Unit Name: Inwood Formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Formal
Usage: Currently in use
Age Interval: Early Silurian - middle Silurian (443.7 - 422.9 ma)
Province/Territory: Manitoba

Originator: Stearn, C.W., 1956.

Type Locality:
Building Products and Lime Company Quarry, 1 km (0.6 mi) north of Inwood, Manitoba (Lsd. 4, Sec. 4, Twp. 11, Rge. 1WPM). Stearn indicated that only a part of the lower member is exposed as this locality.

Stearn reported a total thickness of 12.8 m (42 ft) at Grand Rapids, and indicated a comparable thickness for the southern Interlake area. Outcrops are limited because of the generally recessive nature of the unit, but occur from the Inwood Quarry area as far north as Lake St. Martin. The only other occurrence reported by Stearn is at Grand Rapids, but core hole data and subsequent mapping indicate that the Inwood occurs throughout the Manitoba outcrop belt, although differentiation between the Inwood and overlying Moose Lake is not always possible. The name is applied only in the outcrop belt of southwestern Manitoba.

Stearn subdivided the Inwood into two units. In the south Interlake area the lower unit consists of pale yellowish grey to white, very fine-grained to aphanitic, thin-bedded, in part stromatolitic dolomite and associated intra-formation breccias. In places faintly laminated domes several feet across are flanked by fragmental fossiliferous dolomites. A single small coral bioherm was noted. The upper unit consists of massive to thick bedded, fossiliferous calcarenite. To the north, at Grand Rapids a silty marker beds occurs at the top of the Inwood, and the lower 9 m (30 ft) consists of soft, earthy, highly argillaceous, fossiliferous dolomite. Some question exists as to the precise correlation of the two outcrop areas, and considerable difference in lithology of the lower Inwood is evidence between the northern and southern areas of occurrence.

The contact of the Inwood Formation with the underlying fossiliferous dolomite of the Fisher Branch Formation is sharp but conformable; the contact with the overlying aphanitic Moose Lake Dolomite is conformable and transitional. In the subsurface of southwestern Manitoba and Saskatchewan equivalent strata comprise a portion of the lower Interlake Group of Porter and Fuller (1959), or the upper part of the Strathclair and the lower part of the Brandon Formation of the Interlake Group (King in: Cowan, 1971). The sandy, argillaceous marker at the top of the Inwood Formation probably correlates with the U-marker of Porter and Fuller, 1959.

Other Citations:
Baillie, 1951; King, in: Cowan, 1971; Porter and Fuller, 1959; Stearn, 1956.

Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: H .R. McCabe
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 29 Apr 2003