Coupled detrital sanidine and zircon data, combined with sedimentological and stratigraphic observations, provide temporal constraints on the post-Laramide paleogeographic and structural evolution of the eastern Uinta Mountains region from the late Eocene to late Miocene (ca. 36–8 Ma). Maximum depositional ages (MDAs) calculated from detrital zircon U-Pb and detrital sanidine 40Ar/39Ar ages indicate that the most significant Paleogene fluvial system in the region, represented by the Bishop Conglomerate, existed from 36 to 27 Ma. The abundance of red sandstone and gray limestone clasts, paleocurrent directions, and the large number of Grenville-age detrital zircons suggest that the Uinta Mountain Group (UMG) facies of the Bishop Conglomerate are tributaries that flowed radially away from the crest of the Uinta Mountains. To the north of the Uinta Mountains, these rivers joined a mainstem river in southwestern Wyoming represented by the Bishop Conglomerate Firehole Canyon (FC) facies. This facies consists of rounded cobble- to pebble-sized quartzite clasts with minor quantities of volcanic rocks, has westward paleocurrent directions, and abundant young (younger than 40 Ma) detrital zircon and sanidine grains. Detrital sanidine age and geochemical data suggest that these young detrital grains are tephra that originated from the Basin and Range volcanic field, which was subsequently reworked into Bishop Conglomerate sediments. The more regional headwaters of the mainstem river could have been located east of the Uinta Mountains, or in the Challis and Absaroka volcanic fields and the Wind River Mountains located to the northwest of the region. The question of whether the FC facies of the Bishop Conglomerate represents part of an integrated river system that was a precursor to the Platte River remains unresolved.
Extensional collapse of the eastern Uinta Mountains was marked by the cessation of Bishop Conglomerate fluvial deposition and the onset of Browns Park Formation sedimentation within the Browns Park graben beginning ca. 25 Ma. Tuffaceous sandstone and siltstone and minor quantities of carbonate accumulated in a mosaic of fluvial and lacustrine environments representing an internally drained basin. Detrital sanidine age and geochemical data for young (younger than 40 Ma) grains also support a volcanic ash-fall origin. Some of the grains originated from the Southern Rocky Mountain volcanic field, and were reworked into Browns Park Formation deposits. New MDAs of Browns Park Formation sediments that unconformably overlie Neoproterozoic UMG rocks in westernmost Browns Park provide evidence for a younger (12–8 Ma) phase of extensional collapse of the eastern Uinta Mountains that was associated with 10–20 km of northwestward-directed lengthening of the Browns Park graben. These data are compatible with models for two stages of post-Laramide epeirogenic uplift of the Uinta Mountains region, including post–12 Ma tectonism that set the stage for subsequent integration of the Green and Colorado Rivers after 8 Ma.