Colorado Group

Colorado Group Stratigraphic range: middle Albian to Santonian PreЄ Є O S D C P T J K Pg N
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)[1]
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.^[2]

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.^[3]

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.^[1]

Oil/gas production

Gas 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 1,000 metres (3,280 ft) in thickness in central Alberta. In central Saskatchewan, it thins to 150 metres (490 ft).^[1]

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 (calcareous)	First White Speckled Shale				[4]
	Medicine Hat Sandstone	Santonian	muddy sandstone and siltstone	14 m (50 ft)	[5]
	Martin Sandy Zone	Santonian	Siltstone, fine grained sandstone and calcareous shale	60 m (200 ft)	[6]
Lower (non-calcareous)	Cardium Sandstone	Turonian to Coniacian	marine sandstone	108 m (350 ft)	[7]
	Bowdoin Sandstone	Turonian	very fine grained sandstone and siltstone, shale	61 m (200 ft)	[8]
	Second White Speckled Shale				[9]
	Phillips Sandstone	Turonian	very fine grained sandstone and siltstone, dark calcareous shale	38 m (120 ft)	[10]

References

1. Lexicon of Canadian Geologic Units. "Colorado Group". Retrieved 2009-03-01.
2. Hague, A. and Emmons, S.E., 1877. Descriptive geology. U.S. geological exploration of the fortieth parallel, v.2.
3. Geological Atlas of the Western Canada Sedimentary Basin - Chapter 20. "Cretaceous Colorado / Alberta Group of the Western Canada Sedimentary Basin". Retrieved 2010-05-12.
4. Lexicon of Canadian Geologic Units. "First White Speckled Shale". Retrieved 2009-03-01.
5. Lexicon of Canadian Geologic Units. "Medicine Hat Sandstone". Retrieved 2009-03-01.
6. Lexicon of Canadian Geologic Units. "Martin Sandy Zone". Retrieved 2009-03-01.
7. Lexicon of Canadian Geologic Units. "Cardium Sandstone". Retrieved 2009-03-01.
8. Lexicon of Canadian Geologic Units. "Bowdoin Sandstone". Retrieved 2009-03-01.
9. Lexicon of Canadian Geologic Units. "Second White Speckled Shale". Retrieved 2009-03-01.
10. Lexicon of Canadian Geologic Units. "Phillips Sandstone". Retrieved 2009-03-01.