The Devonian-Mississippian Boundary in the Alberta Rocky Mountains
Over most of the Alberta Rocky Mountain region, the Palliser formation, containing an Upper Devonian fauna, is overlain by the Exshaw formation. In its fullest development, the Exshaw consists of three members—a thin basal sandstone, overlain by a black shale, which grades into an upper calcareous siltstone. It rests on the Palliser with disconformity ; the upper member grades into the Banff formation of Mississippian age. The Exshaw, though widespread, is not present throughout the region, and in some sections the Banff rests disconformably on the Palliser.
The Exshaw corresponds lithologically with the lower two members of the subsurface Bakken formation, and similar correspondence is shown between the Exshaw and the Sappington formation of southwestern Montana if the Sappington is interpreted to include the black shale below the sandstone.
The Exshaw fauna has been considered Devonian, but faunal evidence of age is inconclusive. Some forms resemble specimens from the Louisiana limestone of Missouri. An early Kinderhookian fauna has been reported from the Sappington.
It is suggested that the Devonian-Mississippian junction lies at the base of the Exshaw formation in the Alberta Rocky Mountains.