EVAPORITE KARST IN THE BLACK HILLS, SOUTH DAKOTA AND WYOMING

Dissolution of gypsum and anhydrite in four stratigraphic units of Pennsylvania to Jurassic age in the Black Hills, South Dakota and Wyoming, has resulted in development of karstic collapse and has affected formational hydrologic characteristics. Subsidence has caused damage to houses and sewage retention sites. Substratal anhydrite dissolution in the Minnelusa Formation has produced breccia pipes and pinnacles, a regional collapse breccia, sinkholes, and extensive disruption of bedding. Anhydrite removal probably dates back to the early Tertiary when the Black Hills was uplifted. Evidence of recent collapse includes fresh scarps surrounding shallow depressions, steep-sided sinkholes more than 60 feet deep, and sediment disruption resulting in contamination of water wells and springs. Proof of sinkhole development to at least 26,000 years ago includes the Vore Buffalo Jump, near Sundance, WY, and the Mammoth Site in Hot Springs, SD. Several collapse sinkholes in the Spearfish Formation west of Spearfish, SD, which contain springs that support fish hatcheries and are used for local agricultural water supply, probably originated by dissolution 500 feet below the surface in the Minnelusa Formation. As the anhydrite dissolution front in the subsurface Minnelusa moves down dip and radially away from the center of the Black Hills uplift, present resurgent springs will dry up and new ones will form as the erosion of the Black Hills progresses. Abandoned sinkholes and breccia pipes, preserved in cross section on canyon walls, attest to the former position of the dissolution front. Collapse features extend as much as 1,000 feet upward into post-Minnelusa rocks, as into the Minnekahta Limestone, for example. The Spearfish Formation, mostly comprising red shale and siltstone, is generally considered to be a confining layer. However, in the lower part of the Spearfish solution openings along gypsum beds and secondary fracture porosity have developed due to considerable expansion during the hydration of anhydrite to gypsum. This part of the Spearfish yields water to wells and springs making it a respectable aquifer. 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, especially in the I-90 urban corridor.