Abstract

Despite recent drilling for hydrocarbon tight-gas resources in conglomerates and sandstones in the Piceance Creek Basin, controversy still exists over the age and lateral extent of the Ohio Creek Formation. This controversy originated from the proposition that these deposits belonged in the late Cretaceous Mesaverde Group. However, recent pollen analyses suggest a late Paleocene age. Here I report the first fossil vertebrate fauna from these deposits, which corroborates the late Paleocene age and further refines it to the early Tiffanian NALMA (Ti3 lineage zone). Identified fossils comprise several genera of mammals and reptiles. Reptilian fossils consist of a dorsal osteoderm of a eusuchian with affinity to Borealosuchus and a partial hyoplastron of the turtle Compsemys. Both genera range from the late Cretaceous through the Paleocene. Mammalian taxa represented are: Gelastops, Haplaletes, Promioclaenus, Colpoclaenus, and Mimotricentes, which range from the Torrejonian into the early part of the Tiffanian NALMA. Presence of the index fossil Nannodectes simpsoni further constrains the deposits to an early Tiffanian age (Ti3 lineage zone). Lithologically the Ohio Creek Formation is characterized by the absence of coal and shale, and the presence of tan mudstones and siltstones spaced between thick and widespread units of clay pebble and chert conglomerates and sandstones. The revised age and unique lithic characteristics of the Ohio Creek Formation excludes it from the late Cretaceous Mesaverde Group. Revising the biostratigraphic zonation of the late Cretaceous and early Tertiary strata of the Piceance Creek Basin is important for understanding the complex tectonic history of the Rocky Mountain region.