Basin-margin depositional environments of the Fort Union and Wasatch Formations in the Buffalo-Lake De Smet area, Johnson County, Wyoming

The Paleocene Fort Union and Eocene Wasatch Formations along the east flank of the Bighorn Mountains in the Buffalo-Lake De Smet area, Wyoming, consist of continental alluvial fan, braided stream, and poorly drained alluvial plain deposits. The Fort Union conformably overlies the Cretaceous Lance Formation, which is marine in its lower units and nonmarine in its upper part.

The formations dip steeply along the western margin of the study area and are nearly horizontal in the central and eastern portions. This structural configuration permits the reconstruction of depositional environments as an aid to understanding: (1) the evolution of the Bighorn uplift and its effects on the depositional patterns marginal to the uplift during Paleocene and Eocene time and (2) the changing depositional environments basinward from the margin of the uplift during a relatively small period of time in the Eocene.