Frontier Formation, Southwest Powder River Basin, Wyoming

ABSTRACT The Upper Cretaceous Frontier formation is a conspicuous sandy unit in the predominantly fine-grained rocks of the Colorado group in Wyoming. The Frontier formation is an important oil-producing formation in Wyoming. The formation was studied in detail in the southwest Powder River Basin, Wyoming, in order to determine the paleogeography and the structural and depositional history of the area during Frontier time. Surface and subsurface occurrences of the formation are correlated, and the formation is divided into four members. The results of laboratory investigation of the texture, mineral content, and lithologic character of the rocks are combined with surface and subsurface data and are presented in charts, sections, and maps which aid in the interpretation of the sedimentation. The southwestern part of the Powder River Basin had essentially the same shape as it has at the present during Frontier time, and the outline of the present structural pattern was apparent as early as Third Wall Creek time. The sediments were deposited in a shallow-water marine environment on a sea bottom that had a varied topography. Sediments entered the area principally from the southwest, but there were minor amounts of sediments that came from the northwest and from the south. There probably was local uplift in the south during the deposition of the First Wall Creek member, and older Mesozoic rocks are the most probable immediate source of Frontier sediments. The Frontier formation reveals a history of regular variation in water salinity and sediment supply. The basal bentonite of the Frontier formation is correlated with the Clay Spur bentonite of the Black Hills, and the concretions in the First Wall Creek member are correlated with the Greenhorn limestone of the eastern Powder River Basin.