A new stratigraphic framework for the upper Colorado Group (Cretaceous) in southern Alberta and southwestern Saskatchewan, Canada

Extensive marine shales and shallow-marine sandstones of the Cretaceous Upper Colorado Group represent one of the last thick, informally named, stratigraphic intervals in the southern part of the Western Canada Sedimentary Basin. Two regionally-mappable formations in southern Alberta and southwestern Saskatchewan are introduced: The Carlile Formation (Turonian) overlying the Second White Specks Formation, and the Niobrara Formation (Coniacian-Santonian) underlying the Milk River Formation. Both names are extensions of lithologically-similar and laterally-equivalent strata in adjacent parts of the Interior Seaway in Canada and the United States. The boundary between the Carlile and Niobrara formations is recognized at a distinct zone (3 to 15 m thick) of 17 bentonites in the lower part of the Niobrara Formation, whereof two are argon-argon dated to 89.19 (+ or -0.51) and 89.40 (+ or -0.31). Regional variations in lithology, petrophysics and geochemistry of the two new formations make it possible to further subdivide these into formal subunits. The Carlile Formation is subdivided into the informal lower, middle and upper units. The Niobrara Formation is formally subdivided into three mappable members in ascending order: the shaly, non-calcareous Verger Member, the sandstone-rich Medicine Hat Member, and the shaly calcareous First White Specks Member. An outcrop reference section of the Carlile Formation is designated at Deer Creek (east of West Butte) in the Sweetgrass Hills of north-central Montana. The single core cut from the Carlile Formation in southern Alberta and southwestern Saskatchewan is located at 13-20-17-7W4 and includes twelve metres of the upper part of the formation. It is used as a reference section for the boundary between the Carlile and Niobrara formations. A reference core of the Niobrara Formation is located at 4-16-22-15W4, which also is designated as the type section of the Verger, Medicine Hat and First White Specks members. An outcrop section at the Ghost River Dam Spillway, west of Calgary, serves as the outcrop reference section for the Medicine Hat Member and is correlated to the subsurface using wireline logs and foraminiferal biostratigraphy.