Colorado Group
The Colorado Group, also called the Colorado shale, is a stratigraphical unit of Cretaceous age in the Western Canadian Sedimentary Basin.
It was first described in the Rocky Mountains front ranges of Colorado by A. Hague and S.E. Emmons in 1877.
Lithology
The Colorado Group consists of shale for the most part, and incorporates conglomerate, sandstone and siltstone, and beds of chalk, chalky limestone, coquinas, phosphorite and concretionary beds including calcite, siderite and pyrite.The lower part includes the following sandstone members: Phillips Sandstone, Bowdoin Sandstone and Cardium Sandstone in the non-calcareous shale unit. The upper part includes the Martin Sandy Zone and Medicine Hat Sandstone.
Oil/gas production
is produced from the sandstone members in southern Alberta, southern Saskatchewan and in Montana, such as in the Bowdoin gas field.Distribution
The Colorado Group occurs in the sub-surface throughout southern and central Alberta, western and central Saskatchewan. It is found in outcrops along the south-western edge of the Canadian Shield, as well as in the front ranges of the Rocky Mountains of Colorado. The sediments of the Colorado group exceed in thickness in central Alberta. In central Saskatchewan, it thins to.The Colorado Group ranges in age from middle Albian to Santonian.
Relationship to other units
The Colorado Group is overlain by the Montana Group and underlain by the Dakota Group in the Williston Basin of the western Great Plains. It is unconformably overlain by the Lea Park Formation shale and unconformably underlain by the Blairmore, Mannville or Swan River Group in Western Canada.The lower part is equivalent with the Ashville Formation in eastern Saskatchewan and southern Manitoba, with the upper part corresponding to the Vermillion River Formation and Favel Formation. It is equivalent to the sum of Crowsnest Formation, Blackstone Formation, Cardium Formation, and the lower Wapiabi Formation of the Alberta Group in the Canadian Rockies foothills. It correlates with the upper Smoky Group, Dunvegan Formation, Shaftesbury Formation, Paddy Member and Labiche Formation in northern Alberta. The Colorado Group was previously named Lloydminster Shale in the Lloydminster region, but the term is now obsolete.
The Colorado Group is divided in Canada 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.
Subunits
The Colorado Group includes the following sub-units, from top to bottom:| Subdivision | Sub-unit | Age | Lithology | Max Thickness | Reference |
| Upper | First White Speckled Shale | Santonian | olive-black, chalk-speckled, calcareous and carbonaceous shale; minor shaly limestone | ||
| Upper | Medicine Hat Sandstone | Santonian | muddy sandstone and siltstone | ||
| Upper | Martin Sandy Zone | Santonian | Siltstone, fine grained sandstone and calcareous shale | ||
| Lower | Cardium Sandstone | Turonian to Coniacian | marine sandstone | ||
| Lower | Bowdoin Sandstone | Turonian | very fine grained sandstone and siltstone, shale | ||
| Lower | Second White Speckled Shale | Turonian | olive-black, chalk-speckled, calcareous and carbonaceous shale; minor shaly limestone | ||
| Lower | Phillips Sandstone | Turonian | very fine grained sandstone and siltstone, dark calcareous shale |