GEOLOGY OF THE MOUNT COOLIDGE AND CICERO PEAK QUADRANGLES, BLACK HILLS, SOUTH DAKOTA

Detailed mapping in a 40 square mile area southeast of Custer, South Dakota, provides information on the Proterozoic structure and stratigraphy in the portion of the Black Hills south and east of the Stockade Lake Thrust (Redden et al., 2001). These rocks have previously been mapped as undifferentiated sillimanite-grade quartzite and schist, and are thought to be younger than the rocks to the west of the fault (Dewitt et al., 1989). Mappable units consist of 1. (oldest) quartz-biotite-sillimanite-muscovite schist interbedded with quartzite; 2. massive, pure quartzite; and 3. pure and impure quartzite interbedded with a variety of schists, commonly containing significant amounts of plagioclase and microcline. Small intrusive bodies of metagabbro and pegmatitic Harney Peak Granite occur throughout the area. This interpretation of the stratigraphy is the simplest, and suggests that the dominant structure in the area is an anticline, which repeats the massive quartzite on its limbs. The anticline’s axis trends in a sinuous, z-shaped pattern across the region. Where it trends east-west, units are thickened and contain minor folds; where it trends north-south, units are thinned, and contain pronounced stretching lineations. Minor folds with axial planar foliation are concentrated near changes in trend; a rare second foliation occurs only in these areas. Minor folds that deform foliation are concentrated in the east-west trending portion of the structure. The youngest structures are a widely dispersed set of high strain zones in quartzite beds. These strike north to northeast and dip steeply to the east. Lack of discernable offset across these zones and an interpretation of muscovite textures within them suggest they are pure shear zones. Dewitt, E., Redden, J.A., Busher, D., and Wilson, A.A., 1989, Geologic Map of the Black Hills Area, South Dakota and Wyoming: U.S. Geological Survey, Miscellaneous Investigations Series, Map I-1910. Redden, J.A., Nonnast, D., and Siren, D., 2001, Geologic Map of the Custer Quadrangle, South Dakota: South Dakota Geological Survey, Geologic Quadrangle Map 4.