ABSTRACT: Sequence Stratigraphy and Controls on Sandstone Distribution, Frontier Formation, South-Central Wyoming

Barbara L. Mieras

In south-central Wyoming, the Middle Cretaceous Frontier Formation is transected by an areally and temporally extensive erosional surface. The unconformity represents the sequence boundary between the regressive phase of the Cenomanian-Turonian Greenhorn Cycle and the transgressive phase of the Turonian-Coniacian Niobrara Cycle. It also demarcates a transition from tectonically dominated distribution of sandstones in the Belle Fourche equivalent strata below the unconformity to complexly controlled, cyclic distribution of sandstones in the Wall Creek Member above the unconformity.

A third Frontier unit, the unconformity-bounded Emigrant Gap Member, is discontinuously pocketed between the Belle Fourche strata and the Wall Creek Member. Locally, it contains marginal marine or nonmarine strata atypical of the marine-dominated Frontier of the region. Its original distribution was apparently partially dependent on subtle, pre-existing topography developed during lower sea levels at the sequence boundary.

Interpretations of the interaction of controls on sandstone distribution throughout the Frontier Formation hinge on the ability to regionally correlate the deposits of specific I to 3 million year long third-order depositional cycles within the Frontier. Biostratigraphic collections made in the area have yielded specimens of 65-70% of the inoceramid and ammonite zonal indices for the time span of Frontier deposition. These reference points make possible the delineation of chronostratigraphic units of approximately 0.5 million years duration. The resulting regional correlations form the basis for the reconstruction of depositional sequences within the Frontier Formation.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990