Unit Name: Three Forks Group
Unit Type: Lithostratigraphic
Rank: Group
Status: Formal
Usage: Currently in use
Age Interval: Famennian (372.2 - 358.9 ma)
Province/Territory: Manitoba; Saskatchewan; Montana; North Dakota

Originator: Peale, 1893 for the Three Forks Shale; Christopher, 1961, p. 16. elevated to group status

Type Locality:
Outcrops near Three Forks, southern Montana.

The thickness range is from 35 to 80 m (115 to 262 ft), extending southward from the post-Mississippian erosion edge (south of Saskatoon) throughout the Williston Basin.

Locality Data:
Thickness(m): Minimum 35, Maximum 80.

The Torquay Formation is composed of brown carbonates (mainly dolomite) and shales of various hues, overlain locally by mudstones of the Big Valley Formation, or black bituminous shales of the Bakken Formation. The Bakken comprises two radioactive black shale beds, between which there is a calcareous siltstone. Sparse fauna.

Conformably overlies carbonates of the Saskatchewan Group, and is disconformably overlain by limestone of the Madison Group. Equivalent strata in Alberta are the Wabamun Group and Exshaw Formation.

Peale (1893, p. 29) introduced the term "Three Forks Shale" for argillaceous and calcareous strata sandwiched between Mississippian and Devonian carbonates in southern Montana. Haynes (1916) described similar strata as the Three Forks Formation. Sloss and Laird (1947) and Wilson (1955) outlined different limits to the formation; Sandberg and Hammond (1958) and Christopher (1961) recognized Peale's boundaries. Detailed analysis of the problem by Christopher (1961) resulted in this elevation of the formation to "group" status. The group comprises the Torquay, Big Valley and Bakken formations, the last of which is almost entirely Mississippian in age.

Other Citations:
Brindle and Guliov, 1965; Christopher, 1961; Haynes, 1916; Kents, 1959; Lethiers, 1978; Peale, 1893; Sandberg, 1965; Sandberg and Hammond, 1958; Sloss and Laird, 1947; Wilson, 1955; Nicolas, 2012.

Christopher, J.E., 1961. Transitional Devonian-Mississippian formations of southern Saskatchewan. Saskatchewan Dept. Min. Res. Rept. 66.
Haynes, W.P., 1916. The fauna of the Upper Devonian in Montana; Part 2, The stratigraphy and brachiopoda. Ann. Pittsbugh Carnegie Museum, v. 10, p. 13-54.
Nicolas, M. P. B. 2012: Stratigraphy and regional geology of the Late Devonian- Early Mississippian Three Forks Group, southwestern Manitoba (NTS 62F, part of 62G, K); Manitoba Innovation, Energy and Mines, Manitoba Geological Survey, Geoscientific Report GR2012-3, 92 p.
Peale, A.C., 1893. The Paleozoic section in the vicinity of Three Forks, Montana. U.S G.S., Bull. 110, p. 9-56.
Sandberg, C.A. and Hammond, C.R., 1958. "Devonian System in Williston Basin and central Montana"; American Association of Petroleum Geologists (AAPG), AAPG Bulletin, vol. 42, pp. 2293-2334.
Sloss, L.L. and Laird, W.M., 1947. Devonian stratigraphy of central and northwestern Montana; American Association of Petroleum Geologists (AAPG), AAPG Bulletin, vol. 31, pp. 1404-1430.
Wilson, J.L., 1955. Devonian correlations in northwestem Montana. 6th Ann. Field Conf., Billings Geol. Soc. Guidebook, p. 70-77.

Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: C.E. Dunn; K.R. Milner; Michelle Nicolas
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 17 Feb 2016