ABSTRACT: Cretaceous Formations of the Hanna Basin, South-Central Wyoming
E. A. Merewether
Cretaceous sedimentary rocks of the Cordilleran foreland in the Hanna basin of south-central Wyoming comprise continental facies as well as marine facies of the shelf, slope, and basin. Some chronostratigraphic units in this sequence are laterally intergradational lithofacies that accumulated in several depositional environments and have been assigned to two or more formations. The nomenclature of this composite sequence was derived mainly from outcrops at scattered locations on the western flank of the basin. Both regionally and locally, different names have been applied to the same strata. The rocks are being studied to prepare a geologic history of the region for the ongoing Western Interior Cretaceous (WIK) Project of the Global Sedimentary Geology Program of the Inte national Union of Geological Sciences.
In the western part of the Hanna basin, Cretaceous strata are assigned, in ascending order, to the Aptian and Albian Cloverly Formation; Albian and Cenomanian(?) Thermopolis Shale; Cenomanian Mowry Shale; Cenomanian to Coniacian Frontier Formation; Coniacian Sage Breaks Shale; Coniacian and Santonian Niobrara Formation; Santonian(?) and Campanian Steele Shale; Campanian Mesaverde Group; Campanian and Maastrichtian Lewis Shale; Maastrichtian, Fox Hills and Medicine Bow formations; and a Maastrichtian, lower part of the Ferris Formation. This sequence is about 19,000 ft (5790 m) thick. Regional unconformities at the base of the Cloverly, within the Cloverly, Thermopolis, and Frontier, and at the base of the Pine Ridge Sandstone of the Mesaverde Group probably reflect subaerial and nears oremarine erosion associated with eustatic falls and regional uplifts.
AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990