Colorado Group

Colorado Group
Stratigraphic range: middle Albian to Santonian PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Sub-units	Colorado (type) and Kansas: Graneros, Greenhorn, Carlile, Niobrara Canada: Phillips Sandstone, Second White Speckled Shale, Bowdoin Sandstone, Cardium Sandstone, Martin Sandy Zone, Medicine Hat Sandstone, First White Speckled Shale
Underlies	Montana Group, Belly River Group, Lea Park Formation, Milk River Formation
Overlies	Dakota Group, Blairmore Group, Mannville Group, Swan River Group
Thickness	more than 1,000 metres (3,280 ft)[1]
Lithology
Primary	Shale, Chalk, Sandstone
Other	Siltstone, Conglomerate, Limestone, concretionary beds
Location
Region	Alberta,  Saskatchewan,  Montana,  Wyoming,  Colorado
Country	US,  Canada
Type section
Named for	Colorado, specifically for the hogbacks and plains facing the Front Range of that state[2][1]
Named by	F.V. Hayden[2][3]

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. The surface outcrop of this group produces distinctive landforms bordering the Great Plains and it is a significant feature of the subsurface of the Denver Basin and the Western Canadian Sedimentary Basin. These formations record important sequences of the Western Interior Seaway. As the geology of this seaway was studied, this name came to be used in states beyond Colorado but later was replaced in several of these states with more localized names.

The USGS convention has been to use Colorado Group where the rocks are further divided into formations, Colorado Formation where no beds are developed enough to be mapped as formations, and Colorado Shale where the unit is composed of little more than shale with no distinctive structures (such as in north-central Montana).^[4]