Colorado is a geologic name applied to certain rocks of Cretaceous age in the North America, particularly in the western Great Plains.

This name was originally applied to classify a group of specific marine formations of shale and chalk known for their importance in Eastern Colorado.

Shales of middle Albian to Santonian age are distributed throughout much of the former extent of the Western Interior Seaway, including broadly from Arizona, to Iowa and Alberta.

From Iowa to Arizona, the lithology is remarkably consistent and the bentonites and rhythmic chalk beds of the upper Greenhorn especially are geologic events that can be traced over that distance.

United States sub-unit classifications have generally derived from the related Colorado and Kansas classifications, from top to bottom: The Colorado Group in Canada, which retains the Santonian-time Pierre Shale correlations, is divided into an upper part which is calcareous, and a lower part, which is non-calcareous.

The sub-units are defined at the base of two regional markers, called First and Second White Speckled Shale characterized by coccolithic debris.