THE ROLE OF SANDSTONE IN THE DEVELOPMENT OF AN OZARK KARST SYSTEM, SOUTHEASTERN MISSOURI

Detailed 1:24,000-scale geologic mapping in the Ozarks of southeastern Missouri has placed cave, spring, and sinkhole development in the context of a geologic framework. One major observation documented by geologic mapping is that dissolution generally takes place just beneath sandstone horizons in the Upper Cambrian and Lower Ordovician dolomite of this region. Although rocks of the Ozark region have systematic and pervasive vertical joints, bedding planes and sequences of impermeable and soluble rocks control the development of conduits. Ninety-five percent of caves in the study area occur beneath sandstone in the Eminence Dolomite, the Gunter Sandstone Member of the Gasconade Dolomite, and sandstone of the Roubidoux Formation. Also, the major sinkhole plains in the Salem Plateau occur in the lower part of the Roubidoux where sinkholes are rimmed with sandstone that has collapsed into voids in the Gasconade Dolomite below. Recent cave diving by the Ozark Cave Diving Alliance into Alley Spring, a large (130 cfs) spring along the Jacks Fork in the Ozark National Scenic Riverways, shows that although the spring discharges from the middle part of the Gasconade in the river valley, the cave passage descends to just beneath the Gunter Sandstone Member where the cave levels out. Artesian ground-water conditions create the upward movement of ground water from beneath the sandstones to the large springs that this area of the Ozarks is noted for. We hypothesize that the 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 dolomite beds.