EVAPORITE KARST IN THE UNITED STATES--EAST VS WEST

Evaporite rocks, including gypsum, anhydrite, and halite, underlie extensive areas of the lower forty eight United States. Both carbonates and sulphates behave differently in response to dissolution under different climatic regimes. Low ground-water tables and decreased ground-water circulation in the semi-arid to arid west does not favor rapid carbonate dissolution and development of karst, whereas more soluble evaporites have developed extensive karst in the subsurface. Conversely, in the humid eastern United States, evaporites are dissolved much more readily and actively than are carbonate rocks, and any near-surface karst features are rapidly obliterated. Dissolution of evaporites in many areas of the semi-arid western United States has resulted in ground subsidence and has affected formational hydrologic characteristics, resulting in hazards similar to those in karstic carbonate rocks. Dissolution of these evaporites and collapse of overlying non-soluble strata has resulted in intrastratal collapse breccia, breccia pipes, and sinkholes. In gypsum and anhydrite deposits in rocks ranging from Pennsylvanian to Jurassic in age in the Black Hills of South Dakota, karst development progresses along a down-dip dissolution front forming new resurgent springs while leaving behind abandoned sinkholes, breccia pipes and caves. The Holbrook Basin of Arizona is an example of the surface expression of collapse due to removal of salt at depth. Thick halite, anhydrite, and sylvite, interbedded with Permian clastic redbeds, have been removed resulting in the development of presently active collapse structures in overlying non-soluble clastic rocks. Sinkholes in quartz sandstone, thus produced, can be termed “karst”, even though they themselves were not directly removed by solution. The differences between karst in carbonate and evaporite rocks in the humid eastern United States, and those in the semi-arid to arid western United States are delimited approximately by a zone of mean annual precipitation of 32 inches.