Abstract

The Uinta Formation, Uinta Basin, Utah, is the type formation for the Uintan North American Land Mammal Age (NALMA). Lack of a high-resolution stratigraphic section in the Uinta Formation has led to confusion regarding the nature of and position of the contact between Uinta B and Uinta C rocks. Historically, this lithostratigraphic boundary has been defined in various ways, which has also affected our understanding of the early Uintan to late Uintan faunal transition. We describe a new high-resolution stratigraphic section in the upper intervals of the Uinta Formation, in the eastern Uinta Basin of Uintah County. Since 1994, fossil collection in this region by parties from Washington University has resulted in more than 200 productive fossil vertebrate localities, with most of these localities correlated to the new stratigraphic section. The section was measured from the lowest productive fossil mammal locality to the contact between the Uinta Formation (UF) and the overlying Duchesne River Formation (DRF) in the Devil’s Playground region and the section was linked using marker beds to the western regions of the study area where it was measured to the UF-DRF contact at the top of Deadman’s Bench. The stratigraphic section was used to investigate the utility of the “Amynodon Sandstone” unit as a lithologic boundary between Uinta B and Uinta C rocks, and to evaluate previous perceptions of the early to late Uintan mammal faunal transition. Uinta B rocks are characterized by gray-green mudstones interrupted by golden sheet sandstones and the transition from this lithology to the fine-grained red and orange claystones distinctive of Uinta C rocks occurs at 137 to 140 m. The Uinta B-C lithological transition is 73 m above the “Amynodon Sandstone” unit as measured in our section. A preliminary biostratigraphic analysis suggests that the early to late faunal transition occurred 10–20 m below the lithologic transition.