ABSTRACT: Cretaceous Stratigraphy of the Old Woman Anticline Area, Southeast Powder River Basin, Wyoming

David M. Wheeler, Paul E. Devine

Cretaceous stratigraphy in the Old Woman Anticline area is being studied as part of the Western Interior Cretaceous (WIK) Project of the Global Sedimentary Geology Program. Emphasis is on recognition of unconformities and correlation of depositional sequences. Lower Cretaceous units include the Lakota Sandstone (Aptian) and Fall River Formation, Skull Creek Shale, Muddy Sandstone, and Shell Creek and Mowry Shales (Albian). The Upper Cretaceous consists of the Belle Fourche Shale (Cenomanian), Greenhorn Limestone (Cenomanian-Turonian), Carlile Shale (Turonian), Niobrara Formation (Coniacian-Santonian), Pierre Shale (Campanian-Maestrichtian), and Fox Hills Sandstone and Lance Formation (Maestrichtian).

The top of the Skull Creek Shale is a subaerial unconformity including valleys filled with fluvial and estuarine deposits of the basal Muddy Sandstone. Upper Muddy shallow-marine sandstones interfinger with the offshore marine Shell Creek Shale to the west. Unconformities also occur at the base of the Turner Sandy Member of the Carlile Shale, an eastern offshore equivalent of the Wall Creek Sandstone, and at the top of the Sage Breaks Shale Member of the Carlile.

Thin basinal deposits of the Gammon Ferruginous Member of the Pierre Shale are equivalent to thick shelf-slope deposits to the west that include the Shannon Sandstone Member of the Steele Shale. The Sharon Springs and Mitten Black Shale Members of the Pierre, and additionally the Sussex Sandstone Member of the Steele Shale, comprise a second episode of shelf-slope-basin deposition. The Ardmore Bentonite bed is a time-stratigraphic marker near the base of the second sequence that demonstrates the depositional topography of these units.

The Red Bird Silty Member and lower unnamed shale of the Pierre are offshore equivalents of the Parkman Sandstone and Marine Shale and Teapot Sandstone Members, respectively, of the Mesaverde Formation. A subaerial unconformity within the Teapot has not been recognized in the Old Woman Anticline area.

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