CENOZOIC COLLAPSE OF THE EASTERN UINTA MOUNTAINS AND DRAINAGE EVOLUTION OF THE UINTA MOUNTAIN REGION

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 Mountain 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 reddish sandstone and gray limestone clasts and paleocurrent directions 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. 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, westerly paleocurrent directions, and abundant young (<40 Ma) detrital zircon and sanidine grains. The more regional headwaters of the mainstem river could have been located east of the Uinta Mountains, or in the Challis/Absaroka volcanic fields and the Wind River Mountains located to the northwest of the region.

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. New MDAs of Browns Park Formation sediments that unconformably overlie Neoproterozoic Uinta Mountain Group 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 Mountain region, including post- 12 Ma tectonism that set the stage for subsequent integration of the Green and Colorado rivers after 8 Ma.