Unit Name: Old Fort Island Formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Formal
Usage: Currently in use
Age Interval: Early Cambrian ? - Middle Cambrian ? (542 - 499 ma)
Age Justification: Very sparse fauna; trilobites (Bonnia-Olenellus Zone) and the trace fossil Skolithos.
Province/Territory: Northwest Territories

Originator: A.W. Norris, 1965.

Type Locality:
Old Fort Island, near Wrigley Point, about mid-way up the west shore of the North Arm of Great Slave Lake.

Distribution:
The formation occurs along the western edge of the exposed Canadian Shield, from Great Slave Lake at least to Great Bear Lake. It is 7.5 m (23 ft) thick at the type section. Maximum thickness is estimated to be 30 m (91 ft).

Lithology:
Mainly white to grey quartzose sandstone, thin- to thick-bedded and generally cross-bedded, locally conglomeratic. Minor green and red siltstone and green shale.

Relationship:
The Old Fort Island Fm rests nonconformably upon a variety of Precambrian (Aphebian and Helikian) formations. Its contact with overlying Paleozoic rocks is concordant, and may be conformable or disconformable. It is the stratigraphic equivalent to the Backbone Ranges Fm.

History:
The Old Fort Island forms the basal strata of the Paleozoic succession along the western edge of the exposed Canadian Shield, from Great Slave Lake northward. In the region containing the type section, it can only be dated as Middle Ordovician or older. Farther north, the basal Paleozoic sandstone, to which the same name is given, underlies beds with a Middle Cambrian fauna probably assignable to the Glossopleura Zone. As remarked by Balkwill (1971), the Old Fort Island, as a basal transgressive sand unit, may be expected to differ in age from place to place.

Other Citations:
Aitken, Macqueen and Usher, 1973a; Balkwill, 1971; Balkwill and Yorath, 1970b; A.W. Norris, 1965.

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: J.D. Aitken; D.W. Morrow
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 29 Apr 2003