FAULTING IN THE CICERO PEAK QUADRANGLE, BLACK HILLS, SOUTH DAKOTA

Mapping in the Cicero Peak Quadrangle southeast of Custer, SD, gives information on Proterozoic metamorphism and deformation. Three faults - Grand Junction (GJF), Laughing Water Creek (LWCF), and Stockade Lake (SLF) - can be traced from the north (Redden et al., 2001) into the Cicero Peak Quadrangle, where all trend N-S in a 1 km-wide zone. A previously unknown, as yet unnamed fault marked by mylonitized quartzite lies between the SLF and the LWCF. In sum, these faults separate a shelf sequence of siliceous schists, quartzites, and minor calc-silicates, from a deeper water turbidite sequence of quartz-biotite schists.

Different metamorphic mineral assemblages occur on either side of the SLF. These may be due to subtle composition differences, although the gross compositions of the rocks are similar. Staurolite is common west of the fault and not present to the east. Sillimanite pseudomorphs of andalusite are common to the west, and not present in the east, although sillimanite is abundant. A dramatic decrease in pegmatite abundance across the SLF from west to east (e.g. Norton and Redden, 1990) has been ascribed to resistance of quartzite layers to the east to intrusion. Alternatively, it may result from different thermal regimes across the faults, as the bulk of the rock to both the east and west of the SLF is schist. Cumulatively, this evidence suggests the possibility of different metamorphic histories on either side of the SLF.

Evidence suggests motion on the faults during both prograde and post-peak metamorphism. A stretching lineation seen only in quartzite along the SLF parallels a lineation defined by sillimanite pseudomorphs of andalusite in schist between the SLF and the unnamed fault, suggesting motion along the SLF during prograde metamorphism. However, preservation of deformation microstructures in the quartzite along the unnamed fault suggests post-metamorphic-peak motion along this zone.

Norton, J.J. and Redden, J.A., 1990, Relations of zoned pegmatites to other pegmatites, granite, and metamorphic rocks in the southern Black Hills, South Dakota. Am. Min. v. 75, pp. 631-655.

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.