Joint Meeting: University of Wyoming Department of Geology and Geophysics Wyoming Geological Association Geological Survey of Wyoming: April 2-4, 1982 Laramie, Wyoming: Subsurface Practices in Geology and Geophysics Abstracts of Papers - Compiled by James R. Steidtmann

Structural movements in southwestern Wyoming and adjoining areas and changes in sea level during mid-Cretaceous time are reflected by the regional stratigraphy of the Frontier Formation. The Frontier consists mainly of clastic rocks that were deposited in marine and nonmarine environments during Albian, Cenomanian, Turonian, and Coniacian time. Thicknesses of the formation range from less than 50 m (164 ft) on the northern flank of the Uinta Mountains, to more than 300 m (984 ft) near the southern boundary of the Granite Mountains, and to more than 600 m (1,968 ft) on the east side of the Overthrust belt. In most of southwestern Wyoming, the Frontier conformably overlies marine beds of Early Cretaceous age, either the Mowry Shale or the Aspen Shale, and is conformably overlain by the marine Hilliard Shale or Cody Shale of Late Cretaceous age.

The Frontier on the Rawlins uplift and the Casper arch is composed of three disconformable members which have been called, in ascending order, the Belle Fourche Member, unnamed member, and Wall Creek Member. In the Overthrust belt of southwestern Wyoming, the formation consists of, from oldest to youngest, the Chalk Creek Member, Coalville Member, Allen Hollow Member, Oyster Ridge Sandstone Member, and Dry Hollow Member of Hale and Myers.

The truncation of the Belle Fourche Member in much of Wyoming and the southward thinning of the member in the vicinity of the Moxa arch and the Rock Springs uplift were caused by regional uplift, which was centered in the eastern part of the Uinta Mountains in early to middle Turonian time. Regional subsidence during the middle Turonian was accompanied by a marine transgression and deposition of the unnamed member and laterally equivalent strata. The areal distribution of the nonmarine rocks in the unnamed member indicates that a northwest-trending area along the southern flank of the Granite Mountains was emergent during middle Turonian time. Erosion of the top of the unnamed member was caused mainly by early late Turonian uplift in the vicinity of the Casper arch, Shirley basin, and Hanna basin. The subsequent deposition of the Wall Creek Member and at least the upper part of the Dry Hollow Member was associated with a regional marine transgression, which was caused by regional subsidence and a rising sea level in the late Turonian and Coniacian.