Summary

World-class examples of two dynamic sandstone facies, a bayhead delta and an overlying estuary-mouth bar, occur in the lenticular Frewens Castle sandstone of the Belle Fourche Member of the Frontier Formation in the western Powder River Basin. The vertical sequence of this paleovalley-fill is unusual in that the bayhead-delta is abruptly overlain by the estuary-mouth bar, probably as a result of rapid transgression. Neither of these sand-rich facies have previously been well documented in the western U.S. Sandstones and mudstones comprising the bayhead-delta coarsen upward and are about 100 ft thick; sandstones of the estuary-mouth bar are coarse grained and 20 to 40 ft thick. The deltaic beds were tidally deposited within a paleovalley that was eroded into estuarine mudstones. Beds of the estuary-mouth bar are 5-10 ft thick and are composed of steep foresets with highly variable dip-directions. This paleovalley-fill in the Powder River Basin has recently been correlated with age equivalent, dominantly fluvial channel-fill sandstones located 50 miles to the west in the Big Horn Basin. The intervening area contains older rocks of the Big Horn Mountains. The age of the valleys in both basins is constrained by distinctive fauna which occur almost immediately above the valley fills in both basins.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah