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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 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 5 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 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 GEOSPHERE | V ol u m e 1 4 | Nu mber 1 A sl a n et al.

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