GYPSUM-ANHYDRITE KARST IN THE BLACK HILLS, SOUTH DAKOTA-WYOMING

Dissolution of gypsum and anhydrite in four stratigraphic units has affected their hydrologic characteristics and has resulted in subsidence hazards in several areas of the northern Black Hills, SD-WY. Dissolution of gypsum in the Spearfish Formation (Permian-Triassic) and in the Gypsum Spring Formation (Jurassic) has resulted in collapse and development of sinkholes in urbanized areas between Rapid City and Spearfish. Anhydrite dissolution at depth in the Minnelusa Formation (Pennsylvanian-Permian) has produced breccia pipes and pinnacles, some of which extend more than 300 m (1,000 ft) into overlying strata; a regional collapse breccia; sinkholes; and extensive disruption of bedding. Removal of the anhydrite probably began soon after the Black Hills were uplifted (early Tertiary) and continues today. Recent subsidence is evidenced by sinkholes more than 18 m (60 ft) deep, collapse in water wells and natural springs resulting in sediment disruption, and fresh circular scarps surrounding shallow depressions. Three hundred years ago the Native Americans used one sinkhole to trap hundreds of bison. The Mammoth Site sinkhole, Hot Springs, SD, was a natural trap for large mammals about 26,000 years ago. Some sinkholes in the Spearfish Formation are too large to be accounted for by dissolution of the relatively thin gypsum beds within that formation. They probably formed by removal of much thicker anhydrite of the Minnelusa, about 150 m (500 ft) below. Several sinkholes are sites of resurgent springs, including Cox and Mirror Lakes at the State Wildlife Management Area 13 km (8 mi) west of Spearfish that support fish hatcheries and are used for local agricultural water supply. As the anhydrite dissolution front in the subsurface Minnelusa moves downdip and radially away from the center of the Black Hills uplift, these resurgent springs will be replaced by new ones as the geomorphology of the Black Hills evolves. Sinkholes and breccia pipes, preserved in cross section on canyon walls, attest to the former position of the dissolution front. Appreciation of the processes involved in the formation of gypsum karst should be considered in land use planning in this increasingly developed part of the northern Black Hills.