ABSTRACT: Sequence Stratigraphy, Frontier Formation, South-Central Wyoming: Eustatic and Tectonic Influences

MIERAS, BARBARA L., University of Colorado, Boulder, CO

Across much of Wyoming, the middle Cretaceous Frontier Formation is transected by an erosional unconformity of approximately 90 Ma, which represents the second-order sequence boundary between the Cenomanian-Turonian Greenhorn cycle and the Turonian-Coniacian 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 Emigrant Gap Member, is discontinuously pocketed between the Belle Fourche strata and the Wall Creek Member. The Emigrant Gap Member's base apparently represents a third-order sequence boundary near the close of the second-order Greenhorn Cycle.

Basal Wall Creek strata demonstrate a general south-to-north and east-to-west progression of shoreline facies in response to eustatic sea level rise. But Wall Creek strata, compared with Belle Fourche

strata, also illustrate significant differences in basin geometry between the Greenhorn and Niobrara cycles, as well as important effects of local tectonism. Wall Creek strata in south-central Wyoming are variable in thickness and in sandstone content. Locally, they are at least as thick as temporally equivalent strata in the Cretaceous foredeep region of western Wyoming and represent shallow-water to emergent depositional environments. The Belle Fourche, Emigrant Gap, and Wall Creek members of the Frontier Formation in south-central Wyoming comprise a coarsening-upward succession that crosses the second-order sequence boundary and continues well into the transgressive phase of the Niobrara cycle, suggesting tectonic influence on local Frontier deposition.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)