Unit Name: Bearpaw Formation
Unit Type: Lithostratigraphic
Usage: Currently in use
Age Interval: Campanian - Maastrichtian (83.5 - 65.5 ma)
Age Justification: Fossils are critical to understanding the stratigraphy of the Bearpaw Formation, and Bearpaw molluscs (baculitid, scaphitid, and placenticeratid ammonites, bivalves, gastropods, and scaphopods) are among the best known fossils of the southern interior plains. The formation also carries rare bryozoans, articulate and inarticulate brachiopods, annelids, rudistid and sepiod molluscs, and ophioderms. The base of the formation lies as low as the ammonite zone of Baculites gregoryensis in mid-Saskatchewan, and climbs to lie in the zone of B. compressus across the plains of southern Alberta. The top of the formation lies mostly in the zones of B. baculus and B. grandis. A sequence of foraminiferal faunas has been described from the formation. Most of the component species are arenaceous-walled, but a well diversified and rich arenaceous- and calcareous-walled assemblage characterizes the middle shaly beds. In terms of the foraminiferal zonal scheme for the southern interior plains the base of the fully developed formation lies in the zone of Eoponidella linki and the top high in the zone of Haplophragmoides excavata. Ostracodes occur rarely. The disarticulated remains of fishes, mososaurs, plesiosaurs, a hadrosaur reptile, avians and other vertebrates have been reported from the formation, as have logs and branches of coniferous trees, diatoms, dinoflagellates, pollen grains and spores.
Province/Territory: Alberta; Saskatchewan; Montana
Originator: Hatcher and Stanton, 1903; Stanton and Hatcher, 1905.
The Bearpaw Mountains of north-central Montana (48 deg 15'N, 109 deg 30'W) where the "Bearpaw shales ... are well developed around the northern, eastern and southern borders" (Hatcher and Stanton, 1903, p. 212).
Widely distributed through southern Alberta and southwestern Saskatchewan, where it forms much of the bedrock surface, the Bearpaw Formation can be traced from Montana as far northwards as Edmonton, with an outlier believed to exist in the Buffalo Head Hills of northern Alberta, and from the Rocky Mountain foothills as far eastwards as Regina. Thicknesses are variable, but because of its wedge-like form generally decrease northwards and westwards. Originally estimated to be about 183 m (600 ft) at the type locality and 230 m (750 ft) in the southern interior plains of Canada, the latter figure would be conservative even as an average. About 350 m (1148 ft) would be a more appropriate figure for the thickness in the Qu'Appelle and South Saskatchewan River valleys of south Saskatchewan, 300 m (984 ft) in the Alberta-Saskatchewan borderland, 150 m (492 ft) in the western part of the outcrop belt between Bassano and Castor in southern Alberta, and 30 m (98 ft) in the subsurface between Calgary and Drumheller in southern Alberta. The formation retains a notable thickness of about 290 m (951 ft) in the Crowsnest River valley of the southernmost Rocky Mountain foothills.
Dark grey clays, claystones, silty claystones, shales, silts and siltstones, with subordinate brownish grey silty sands, sands and sandstones; numerous concretionary beds, and thin beds of bentonite are the principal rock of sediment types of the Bearpaw Formations The argillaceous rocks have been variously described; originally called "clay shales", this term has been perpetuated by some subsequent workers, but most have referred to them as shales. They do, in fact range from blocky clays and claystones to true shales. Generally they have limited fissility, poor induration and a substantial, if variable, content of silt and sand. They contain numerous bedded concretions, some calcareous, others noncalcareous and sectarian and others again composed of mixed rusty iron oxides. The arenaceous rocks, composed mainly of weakly cemented, fine to medium sized grains of quartz, feldspars, chert, glauconite, various nonopaque heavy mineral and iron oxides, form widely traceable units in many parts of the plains. In some parts they occur only near the base and top of the formation; in others they alternate with argillaceous units throughout. Commonly clayey and silty near the base, the arenaceous units are usually thick-bedded to massive, display an increase in grain size and "purity" in upward sequence, and may be capped by a prominent bed of hard, resistant sandstone. They carry various kinds of bedded concretions, some large, calcareous and fossiliferous, others small, composed of mixed iron oxides and unfossiliferous. Thin beds and partings of bentonite (altered volcanic ash) occur throughout the formation but are concentrated in the middle shaly beds. Marine foraminifers and molluscs also are recurrent throughout, and banks and carpets of rock-forming inoceramid, mytilaceid, ostreid and pteriaceid bivalves are present locally. The argillaceous units suggest the prevalence of open sea conditions of the deposition; the arenaceous units point to episodes of shallowing, perhaps accompanied by regression; the highest beds of some sandy units providing evidence of deposition in brackish waters of the intertidal zone.
in apparently conformable relationship the Bearpaw Formation succeeds the Belly River Formation in the southern Rocky Mountain Foothills, and the Judith River Formation eastwards across the plains. It is apparently conformably succeeded by the Blood Reserve and the overlying St. Mary River Formation in southwestern Alberta, by the Horseshoe Canyon Formation (in places with a thick series of transitional beds) in the main outcrop of the southern Alberta Plains between the Bow River and Battle River valleys, and by the sequence of thin units known, in ascending order as the Eastend, Whitemud and Battle formations in the Cypress Hills of the Alberta-Saskatchewan borderland, in the South Saskatchewan River valley and other parts of southwestern Saskatchewan. From mid-Saskatchewan the Bearpaw Formation thins westwards by facies change between its lowest and highest beds of marine silty clays and sands and continental to marginal-lmarine sandy beds of the underlying the overlying formations. Eastwards it loses it identity where the underlying Judith River is replaced by the Pierre Shale. There it loses its sandy members by facies change into silty clays and becomes the upper part of the Pierre Shale (formerly Riding Mountain Formation) in the eastern plains of Saskatchewan and the Manitoba escarpment. Northwards from southwestern Saskatchewan the Bearpaw Formation is thinned by pre-Pleistocene erosion and disappears completely south of the North Saskatchewan River valley. Only to the northwest can it be traced as a diminishing wedge as far as Edmonton, where it ultimately passes completely by facies change into the Judith River Formation below and the Edmonton Group above. Further northwest the Judith River Formation and Edmonton Group comprise the Wapiti Formation which, in the Foothills is equivalent of the Brazeau Formation. From its type area in the Bearpaw Mountains the Bearpaw Formation can be traced across northern Montana, where it displays similar stratigraphic relationships to those in Alberta and Saskatchewan. West of Great Falls the formation wedges to the point of disappearance by facies change with the underlying Judith River Formation and overlying Horsethief Sand, St. Mary River and Hell Creek formations. Beyond the point of disappearance the nonmarine formations coalesce to form the Two Medicine Formation, which includes thick volcanic products. Eastwards from the type locality, to the border with North Dakota the Bearpaw Formation persists, underlain by the Judith River Formation (in marine sandy facies) and succeeded by the Fox Hills Sandstone and overlying Hell Creek Formation. The Judith River Formation disappears in the borderland with North Dakota, beyond which the Bearpaw Formation becomes the upper part of the Pierre Shale.
Caldwell, 1968; Caldwell et al., 1978; Clark, 1931: Douglas, 1942; Fraser et al., 1935; Furnival, 1946; Given and Wall, 1971; Hatcher and Stanton, 1903; Jeletzky, 1968, 1971; Lines, 1963; Link and Childerhose, 1931; North and Caldwell, 1970; Russell, 1948; Russell and Landes, 1940; Stanton and Hatcher, 1905; Wall and Rosene, 1977; Wall and Singh, 1975; Sweet and Hills, 1971; Warren, 1934; Williams and Burk, 1964; Williams and Dyer, 1930.
Hatcher, J.B. and Stanton, T.W., 1903. The stratigraphic position of the Judith River beds and their correlation with the Belly River beds. Science, no. 5, v. 18, p. 211-212.
Stanton, T.W. and Hatcher, J.B., 1905. Geology and paleontology of the Judith River Beds; United States Geological Survey (USGS), Bull. 257.
Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: W.G.E. Caldwell; R.J. Hawes
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 19 Mar 2009