First White Specks
| Colorado Group Stratigraphic range: middle Albian to Santonian |
|
|---|---|
| Type | Geological formation |
| Sub-units | Medicine Hat Sandstone, Martin Sandy Zone, Second White Speckled Shale, Cardium Formation, Bowdoin Sandstone, First White Speckled Shale, Phillips Sandstone |
| Underlies | Montana 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) |
| Lithology | |
| Primary | Shale, Sandstone |
| Other | Siltstone, Conglomerate, Chalk, Limestone, concretionary beds |
| Location | |
| Region |
 Alberta,  Saskatchewan,  Montana,  Wyoming,  Colorado
|
| Country |
 US,  Canada
|
| Type section | |
| Named for | Colorado River |
| Named by | Hague and Emmons, 1877 |
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.
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 (below the Second White Speckled Shale), Bowdoin Sandstone and Cardium Sandstone in the non-calcareous shale unit. The upper part includes the Martin Sandy Zone and Medicine Hat Sandstone.
Gas is produced from the sandstone members in southern Alberta, southern Saskatchewan and in Montana, such as in the Bowdoin gas field.
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 1,000 metres (3,280 ft) in thickness in central Alberta. In central Saskatchewan, it thins to 150 metres (490 ft).
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