y n ds 5 , K arl E. K arlstr o m 6 , and Ma H eizl e r 7 c a ni c a cti vit y ( e. g., F a n et al., 2 0 1 5). the analysis have been overesti mated, but the mean age may be considered 1 D e pt. of Physical and Environ mental Sciences, Colorado Mesa University, Grand Junc o n, C O 81501 Detrital mineral ages provide an esti mate of the M D A of a clastic deposit r eli a bl e. 2 D e pt. of G e osci e nc es, C ol or a d o St at e U ni v ersit y, Ft. C olli ns, C O 8 0523 (e.g., Dickinson and Gehrels, 2009; May et al., 2013), assu ming that it did not 3 Utah Geological Survey, Salt Lake City, UT 84114 -610 0

4 Exxon Mobil Explora on Co mpany, 2 2777 Spring woods Village Park way, Spring, TX 77389 undergo te mperatures that would have enabled ther mal diffusion of radiogenic

5 Wyo ming Geological Survey, Lara mie, W Y 8 2072 daughter product. For zircon, diffusive loss of daughter product occurs above BISHOP CONGLO MERATE 6 D e pt. of Earth and Planetary Sciences, University of Ne w Mexico, Albuquerque, N M 87131 te mperatures of ~800–1000 ° C ( Cherniak, 2010), and for sanidine, it is at least 7 N e w Mexico Bureau of Geolo gy and Mineral Resources, Ne w Mexico Te c h , Socorro, N M 87801 250 ° C ( Mc Dougall and Harrison, 1999). While it may see m straightfor ward to The Bishop Conglo merate was studied in detail north of the Uinta Mountains 4 Supple mental Ite m 4. Su m mary of maxi mu m depo - identify the youngest age, the uncertainty (often attributable to rando m error in south western Wyo ming and along the peri meter of the outcrop belt of the sitional age calculations for Bishop Conglo merate and or detection li mits) surrounding any individual analysis may co mplicate a de - Bro wns Park For mation in northeastern Utah and north western Colorado (Fig. Bro wns Park For mation sa mples using both detrital cision. In addition, young zircons (younger than 100 Ma) may carry a larger 3 B). This unit unconfor mably overlies Eocene and older units in these areas. zircon U-Pb and sanidine 4 0 Ar/ 3 9 Ar data. Please visit htt p: / / d oi .or g / 1 0 .1130 / G E S01523 .S4 or t h e f ull-t e xt uncertainty (as a percentage of age) because it beco mes increasingly difficult The majority of the outcrops are represented by poorly consolidated conglo m - article on w w w .gsapubs .org to vie w Supple mental to distinguish the radiogenic *Pb co mponent fro m the co m mon background eratic sandstone and sandy conglo merate with boulder- to pebble-sized clasts It e m 4. Pb. Co mplicating matters, the concentration of radiogenic Pb (* 2 0 6 Pb) depends do minated by red sandstone and light gray li mestone and sandstone derived on the concentration of 2 3 8 U, so so me zircons should ostensibly have higher fro m the Uinta Mountains, si milar to the co mposition of the extensive outcrops precision than others, and those analyses should be considered more robust of Bishop Conglo merate on the south flanks of the Uinta Mountains ( Hansen, than others. Because of mag ma syste m co mplexities and zoning, a collection 1986; Ko wallis et al., 2005). Specifically, the red sandstones are re worked fro m

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T A B L E 1. MAXI MU M DEPOSITIONAL AGE CALCULATIONS FOR BISHOP CONGLO MERATE AND BR O W NS PARK F OR MATI ON DETRI T A L ZI R C O N A N D S A NI DI N E D A T A Yo u n g e st zi r c o n D etrit al s a ni di n e Yo u n g e st s a ni di n e D etrit al zir c o n M D A** M S W D* f o r U- Pb age M D A** 4 0 Ar/ 3 9 Ar a g e M S W D f o r S a m pl e l o c ati o n a n d a b br e vi ati o n* U nit † F a ci e s § Lit h ol o g y ( Ma) zircon M D A ( Ma) ( Ma) ( Ma) s a ni di n e M D A Fi r e h ol e C a n y o n, W Y ( F C) Tbc UMG sandstone 3 4. 7 ± 4. 7 ( n = 3) 1 2 3 3. 4 ± 0. 9 P o wder Wash, C O ( P W) Tbc UMG sandstone 3 2. 2 ± 4. 4 ( n = 3) 9. 8 2 9. 8 ± 1. 3 El k S pri n g s, C O ( E L K) Tbc UMG conglomerate 2 9. 8 ± 0. 9 ( n = 1 2) 0. 4 6 2 9. 4 ± 1. 2 3 4. 9 ± 0. 1 4 ( n = 9) 3 0. 6 ± 0. 1 1 1 1 . 5 Sand Wash, C O ( S W) Tbc UMG sandstone 2 7. 1 ± 1. 2 ( n = 2) 0. 3 5 2 6. 8 ± 1. 7 Gr e e n Ri v er, W Y ( G R AT) Tbc FC conglomerate 3 5. 5 ± 0. 6 ( n = 2 2) 0. 5 3 0. 9 ± 5. 9 Fi r e h ol e C a n y o n, W Y ( F C) Tbc FC sandstone 3 3. 4 ± 0. 3 ( n = 2 6) 0. 6 1 3 1. 6 ± 1. 9 Bi tt er Cr e e k, W Y ( B C) Tbc FC conglomerate 3 4. 0 ± 1. 8 ( n = 6) 6 3 2. 5 ± 0. 9 3 3. 4 ± 0. 1 6 ( n = 8) 3 2. 3 ± 0. 0 7 2 0 S o ut h B a xt er, W Y ( S B) Tbc FC conglomerate 3 3. 9 ± 0. 6 ( n = 1 0) 4. 5 3 1. 3 ± 1. 9 A nt el o p e B ut t e , W Y ( A B) Tbc SAND tuff 3 1. 2 ± 0. 6 ( n = 2 5) 3 3 0. 0 ± 0. 7 Upper Crouse C a n y on, UT (UCC) Tbc SAND sandstone 2 7. 0 ± 0. 3 ( n = 1 3) 1. 1 2 4. 3 ± 5. 0 T a yl or Fl at, U T ( T F) T b p sandstone 8. 4 ± 0. 3 ( n = 1 6 ) 0 . 6 3 7 . 8 ± 0. 7 J e s s e E wi n g C a n y o n, U T ( J E C) T b p t uff 9. 2 ± 1. 4 ( n = 3) 0. 7 9 8. 4 ± 2. 0 Cro us e C a n y o n, U T ( C C) T b p t uff 8. 9 ± 0. 6 ( n = 1 7 ) 0 . 4 1 6 . 4 ± 4. 5 L o d ore C a n y o n, C O ( L C) T b p sandstone 1 8. 5 ± 1. 8 ( n = 4) 1. 5 1 7. 8 ± 1. 3 V e r milli o n Cr e e k, C O ( V C) T b p t uff 1 3. 7 ± 2. 9 ( n = 4) 0. 3 7 1 2. 7 ± 7. 0 John Weller Mesa, C O (J W M) T b p sandstone 3 0. 7 ± 1. 7 ( n = 9) 4. 1 1 8. 8 ± 0. 7 2 7. 5 ± 0. 3 1 ( n = 8) 1 6. 6 ± 0. 0 5 2 7 Littl e S n a k e Ri v er ( L S R) T b p t uff 2 4. 5 ± 1. 1 ( n = 4) 0. 1 9 2 4. 1 ± 3. 3 * Abb r e vi ati o n s: W Y — W y o ming; C O — Colorado; UT — Utah (see Fi g. 3 B f or l o c ati o n s; d et ail e d l o c ati o n s ar e r e p o rt e d i n S u p pl e m e nt al It e m 1 [ s e e t e xt f o otnote 1]). MS W D — mean squa r e of w ei g ht e d d e vi at e s. † Tbc — Bishop Conglo merat e, Tbp —Bro wns P a r k F o r m ati o n. § U M G — Ui nt a M o u nt ai n Gr o u p f a ci e s, F C — Fir e h ol e C a n y o n f a ci e s, S A N D — s a n d y f a ci e s. ** M DA — maxi mu m depositional age based on weighted a v e r a g e t o ol i n I s o pl ot ( L u d wi g, 2 0 1 2). U n c e rt ai nti e s ar e r e p o rt e d at t w o st a n d a r d d e vi ati o n s ( 2 σ ); n — n u m b er of d etrit al zir con grains used t o c al c ul a t e t h e m a xi m u m d e p o siti o n al a g e.

the Neoproterozoic Uinta Mountain Group ( U M G) and light gray li mestones minor quantities of quartzite and chert clasts (Fig. 5 B). Where ce mented, the and sandstones are derived fro m Paleozoic carbonates and siliciclastic units. Bishop Conglo merate is crudely bedded, and contains lenses of cross-strati - We identified and described three facies that co mpose the Bishop Conglo m - fied sandstone. Paleocurrents measured fro m i mbricated gravel clasts and erate: (1) U M G facies, (2) Fire h ole Ca ny o n (F C) facies, a n d (3) sa n dy facies. T he trough cross- strata at Firehole Canyon located north of Miller Mountain sho w U M G and F C facies locally interfinger in south western Wyo ming, and the sandy a predo minant northeast ward paleoflo w direction (Fig. 4). Where exposed, the facies, w hic h is rare d ue t o its fria ble text ure, overlies ol der U M G facies de p osits. basal contact of the U M G facies is strongly erosional with several meters of local relief, and basal elevations of the conglo merate decrease to the north by U M G Facies Sedi mentology and Co mposition ~600 m over a distance of ~35 k m (average gradient of ~1°).

The U M G facies of the Bishop Conglo merate is the predo minant type FC Facies Sedi mentology and Co mposition within the southern Green River Basin region, ar moring the north-sloping s urfaces of Miller, Little, a n d Pi ne M o u ntai ns (Fi g. 4). Overall, t his facies re p - The F C facies of the Bishop Conglo merate is a ne wly recognized, rare but resents a series of lenses of north ward-thinning coarse-grained wedges that paleogeographically significant facies of the southern Green River Basin re - unconfor mably overlie Late Cretaceous–Eocene rocks of the Green River Basin gion. The facies is represented by a roughly east- west–trending outcrop belt and the adjacent Rock Springs uplift (Fig. 5 A). The U M G facies also crops out located in the saddle bet ween Aspen and Miller Mountains, and extends as far over areas of the western Sand Wash Basin (Po wder Wash), the eastern Uinta west as Green River, Wyo ming (Fig. 4). Si milar to the U M G facies, F C facies Mountains (Elk Springs), and within Bro wns Park ( Sand Wash), west of the strata unconfor mably overlie Late Cretaceous–Eocene rocks. The best expo - Littl e S n a k e Ri v er ( Fi g. 4). T hi s f a ci e s i s al s o at Littl e M o u nt ai n ( Ut a h) a n d al o n g sures of the F C facies are at Firehole Canyon, the saddle bet ween Miller and parts of the south flank of the Uinta Mountains ( Hansen, 1986). Aspen Mountains, and on the bluffs south of Green River, Wyo ming. Outside Outcrops of the U M G facies range fro m ~3 to 30 m in thickness and are of the southern Green River Basin, F C facies outcrops are also present near do minated by moderately to poorly sorted, boulder- to pebble-sized clasts of Po wder Wash. At this location, northeast ward-thinning U M G facies pass later - subangular to subrounded reddish sandstone and light gray li mestone with all y ( e a st w ar d) i nt o F C f a ci e s ( Fi g. 3 B).

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Modern Green River D Z s a m pl e Rock S pri n gs

Green River G R A T

Aspen

N M o u nt ai n A B

W E S B F C

2 2 2° S B C

N 4 0°

W E Mill er M o u nt ai n

S

Fl a mi n g Gorge R es er v oir

Littl e Pi n e M o u nt ai n M o u nt ai n

Figure 4. Geologic map of the southern Rock Springs uplift in south western Wyo ming. Locations of paleocurrent data for the U M G ( Uinta Mountain Group) and F C (Firehole Canyon) facies of the Bishop Conglo merate are sho wn. Detrital zircon sa mple and F C and U M G abbreviations are as in Figure 3 B. Location of modern Green River detrital zircon sa mple (black star) is also sho wn.

Sedi mentary rocks of the F C facies range fro m ~2 to 6 m in thickness and clasts (Fi g. 5 C). A s mall b ut si g ni fica nt q ua ntity of a p ha nitic t o p or p hyritic are do minated by moderately sorted, pebble- to cobble-sized, rounded to light gray, gray, and dark green andesitic pebbles are also present in the subrounded conglo merate with minor sandstone lenses. In addition to the F C facies, but these types of volcanic clasts are co mpletely absent fro m s maller caliber of the clasts and the greater percentage of rounded clasts, t h e U M G f a ci e s. the FC facies differs markedly in co mposition fro m the U M G facies. FC Carbonate-ce mented conglo merate beds of the FC facies near Firehole facies consist al most exclusively of black, gray, bro wn, and green quartzite Canyon consist of crudely-bedded conglo merate with rare lenses of trough

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A B Bishop Conglo merate

Figure 5. Bishop Conglo merate and Bro wns Park For mation field photographs fro m south western Wyo ming and northeast - ern Utah. ( A) Well-ce mented, horizontally bedded Bishop Conglo merate overlies gently dipping strata of the Eocene Green River For mation at Firehole Canyon (loca - Gree n Rive r F m. tion F C in Fig. 4). Thickness of the Bishop Conglo merate outcrop is ~5 m. ( B) Out - crop photograph of U M G ( Uinta Mountain Group) facies at Firehole Canyon sho w - ing abundant reddish sandstone cobbles eroded fro m outcrops of the Neoprotero - zoic Uinta Mountain Group (pen for scale). C D ( C) Outcrop photograph of F C facies at Firehole Canyon. Clasts are chiefly rounded quartzite and i mbrication sho ws paleoflo w directi o n ( w hite arro w) fro m ri g ht t o left i n the photograph (pen for scale). ( D) Thinly b e d d e d t o l a mi n at e d vitri c t uff i n t h e Ta yl or Flat area of western Bro wns Park (location TF in Fig. 3 B). Bedding indicates that the tuff is re worked and probably accu mulated i n sta n di n g water ( pers o n for scale).

cross-bedded sandstone. The sandstone lenses are distinct channel-for m Sandy Facies Sedi mentology and Co mposition bodies that are 2–3 m thick. Paleocurrents measured fro m i mbricated gravels and trough cross-strata at Firehole Canyon sho w a predo minant south west - North of the Uinta Mountains, sandy facies of the Bishop Conglo merate ward paleoflo w direction, opposite to that of the U M G facies paleocurrents are relatively rare and poorly exposed. Where present, the sandy facies gen - (Fig. 4). The basal contact of the F C facies is strongly erosional with several erally overlie the U M G facies. In the southern Green River Basin, the sandy meters of l ocal relief, a n d basal elevati o ns of t his c o ntact decrease t o t he west facies are best exposed at Antelope Butte, which is located on the south west by ~90 m over a distance of ~30 k m. flank of Aspen Mountain (Fig. 4). At Antelope Butte, as much as 60 m of inter -

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bedded tuffaceous siltstone and sandstone and minor interbeds of siliceous depositional syste m. The transition fro m fluvial coarse-grained facies of the li mestone overlie the bouldery Bishop Conglo merate (Love and Black mon, Bishop Conglo merate to finer grained facies of the Bro wns Park For mation 1962; Kirschbau m, 1986). Love and Black mon (1962) mapped these deposits has been interpreted to reflect collapse of the eastern Uinta Mountains and as part of the Bishop Conglo merate, but were uncertain about this assign ment for mation of the Bro wns Park syncline ( Hansen, 1986). Intrafor mational un - given the si milarity bet ween these deposits and tuffaceous sandstones of the confor mities and faults that cut the Bro wns Park For mation sho w that defor - Bro wns Park For mation. mation continued during and after the ti me represented by the Bro wns Park T he ot her set of ex p os ures of t he sa n dy facies is i n t he easter n Ui nta M o u n - For mation. tains. In upper Crouse Canyon ( U C C of Fig. 3 B), mapping (by Sprinkel, 2006) sho wed that south-trending Bishop Conglo merate paleovalleys are filled, in part, by tuffaceous, locally cross-bedded, friable sandstone and siltstone. DETRITAL ZIRCO N A ND SA NIDI NE GEOCHRO NOLOGY These beds overlie Neoproterozoic through Eocene bedrock and although poorly exposed, superficially rese mble tuffaceous sedi mentary rocks of the Detrital zirc o n a n d sa ni di ne data were use d t o (1) f urt her c haracterize t he Bro wns Park For mation located ~10 k m northeast in Bro wns Park. The sandy different facies of the Bishop Conglo merate, (2) evaluate the provenance facies is also associated with classic outcrops of the Bishop Conglo merate on of young (younger than 40 Ma) detrital grains in the Bishop Conglo merate the Dia mond Plateau near Vernal, Utah. and Bro wns Park For mation, (3) co mpare detrital zircon and sanidine results, and (4) constrain the ti ming of key events using M D As of zircon and sani - di n e s a m pl e s. BRO WNS PARK FOR MATION

In the study area, deposits of the Bro wns Park For mation as much as 500 m Bishop Conglo merate Facies thick pri marily crop out in the Bro wns Park syncline, along the eastern margin of the Uinta Mountains, and in the southern Sand Wash Basin (Fig. 3 B). The Co mparison of the detrital zircon data of the U M G and F C facies clearly b e st e x p o s ur e s cr o p o ut i n t h e Br o w n s P ar k s y n cli n e, w hi c h r e pr e s e nt s a gr a - s h o ws t hat t he pr ove na nce of t he t w o facies differs (Fi g. 6 A). W hile Arc hea n ben localized over the northern edge of the eastern Uinta Mountains range (2700–2600 Ma), Paleoproterozoic (1850–1750 Ma), Mesoproterozoic (ca. front, presu mably related to extensional collapse along preexisting reverse 1450 Ma) and Grenville orogeny (1200–1000 Ma) detrital zircon U-Pb ages fa ults ( Ha nse n, 1986; St o ne, 1993; Bra dley, 1995; S pri nkel, 2006). I n t he Br o w ns are well re prese nte d i n b ot h facies, detrital zirc o n grai ns of Gre nville or o g - Park syncline, the Bro wns Park For mation consists chiefly of gray, interbed - eny age are more abundant (39 % of the grains analyzed) in the U M G f a ci e s de d, t ufface o us sa n dst o ne, siltst o ne, a n d le nses of li g ht gray, vitric t uff (Izett, co mpared to the F C facies sa mple. Another note worthy difference is the 1975; H o ney a n d Izett, 1980; L uft, 1985) (Fi g. 5 D). Al o n g t he easter n mar gi n of Archean grain ages (Figs. 7 A, 7 B). The F C facies contains a ca. 2650 Ma age Bro wns Park bet ween Ver million Creek and the Little Snake River, the Bro wns mode that is absent fro m the U M G facies. The F C facies sa mple also has Park For mation confor mably overlies the Bishop Conglo merate, and both units Early Cretaceous (107–90 Ma) detrital zircon grains that are absent fro m the dip steeply to ward the axis of the Bro wns Park syncline. Else where, near the U M G facies sa mple (Fig. 6 B). The most striking difference, ho wever, is rep - western margin of Bro wns Park, Bro wns Park strata are relatively flat lying, resented by the over whel ming quantity of grains younger than 80 Ma in and onlap gently dipping Neoproterozoic U M G sandstones. Zircon fission t h e F C facies (47 %) co mpared to the U M G facies (11 %) (Fig. 6). The majority track ages fro m tuffaceous material indicate that the Bro wns Park For mation of these young grains are younger than 40 Ma. These differences in the a c c u m ul at e d fr o m c a. 2 5 t o 8 M a i n t h e Br o w n s P ar k s y n cli n e (I z ett et al., 1 9 7 0; U-Pb age populations of the U M G and F C facies are further confir med by a I z ett, 1 9 7 5; L uft, 1 9 8 5). Kol molgorov- S mirnov ( K- S) test that sho ws that the t wo facies are statisti - The lenticular geo metry and the presence of well-preserved bedding in - c all y diff er e nt ( p = 0. 0 0). dicate that Bro wns Park For mation tuffs are re worked ash fall that probably accu mulated in shallo w ponds ( Hansen, 1986). Lenses of micritic, siliceous li mestone, which are also interpreted as shallo w ponds, are present locally Bro wns Park For mation and interfinger with tuffaceous units. Conglo meratic beds are 1–5 m thick and consist of clasts of locally derived reddish sandstone of the Uinta Moun - Detrital zircon and sanidine ages for grains younger than 60 Ma in the tain Group. The conglo meratic beds for m discontinuous clastic wedges that Bro wns Park For mation sa mples range fro m 57 to 8 Ma, and all sa mples sho w interfinger with sandy and silty facies along the northern margins of western - major U-Pb age modes bet ween 39 and 24 Ma (Figs. 8 A–8E). When arranged in most Bro wns Park. Collectively, these deposits represent a mosaic of lacus - stratigraphic order, stratigraphically younger Bro wns Park For mation sa mples trine and fluvial environ ments that are consistent with an internally drained sho w successively younger U-Pb age distributions.

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A B

1000–1200 1750–1850 2600–2700 D etrit al zi rc o n d at a f o r 1450 U M G F a ci es ( n = 3 7 6)

and F C Facies (n = 367) ytilibaborPegAdezilamroN 223–23 7 3 × V. E.

B 2 9 LEGEND

A g e Gr ai n % of U M G % of F C 4 7 i nt er v al p o p ul ati o n c at e g o r y F a ci es Gr ai ns F a ci es Gr ai ns D etrit al zir c o n d at a f or Y ounge r La r a mi d e U M G Facies (n=46) t ha n Orogen y t o 1 1 4 7 9 1 and F C Facies (n=196) 8 0 M a Prese n t 91–107 P ost -A p p a l a c hi a n 9 0 1 0 7 9 7 80–330 Orogen i e s t o 3 1 1 M a La r a mi d e Or o ge n y

330–76 0 A p p a l a c hi a n 2 2 M a Or o ge ni es

760–90 0 Neop roterozoic 1 0 N or a mli z e d A g e Pr o b a bilit y M a Gr e n vi ll e 900–1300 3 9 1 5 M a Orogen y

1300– M e soproterozoi c 2 2 7 3 4 1650 Ma 1650– 4 7 Paleoproterozoic 1 7 1 0 < 8 0 2500 Ma 2 5 35 45 55 65 75 85 95 105 1 1 5 Age ( Ma) Ol d er t h a n Ar c h e a n 5 8 2500 Ma 0 500 1000 1500 2000 2500 3000 3500 4000 Age ( Ma)

Figure 6. ( A) Co mposite age probability curves co mparing detrital zircon U-Pb data for U M G ( Uinta Mountain Group; red curve) and F C (Firehole Canyon; blue curve) facies of the Bishop Conglo merate. Each co mposite curve represents data fro m four separate sa mple sites. The U M G curve includes data fro m sa mples F C, A B, P C, and EL K (locations sho wn in Fig. 3 B). The F C curve includes data fro m sa mples G R AT, B C, S B, and F C (locations sho wn in Fig. 3 B). Abbreviations as in Figure 3. ( B) Detrital zircon U-Pb data for Cenozoic and Cretaceous (younger than 115 Ma) grains. Nu mbers in bold represent age ranges of detrital zircon U-Pb age distributions in Ma; n = total nu mber of grains analyzed, and vertical exaggeration of both curves is 3×.

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0. 0 1 8

0. 0 1 6 2656 A 2757 T b c, F C f a ci es, n =14 4 0. 0 1 4 B 2 7 1 1 0. 0 1 2 T b c, U M G f a ci es, n =31 1

ytilibaborPevitaleR 2633

0. 0 1 Figure 7. Nor malized probability curves co mparing detrital zircon age distribu - tions for Archean and Proterozoic (2900– 900 Ma) grains for the F C (Firehole Can - 0. 0 0 8 yon) and U M G ( Uinta Mountain Group) facies. ( A, B) Bishop Conglo merate (Tbc). (C) Modern Green River. The FC and U M G 0. 0 0 6 curves are co mposites of the sa me sa m - ples as in Figure 6. Sa mple locations are s h o w n i n Fi g ur e s 3 B a n d 4.

0. 0 0 4

0. 0 0 2 C G r e e n Ri v e r, n = 7 8 0 9 0 0 1400 1900 2400 2900 Age ( Ma)

Modern Green Rive r Tbc U M G facies T bc F C facies

Maxi mu m Depositional Ages detrital zirc o n U- P b M D A of 27.6 ± 0.7 Ma calc ulate d by Fa n et al. (2015) f or the Bishop Conglo merate outcrop along the nearby Little Snake River (Figs. Bishop Conglo merate 1 0 A, 1 0 B). The zircon U-Pb M D A for the sandy facies of the Bishop Conglo merate Detrital zircon and sanidine M D As of the Bishop Conglo merate confir m at A ntel o pe B utte (31.2 ± 0.6 Ma) is c o nsiste nt wit h its strati gra p hic p ositi o n t hat t his u nit is late E oce ne–early Oli g oce ne (Fi g. 9; Ta ble 1). I n t he s o ut her n above the other t wo facies, which have older M D As (Fig. 9). The zircon U-Pb Green River Basin, the zircon U-Pb M D A for the U M G facies is 34.7 ± 4.7 Ma M D A in the eastern Uinta Mountains (upper Crouse Canyon) is 27.0 ± 0.3 Ma, ( Fir e h ol e C a n y o n; Fi g. 9), a n d t h e a g e r a n g e f or t h e F C f a ci e s i s 3 5. 5 ± 0. 6 – 3 3. 4 ± which is broadly consistent with the 26.2 ± 0.7 Ma K- Ar age of a tuff located 0.3 Ma ( G R AT, Firehole Canyon; Fig. 9). The M D A esti mates for the U M G and ~90 m above the base of the Bishop Conglo merate on the Dia mond Plateau F C facies overlap, consistent with the observed stratigraphic interfingering of near Vernal, Utah ( Da mon, 1970) (Fig. 9). In su m mary, the Bishop Conglo m - t hese t w o u nits. erate detrital zircon U-Pb-based age esti mates (ca. 36–27 Ma) are re markably In the Bro wns Park and western Sand Wash Basin regions, the zircon U-Pb c o nsiste nt wit h t he 4 0 Ar/ 3 9 Ar (ca. 34–30 Ma) and K- Ar (29–26 Ma) age esti mates M D As for the U M G facies range fro m 32.2 ± 4.4 to 27.1 ± 1.2 Ma (Po wder Wash, of Bishop Conglo merate tuffs fro m the southern flank of the Uinta Mountains Sand Wash; Fig. 9). The 27.1 ± 1.2 Ma age at Sand Wash overlaps with the ( D a m o n, 1 9 7 0; H a n s e n, 1 9 8 6; K o w alli s et al., 2 0 0 5).

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1. 4 8

1. 2 3 6 A 2 9 D Z , T b p, T a yl o r Fl at, n = 1 9 B 1 3 2 8 D Z , T b p, V e r m illi o n Cree k, n =21 1 3 4 Figure 8. Nor malized probability curves 1 6 2 3 4 2 4 5 co mparing detrital zircon (black curves) C D S, T b p, J o h n W ell e r, n =45 3 1 ytilibaborPevitaleR 3 5 and sanidine (red curves) age spectra 1 8 4 3 5 7 for young (0–60 Ma) grains for selected D Z , T b p, J o h n W ell e r, n =36 0. 8 D Bro wns Park For mation and Bishop Con - 3 5 2 4 5 0 glo merate sa mples. Sa mple locations are E D Z , T b p, Littl e S n a k e R, n = 1 9 3 4 sho wn in Figure 3 B. Paired sa mples C- D, F- G, a n d H-I re prese nt detrital zirc o n U- P b age and detrital sanidine 4 0 Ar / 3 9 Ar age 0. 6 data fro m the sa me sa mple. Nu mbers in bold are ages (in Ma) of major U-Pb and 3 9 4 6 4 0 A r / 3 9 Ar age distributions; n —nu mber F D S, T b c, El k Springs, n =20 2 9 of grai ns use d i n t he pro ba bility pl ot. DZ —detrital zircon, D S —detrital sani - 0. 4 dine, Tbp —Bro wns Park For mation, Tbc — Bishop Conglo merate. 3 8 4 7 G D Z , T b c, El k Springs, n =12 3 3 3 5 0. 2 4 6 H D S, T b c, Bitt e r C r e e k, n =16 4 7 3 4 I D Z , T b c, Bitt e r Cree k, n =33 0 0 1 0 2 0 3 0 4 0 5 0 6 0 Age ( Ma)

Bro wns Park For mation The youngest M D As of Bro wns Park For mation sa mples are fro m tuffa - c e o u s silt st o n e a n d vitri c t uff l o c at e d w e st of V er milli o n Cr e e k ( Fi g. 9; T a bl e 1). Detrital zircon and detrital sanidine M D As of the Bro wns Park For mation Ot her t ha n t he ca. 14 Ma Ver milli o n Creek t uff, t he ol dest a ge esti mate (9.2 ± confir m that the unit is late Oligocene to Miocene (ca. 25–8 Ma) (Izett, 1975; 1.4 Ma) fro m this area co mes fro m the lo wer most of the t wo pro minent vitric Luft, 1985), and suggest that the youngest sedi ments of the graben fill are con - tuffs exposed at Jesse E wing Canyon. This tuff is probably the one reported centrated in western most Bro wns Park along the Colorado- Utah border (Fig. 9; in Winkler (1970), and is dated to 11.8 ± 0.4 Ma using volcanic glass and K- Ar Table 1). The oldest zircon U-Pb M D A (24.5 ± 1.1 Ma) is fro m a tuff near the geochronology. The other t wo sa mples ( Crouse Canyon, Taylor Flat) repre - base of the Bro wns Park For mation, which is located ~20 m above the Bishop se nt a ge esti mates t hat are fr o m strati gra p hic p ositi o ns t hat are y o u n ger t ha n Conglo merate ( M D A = 27.6 ± 0.7 Ma; Fan et al., 2015) (Fig. 10 B). This tuff was the Jesse E wing Canyon tuff. The Taylor Flat sa mple is fro m a s mall re mnant date d previ o usly t o 24.8 ± 0.8 Ma usi n g K- Ar dati n g met h o ds (Izett, 1975). A zir - of the Bro wns Park For mation (elevation 1865 m) that is preserved beneath con U-Pb M D A of 18.5 ± 1.8 Ma was obtained on pebbly tuffaceous sandstone er osi o nally resista nt river gravel of t he a ncestral Gree n River ( C o u nts, 2005), directly overlying probable Bishop Conglo merate near Lodore Canyon. A bed - a n d t he sa m ple is fr o m a t ufface o us siltst o ne t hat overlies t he Ne o pr oter ozoic ded vitric tuff near Ver million Creek produced an M D A of 13.7 ± 2.9 Ma. This Uinta Mountain Group. This re mnant is significantly higher in elevation than t uff w a s d at e d pr e vi o u sl y t o 9. 1 ± 1. 0 M a (I z ett, 1 9 7 5) a n d 9. 9 ± 0. 4 M a ( N a e s er any nearby Bro wns Park For mation outcrop, and probably represents the best et al., 1980) using fission track methods. Because ther mal heating can anneal esti mate for the mini mu m age (8.4 ± 0.3 Ma) of the Bro wns Park For mation tracks, the fission track data probably represent a mini mu m depositional age. i n t h e r e gi o n.

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Legend

A n t el o p e B u tt e Mi o c e n e Br o w n s P a r k F m . a n d e q ui v al e nt u nit s 3 1. 2 ± 0 . 6 M a Oli g o c e n e Bis h o p C o n gl o me r at e S o ut h B a xt e r G R AT D et rit al Z i r c o n S a m p l es 3 3. 9 ± 0 . 6 M a 3 5. 5 ± 0 . 6 M a Br o w n s P a rk For m ati o n Sa n d y F a ci es of t h e Bis h o p C o n gl o me r at e F C F a ci es of t h e Bishop Conglo me r at e Fi r e h ol e C a n y o n U M G F a ci es of t h e Bis h o p C o n gl o me r at e 3 4. 7 ± 4 . 7 M a ( U M G f acies) 3 3. 4 ± 0. 3 M a ( F C facies) Bitt e r Cree k M e s a 3 4. 0 ± 1 . 8 M a

J. E w i n g Canyon Cro use Canyon T a yl o r Fl at 9. 2 ± 1 . 4 M a 8. 9 ± 0 . 6 M a P o w d er Wash 8. 4 ± 0 . 3 M a 3 2. 2 ± 4 . 4 M a U p p e r Cro use Canyon V e r m illi o n C r e e k F T a g e of 2 7. 0 ± 0 . 3 M a 1 3. 7 ± 2 . 9 M a 9. 1 ± 1 . 0 M a S a n d W a s h Ui nt a M o u nt ai ns Iz et t ( 1975) 2 7. 1 ± 1 . 2 M a T b c d etrit al zir c o n a g e 2 7. 6 ± 0 . 7 M a fro m F a n et al. (2015)

3 0. 5 4 ± 0 . 2 2 M a Lo dore Canyon 4 0 Ar/ 3 9 Ar Ages fro m 1 8. 5 ± 1 . 8 M a K o w allis et al. ( 2 0 0 5 ) 3 4. 0 7 ± 0 . 0 4 M a K- Ar a g e of Littl e S n a ke Ri v e r 2 4. 8 ± 0 . 8 M a J o h n We ll e r Mes a * 2 4. 5 ± 1 . 1 M a Iz et t ( 1975 ) 3 0. 7 ± 1 . 7 – 1 8. 8 ± 0 . 7 M a El k S p ri n g s 2 9. 8 ± 0 . 9 M a

Figure 9. Outcrop map and su m mary of Bishop Conglo merate and Bro wns Park For mation (F m.) maxi mu m depositional age ( M D A) data. Data are also presented in Table 1. Radio metric age esti - mates of the Bishop Conglo merate fro m Ko wallis et al. (2005) and Fan et al. (2015) and of the Bro wns Park For mation fro m Izett (1975) are also sho wn. FT —fission track. Note that the M D A sho wn for John Weller Mesa (30.7 ± 1.7 Ma) was calculated using the youngest group of zircon U-Pb ages, but the youngest grain (18.8 ± 0.7 Ma), which was an outlier in this set of young grains, is also sho wn. Legend and detrital zircon sa mple abbreviations are as in Figure 3 B. Sources: Esri —https:// w w w .e sri .co m/; US GS — U.S. Geological Survey; N O A A — U.S. National Oceanic and At mospheric A d mi ni str ati o n.

C o m p ari s o n of D etrit al Zir c o n a n d S a ni di n e D at a The young (younger than 60 Ma) detrital zircon and sanidine ages fro m the Bro wns Park For mation sa mple (John Weller Mesa) also sho w si milarities, and For the Elk Springs and Bitter Creek Bishop Conglo merate sa mples (Figs. a ge distri b uti o ns cl uster bet wee n 35 a n d 28 Ma (Fi gs. 8 C, 8 D). Detrital sa ni di ne 8F–8I), the young (younger than 60 Ma) detrital zircon and sanidine age distri - data sho w a younger (16 Ma) age distribution co mpared to the detrital zircon butions are si milar with peaks of 35–33 Ma, 39–38 Ma, and 47–46 Ma. The main data (18 Ma). B ot h t he zirc o n U- P b a n d sa ni di ne 4 0 Ar/ 3 9 Ar M D A s ( zir c o n = 3 0. 7 ± difference bet ween the zircon U-Pb and sanidine 4 0 Ar/ 3 9 Ar M D As for the Bishop 1.7 Ma; sanidine = 27.5 ± 0.3 Ma) for the John Weller Mesa are older than the Conglo merate is the precision of the age esti mates (Table 1). For exa mple, the y o u n gest detrital grai ns (zirc o n = 18.8 ± 0.7 Ma; sa ni di ne = 16.6 ± 0.05 Ma) i n d etrit al s a ni di n e M D A of t h e Bitt er Cr e e k s a m pl e i s 3 3. 4 ± 0. 1 6 M a, w h er e a s t h e the sa mple (Table 1). The John Weller Mesa sa mple is located in the upper detrital zircon M D A of the sa me sa mple has a precision of ±1.8 m.y. portion of the preserved Bro wns Park For mation section, and represents a sig -

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D et rit al zi rc o n a g e of A 2 7. 1 ± 1. 2 M a B Bishop Conglo me r at e Bro wns P a r k F m .

W a s a tc h F m . Bri d g e r F m .

D et rit al zi rc o n a g e of Bishop Conglo me r at e 2 4. 5 ± 1. 1 M a D et rit al zi rc o n a g e of 2 7. 6 ± 0. 7 M a ( F a n et al., 2 0 1 5)

Figure 10. Field photographs of i mportant stratigraphic and structural relationships in the Bro wns Park region of north western Colorado. ( A) Sand Wash angular unconfor mity, looking southeast. Gray mudrock of the Bridger For mation (F m.) dips north-northeast and is unconfor mably overlain by basal conglo merate and sandstone of the Bishop Conglo merate, which dips in the opposite direction (south west) to ward the axis of the Bro wns Park graben. Paleocurrent measure ments fro m i mbricated sandstone clasts in the Bishop Conglo merate sho w northeast-directed paleoflo w, a way fro m the eastern Uinta Mountains. The Uinta Mountains were the source of the light reddish-bro wn sandstone and reddish sandstone clasts that do minate the Bishop Conglo merate at this outcrop. The red dot is located near the detrital zircon sa mple site for Sand Wash [ Bishop Conglo merate U M G ( Uinta Mountain Group) facies]. See Figure 3 B for location; note people for scale. ( B) Little Snake River angular unconfor mity, looking north west. Where the Little Snake River enters Bro wns Park, beds of the light gray Bro wns Park For mation and the underlying reddish Bishop Conglo merate dip south - west to ward the axis of the Bro wns Park graben, and unconfor mably overlie more gently dipping strata of the Eocene Wasatch For mation. At this location, the Bishop Conglo merate is only 2–3 m thick and consists of a pebbly sandstone with abundant red sandstone clasts. Fan et al. (2015) reported an age of 27.6 ± 0.7 Ma fro m this locality as the basal age of the Bro wns Park For mation, but this red c o n gl o m er ati c u nit u n d erl yi n g t h e t uff a c e o u s Br o w n s P ar k F or m ati o n al o n g t h e Littl e S n a k e Ri v er i s g e n er all y r e g ar d e d a s p art of t h e Bi s h o p C o n gl o m er at e ( H a n s e n, 1 9 8 6). T h e y o u n g er Littl e S n a k e River detrital zirc o n sa m ple was ac q uire d fro m a li g ht gray vitric tuff near t he base of t he Bro w ns Park For mati o n. A K- Ar a ge of 24.8 ± 0.8 Ma for t his tuff was re p orte d by Izett (1975).

nificantly higher position stratigraphically than nearby Bishop Conglo merate youngest bedrock unit in the region was probably the Eocene Bridger For ma - o utcr o ps, s o it is likely t hat t he M D As f or t his sa m ple are ol der t ha n its de p o - tion, which has a mini mu m age of ca. 47 Ma ( Murphey and Evanoff, 2007). It siti o n al a g e. is also unlikely that grains of this young (younger than 40 Ma) age were trans - ported by rivers draining hinterlands co mposed of Oligocene and younger rocks, and deposited subsequently in south western Wyo ming and north west - DISCUSSIO N ern Colorado. Rocks of this age are abundant in the southern Basin and Range, but barriers such as the Wyo ming fold and thrust belt (Fig. 1) make a fluvial V ol c a ni c A s h- F all Ori gi n f or Y o u n g er t h a n 4 0 M a D etrit al connection bet ween the regions unlikely. Instead, the younger than 40 Ma de - Zircon and Sanidine Grains trital zircon and sanidine grains in the Bishop Conglo merate and Bro wns Park For mation probably represent a co mbination of Oligocene through Miocene The abundance of younger than 40 Ma detrital zircon and sanidine grains volcanis m, ash-fall deposition, and subsequent fluvial re working. This inter - in the Bishop Conglo merate and Bro wns Park For mation sa mples (Fig. 8) can - pretation is supported by a co mbination of stratigraphic observations and age not be explained si mply by traditional explanations for detrital grain spectra esti mates of Ce n ozoic volca nic activity i n t he wester n U. S. involving bedrock erosion and fluvial transport. For exa mple, during the ti me Figure 8 (sa mples A, B, D, E, G) sho ws that for a co mposite stratigraphic fra me (ca. 36–27 Ma) represented by the Bishop Conglo merate, there were no succession of the Bishop Conglo merate and Bro wns Park For mation de trit al nearby late Eocene– Oligocene outcrops to explain the abundant younger than zircon sa mples, the age of the youngest U-Pb age distributions decrease sys - 40 Ma detrital zircon and sanidine grains (Love and Christiansen, 1985). The te matically at s uccessively y o u n ger strati gra p hic levels. T his patter n is c o nsis -

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tent with episodic Bishop Conglo merate and Bro wns Park For mation deposi - mation detrital zircon age distributions is expected if the flux curve were to tion acco mpanied by periodic ash-fall deposition and fluvial re working. This incorporate data for the Snake River Plain volcanic field, which ranges fro m ca. interpretation is further supported by co mparing the ti ming of Cenozoic igni m - 1 6 t o 6 M a ( P er ki n s et al., 1 9 9 8; P er ki n s a n d N a s h, 2 0 0 2). B a s e d o n pr o xi mit y t o brite eve nts wit h t he ge neral a ges of t he y o u n g detrital grai ns. t he st u dy area, it is likely t hat t he y o u n ger t ha n 20 Ma detrital zirc o n a n d sa ni - Figure 11 co mpares the ti ming and volu me of Oligocene and Miocene ig - dine grains in the Bro wns Park For mation (Fig. 8, sa mples A– D) were derived ni m brites ( Perki ns et al., 1998; Cat her et al., 20 09; Best et al., 2013; Li p ma n ori gi n all y a s a s h f all fr o m t hi s v ol c a ni c fi el d. and Bach mann, 2015) with detrital zircon data for the Bro wns Park For mation Previous studies have suggested that Basin and Range volcanis m was a and the Bishop Conglo merate. There is a good correspondence bet ween the pri mary source for both the Bro wns Park For mation (Luft, 1985) and Bishop ti ming of the Cenozoic igni mbrite flux for western North A merica and the ages Conglo merate ( Ko wallis et al., 2005) tephra. For exa mple, geoche mical studies of the do minant detrital zircon age distributions in the Bishop Conglo merate, and radio metric dating of a distal tuff bed in the Bishop Conglo merate near a n d t o a lesser exte nt, i n t he Br o w ns Park For mati o n (Fi g. 11 A). A n eve n better Vernal, Utah, suggested that the Cotton wood Wash igni mbrite (age 31.13 Ma, match bet ween the igni mbrite eruptive flux curve and the Bro wns Park For - esti mated volu me of 2000 k m 3 ; Fi g. 11 B) was t he s o urce of t he t uff ( Ko wallis

0. 2 5 4 0 A I g ni m b rit e fl u x ni g I ytilibaborPevitaleR

( Cathe r et al., 2009 ) bm 0. 2 3 5 B r o w ns Par k F m . 2 9 3 0 e rit d et rit al zi rc o n ages ( n =89) t e v pti u r e 0. 1 5 1 8 3 3 2 0 Fi g ure 11. ( A) N or malize d pro ba bility c urves 0. 1 m k x( u fl of young (younger than 40 Ma) detrital Bis h o p Conglo merate zircon U-Pb ages for all Bro wns Park For - mation sa mples (black curve) and Bishop d et rit al zi rc on ages (n =183) 1 0 Conglo merate sa mples (gray curve; n is 0. 0 5 3 nu mber of zircon grains used to construct k/ the curves. These curves are co mpared ry to the Cenozoic eruptive flux curve for

) western North A merica ( Cather et al., 0 0 2009). ( B) Su m mary of ti ming of Oligo - 1 5 2 0 2 5 3 0 3 5 4 0 cene– Miocene igni mbrite eruptions of the )

3 7000 Sapinero Basin and Range (blue curve) and South - B C ar pe nter Ri d g e 2 8. 4 Pancake Su m mit mk(emulovtinuetirbmingI ern Rocky Mountain (red curve) volcanic 6000 2 7. 6 Fis h C y n 3 5. 3 fiel ds. Data are fro m Perki ns et al. (1998), 2 8. 2 Co on woo d Best et al. (2013), and Lip man and Bach - 5000 3 1. 1 3 mann (2015). Radio metric ages of i mpor - Stone Cabi n tant igni mbrites are sho wn as blue nu m - 4000 Badger 3 5. 8 bers for t he Basi n a n d Ra n ge volca nic fiel d 3 4. 0 and red nu mbers for the Southern Rocky 3000 M o u nt ai n v ol c a ni c fi el d. 2000 Bonanz a 3 3. 2 W all M t 1000 3 7. 3 0 1 5 2 0 2 5 3 0 3 5 4 0 Age ( Ma)

B asi n a n d R a n g e v ol c a ni c fi el d Southern Rocky Mountain volcanic field

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et al., 2005). C o m paris o n of t he detrital zirc o n U- P b a ge distri b uti o ns wit h t he detrital sa ni di ne data s u g gest t hat t he S R M VF, al o n g wit h t he S nake River Plai n ti ming of Basin and Range and San Juan Mountain igni mbrite events suggests and Basin and Range volcanic fields, was a source for a portion of the Bro wns only a general correspondence (Fig. 11 B). Ho wever, the precision of the de - Park For mati o n te p hra. trit al s a ni di n e 4 0 Ar/ 3 9 Ar geochronology provides a better means for correlating Prior studies have proposed that Oligocene– Miocene volcanis m played Bishop Conglo merate and Bro wns Park For mation sanidine grains with spe - a significant role in the accu mulation of fine-grained Cenozoic units in the cific caldera-for ming eruptions (Fig. 12). Detrital sanidine ages and K/ Ca data western U. S. based on a co mbination of radio metric dating, analysis of vol - for the Bishop Conglo merate Bitter Creek sa mple match well with the age and canic glass che mistry, and detrital zircon studies ( Blaylock, 1998; Larson and K/ Ca data for tuffs for the Stone Cabin Tuff in the Basin and Range Central Evanoff, 1998; Fan et al., 2015; Ro wley and Fan, 2016). Larson and Evanoff Nevada igni mbrite field ( C NIF) ( Best et al., 2013), and so me what well with the (1998) used radio metric ages to broadly correlate Oligocene White River For - age and K/ Ca data for the Basin and Range Pancake Su m mit Tuff of the C NIF mation tuffs located in eastern Wyo ming and western Nebraska with Basin ( Best et al., 2013) (Fig. 12 A). The K/ Ca data for the Wall Mountain Tuff of the and Range volcanis m. Blaylock (1998) refined tephra correlations of Larson Southern Rocky Mountain volcanic field ( S R M VF), ho wever, do not match well and Evanoff (1998) using detailed geoche mical analyses of co mag matic min - with the Bishop Conglo merate detrital sanidine data. The age and K/ Ca data erals, but also concluded that the southern Basin and Range was the source for the Bro wns Park For mation John Weller Mesa sa mple matches well with of several White River For mation tephra units. Fan et al. (2015) used overall data for the Carpenter Ridge, Fish Canyon, Sapinero Mesa, and Badger Creek si milarities in the ti ming of Oligocene volcanis m and M D As of detrital zircon Tuffs of the S R M VF (Fig. 12 B). These results confir m that Basin and Range sa mples to explain the abundance of Oligocene detrital zircon grains in tuffa - volcanis m contributed to the Bishop Conglo merate sedi mentation ( Ko wallis ceous fluvial deposits in Wyo ming and Nebraska. This study suggested, ho w - et al., 20 05). C o ntrary t o previ o us i nter pretati o ns (e. g., L uft, 1985), h o wever, t he ever, that the Sierra Madre Occidental volcanic field in Mexico may have been

A Bishop Conglo merate, Bitter C k B B r o w ns P a r k F m., J o h n We ll e r Mesa

Figure 12. Detrital sanidine 4 0 Ar / 3 9 Ar and tuff geochronology. ( A) Detrital sanidine ( D S) data fro m the 3 m.y. interval 37.5– 34.5 Ma for the Bishop Conglo merate [ Bitter Creek ( B C) sa mple, location sho wn in Fig. 3 B] co mpared to possible caldera sources fro m the Basin and Range (Stone Cabin, Pancake Su m mit) and Southern Rocky Mountain ( Wall Mountain) igni mbrite units. Bishop Conglo merate detrital sani - dine ages and K/Ca values match well with data for the Stone Cabin and Pan - cake Su m mit Tuffs, but K/C A values do not match those of the Wall Mountain Tuff. ( B) Detrital sanidine data fro m 36 to 26 Ma for the Bro wns Park For mation (Tbp; John W ell er M e s a s a m pl e; l o c ati o n s h o w n i n Fi g. 3 B) co mpared to data for Southern Rocky Mountain igni mbrite units. Bro wns Park For mation detrital sanidine ages and K/ Ca values match well with data for Southern R o c k y M o u nt ai n i g ni m brit e u nit s i n cl u di n g the Sapinero Mesa, Fish Canyon, Carpenter Ridge, and Badger Creek Tuffs. Ck — Creek; N —nu mber of analyses.

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the source for the Oligocene detrital zircon grains. Figure 13 su m marizes our Repetition of this sequence of events produced deposits with successively ne w interpretations of Oligocene– Miocene ash-fall deposition based on the younger detrital zircon grains, and further suggests that M D As for the Bishop data and discussion presented in this study. In su m mary, far-traveled tephra Conglo merate and Bro wns Park For mation are close to true depositional ages, that mantled the landscapes of south western Wyo ming, northeastern Utah, as has been suggested for other Cenozoic units in the western U. S. that accu - and north western Colorado during the latest Eocene through Miocene was mulated in regions of se micontinuous igni mbrite volcanis m (Fan et al., 2015; re worked subsequently into Bishop and Bro wns Park depositional syste ms. Ro wley and Fan, 2016).

1 1 4° 1 0 4° S na k e Ri v e r v ol c a ni c fi el d 1 6- 6 M a

Oli g o c e n e W hit e Ri v e r F m . 3 6- 3 0 M a St u d y area 4 2°

P S W M B C S C B S F C C R U T C O So ut her n B asi n a n d Ra ng e A Z N M So ut her n Roc k y Mt 3 7° v ol c a ni c fi el d v ol c a ni c fi el d 3 6- 1 8 M a 3 7- 2 3 M a

0 5 0 1 00 200 k m

Figure 13. Map sho wing Oligocene– Miocene volcanic centers including the Snake River, southern Basin and Range, and Southern Rocky Mountain volcanic fields (solid and dashed red lines), paleo - wi n d dir e cti o n s ( s oli d a n d d a s h e d r e d arr o w s), t h e a p pr o xi m at e di stri b uti o n of t uff a c e o u s Oli g o c e n e W hit e Ri v er F or m ati o n d e p o sit s ( bl u e s oli d li n e), a n d t h e st u d y ar e a ( bl a c k b o x). Bl a c k cir cl e s mark the approxi mate locations of calderas associated with igni mbrite units referred to in the text. Abbreviations refer to igni mbrite units associated with the calderas: B — Bonanza, B C — Badger Creek, CR —Carpenter Ridge, FC —Fish Canyon, PS —Pancake Su m mit, S —Sapinero Mesa, SC —Stone Cabin, and W M — Wall Mountain Tuffs. Data were co mpiled fro m Best et al. (1989, 2013), Larson and Evanoff (1998), S wisher and Prothero (1990), Obradovich et al. (1995), Perkins et al. (1998), Perkins and Nash (2002), and Lip man and Bach mann (2015). AZ — Arizona; C O — Colorado; N M — Ne w Mexico; UT — Utah.

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Provenance and Paleogeography during the Paleogene cene Washakie For mation conglo meratic sandstones ( Adobe To wn Me mber) that crop out in the Washakie Basin to the east of the F C facies outcrop belt Differences in the distribution, paleocurrents, and co mposition of the have minor quantities of inter mediate volcanic clasts. It is conceivable that Bishop Conglo merate U M G and F C facies suggest that these deposits repre - a west-flo wing Bishop Conglo merate river re worked conglo merate of the se nt tri b utaries a n d a mai nste m river, res pectively, of a latest E oce ne t o early Washakie For mation to produce the minor volcanic co mponent observed in Oligocene fluvial syste m. Previous investigators have sho wn that the Bishop t h e F C f a ci e s ( Fi g. 1 4). Conglo merate ( U M G facies) represents a series of rivers that drained radially The absence of granitic clasts in the F C facies conglo merate is i mportant a way fro m the Uinta Mountains ( Bradley, 1936; Hansen, 1986). The abundance because granitic clasts would be expected for a river draining either the Wind of reddish sandstone clasts and Grenville-age grains in the U M G facies con - River or Sierra Madre Mountains (Fig. 14). Detrital zircon data provide a pos - fir ms that the provenance was the Neoproterozoic U M G in the nearby Uinta sible link bet ween the provenance of the F C facies and the Wind River Moun - Mountains. Ne w nu merical age constraints sho w that this river syste m flo wed tai ns. T he F C facies c o ntai ns a greater a b u n da nce of ca. 2650 Ma detrital zirc o n along the north flank of the Uinta Mountains ca. 36–27 Ma (Table 1). grains than the U M G facies, and Archean detrital zircon grains are si milarly Pale oc urre nt data a n d t o p o gra p hic relati o ns hi ps i n dicate t hat t he F C facies abundant in the modern Green River, which drains the Wind River Mountains represents a distinctly different river syste m fro m the U M G facies rivers. At (Fi g. 7). Alter natively, t he a bse nce of gra nitic clasts i n t he F C facies c o ul d i n - Firehole Canyon, the U M G and F C facies interfinger, and paleocurrent direc - dicate that base ment rocks were buried by younger sedi ment during the ti me ti o ns f or t he t w o facies are r o u g hly per pe n dic ular t o o ne a n ot her (Fi g. 4), w hic h represented by the Bishop rivers (Fan et al., 2015; Ro wley and Fan, 2016). suggests that this area was a paleoconfluence. At South Baxter, the F C facies I n s u m m ar y, t h e Bi s h o p C o n gl o m er at e r e pr e s e nt s t h e m o st si g ni fi c a nt l at e outcrop belt occupies a distinctive topographic saddle that is bordered to the Eocene– Oligocene river syste m of the Uinta Mountains region, but the inter - south by Miller Mountain, which represents north ward-sloping Bishop Con - pretation of the flo w direction of this river is unresolved. An east- or south - glo merate U M G facies, and to the north by Aspen Mountain, which represents east-flo wing late Eocene–early Oligocene river, as envisioned by Hansen (1986) south ward-sloping Bishop Conglo merate sandstone-do minated gravels a n d Fa n et al. (2015), is a p ossi ble i nter pretati o n f or t he Bis h o p C o n gl o merate F C (Fig. 4). The north-south orientation of the saddle suggests that the F C facies facies, but it would require post- Oligocene, do wn-to-the- west tilting to explain was part of an east- west–trending seg ment of a lo wer order, probable main - the present-day elevations of the conglo merate, and it conflicts with F C facies ste m river, which was fed by Bishop tributaries. The basal elevation of the F C paleocurrent data. A west-flo wing river is consistent with F C facies paleocur - facies decreases ~90 m to the west over a distance of ~20 k m bet ween South rents and conglo merate elevations, but the absence of granitic clasts fro m the Baxter and Firehole Canyon. This observation could indicate that the mainste m Sierra Madre Mountains is proble matic. Lucchitta et al. (2011) and Ferguson river fl o we d t o t he west, b ut t his i nter pretati o n ass u mes t hat t here has bee n n o (2011), as su m marized in Cather et al. (2012), argued for possible north-flo wing significant post- Oligocene defor mation of the F C facies conglo merates. rivers that extended across the Colorado Plateau and possibly into Wyo ming The presence of volcanic clasts and the abundance of rounded quartzite in the vicinity of Po wder Wash (Fig. 14). Ho wever, the suggested ages (late c o b bles i n t he F C facies clearly s h o w t hat t he pr ove na nce of t his facies differs Oli g o ce ne t o Pli oce ne) f or t hese rivers are si g ni fica ntly y o u n ger t ha n t h ose pr o - fro m the U M G facies. Figure 14 su m marizes co mpeting possibilities for the posed for the Bishop river syste m, and deposits of north-flo wing rivers drain - source areas and flo w directions of the Bishop river syste m representing these ing Lara mide uplifts of the Colorado Rocky Mountains should contain granitic facies. Possi ble s o urces of t he q uartzite clasts i n t he F C facies i ncl u de Pr oter o - cl a st s. F or t h e s e r e a s o n s, it i s u nli k el y t h at t h e Bi s h o p ri v er s y st e m i s p art of t h e z oic q uartzite i n t he Sierra Ma dre M o u ntai ns ( Sc ott et al., 2011) as well as Creta - n ort h- fl o wi n g river e nvisi o ne d by L ucc hitta et al. (2011) a n d Fer g us o n (2011). ceous and Paleocene quartzite conglo merates of the Wyo ming fold and thrust belt ( De Celles, 1994) and north western Wyo ming (Lyndsey, 1972). Inter mediate volcanic clasts in the F C facies also have several potential sources. These in - Ti mi n g of C oll a p s e of t h e E a st er n Ui nt a M o u nt ai n s clude upper Cretaceous and Paleocene quartzite conglo merates located near Jackson Hole, Wyo ming, that contain a significant proportion (as much as Detrital zircon U-Pb ages of the Bishop Conglo merate and Bro wns Park 15 %) of inter mediate volcanic clasts (Lyndsey, 1972), and the Eocene Challis For mation sa mples provide ne w constraints on the ti ming of the collapse of and/or Absaroka volcanic fields of Idaho and Wyo ming, respectively. Previous the eastern Uinta Mountains ( Hansen, 1969a, 1984, 1986). Based on strati - st u dies s u g gest t hat a river drai ne d t he C hallis a n d/ or A bsar oka volca nic fiel ds graphic and structural relationships in the Bro wns Park region, Hansen (1986) and flo wed into south western Wyo ming in the Eocene (ca. 50–47 Ma) ( Carroll suggested that the main phase of collapse of the eastern Uinta Mountains et al., 2 0 0 8; D a vi s et al., 2 0 0 8; C h et el et al., 2 0 11). It i s p o s si bl e t h at a s e g m e nt of occurred bet ween deposition of the Oligocene Bishop Conglo merate and the the Bishop river syste m si milarly drained the Challis and/or Absaroka volcanic Miocene Bro wns Park For mation. Ho wever, tilting and faulting of the Bro wns fields, or perhaps it re worked the older volcanic and quartzite conglo merates Park For mation strata sho w that defor mation continued through at least Mio - of north western Wyo ming. Alternatively, Roehler (1973, 1992) noted that Eo - cene ti me. Late Oligocene to early Miocene collapse is based on three key

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Legend Oligocene Bishop Conglo merate C e n o z oi c U nit s M e s o z oi c U nit s P al e o z oi c U nit s Preca mbrian Units River courses supported by gravels and paleovalleys S p e c ul ati v e e a st-fl o wi n g ri v er s y st e m S p e c ul ati v e w e st-fl o wi n g ri v er s y st e m Southeast-flo wing river with River course supported by gravels hea d waters i n t he a n d p al e o v all e y c o m m o n t o Wyo ming Fold and Thrust Belt eit h er ri v er s y st e m a n d p ossi bl y t h e Wind River Mountains West-fl o wi n g ri v e r Roc k wi t h hea d w a t e rs i n t h e ? S p ri n gs Sierra Madre Mountains Wyo ming Fold and Thrust Belt As p e n U plift and Washa ki e B asi n Mt ? Sierra Madre Mill e r Was ha ki e Mountain s Mt B asi n

Ui nt a Mountain s

Figure 14. Map sho wing possible courses of the late Eocene to early Oligocene (ca. 36–27 Ma) Bishop river syste m in the southern Green River Basin of Wyo ming and adjacent areas. The river syste m could have flo wed southeast across south western Wyo ming with head waters in the Wyo ming fold and thrust belt and Wind River Mountains (red lines). Alternatively, the river syste m could have flo wed west with sources in the Sierra Madre Mountains (blue lines).

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observations. First, the Bishop Conglo merate deposits extend radially a way ( Hansen, 1984, 1986). The barbed drainages consist of south ward-draining fro m the structural axis of the eastern Uinta Mountains, including areas be - Bishop Conglo merate paleovalleys reincised by modern strea ms. The modern yond the present-day Bro wns Park graben (Fig. 15). This observation clearly strea ms drain south ward fro m their head waters, but then turn sharply to the de monstrates that the graben did not exist at the ti me of Bishop Conglo m - north where they join the Green River in present-day Bro wns Park. Hansen erate deposition. Second, a spectacular series of barbed drainages present (1986) p oi nte d o ut t hat t his reversal i n drai na ge directi o n clearly p ost date d t he along the south flank of the eastern Uinta Mountains, especially near Dia mond Bishop Conglo merate, and suggested that collapse of Oligocene topography, Plateau, provide additional evidence of post– Bishop Conglo merate collapse o n t he or der of several h u n dre d meters ( Bra dley, 1936; Ha nse n, 1986), i nitiate d

U T C O

9. 2 3 2. 2 1 1. 8 P W 8. 4 8. 3 8. 9

U C C 2 7. 0 1 3. 7 1 8. 5 9. 9

S W 2 7. 1 31–19* 2 7. 6 D P 2 4. 5 3 0. 5 8. 6 2 4. 8 L S R

South-flo w i n g Bi s h o p ri v e rs at Di a m o n d Pl at e a u

1 0 k m

Y P 3 4

Figure 15. Satellite i mage of the Bro wns Park area in northeastern Utah ( UT) and north western Colorado ( C O) sho wing possible Oligocene Bishop Conglo merate river courses ( white dashed lines with arro ws pointing in the direction of paleoflo w). White nu mbers are age esti mates (in Ma) for the Bishop Conglo merate based on detrital zircon U-Pb maxi mu m depositional ages ( white dots) and radio metric ages ( white stars). Radio metric 4 0 Ar / 3 9 Ar age esti mates are fro m Ko wallis et al. (2005). Yello w nu mbers are age esti mates (in Ma) for the Bro wns Park For mation (F m.) based on detrital zircon U-Pb maxi mu m depositional ages (yello w dots) and a co mbination of fission track and K- Ar age esti mates (yello w stars). K- Ar and fission track data are fro m Izett (1975) and Luft (1985). DP — Dia mond Mountain Plateau, YP —Ya mpa Plateau, UCC —upper Crouse Canyon, S W —Sand Wash, LSR —Little Snake River, P W —Po wder Wash. I mage fro m Google Earth.

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the for mation of the Bro wns Park graben. Third, the transition fro m Bishop in Fig. 15), the presence of Bishop Conglo merate northeast of the Bro wns Park Conglo merate to Bro wns Park sedi mentation acco mpanied the collapse of the region sho ws that this strea m syste m predates collapse of the eastern Uinta eastern Uinta Mountains ( Hansen, 1986). In this scenario, late Oligocene to Mountains. A ne w 32.2 ± 4.4 Ma M D A for these sedi ments provides a maxi - early Miocene Bro wns Park sedi ments began to accu mulate in Bro wns Park mu m age for the ti ming of collapse. Of even greater i mportance, ho wever, is due to a co mbination of changes in topography, disruption of the Bishop Con - the ne w age esti mate and structural relations at Sand Wash (Fig. 10 A; S W in glo merate drainage net works, and the for mation of the Bro wns Park graben, Fig. 15). At this location, Bishop Conglo merate is represented by i mbricated which provided acco m modation space for Bro wns Park sedi ments. Bro wns sandy conglo merates and conglo meratic sandstones with a detrital zircon Park For mation deposits are si milarly concentrated in grabens else where in M D A of ca. 27.1 ± 1.2 Ma. T his a ge is c o nsiste nt wit h a si milar detrital zirc o n north western Colorado and along the Colorado- Wyo ming border (Izett, 1975; M D A of 27.6 ± 0.7 Ma reported by Fan et al. (2015) for the Bishop Conglo mer - L uft, 1 9 8 5; B uf fl er, 2 0 0 3). ate along the nearby Little Snake River (Figs. 10 B and 15). I mbricated clasts Bro wns Park For mation facies and age relationships in the Bro wns Park within the Bishop Conglo merate indicate northeast-directed paleoflo w of the graben further support the idea that the transition fro m coarse-grained Bishop Bishop river syste m at Sand Wash. Ho wever, the Bishop Conglo merate beds Conglo merate fluvial deposition to fine-grained, tuffaceous Bro wns Park sed - at Sand Wash dip south west, to ward the structural axis of the Bro wns Park i mentation is consistent with extensional collapse in the late Oligocene. In graben. Collectively, these observations suggest that collapse of the eastern the vicinity of the Little Snake River, excellent exposures of the lo wer Bro wns Uinta Mountains and cessation of Bishop Conglo merate deposition occurred Park For mation sho w significant nu mbers of siliceous, micritic, and so me - s o m eti m e aft er c a. 2 8 M a. ti m e s oolitic li mestone beds within the lo wer most 100 m of the for mation. The A n a d diti o nal c o nstrai nt o n t he ti mi n g of t he c olla pse of t he easter n Ui nta li mestones along with the presence of broad lenses of re worked tuff suggest Mountains is provided by a ne w age esti mate of the Bishop Conglo merate that shallo w ponds existed during the ti me represented by lo wer Bro wns Park sandy facies in upper Crouse Canyon ( U C C in Fig. 15). In this area, a fe w tens sedi mentation. Thick, lenticular fluvial sand bodies are conspicuously absent of meters of friable, light gray tuffaceous sandstone and siltstone partially in these laterally continuous exposures. Collectively, these observations sup - fill a Bishop Conglo merate paleovalley, which is part of the Dia mond Plateau port the interpretation of internal drainage within a closed basin during the dendritic channel net work sho wing south west-directed paleoflo w. The color, early stages of Bro wns Park sedi mentation in response to the for mation of the texture, and friable consistence of the deposits rese mble those of the Bro wns Bro wns Park graben. Alternatively, the transition fro m Bishop Conglo merate to Park For mation, but these sandy units are mapped as Bishop Conglo merate Bro wns Park sedi mentation could reflect changes in cli mate, and conco mitant ( Sprinkel, 2006). The detrital zircon M D A of the upper Crouse Canyon sand - changes in sedi ment co mposition and caliber. Ho wever, the age constraints stone is 27.0 ± 0.3 Ma. This age esti mate is older than the oldest Bro wns Park presented in this study suggest that the transition fro m Bishop Conglo merate For mation age in the Bro wns Park graben, overlaps in age with conglo mer - fluvial conglo meratic to Bro wns Park fluvial-lacustrine deposition occurred at atic facies of the Bishop Conglo merate along the Little Snake River and at differe nt ti mes i n differe nt places, as disc usse d i n t he f oll o wi n g secti o n, a n d Sand Wash, and is only slightly younger than a radio metric age esti mate (ca. changes due to cli mate are likely to be synchronous across a region. 30.5 Ma) for sandy, tuffaceous deposits overlying conglo merates of the Bishop Conglo merate on the Dia mond Plateau ( Ko wallis et al., 2005; DP in Fig. 15). Ko wallis et al. (2005) dated the main body of the Bishop Conglo merate to ca. A g e C o n str ai nt s 34 Ma ( YP in Fig. 15). Collectively, these observations suggest that the tran - sition fro m coarse-grained to fine-grained Bishop Conglo merate deposition Ne w detrital zircon M D As constrain the ti ming of the structural and geo - occurred ca. 34–27 Ma, along the southeastern flank of the Uinta Mountains. morphic changes to after the late Oligocene. Exposures of the lo wer Bro wns Along the northeastern flank of the Uinta Mountains in south western Wyo - Park For mation along the northern flank of the Little Snake River valley (Fig. ming, this transition occurred at approxi mately the sa me ti me, ca. 34–31 Ma 10 B) include a vitric tuff located ~20 m above the base of the Bishop Con - (Fig. 9). In contrast, the accu mulation of sandy, tuffaceous sedi ment ( Bro wns gl o m er at e ( L S R i n Fi g. 1 5). T h e d etrit al zir c o n M D A of 2 4. 5 ± 1. 1 M a s u p p ort s a Park For mation) began ca. 28–25 Ma in Bro wns Park. The ti me transgressive previ o us K- Ar ra di o metric a ge of 24.8 ± 0.8 Ma (Izett, 1975) fr o m a p pr oxi mately nat ure of t his lit h ol o gic tra nsiti o n s u p p orts t he i dea t hat c olla pse of t he easter n the sa me location. These data indicate that lo wer Bro wns Park For mation sedi - Uinta Mountains beginning so meti me after 28 Ma facilitated the accu mulation ments began to accu mulate by ca. 25 Ma, and based on the reasoning out - of tuffaceous sedi ments in the Bro wns Park graben. lined previously, ca. 25 Ma represents the mini mu m age for collapse of the The distribution of Bro wns Park age esti mates in Figure 15 sho ws an eastern Uinta Mountains. Subsequent defor mation has tilted both the Bishop ano malous cluster of young (younger than 12 Ma) ages in western most Conglo merate and Bro wns Park For mation beds to the south west (Fig. 10 B). Bro wns Park along the Green River upstrea m of Lodore Canyon. In this re - The youngest Bishop Conglo merate sa mples provide additional con - gion, late Miocene Bro wns Park sedi ments unconfor mably onlap Neoprotero - strai nts o n t he ti mi n g of str uct ural c olla pse i n t he re gi o n. At Po w der Was h ( P W zoic U M G sandstone beds; the Bishop Conglo merate is absent. The absence

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of older (older than 12 Ma) Bro wns Park For mation and Bishop Conglo merate CO NCLUSIO NS i n t his area raises t he p ossi bility t hat t his re gi o n of wester n m ost Br o w ns Park collapsed more recently than areas located southeast of Lodore Canyon and Ne w sedi mentological observations and a co mbination of detrital zir - Ver million Creek where older sedi ments are present (Fig. 15). One scenario is con and sanidine provenance data and age constraints provide insights on that collapse of the eastern Uinta Mountains occurred in a series of phases, the paleo g e o graphic and structural evolution of the Uinta Mountains region the earliest (ca. 28–25 Ma) affecting areas bet ween Ver million Creek and the during the late Eocene to late Miocene (ca. 36–8 Ma). During the ti me repre - Little Snake River. Later in ti me, western most Bro wns Park collapsed, leading sented by the Bishop Conglo merate (ca. 36–27 Ma), tributary strea ms drained to the onset of a younger phase of Bro wns Park sedi mentation ca. 12–8 Ma. radially a way fro m the crest of the Uinta Mountains and, in the southern Green This possibility could explain why it took the upper Green River until so me - River Basin, joined a mainste m river syste m. The flo w direction of this ancient ti me bet ween 8 and 2 Ma to integrate through this region ( Aslan et al., 2013). m ai n st e m ri v er i s u nr e s ol v e d. It i s p o s si bl e t h at it ori gi n at e d i n t h e C h alli s a n d Further more, the difference in the age esti mates of the Bro wns Park For ma - Absaroka volcanic fields and Wind River Mountains, flo wed southeast ward tion deposits in western most Bro wns Park (ca. 12–8 Ma) and lithologically across south western Wyo ming, and then veered east ward across the south si milar Bishop Conglo merate sedi ments in upper Crouse Canyon (ca. 27 Ma) fla nk of t he Rock S pri n gs u plift. Alter natively, t he river c o ul d have ori gi nate d argues against the possibility that the Green River’s course across the east - along the western margin of the Sierra Madre Mountains in Wyo ming and ern Uinta Mountains through Lodore Canyon was due to strea m capture of flo wed west ward across the south flank of the Rock Springs uplift to ward the a superi mposed river. If the Bro wns Park graben had been co mpletely filled Bridger Basin. In either case, the lack of base ment clasts is co mpatible with to the level of Bishop Conglo merate paleovalleys that flank the graben, then t he i nter pretati o n t hat it was n ot re gi o nally i nte grate d. late Miocene Bro wns Park For mation rather than ca. 27 Ma sandy facies of the Detrital sanidine and zircon data sho w that the Bishop Conglo merate and Bishop Conglo merate would fill the paleovalleys. Instead, the ne w age esti - Bro wns Park For mation contain significant quantities of re worked Cenozoic tephra. mates lend support to the strea m capture model postulated by Pederson and Co mparisons bet ween the ages of young (younger than 40 Ma) detrital sanidine Hadder (2005) that invokes a co mbination of head ward erosion and super - 4 0 Ar/ 3 9 Ar and zircon U-Pb age distributions and previously published radio m etri c i mposition by the Green River. age and che mical data for western North A merica Cenozoic igni mbrites support t h e i nt er pr et ati o n t h at t h e s o ut h er n B a si n a n d R a n g e v ol c a ni c fi el d w a s a n i m p ort - ant source of Bishop Conglo merate tephra. By co mparison, the Southern Rocky Regional Co mparisons Mountain, and probably the Snake River Plain, volcanic fields were i mportant s o ur c e s of Br o w n s P ar k F or m ati o n t e p hr a. T h e gr o wi n g r e c o g niti o n t h at d etrit al zir - The i mportance of Neogene tectonis m in modifying earlier uplifts is also c o n gr ai n s c a n ori gi n at e a s f ar-tr a v el e d v ol c a ni c a s h f all h a s i m p ort a nt i m pli c ati o n s seen in nu merous areas. To the north of the Uinta Mountains, in central for provenance interpretations based on detrital mineralogy. Wyo ming, the southern margin of the Wind River Mountains is marked by Detrital zircon M D As constrain the ti ming of collapse of the eastern Uinta east-southeast– west-north west nor mal faults (e.g., Sales, 1983; Sutherland and Mountains, which was part of a more regional episode of extensional tec - Hausel, 2006). Precise age control for the ti ming of fault move ment is inferred tonis m. Bro wns Park For mation sedi ment accu mulation in the Bro wns Park to be prekine matic to synkine matic to the deposition of the Miocene Split Rock graben marked the onset of collapse ca. 28–25 Ma. The abundance of young For mation. To the east of this range, the for mer structural cul mination of the (12–8 Ma) tuffaceous Bro wns Park sedi ment in western most Bro wns Park sug - Granite Mountains also foundered in north-northeast–south-south west exten - gests that this subregion had a later phase of collapse (post–12 Ma), which sion during deposition of the Miocene Moonstone For mation (see Sutherland f oll o we d t he i nitial str uct ural f o u n deri n g. T his fi nal p hase of c olla pse exte n de d and Hausel, 2003). The age of the Moonstone For mation is constrained by t h e e a st- w e st –tr e n di n g Br o w n s P ar k gr a b e n f art h er w e st, a n d li k el y f a cilit at e d vertebrate paleontology, U-Pb zircon geochronology, and magnetostratigraph - the establish ment of the ancestral Green River, which eventually was inte - ically ( Pr ot her o et al., 2008). I n a d diti o n, t here are east-s o ut heast– west- n ort h - grated across the Uinta Mountains through Lodore Canyon after 8 Ma. west–striking nor mal faults on the Rock Springs uplift (Love and Christiansen, 1985) and Piceance Basin ( Cashion, 1973) that currently lack age control, but may be associated with Miocene extension. For all of these locations and syn - ACKNO WLEDG MENTS tectonic deposits, regional uplift and drainage evolution may have been driven N ati o n al S ci e n c e F o u n d ati o n ( N S F) gr a nt E A R 111 9 6 3 5 a n d f u n di n g fr o m t h e C ol or a d o M e s a U ni - versity Unconventional Energy Center (to Aslan) supported this research. Karlstro m ackno wledges by post- Oligocene mantle convection in the Rocky Mountain region ( Karlstro m support fro m NSF grant E AR-1348007 fro m the Tectonics Progra m. Thanks to Steve May and et al., 2 0 1 2; R o s e n b er g et al., 2 0 1 4), d el a mi n ati o n of lit h o s p h eri c m a ntl e b e n e at h Exxon Mobil Upstrea m Research Co mpany for financial support (to Becker), and George Gehrels in western Utah (Levander et al., 2011) and central Colorado ( Hansen et al., and Mark Pecha for detrital zircon analysis analytical support. Colorado Mesa University students Erinn Fought and Doug Nichols helped collect field data, and Lori Stead man of the Utah Geologi - 2013), and a co mbination of regional plate-related(?) extension plus m a ntl e cal Survey helped with several of the figures. Co m ments by an associate editor and revie ws by f orci n gs affecti n g t he mar gi ns of t he C ol ora d o Platea u ( Ricketts et al., 2016). Majie Fan and an anony mous revie wer i mproved the content of this paper.

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GEOSPHERE | V ol u m e 1 4 | Nu mber 1 A sl a n et al. | C e n o z oi c c oll a p s e of t h e e a st er n Ui nt a M o u nt ai n s 1 4 0 Do wnloaded fro m https://pubs.geoscience world.org/gsa/geosphere/article-pdf/14/1/115/4034955/115.pdf by Univ Ne w Mexico user on 19 Dece mber 2018