GEOLOGIC MAP OF THE BLACKHAWK QUADRANGLE, BLACK HILLS OF SOUTH DAKOTA (1:24,000-SCALE)

Rapid urbanization of the Black Hills Development Corridor parallels U.S. Interstate 90 along the eastern and northeastern flank of the Black Hills uplift where development of one- to five-acre housing tracts in the Blackhawk Quadrangle is currently the most active in the entire state. A basic tool to assist in natural resource and geologic hazard evaluation of such areas is 1:24,000-scale geologic maps: Members of the South Dakota School of Mines and Technology, with technical and financial assistance from the West Dakota Water Development District, the South Dakota Geological Survey, and the U.S. Geological Survey (EDMAP Project) are currently preparing geologic atlases of quadrangles along the corridor, including that of Blackhawk.

A gently ENE-dipping panel of Mississippian through Permian marine sandstone, mudstone, and carbonate rocks and Triassic to upper Cretaceous marine and continental sandstone and shale is exposed in WNW-trending ridges (Paleozoic strata and Cretaceous sandstone) and valleys (Triassic and Cretaceous mudstones). Cenozoic alluvial and terrace deposits parallel Boxelder and Elk Creeks and indicate a complicated example of stream piracy by Boxelder Creek. The highest elevations within the quadrangle contain remnants of probable White River (Oligocene) Group gravel. North-northwest-trending, west-vergent, anticline-syncline pairs deform the homoclinal dip of the east flank of the Laramide-age Black Hills uplift, including the Piedmont and Boxelder anticlines within this quadrangle. Exploratory petroleum tests on the Piedmont anticline were unsuccessful. Systematic fractures of probable Laramide age show a strong ENE trend.

Urban development is currently covering numerous deposits of sand and gravel, limestone, gypsum, and sandstone. The value of such commodities, and the effect of their long-term loss on the local economy, is rarely considered in the frantic rush to change rural land to urban and to increase the tax base for city and county governments. An additional, and potentially very significant factor, relates to protection of the recharge areas of local aquifers (Madison, Minnelusa, Minnekahta, Lakota), for which there is presently little restriction. Geologic hazards include flood plains, sink holes (Spearfish Fm), and landslides (Jurassic shale).