Permo-Pennsylvanian Formations Between Laramie Mountains, Wyoming, and Black Hills, South Dakota

Abstract

In the Laramie Mountains-Black Hills area of Wyoming and South Dakota the strata that lie above the Mississippian Madison-Guernsey-Pahasapa limestone and below the Middle Permian Opeche shale belong to the Fairbank, Casper, Hartville, and Minnelusa formations. The Fairbank, of regional extent northeast of the Laramie Mountains, underlies the other formations and represents chiefly reworked residual mantle derived from Mississippian limestone. The most complete stratigraphic record of the post-Fairbank formations is found in the subsurface Minnelusa formation, which accumulated in a trough here termed the Lusk embayment. In the subsurface the Minnelusa averages about 1,050 feet thick, and consists, in ascending order, of limestone and red shale averaging 225 feet thick, dolomite and sandstone, 375 feet, and anhydrite, sandstone, and dolomite, 450 feet. The surface Minnelusa of the Black Hills is considerably thinner, because most of the lower limestone-shale part of the section is absent through overlap, and because anhydrite in the upper part of the section has been removed by solution. The Hartville is the lateral equivalent of the subsurface Minnelusa, except that it contains breccias instead of evaporites in the upper p rt. The Casper, a shelf section of sandstone and dolomite, merges into the middle and upper parts of the Hartville formation. The absence of the lower part of the post-Fairbank section in the Laramie Mountains and its thinness in the Black Hills, and the precipitation and penecontemporaneous removal of anhydrite from the upper part of the section in the Black Hills and the Hartville uplift suggest that subsidence during Pennsylvanian and Early Permian time was somewhat uneven and spasmodic.