ABSTRACT: Cretaceous Rocks in the Wind River Basin, Central Wyoming

William R. Keefer

Strata of Cretaceous age are well exposed at many locations around the margins of the Wind River basin in central Wyoming and have been penetrated by numerous wells in the basin interior. The formations represented are: Cloverly (Albian? and Aptian?), Thermopolis (Albian), Mowry (Cenomanian), Frontier (Coniacian to Cenomanian), Cody (early Campanian to Coniacian), Mesaverde (late and middle Campanian), Lewis and Meeteetse (early Maastrichtian), and Lance (late and middle Maastrichtian). Thicknesses of individual formations vary considerably; aggregate average thickness of the Cretaceous sequence in the central part of the basin is about 10,500 ft (3200 m). Pronounced facies changes are present within most formations, but few unconformities are evident. Clastic rock types redominate.

The Cretaceous sequence in central Wyoming reveals a complex history of marine and nonmarine sedimentation, with the final withdrawal (eastward) of the epicontinental sea recorded in the interfingering sandstones and shales of the Cody, Mesaverde, Lewis, and Meeteetse formations. Nonmarine strata of the Lance Formation reflect the initial tectonic movements of the Laramide orogeny in latest Cretaceous time that are manifested by downwarping of the easttrending basin trough and broad uplift of some segments of the surrounding mountain areas. In the northeastern part of the Wind River basin, where subsidence was greatest, more than 5000 ft (1525 m) of clastic, basin-fill sediments of the Lance accumulated. Similar depositional conditions continued into early Tertiary time and strata of he succeeding Fort Union Formation (Paleocene) rest conformably on uppermost Cretaceous rocks throughout most of the basin interior. Angular and (or) erosional unconformities separate the Tertiary and Cretaceous sequences locally along the basin margins.

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