Devonian strata of the Williston basin area of Saskatchewan, Manitoba, North Dakota, and eastern Montana are divided into four major lithologic units that are, in ascending order, Elk Point group, Manitoba group, Saskatchewan group, and Qu'Appelle group.

The Elk Point group unconformably overlies Silurian and consists of broad areas of shelf-type carbonates, including reef strata (Winnipegosis formation) that enclose a large evaporite basin extending westward into Alberta. Salt as much as 650 feet thick (Prairie evaporite) occurs in the center of the basin. In northwestern North Dakota about 300 feet of dark-colored dense argillaceous limestone underlies the salt, indicating an euxinic depositional environment in this area prior to evaporite deposition.

The Manitoba group is characterized by repetitive sequences of carbonate strata punctuated by thin persistent shaly beds. A complete sequence consists of shale and argillaceous limestone that grades upward to light-colored bedded limestone overlain by a fragmental and reefoid zone. Evaporites commonly mark the upper member of a sequence. The basal sequence of the group exposed in the outcrop area is named the Dawson Bay formation.

The Saskatchewan group consists of predominantly carbonate strata that exhibit little lithologic or thickness variation throughout the area. Widespread fragmental carbonate zones indicate long periods of tectonic stability and uniform depositional conditions. Lithofacies trends suggest that the group had a wider areal extent than indicated by the present erosional limit. A widespread biostrome horizon at the top of the group is named the Nisku formation and is correlated with the Nisku member of the Winterburn formation in central Alberta.

The Qu'Appelle group is composed of red shales and siltstones with erratic occurrences of anhydrite and carbonates. Toward the west the evaporite content increases, and seemimgly the evaporitic environment that gave rise to the Potlatch anhydrite in southeastern Alberta also influenced the deposition of the Qu'Appelle group.

The Devonian is overlain by widespread black shale deposited in an euxinic environment that initiated Mississippian deposition. In parts of Manitoba where Mississippian strata have been removed by erosion, Jurassic redbeds and anhydrite overlie eroded Devonian. Beyond the limit of Jurrassic deposition the Devonian is overlain by sandstones and shales of Cretaceous age.

The Williston basin area includes more than 100,000 square miles of Devonian marine carbonate strata greater than 1,000 feet thick that contain many favorable aspects for future petroleum exploration. Fragmental zones, biostromes, and bioherms occurring at several horizons form potential reservoirs, and in places, where associated with structure, contain commercial quantities of oil. Pre-Mississippian erosion with subsequent Mississippian overlap in southern North Dakota and eastern Montana suggests that favorable truncation traps may be present in these areas.