Unit Name: Blairmore Group
Unit Type: Lithostratigraphic
Usage: Currently in use
Age Interval: Early Cretaceous (145.5 - 99.6 ma)
Province/Territory: Alberta; British Columbia
Originator: Leach, 1914.
No type section was designated. A section on Ma Butte, 14 km (8.75 mi) northeast of Blairmore, Alberta, mentioned by Leach (1912) and described by Norris (1964) is the principal reference section, although the lower part of the group is no longer well exposed. The group has been divided into four formations in the type area: Cadomin, Gladstone, Beaver Mines and Ma Butte, all of which are defined by type sections.
The thickness at the Ma Butte section is 635 m (2,083 ft) (Norris, 1964). The maximum reported thickness, in the Fernie Basin is estimated at 2,000 m (6,560 ft). A distinct west to east thinning occurs, with thicknesses of about 300 m (984 ft) reported from the easternmost foothills. To the northwest, along the foothills thicknesses of 400 to 600 m (1,312 to 1,968 ft) prevail. Use of the name Blairmore is confined to the Fernie Basin and the Alberta Foothills between the International Boundary and the Smoky River.
The basal unit, the Cadomin Formation is typically a very resistant, siliceous pebble conglomerate, but includes beds of quartzose sandstone and, in some sections, particularly in the eastern foothills is entirely quartzose sandstone. The lower part of the overlying redefined Gladstone Formation is a series of interbedded grey mudstone to sandstone, the proportions of which are very variable. Sandstones, which rarely exceed fine grain size often exhibit a distinct upward decrease in grain size. The upper Gladstone Formation is characterized by dark grey, argillaceous limestone and fossiliferous calcareous shale. North of the Clearwater River (52° N) limestone beds are rare or absent. The Beaver Mines Formation in the south, and the Mountain Park Formation in the north are composed of interbedded mudstone to very fine-grained sandstone with subordinate, but prominent coarser and thicker sandstone units with abrupt bases and fining-upward grain size. Conglomerate beds are a minor constituent. The Malcolm Creek Formation, confined to the area north of Waiparous Creek (51 deg 20'N) consists of the marine mudstone Moosebar Member, overlain by the prominent sandstone dominated Torrens Member and the coal bearing Grande Cache Member. The Ma Butte Formation consists of mudstone to very fine-grained sandstone, with subordinate coarser sandstone and conglomerate beds. Tuffaceous mudstones are common in the upper part of the formation in the type area, but disappear to the northwest along the foothills. The entire formation is absent north of the Clearwater River. Shades of red and green, often mottled, are very common south of the Bow River. Grey predominates to the north, except in the Mountain Park Formation, where greenish grey is the dominant color in more southerly sections and is more prominent upward. The proportion of the section with a prominently greenish color decreases northward and is confined to a subdued shade of greenish grey in the uppermost part of the formation in the Smoky River region.
The Blairmore Group overlies the Kootenay Group disconformably. It is overlain gradationally by the Crowsnest Formation in the type area and as far north as about 50 deg 05'N. To the northwest the Blackstone Formation rests abruptly and disconformably on the Blairmore Group. The group is equivalent to the Bullhead Group and lower part of the Fort St. John Group in northeastern British Columbia, and to the Mannville Group in the Alberta Plains. Correlatives in northwestern Montana are: the Cut Bank Sandstone with the Cadomin Formation; the lower Kootenai Formation, including the Draney Limestone with the Beaver Mines Formation; and the lower Blackleaf Formation with the Ma Butte Formation.
Leach (1914) included in his Blairmore Formation all of the strata between a prominent conglomerate bed (now the Cadomin Formation) and the Crowsnest Volcanics. Rose (1917) included the conglomerate bed in the Blairmore Formation. Douglas (1950) elevated Blairmore to group status. Mellon (1967) included the Crowsnest Volcanics as a member of the Blairmore Group, but because this is contrary to the work of all other geologists in the type area Norris (1978) and McLean (1980) reverted to the definition of Rose (1917). South of 51° N the Pocaterra Creek Member, found below the Cadomin Formation in some areas is included in the Blairmore Group. Use of the Blairmore Group traditionally has been confined to the southern Alberta Foothills as far north as the North Saskatchewan River. Similarities in lithology but differences in nomenclature to the north suggest that extension of the name Blairmore Group to the vicinity of the Smoky River would emphasize the elements of continuity in lower Cretaceous stratigraphy in the foothills (McLean, 1980).
Douglas, R.J.W., 1950. Callum Creek, Langford Creek and Gap map-areas, Alberta. Geological Survey of Canada, Memoir 255.
Leach, W.W., 1912. Geology of the Blairmore Map-area, Alberta; Geological Survey of Canada, Summary Report 1911, pp. 192-200.
Leach, W.W., 1914. Blairmore map-area, Alberta; Geological Survey of Canada, Summary Report 1912, p. 234. with Map 107A, Blairmore, Alberta, Scale: 1 inch to 2 miles.
McLean, J.R., 1980. Lithostratigraphy of the Lower Cretaceous coal-bearing sequence, Foothills of Alberta; Geological Survey of Canada, Paper 80-29.
Mellon, G.B., 1967. Stratigraphy and petrography of the Lower Cretaceous Blairmore and Mannville groups, Alberta Foothills and Plains; Res. Counc. Alberta, Bulletin 21.
Norris, D K., 1978. "Ma Butte, Coleman (Lower Cretaceous)", in, Field guide to rock formations of southern Alberta, Ollerenshaw, N.C. and Hills, L.V. (Eds.); Can. Soc. Petrol. Geol., p. 66-69.
Norris, D.K., 1964. The Lower Cretaceous of the southeastern Canadian Cordillera; Bulletin of Canadian Petroleum Geology, vol. 12, Field Conf. Guidebook issue, pp. 512-535.
Rose, Bruce, 1917. Crowsnest coal field, Albenta; Geological Survey of Canada, Summary Report 1916, pp. 107-114.
Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: J.R. McLean
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 31 Mar 2010