Stratigraphy of Some Lower Cretaceous Rocks of Black Hills Area

Abstract

The Skull Creek shale, the Newcastle sandstone, and the lower part of the Mowry shale are considered to be Lower Cretaceous in age correlated by Foraminifera with rocks of Kansas, Oklahoma and Texas. Structural, mineralogical, and biological parameters indicate that the sandstone was deposited toward the close of the Lower Cretaceous as a chain of lenticular, discontinuous offshore bars rimming a low, forested landmass of smaller area than the present Black Hills. Periodic forest fires produced mineral charcoal which was incorporated in the sediments. Deposition of the offshore bars was synchronous only in the gross aspect, and their upper surfaces ranged from a few feet above to a few feet below sea-level with sufficient temporal and spatial expression to result in the existence of a brackish, lagoonal environment which supported specialized foraminiferal assemblages. Migration of these sand bodies occurred laterally and through time with resulting rapid lensing and interfingering conditions both parallel with and normal to the trend of the ancient shore line. The Newcastle sandstone and the lower part of the Mowry shale are considered subordinate to the Skull Creek formation and are assigned member rank.