Type Locality:

Type section is that of Milner and Thomas (1954). The lower member of the Vanguard Formation is defined in the Tidewater Rush Lake Crown No. 1 well, in Lsd. 3, Sec. 5, Twp. 18, Rge. 10W3M, southwestern Saskatchewan, between 1,037 and 1,083 m (3,402 and 3,552 ft).

Distribution:

The Rush Lake Shale is defined wherever the overlying Roseray Formation is mapped, i.e., in an area of southwestern Saskatchewan, between about 105 deg W and 109 deg W and from 51 deg N in Saskatchewan to 48 deg N in northern Montana. Thickness ranges between 75 and 45 m (246 and 148 ft).

Lithology:

Dark green-grey calcareous shale, calcareous mudstone, and light green-grey mudstone; Rhynchonella gnathopora Meek, Gryphaea nebrascensis, Meek et Hayden, Kepplerites and Arctocephalites are characteristic of the formation.

Relationship:

The basal contact is that described for the Vanguard Formation, i.e., a calcite cemented sandstone with a conglomerate of pelecypod shells (Gryphaea) resting erosionally on the Shaunavon Formation. The formation grades upward into a sandy facies transitional to the Roseray Formation. It also becomes younger eastward by virtue of Roseray clinobeds grading into shales as they slope downward into the Rush Lake Formation. Beyond the body of the Roseray Formation the Rush Lake becomes the Rierdon Shale of Alberta by virtue of a late Vanguard unconformity terminating the Roseray in the west. To the south and southeast (Montana, North Dakota and eastern Saskatchewan) the formation is equivalent to the lower half of the Rierdon Subgroup (Christopher, 1984).

Other Citations:

Christopher, 1974, 1984; Milner and Thomas, 1954.