Unit Name: Mount Cap Formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Formal
Usage: Currently in use
Age Interval: Early Cambrian - Middle Cambrian (542 - 499 ma)
Age Justification: Trilobites that belong to the Bonnia-Olenellus, (?) Plagiura-Poliella, Albertella and Glossopleura zones of the Lower and Middle Cambrian have been collected from Mount Cap Fm. Paleontological work at the type section remains to be done, however, trilobites that belong to the Bonnia-Olenellus zone have been recorded here. The inarticulate brachiopod Micromitra sp. and the ichnofossil Skolithos sp. are also commonly encountered.
Province/Territory: Northwest Territories

Originator: M.Y. Williams, 1922.

Type Locality:
Located in Franklin Mountains on the southwest flank of Cap Mountain (Wrigley map sheet 95-0) approximately 22 km (14 mi) northeast from the town of Wrigley; 63 deg 23'N, 123 deg 11'W.

Distribution:
Mount Cap Fm is widely but discontinuously distributed in the northern interior plains. In western Great Slave Plain area the formation attains a thickness of 65 m (213 ft) and is predominantly calcareous and shaly. In Great Bear Plain area it varies in thickness from 30 to 75 m (98-246 ft) and is consistently shalier than in Franklin and Mackenzie mountains to the west where it attains thicknesses of 210 m and 100 m (689 and 328 ft) respectively. In Colville Hills and Horton Plain area the formation is 70 to 260 m (230-853 ft) thick and sandier than in the Franklin and Mackenzie mountains.

Lithology:
Sandstone and shale interbeds with short intervals of interbedded limestones and dolostones. Sandstones are greyish-green in fresh surface, brown-red weathering, quartz arenites, commonly fine- to coarse-grained, well-sorted, well rounded and contain glauconite pellets. Shales are dark grey or green in fresh surface, grey, green or brown weathering, may be calcareous and possess limestone nodules. Sandstones and shales are characteristically parallel and ripple cross-laminated, bioturbated and burrowed; many sandstone beds fine upward. Carbonates are grey, green in fresh surface, brown weathering, quartzose, micritic, bioclastic, bioturbated, burrowed, parallel and ripple cross-laminated.

Relationship:
In Mackenzie Mountains the formation rests unconformably on Precam-brian sediments down to Lower Katherine Group. In Franklin Mountains, the lower contact is conformable with Mount Clark Fm. Elsewhere, in the northern interior plains, the formation is in conformable contact with the underlying Old Fort Island Fm; both occurring within paleodepressions between knobs and ridges in the underlying Precambrian surface. Mount Cap Fm is overlain everywhere by Saline River Fm. The contact between the two formations is unconformable in westernmost Slave Plain, Franklin and Mackenzie mountains, and seemingly conformable every-where else. The lower part of this formation is referred to as the Alder Fm in the older literature. The Clemis Fm is equivalent to the upper part of the Mount Cap Fm. In the subsurface west of Lac La Martre in the southwestern part of the Great Slave Plain the term La Martre Falls Fm is applied to erosional remnants of the Mount Cap Fm, Saline River Fm and the Franklin Mountain Fm equivalents (Meijer-Drees, 1975).

Other Citations:
Aitken and Cook, 1974a, 1974b; Aitken, Macqueen and Usher, 1973a; ASPG, 1960; Balkwill, 1971; Balkwill and Yorath, 1970a, 1970b; C.R. Barnes et al., 1974; Cook, 1975; Cook and Aitken, 1969, 1976; Davis and Willott, 1978; Douglas and D.K. Norris, 1963; Gabrielse, 1967; Haimila, 1975; Macqueen and MacKenzie, 1973; Meijer-Drees, 1975; A.W. Norris, 1965; Tassonyi, 1969; M.Y. Williams, 1922, 1923, 1963.

Source: CSPG Lexicon of Canadian Stratigraphy, Volume 2, Yukon Territory and District of Mackenzie; L.V. Hills, E.V. Sangster and L.B. Suneby (editor)
Contributor: F.F. Krause; A.E. Oldershaw
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 29 Apr 2003