OVERVIEW OF EVAPORITE KARST IN THE GREATER PERMIAN EVAPORITE BASIN

The Greater Permian Evaporite Basin (GPEB) embraces about 500,000 km² in southwestern United States, including parts of Texas, New Mexico, Oklahoma, Kansas, and Colorado. Permian-age gypsum/anhydrite, salt, and other evaporites are interbedded mainly with thick red-bed clastics and thinner carbonates. The evaporite sequence (500–1,500 m thick) is one of the thickest accumulations of evaporites in the world. Bedded salt and gypsum are extensively dissolved, both naturally and by human activities: this has resulted in many karst areas that can pose hazardous conditions, including sinkholes and caves developed naturally in gypsum, and large collapse sinks caused by dissolution of salt beds deep underground.

Natural gypsum sinkholes and karst features are widespread in West Texas and southeast New Mexico, where sinkholes range from 1–100 m across and generally overlie caverns less than 100 m deep. Another major gypsum-karst area is on the east flank of the GPEB, in western Oklahoma and north-central Texas, where sinkholes typically are 1–30 m wide and 3–25 m deep. In southwestern Kansas, gypsum units locally contain sinkholes and other karst features.

In most parts of the GPEB there is evidence of previous and on-going dissolution of subsurface Permian salts, resulting in brine springs, salt flats, sinks, collapse structures, and subsidence troughs that are now commonly filled with post-Permian sediments. Salt springs are present along portions of the Pecos, Brazos, Red, Cimarron, and Arkansas River systems. Prehistoric salt-dissolution collapse zones are present in all five states of the GPEB, chiefly above salt-dissolution zones in the Delaware Basin and the Texas Panhandle.

Human activities have produced some of the most dramatic evaporite-karst features in the GPEB. Boreholes drilled into or through bedded salt can allow (either intentionally or inadvertently) unsaturated water to encounter and dissolve the salt and create large underground cavities in the salt. If the dissolution cavity is large enough and shallow enough, successive roof failures can cause the water-filled “void” to migrate upward; this can result in land subsidence or catastrophic collapse. Human activities that may lead to local salt dissolution and sinkhole development in the GPEB are solution mining of salt and petroleum activities.