Colorado Shale of Central and Northwestern Montana and Equivalent Rocks of Black Hills

Abstract

The Colorado shale of central and northwestern Montana and the equivalent rocks on the north flank of the Black Hills are dominantly dark gray marine shales. It has been customary to treat the Colorado shale as a single formation, with one or two members. More detailed studies show that the Colorado shale is divisible into many lithologic units that can be correlated with the standard sequence in the Black Hills. The formations in the Black Hills that are equivalent to the Colorado shale are the Fall River sandstone, Skull Creek shale, Newcastle sandstone, Mowry shale, Belle Fourche shale, Greenhorn formation, Carlile shale, and Niobrara formation. Each formation and the equivalent part of the Colorado shale are briefly described, compared, and contrasted, and the importa t fossils are listed.