Unit Name: Exshaw Formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Formal
Usage: Currently in use
Age Interval: late Famennian - middle Tournaisian (364.3 - 349.9 ma)
Age Justification: Conodonts, locally common within calcareous concretions of the black shale member indicate the Devonian/Carboniferous boundary lies within that unit at some localities (Macqueen and Sandberg, 1970; Richards and Higgins, 1988).
Province/Territory: Alberta; British Columbia; Northwest Territories; Montana

Originator: Warren, 1937.

Type Locality:
Jura Creek, 3.2 km (2 mi) north of Exshaw, Alberta; 51°05'29"N, 115°09'29"W; NTS 82O/3; eastern Rocky Mountain Front Ranges, southwestern Alberta (Macqueen and Sandberg, 1970; Richards and Higgins, 1988).

Distribution:
The Exshaw underlies most of the western interior plains from northern Montana into southwestern District of Mackenzie, but is absent in much of the Peace River Embayment. In the eastern Cordillera it is generally present from southeastern British Columbia to 52°30'N, but from 52°30'N to 54 deg 25'N it occurs only locally. The Exshaw, 46.7 m (153 ft) thick at its type section is generally between 7 and 50 m (23 and 164 ft) thick and is thickest in the eastern Rocky Mountain Front Ranges and foothills west of Calgary.

Locality Data:
Thickness(m): Minimum 7, Maximum 50.

Lithology:
The Exshaw type section consists of a lower, shale dominated member (9.3 m, 31 ft thick) gradationally overlain by an upper member (37.4 m, 123 ft thick) comprising siltstone and silty limestone (Macqueen and Sandberg, 1970; Richards and Higgins, 1988). Elsewhere in the south both members are generally present, but in most of the central Rocky Mountains only the shale member occurs (Harker and McLaren, 1958). In the southern Peace River Embayment the two members are usually present, but farther north the siltstone member is absent. The lower member comprises anomalously radioactive, brownish black, sparsely fossiliferous shale; a thin (< 10 cm, 4 in) phosphatic, pyritic to sphaleritic, basal sandstone to conglomerate bed is locally present. Bentonite and tuft beds are commonly present in the shale member (Macdonald, 1987). The brown weathering, medium- to very thick-bedded upper Exshaw comprises sparsely fossiliferous, bioturbated, calcareous and dolomitic siltstone with subordinate silty limestone in most of the southern Rocky Mountains. To the east the upper member comprises grey shale grading up into siltstone, sandstone, silty limestone and skeletal to ooid lime grainstone and packstone.

Relationship:
The Exshaw generally overlies upper Famennian strata with probable disconformity, but its basal contact may be locally conformable in the central Rocky Mountains (Harker and McLaren, 1958; Macqueen and Sandberg, 1970; Richards and Higgins, 1988). In the southern Peace River Embayment and southward the Exshaw is generally disconformably overlain by the Banff Formation. To the north it is mainly gradationally overlain by the Banff and grades laterally into that unit. An arbitrary nomenclatural boundary along the axis of the Sweetgrass Arch in southeastern Alberta separates the Exshaw from the lower and middle members of the Bakken Formation, which are jointly equivalent to the Exshaw. In northeastern British Columbia the Exshaw passes southwestward into the Besa River Formation (Pelzer, 1966; Richards et al., 1983).

History:
Warren (1937) named the formation and considered Jura Creek to be the type locality, but he included only the black shale member and did not describe a type section. Clark (1949) redefined the upper contact of the type Exshaw to include the lower 9.1 m (30 ft) of what is currently called the siltstone member (Richards and Higgins, 1988). Harker and McLaren (1958) provided the first adequate description of the stratotype as defined by Clark (1949). Macqueen and Sandberg (1970) redefined the upper contact of the stratotype to include all, rather than only the lower part of the siltstone member.

References:
Clark, Leslie M., 1949. Geology of Rocky Mountain Front Ranges near Bow River, Alberta; American Association of Petroleum Geologists (AAPG), AAPG Bulletin, vol. 33, no. 4 (April), pp. 614-633.
Harker, P. and McLaren, D.J., 1958. The Devonian-Mississippian boundary in the Alberta Rocky Mountains; in, Jurassic and Carboniferous of western Canada; Goodman, AJ. (Ed.). Assoc Petrol. Geol., John Andrew Allan Memorial Volume, p. 244-259.
MacDonald, D.E., 1987. Geology and resource potential of phosphates in Alberta; Alberta Geol. Surv., Alberta Res. Counc., Earth Sci. Rept. 87-2.
Macqueen, Roger W. and Sandberg, Charles A., 1970. Stratigraphy, age and interregional correlation of the Exshaw Formation, Alberta Rocky Mountains; Canadian Society of Petroleum Geologists (CSPG), Bulletin of Canadian Petroleum Geology, vol. 18, no. 1 (March), pp. 32-66.
Pelzer, E.E., 1966. Mineralogy, geochemistry, and stratigraphy of the Besa River Shale, British Columbia; Canadian Society of Petroleum Geologists (CSPG), Bulletin of Canadian Petroleum Geology, vol. 14, no. 2 (June), pp. 273-321.
Richards, B.C. and Higgins, A.C., 1988. Devanian-Carboniferous boundary beds of the Palliser and Exshaw formations at Jura Creek, Rocky Mountains, southwestern Alberta. In: Devonian of the World; McMillan, N.J., Embry, A.F. and Glass, D.J. (Eds.). Can. Soc. Petrol. Geol., Memoir 14, v. 2, p. 399-412.
Richards, B.C., Bamber, E.W., Higgins, A.C. and Utting, J., 1993. Carboniferous, Chapter 4E. In: Sedimentary cover of the North American Craton: Canada; Stott, D.F. and Aitken, J.D. (Eds.). Geol. Surv. Can., v. 5, (also Geol. Soc. Amer., The Geology of North America, DNAG, v. D-1), p. 202-271.
Warren, P.S., 1937. Age of the Exshaw shale in the Canadian Rockies; American Journal of Science, Series 5, volume 33, pages 454-457.

Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: B.C. Richards
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 26 Jan 2011