Abstract: Sequence Stratigraphy of the Bear River Formation on the Prospect Thrust Hanging-Wall, Western Wyoming

ANDERSON, DONNA S.

Two sequences within the Albian Bear River Formation on the hanging-wall of the Prospect Thrust correlate to Dakota-Muddy strata at the northern end of the Moxa Arch in the Green River Basin. Shoreface successions within each sequence show marked changes in progradation directions which indicate depocenter switching in an evolving, early Cretaceous foreland basin.

The base of the older sequence consists of lower shoreface strata (Rusty beds) which onlap the underlying Gannett Group from north to south. These strata deepen upward to a maximum flooding zone in marine shale (Thermopolis Shale). Overlying, easterly prograding lower to upper shoreface successions form the top of the older sequence and correlate to strata of the Dakota Formation on the northern Moxa Arch.

The younger sequence begins at a regional unconformity which truncates underlying "Dakota" shoreface successions. Strata above the unconformity include estuarine and tidal deposits overlain by cyclic carbonates containing pelecypods, gastropods, and stromatolites. Successions of marine shale and sandstone sharply overlie stromatolitic carbonates and deepen upward to a maximum flooding zone in shale. At the southern end of the Prospect Thrust sheet, shale successions above the maximum flooding zone shallow upward into lower shoreface, hummocky cross-stratified sandstone. Lower shoreface sandstones change facies to marine shale at the northern end of the Prospect Thrust sheet, a condition implying a northerly component of shoreface progradation. These lower shoreface to shelf strata, which form the top of the younger sequence, correlate to the Muddy Formation on the northern Moxa Arch.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado