Unit Name: Colorado Group
Unit Type: Lithostratigraphic
Rank: Group
Status: Formal
Usage: Currently in use
Age Interval: middle Albian - Santonian (108.8 - 83.5 ma)
Province/Territory: Alberta; Saskatchewan; Colorado; Iowa; Kansas; Minnesota; Montana; Nebraska; New Mexico; North Dakota; Oklahoma; South Dakota; Utah

Originator: Hague and Emmons, 1877.

Type Locality:
Exposures along the eastern base of the Front Range of the Rocky Mountains in Colorado.

Recognized across central and southern Alberta and the western half of Saskatchewan it is best known from subsurface data, although scattered exposures are found, notably along the northern erosional edge near the southern perimeter of the Precambrian Shield. Maximum thicknesses exceed 1,000 m (3,280 ft) in southern Alberta, minimum values for the complete succession are in the order of 150 m (492 ft) in central Saskatchewan .

Dominantly argillaceous, incorporating subordinate shaly conglomerate, sandstone and siltstone, as well as minor shaly chalk, chalky limestone, bentonite, pelecypod coquinas, accumulations of fish-skeletal debris, nodular phosphorite and concretionary layers of calcite, siderite and pyrite. The Colorado Group is divisible in Canada into an upper, calcareous and a lower, noncalcareous part at the base of the lower of two widespread marker units, the First (upper) and Second (lower) White Speckled Shale. The white speckled markers are characterized by abundant white flakes of coccolithic debris and, together with an intervening unnamed sequence of noncalcareous shale, make up the upper Colorado sequence. Each of the upper Colorado units incorporates one or more sandstone sequences of restricted distribution: the Phillips (Second White Specks) Sandstone of the Second While Speckled Shale; the Bowdoin and Cardium Sandstones of the unnamed noncalcareous shale unit; and the Martin Sandy Zone and Medicine Hat Sandstone of the First White Speckled Shale. The lower Colorado succession comprises upper and lower noncalcareos shale units - the Big River and Joli Fou formations respectively - separated by a prominent northeastward thinning wedge of coarse, siliciclastic strata, the Bow Island-Viking sequence. The Big River shales include a widespread marker unit, the Fish Scale Sandstones which consists of fish-skeletal debris in shaly sandstone layers, spanning the Lower to Upper Cretaceous boundary. Both the Big River and Joli Fou successions are replaced northeastwards by localized sandstone units, the St. Walburg and Spinney Hill sandstones respectively. The lower Colorado sequence remains undifferentiated where the Bow Island-Viking succession and its northern equivalents, the Pelican Sandstone and Flotten Lake Sand are absent.

The Colorado Group is overlain by the Montana Group and overlies the Dakota Group in the type area and throughout most of the northern Great Plains region of the United States. In western Canada the Colorado Group is unconformably overlain by the Lea Park Shale, which forms the basal unit of the Montana Group throughout much of the region; in southwestern locations however, the Milk River Sandstone at the base of the Montana Group appears to be lithologically gradational with the Colorado Group. The unit rests unconformably on sandstones of the Blairmore-Mannville-Swan River sequence. In southern Manitoba and eastern Saskatchewan the lower Colorado succession is termed the Ashville Formation, while the upper and lower calcareous markers are represented by the Boyne Member of the Vermilion River Formation and the Favel Formation respectively; the Morden Member of the Vermilion River Formation corresponds to the unnamed noncalcareous shale. In the Rocky Mountain Foothills of Alberta the units corresponding to the Colorado Group are the Crowsnest Volcanics, Blackstone Formation, Cardium Formation, and the lower part of the Wapiabi Formation, in order of decreasing age. The Alberta Shale of central Alberta includes the Colorado Group and overlying Lea Park Shale. In northern Alberta the Colorado Group is represented by all but the uppermost beds of the Smoky Group and the Dunvegan, Shaftesbury and Paddy formations, and essentially all of the Labiche Formation. The name Lloydminster Shale, introduced by Nauss (1947), includes all lithologic divisions of the Colorado Group and is no longer used.

Term originally applied by King and co-workers (Hague and Emmons, 1877; King, 1878) to the thick shale sequence comprising the Fort Benton Group, Niobrara Division and Fort Pierre Group previously recognized by Meek and Hayden (1861) in the upper Missouri region. White (1878) subsequently restricted usage to include only the Fort Benton and Niobrara units. The Colorado Shale, exposed in the vicinity of the Black Hills was seen to be made up of contrasting lithologic units, many of which were distinctively fossiliferous (Darton, 1909; Rubey, 1930), and became accepted as a standard sequence. Cobban (1952) recognized that the Colorado Shale of central and northwestern Montana was divisible into a number of lithologic units, which he correlated with the standard Black Hills succession. In the Canadian part of the Western Interior early progress in correlating the Colorado sequence was hampered by the general paucity and sporadic distribution of exposure along the southern perimeter of the Precambrian Shield. However, the term gained widespread usage in subsurface studies for the argillaceous succession resting upon the Blairmore-Mannville-Swan River sequence and overlain by Lea Park shales. Badgley (1952) provided one of the earliest descriptions of Lower Cretaceous units in the subsurface of central Alberta referable to the lower part of the Colorado Group.

Other Citations:
Badgley, 1952; Caldwell et al., 1978; Cobban, 1952; Darton, 1909; Hague and Emmons, 1877; King, 1878; McGookey et al., 1978; Meek and Hayden, 1861; Nauss, 1947; North and Caldwell, 1975; Price and Ball, 1971; Rubey, 1930; Rudkin, 1964; Simpson, 1975, 1979a, 1979b, 1979d; White, 1878; Williams and Burk, 1964.

Hague, A. and Emmons, S.E, 1877. Descriptive geology. U.S. geological exploration of the fortieth parallel, v.2.

Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: F. Simpson
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 28 May 2008