ABSTRACT: Cretaceous Formations in the Southwestern Part of the Powder River Basin, Northeastern Wyoming

E. A. Merewether

Cretaceous strata of the Cordilleran foreland in northeastern Wyoming include marine and continental lithofacies that consist of siliciclastic beds along the southwestern margin of the Powder River basin (PRB) and siliciclastic and carbonate rocks near the eastern edge of the PRB. The marine strata accumulated in depositional environments of the shelf, slope, and basin. Chronostratigraphic units in the Cretaceous System of the region enclose laterally intergradational facies, many of which have been assigned to two or more formations. Furthermore, different names have been applied to the same strata both locally and regionally. The nomenclature of this composite sequence was derived mostly from scattered outcrops on the southwestern flank of the PRB. The rocks are being r viewed for a geologic history of that area for the ongoing Western Interior Cretaceous (WIK) Project of the Global Sedimentary Geology Program of the International Union of Geological Sciences.

In the southwestern part of the PRB, the Cretaceous sequence is comprised, in ascending order, of the Aptian and Albian Cloverly Formation; Albian and Cenomanian(?) Thermopolis Shale; Cenomanian Mowry Shale; Cenomanian and Turonian Frontier Formation; Turonian, Coniacian, Santonian, and Campanian Cody Shale; Campanian Mesaverde Formation; and Maastrichtian Lewis Shale, Fox Hills Sandstone, and Lance Formation. The sequence is about 9000 ft (2740 m) thick. Regional unconformities have been identified at the base of the Cloverly; within the Cloverly, Thermopolis, and Frontier; at the base of the Niobrara Member of the Cody; and at the base of the Teapot Sandstone Member of the Mesaverde. The unconformities probably represent subaerial and submarine erosion associated with eustasy and re ional tectonism.

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