ABSTRACT: Cretaceous Stratigraphy, Laramie Basin, Southeastern Wyoming

Randi S. Martinsen

The Laramie Basin, a small (2000 mi²) inter-mountain basin located in the foreland province of southeastern Wyoming, contains over 15,000 ft of Cretaceous age sediments.

Lower Cretaceous strata rest unconformably on Upper Jurassic Morrison Formation, average only 400 ft in thickness, include both nonmarine and marine sediments, and contain at least one sequence-bounding unconformity. This unconformity is at the base of the Muddy Sandstone, has up to 50 ft of relief, and probably correlates with the 97 million year sea-level drop recognized in other areas of the seaway. Strata below the Muddy include the Aptian Lakota Conglomerate, a nonmarine, dominantly braided stream deposit, the Albian Fall River Formation, a transitional marine deposit, and the Thermopolis Shale, a marine deposit also of Albian age. Contacts between each of these formations have been described as sharp and conformable. The Muddy Sandstone consists dominantly of nonmarine valley-fi l deposits sharply overlain by shallow-marine deposits and is conformably overlain by the marine Albian Mowry Shale.

Conformably overlying the Mowry Shale are 6000 ft of Cenomanian through Campanian marine shale and minor sandstone. Formations include the Frontier Formation, Sage Breaks Shale, Niobrara Shale, Steele Shale, and Rock River Formation. A major regionally extensive unconformity occurs within the Frontier and separates the Cenomanian age Belle Fourche Member from the Turonian Wall Creek Member; the middle unnamed member of the Frontier being absent. A second regionally extensive unconformity separates the Rock River Formation from the overlying Campanian Pine Ridge Sandstone of the Mesaverde Group. The Pine Ridge Sandstone is a nonmarine valley fill deposit that varies between 90 and 450 ft in thickness and is believed to be coextensive with the Teapot Sandstone of the Powder River Basin. Up to 3000 ft of marine Lewis Shale, which is not equivalent to the Lewis Shale of southwestern Colorado, sharply overlies the Pine Ridge. The Fox Hills Formation conformably overlies the Lewis and represents the last marine deposition. Latest Cretaceous, Maestrichtian deposits consist of 6000 ft of nonmarine, dominantly fluviatile sediments of the Medicine Bow Formation.

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