ABSTRACT: Cretaceous Stratigraphy of the Black Hills Region

James E. Fox

Composite stratigraphic sections of Cretaceous rocks in both the northern and southern areas of the Black Hills uplift are presented as part of the Western Interior Cretaceous (WIK) Project of the Global Sedimentary Geology Program. Although strata can be readily correlated throughout the area, differences in thickness and facies exist. The Neocomian Lakota Formation, predominantly alluvial, was deposited unconformably on the Jurassic Morrison Formation and is thickest (450 ft) along the southern and western sides of the Black Hills uplift where a major alluvial valley system was located.

With the exception of much of the Inyan Kara Group (Neocomian-lower Albian), Cretaceous strata in the region are primarily marine, the product of cyclic sedimentation in the Western Interior seaway. Ages and correlation of marine strata are based primarily on ammonite range zones established by W. A. Cobban for the Western Interior of the United States. Five major transgressive-regressive cycles are evident, typically with a basal unconformity overlain by thin sandstone that fines upward into shale and sometimes limestone. The stratigraphy is as follows: cycle 1--Fall River Formation and Skull Creek Shale (Albian), 400 ft; cycle 2 --Newcastle Sandstone (Albian), Mowry Shale, Belle Fourche Shale, and lower Greenhorn Formation (Cenomanian), upper Greenhorn and lower Carlile Shale (early Turonian), 1100 ft; cycle 3--Turner Sandy and Sage Breaks Shale Members of the Carlile Shale (late Turonian), 400 ft; cycle 4--Niobrara Formation (Coniacian and Santonian) and lower Pierre Shale (early Campanian), 1150 ft; and cycle 5--upper Pierre Shale (late Campanian and early Maestrichtian) and Fox Hills Sandstone (late Maestrichtian), 2150 ft. The Fox Hills Sandstone is overlain by the continental Hell Creek Formation (latest Maestrichtian).

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