The earliest known comprehensive karst map of the entire United States was published in 1969, based on compilations of William E. Davies of the U.S. Geological Survey (USGS). Various versions of essentially the same map have been published since. The USGS published new digital maps and databases in 2014 depicting the extent of known karst, potential karst, and pseudokarst areas of the United States, including Puerto Rico and the U.S. Virgin Islands. These maps are based primarily on the extent of potentially karstic soluble rock types, and rocks with physical properties conducive to the formation of pseudokarst features. These data were compiled and refined from multiple sources at various spatial resolutions, mostly as digital data supplied by state geological surveys. The database includes polygons delineating areas with potential for karst tagged with attributes intended to facilitate classification of karst regions. Approximately 18% of the surface of the 50 United States is underlain by significantly soluble bedrock. In the eastern United States, the extent of outcrop of soluble rocks provides a good first approximation of the distribution of karst and potential karst areas. In the arid western states, the extent of soluble rock outcrop tends to overestimate the extent of regions that might be considered as karst under current climatic conditions, but the new data set encompasses those regions nonetheless. This database will be revised as needed, and the present map will be updated as new information is incorporated.

Abstract The Mid-Atlantic region hosts some of the most mature karst landscapes in North America, developed in highly deformed rocks within the Piedmont and Valley and Ridge physiographic provinces. This guide describes a three-day excursion to examine karst development in various carbonate rocks by following Interstate 70 west from Baltimore across the eastern Piedmont, across the Frederick Valley, and into the Great Valley proper. The localities were chosen in order to examine the structural and lithological controls on karst feature development in marble, limestone, and dolostone rocks with an eye toward the implications for ancient landscape evolution, as well as for modern subsidence hazards. A number of caves will be visited, including two commercial caverns that reveal strikingly different histories of speleogenesis. Links between karst landscape development, hydrologic dynamics, and water resource sustainability will also be emphasized through visits to locally important springs. Recent work on quantitative dye tracing, spring water geochemistry, and groundwater modeling reveal the interaction between shallow and deep circulation of groundwater that has given rise to the modern karst landscape. Geologic and karst feature mapping conducted with the benefit of lidar data help reveal the strong bedrock structural controls on karst feature development, and illustrate the utility of geologic maps for assessment of sinkhole susceptibility.

Abstract This field trip examines the geology and geohydrology of a dissected part of the Salem Plateau in the Ozark Plateaus province of south-central Missouri. Rocks exposed in this area include karstified, flat-lying, lower Paleozoic carbonate platform rocks deposited on Mesoproterozoic basement. The latter is exposed as an uplift located about 40 mi southwest of the St. Francois Mountains and form the core of the Ozark dome. On day 1, participants will examine and explore major karst features developed in Paleozoic carbonate strata on the Current River; this will include Devil’s Well and Round Spring Cavern as well as Montauk, Round, Alley, and Big Springs. The average discharge of the latter is 276 × 10 6 gpd and is rated in the top 20 springs in the world. Another, Alley Spring, is equally spectacular with an average discharge of 81 × 10 6 gpd. Both are major contributors to the Current and Eleven Point River drainage system which includes about 50 Mesoproterozoic volcanic knobs and two granite outcrops. These knobs are mainly caldera-erupted ignimbrites with a total thickness of 7–8 km. They are overlain by post-collapse lavas and intruded by domes dated at 1470 Ma. Volcaniclastic sediment and air-fall lapilli tuff are widely distributed along this synvolcanic unconformity. On day 2, the group will examine the most important volcanic features and the southernmost granite exposure in Missouri. The trip concludes with a discussion of the Missouri Gravity Low, the Eminence caldera, and the volcanic history of southern Missouri as well as a discussion of geologic controls on regional groundwater flow through this part of the Ozark aquifer.

Cave, spring, and sinkhole development in the Ozarks of south-central Missouri is placed in a geologic framework through detailed geologic mapping. Geologic mapping shows that initial dissolution and inception of cave development is concentrated just beneath sandstone beds within Upper Cambrian and Lower Ordovician dolostone. Although rocks of the Ozarks have systematic and pervasive vertical joints, the development of karst conduits is controlled by bedding planes and stratigraphic variability. In the Salem Plateau of south-central Missouri, sinkholes occur in the lower part of the Ordovician Roubidoux Formation, where sinkholes are rimmed with and contain sandstone that has collapsed into voids in the underlying Ordovician Gasconade Dolomite. Cave diving by the Ozark Cave Diving Alliance into Alley Spring, a large (average flow 3.7 m 3 /s) spring along the Jacks Fork in the Ozark National Scenic Riverways, shows that although the spring discharges from the middle part of the Gasconade, the source of water is a cave passage just beneath the Gunter Sandstone Member of the Gasconade Dolomite. Artesian conditions cause the upward movement of groundwater from cavernous dolostone beneath the sandstone aquitards to the large springs. We hypothesize that sandstone, which is largely impermeable due to silica cementation, acts as a confining unit where hydraulic pressure, combined with mixing of water of differing chemistry, increases dissolution in the underlying dolostone beds.