ABSTRACT

In the Williston Basin, the strata that overlie the Post-Heath (Mississippian) unconformity and underlie the Opeche shale ( middle Permian) previously have been assigned to the Minnelusa formation. This formation oorrelates with the Hartville of eastern Wyoming and western Nebraska. The terminology introduced by Condra, Reed, and Scherer (1940) and used by Agatston (1954) in the Hartville Uplift is recommended for use in the Williston Basin.

A basal sand and shale unit, Fairbank, underlies most of the other Pennsylvanian rocks. The overlying Reclamation and Roundtop groups are respectively a limestone and dolomite and shale and dolomite sequence. These are overlain in succession by the dolomites and sands of the Hayden group and the dolomite, sands and shales of the Wendover-Meek group. An unconformity truncates the Hayden and Wendover-Meek and separates them from the overlying dolomites, anhydrites, and sandstones of the Broom Creek group. This hiatus probably marks the division between the Pennsylvanian and the Permian periods. In South Dapota the sands and shales of the overlying Cassa group are correlated directly with the upper part of the Hartville formation. In North Dakota the salt and silty shale section previously assigned to the Opeche is thought to be a time equivalent of the Cassa. An unconformity separates the Broom Creek and Cassa group in North Dakota.

Fossil evidence, principally from the work in Wyoming, indicates that the Fairbank, Reclamation, Roundtop, Hayden, and Wendover-Meek groups are Pennsylvanian and the Broom Creek and Cassa groups are Permian.

Oil and gas production from Permo-Pennsylvanian strata is very limited, but this may be due to concentration on prospecting from Mississippian production in the Williston Basin rather than non-accumulation in these beds.