Abstract

The Uinta and Piceance Creek basins are closely related early Cenozoic structural and sedimentary basins that were created by Laramide uplifts during latest Cretaceous through Paleocene time. A widespread period of beveling over the entire Piceance Creek Basin and all but the western Uinta Basin took place as the basins formed. During the Paleocene, sedimentation spread to cover most of both basins. Early sediments after the period of beveling in both basins are highly variable, but are commonly conglomeratic, reflecting the active tectonism on nearby Laramide uplifts. By late Paleocene, large shallow lakes and swamps covered much of both basins. This period was followed by a late Paleocene to early Eocene alluvial period, possibly reflecting renewed tectonism on surrounding uplifts, and then by an early Eocene freshwater lacustrine period, which created a large lake in each basin. These lakes may have connected for a brief time across the Douglas Creek arch. A major transgression near the end of the early Eocene marks the beginning of Lake Uinta, which extended as an unbroken lake between the two basins. The history of Lake Uinta is divided into five stages, based on depositional events and changes in water chemistry. The salinity of Lake Uinta increased steadily throughout most of these stages, first killing off the saline sensitive freshwater mollusc population during the first stage and ultimately resulting in the precipitation of nahcolite and halite during the fourth. Lake Uinta appears to have had a broad, shallow shelf area surrounding two much deeper oil-shale depocenters, one in each basin. The Piceance Creek Basin depocenter was several hundred feet deep during most of its existence and received far more saline deposition than did the Uinta. During late Eocene, Lake Uinta gradually filled in, first by volcaniclastics from the Absaroka Volcanic field in Wyoming and later by sediments from local Laramide uplifts.