Type Locality:

Interlake area of Manitoba. Suggested reference outcrop core hole, Manitoba Core Hole M-4-80, Steeprock North, in 10-3-29-10WPM, between 16.6 and 121.2 m (55 and 398 ft).

Distribution:

Extends throughout the Williston Basin area. Thickness ranges from about 50 to 110 m (164 to 361 ft) in the Manitoba outcrop belt to 335 m (1,100 ft) in the central portion of the basin in North Dakota.

Lithology:

Primarily pale yellowish grey to yellowish brown, very finely crystalline to sublithographic dolomites, with coarse fossil fragmental, oolitic, stromatolitic and biohermal interbeds. Within lower Interlake strata (Fisher Branch, Inwood, Moose Lake, Atikameg and East Arm) the relatively monotonous dolomite sequence is interrupted by a series of argillaceous and/or silty to sandy beds (non-sequences, Porter and Fuller, 1959) that are remarkably extensive and can be correlated throughout much of the Williston Basin.

Relationship:

Overlies sharply, and possibly with slight disconformity the dolomites of the Stonewall Formation, and is overlain with gentle angular unconformity by the red shales and argillaceous dolomites of the Devonian Ashern Formation. In the subsurface subdivision of the Interlake is difficult, and alternative subdivisions have been proposed: Strathclair, Brandon, Cedar Lake (King, 1964); Lower, Middle and Upper Interlake (Porter and Fuller, 1958, 1959); Rupert, Hansen, Risser (Saskatchewan Geological Society, 1958); and Strathclair, Fife Lake, Guernsey, Cedar Lake and Taylorton (Jamieson, 1979). In some areas the term Interlake is used as a formation rather than group (Haidl, 1987).

History:

Baillie (1951) defined the Interlake as all Silurian strata overlying the Ordovician Stony Mountain Formation and underlying the Devonian Ashern Formation. He subdivided the Interlake into a lower unit, the Stonewall Formation and a series of informal units, designated in ascending sequence as B, C, D and E. Stearn (1956) determined that the Stonewall type section was Ordovician in age and therefore removed the Stonewall from the Interlake Group. Stearn also proposed formal subdivision of the Interlake into, in ascending sequence the Fisher Branch, Inwood, Moose Lake, Atikameg, East Arm and Cedar Lake formations. Brindle (1960) suggested that the upper part of the Stonewall, above a medial argillaceous marker bed is possibly Silurian in age. McCabe (1980), on the basis of detailed correlations of marker beds in a series of stratigraphic core holes suggested that Stearn's correlation of the Inwood Formation was incorrect and that the Inwood should be abandoned because the type Inwood appears to be stratigraphically equivalent to the Atikameg and Moose Lake formations.

Other Citations:

Baillie, 1951; Haidl, 1987, 1988; Jamieson, 1979; Johnson and Lescinsky, 1986; King, 1964; LoBue, 1982; Magathan, 1987; McCabe, 1980; Osadetz and Haidl, 1989; Porter and Fuller, 1958, 1959; Roehl, 1967; Saskatchewan Geological Society, 1958; Stearn, 1956.