Geological Survey Bulletin 1063—G Geology of the Jewel Cave SW Quadrangle, Custer County, South Dakota

The Black Hills of South Dakota are a range of mountains carved from a domal uplift (fig. 43). They are oval in plan and are 60 miles wide and 125 miles long in the direction of the main north-northwest axis. Precambrian rocks crop out in the central Black Hills, and Paleozoic and younger rocks dip outward from this Precambrian terrane. The Jewel Cave SW quadrangle is in the southwestern part of the Black Hills in Custer County, S. Dak. The quadrangle extends from the southwest side of the outer hogback, which is held up by sandstone of Early Cretaceous age, northeast to the outcrop of the Carboniferous rocks (pl. 20). All the rocks that crop out in the quadrangle are of sedimentary origin.

FIGURE 43.—Index map of the Black Hills showing the location of the Jewel Cave SW quadrangle, and some of the adjoining quadrangles. (click image for a PDF version)

This study was part of the U.S. Geological Survey's continuing study of the geology and ore deposits of the southern Black Hills, which was stimulated by the discovery of uranium deposits in sandstone in the area in 1951 (Page and Redden, 1952; Bell and Bales, 1955). Since that time, twelve 7-1/2—minute quadrangles that blanket the outer hogback from the vicinity of Hot Springs, S. Dak., to Newcastle, Wyo., have been mapped (Mapel and Gott, 1959). The main purpose of most of this work was to determine the geologic relationships that caused the occurrence of the many uranium deposits in sandstones of Early Cretaceous age in this area.

The Jewel Cave SW quadrangle is on the northwest fringe of the Edgemont mining district, and no important uranium deposits have yet been found in the quadrangle. The Inyan Kara Group, which contains most of the known uranium-vanadium deposits in the mining district, crops out only in the southern and eastern parts of the quadrangle. A large part of the present report deals with the petrography and structure of rocks of Pennsylvanian and Permian age from which substantial amounts of evaporite sediments have been leached. It is suspected that the solution of these rocks, and the associated collapse and brecciation of overlying rocks, may have played a role in the genesis of uranium deposits throughout the southern Black Hills.

ACKNOWLEDGMENTS

The author began work in the southern Black Hills in 1952. Since that time he has worked with 12 other geologists of the Survey who have been doing similar work in this area. The writer has benefited greatly from exchange of ideas with these men, and it is impossible to acknowledge specifically the influence that each of them has had on the ideas presented in this report.

George Carter and Thomas Bridge assisted with the mapping during the summers of 1955 and 1956, respectively.