The Steepbank Formation: a paleokarst diamictite deposit in the Athabasca Oil Sands region of northeastern Alberta, Canada

Abstract The name “Steepbank Formation” is proposed for a paleokarst diamictite deposit that is present along the margins of the Middle Devonian Prairie Evaporite Formation in the Western Canada and Williston sedimentary basins, including the eastern portion of the Athabasca Oil Sands region. This poorly lithified diamictite forms a mappable unit that is distinct in both age and lithology from all contiguous formations. The Steepbank consists of clasts of dolostone, limestone, and siltstone up to the size of boulders with a matrix of silty to sandy calcareous mudstone. The material shows little or no evidence of bedding or sorting. Most clasts are angular and their orientation is commonly random. The diamictite formed in response to the dissolution of thick (up to 300 m) sequences of halite, anhydrite, and gypsum in the Prairie Evaporite Formation and the subsequent failure and collapse of interbedded and overlying insoluble strata. The top contact occurs where the intact strata of an overlying formation can be identified and is commonly gradational. The basal contact with the underlying Keg River Formation is sharp. Both contacts are unconformable. Evaporite dissolution and diamicton deposition likely began in late Middle Devonian time, moving down dip toward the west, and are continuing today near the Athabasca River.