Geology of the Inyan Kara Mountain quadrangle, Crook and Weston Counties, Wyoming

The Inyan Kara Mountain quadrangle includes about 215 square miles on the west side of the Black Hills, in Crook and Weston Counties, Wyo. It is about 10 miles south of Sundance, 6 miles east of Upton, and 10 miles northeast of Newcastle, Wyo.

Exposed sedimentary rocks, exclusive of surficial deposits, are about 4,000 feet thick and range in age from Mississippian to Cretaceous. Sandstone, siltstone, and shale make up most of the sedimentary sequence; limestone, gypsum, bentonite, and coal are present in lesser quantities. The Pahasapa limestone of Mississippian age is the oldest formation exposed. It is overlain by the Minnelusa formation of Pennsylvanian and Permian age, followed in turn by the Opeche formation and Minnekahta limestone of Permian age, the Spearfish formation of Permian and Triassic age, the Gypsum Spring, Sundance, and Morrison formations of Jurassic age, and the Lakota, Fall River, Skull Creek, Newcastle, Mowry, Belle Fourche, Greenhorn, and lower part of the Carlile formations of Cretaceous age. A small plug of syenite porphyry that crosscuts all formations from the Pahasapa to the Spearfish crops out at Inyan Kara Mountain in the northern part of the quadrangle.

The sedimentary rocks have a fairly uniform dip of 1 to 3 degrees to the southwest. This simple monoclinal structure is interrupted by sharply folded domes at Inyan Kara and nearby Strawberry Mountains in the northern part of the quadrangle. The Inyan Kant Mountain dome is about 1 1/2 miles across and has about 1,800 feet of structural closure in the outcropping Pahasapa limestone and younger rocks. A fault with a curving trace open to the northeast outlines the fold. Igneous and sedimentary rocks within the area circumscribed by the fault are upthrown as much as 600 feet. The Strawberry Mountain dome is about 11/2 miles across and has about 450 feet of closure; the Minnelusa is the oldest formation exposed on the crest of the fold. Other less sharply folded anticlines or domes involve outcrops of the Opeche, Minnekahta, and Spearfish formations in the northeast corner of the quadrangle; the Sundance formation along Oil Creek in the southeastern part; and the Skull Creek, Newcastle, and Mowry formations near Turner Creek in the southeastern part.

Mineral resources include oil in the Newcastle, Mowry, and Belle Fourche formations at the Osage oil field, which extends about a mile into the southwestern part of the quadrangle ; gypsum in the Gypsum Spring and Spearfish formations ; small amounts of bituminous coal in the Lakota formation ; bentonite in the Newcastle, Mowry, and Belle Fourche formations; and sand and gravel from stream terraces.