EVAPORITE KARST PROBLEMS IN THE UNITED STATES

Evaporites are the most soluble of common rocks; they are dissolved readily to form the same types of karst features that commonly are found in limestones and dolomites. Evaporites, including gypsum (or anhydrite) and salt, are present in 32 of the 48 contiguous United States, and they underlie about 35–40% of the land area. Evaporite-karst features typical in outcrops include sinkholes, caves, disappearing streams, and springs, whereas other evidence of active evaporite karst includes surface-collapse structures and saline springs or saline plumes that result from salt dissolution. Many evaporites also contain evidence of paleokarst, such as dissolution breccias, breccia pipes, slumped beds, and collapse structures. All these natural karst phenomena can be sources of engineering or environmental problems. Dangerous sinkholes and caves can form rapidly, or preexisting karst features can be reactivated and open up (collapse) under certain hydrologic conditions or when the land is put to new uses. Human activities also have caused development of evaporite karst, primarily in salt deposits. Boreholes (petroleum tests or solution-mining oprerations) or underground mines may enable unsaturated water to flow through or against salt deposits, either intentionally or inadvertently, thus allowing development of small to large dissolution cavities. If the dissolution cavity is large enough and shallow enough, successive roof failures can cause land subsidence and catastrophic collapse. Evaporite karst, both natural and human-induced, is far more prevalent than commonly believed.