Mississippian sedimentary rocks within the western Canada and Williston basin region are predominantly shallow-water marine deposits; carbonate rocks predominate on the south and argillaceous content increases northward. In most of the area, sedimentation was nearly continuous from Devonian into Pennsylvanian time. The terminal phase of Mississippian deposition was dominated by terrigenous clastics throughout the area. The original depositional extent was far greater than the area of sedimentary rocks remaining today; a significant part of the present Precambrian shield probably was covered by Mississippian sediments. Westward thickening in the disturbed belt indicates an approach to geosynclinal conditions postulated for the Cordilleran area.

Deep erosion at the end of Paleozoic time led to the partial separation and more complete preservation of Mississippian rocks in three distinct areas: the Williston basin, southern Alberta, and northern Alberta-British Columbia. Laramide tectonics created a fourth distinct area in the Rocky Mountains. Within each area, different nomenclature and independent interpretations of basinal history have been developed. Major barriers to the complete understanding of Mississippian sedimentation have been the tendency to regard each region as a depositional entity and failure to recognize that the presently known sedimentary rocks represent only a part of their original depositional extent. Partial coincidence of these pseudogeologic provinces with political boundaries has further hindered the development of broader regional concepts.

Within this region, Mississippian sedimentary rocks contain recoverable hydrocarbon reserves in excess of 2 billion bbl of oil and 15 trillion cu ft of gas, which are present in four types of trap. Unconformity traps, containing the largest reserves, are in southern Alberta and along the northeast flank of the Williston basin. Two other types of trap important in the Williston basin are features formed by the combined effects of salt solution and erosion, and tectonic folds. Production also is found in overthrust structures along the eastern margin of the Cordillera in southern Alberta.