Abstract

Across much of Wyoming, the mid-Cretaceous Frontier Formation is transected by an erosional unconformity of approximately 90 Ma, which forms the major sequence boundary between the Greenhorn Cycle and the Niobrara Cycle of the Western Interior Basin. In south-central Wyoming, the unconformity marks a transition from sandstone-poor deposition in the underlying Belle Fourche strata to sandstone-rich deposition in the overlying Wall Creek Member. A third Frontier unit, the member of Emigrant Gap, is discontinuously-pocketed between the Belle Fourche strata and the Wall Creek Member. Its base is also a sequence boundary, but the disconformity is contained within the rocks of the Greenhorn Cycle. The Belle Fourche, Emigrant Gap, and Wall Creek Members of the Frontier Formation in south-central Wyoming comprise an overall coarsening-upward succession which crosses the 90 Ma sequence boundary and continues well into the transgressive phase of the Niobrara Cycle, suggesting local tectonic control on Frontier deposition.

Significant surfaces such as sequence boundaries or marine flooding surfaces display different characteristics in different geographic and stratigraphic locations within the Frontier. Recognition of these surfaces is critical in constructing a chronostratigraphic framework for applying sequence stratigraphy to the study of Frontier facies architecture.