Abstract

For over 100 years the Frontier Formation has been investigated in the Bighorn Basin in northern Wyoming and southern Montana. Recently, the U. S. Geological Survey (USGS) has had the opportunity to synthesize existing studies, analyze the latest subsurface information, and develop a stratigraphic framework for the central and southern part of the Bighorn Basin in Wyoming. The framework incorporates surface to subsurface correlations, projects biostratigraphic control from outcrop studies into the subsurface, and relates the ages to specific horizons mapped in the subsurface. As defined here, the Frontier Formation is part of an interval that is about 450 ft thick in the western part of the basin and about 1,075 ft thick in the southeastern part. This interval is mostly of Cenomanian age with a significant disconformity in the upper part, which is overlain by strata of Turonian and Coniacian age.

The main subsurface correlation horizons are marine-flooding surfaces and condensed sections identified on the basis of geophysical log character and confirmed in core and on outcrop. Marine-flooding surfaces are identified by the transposition of marine shale facies over nearshore-marine sandstone facies, signifying a deepening event and relative sea-level rise. Condensed sections are interpreted for the accumulation of bentonite (altered volcanic ash deposits) deposited in sufficient quantity to register as high gamma-ray responses on geophysical logs and these responses are correlated with specific ashes found on outcrops and in cores. The subsurface mapping resulted in identifying ten surfaces that can be correlated throughout parts or all of the study area with varying degrees of certainty.