Speleogenesis in Comal County, Texas
Beck, Barry Frederick
Wilson, James Lee
Master of Arts
Caves in Corral County, Texas, were examined with regard to their lithologic and geomorphic setting, paying particular attention to controls such as stratigraphic variation, topography, jointing, and ground-water flow. In the light of these data, a new classification of caves is proposed based upon the mode of water flow involved in their formation. Influent caves are formed by water flowing from the surface to the ground-water table. Effluent caves are formed by water flowing from the ground-water reservoir to the surface. Conduit caves are formed principally by phreatic flow with little, or no, surface relationship. This classification is extremely useful in identifying the factors involved in speleogenesis when used in conjunction with areal maps of the aforementioned controls. Effluent caves in the lower Glen Rose Formation are localized within a massive, fossiliferous aquifer and oriented generally down-dip, thus substantiating Gardner's (1935) theory of speleogenesis. Influent caves in both the upper and lower Glen Rose Formation are developed in areas with low surface gradient and consequently high infiltration. They develop vertically until the water reaches a suitable calcareous stratum which conducts it away laterally. A later change of conditions may cause further deepening and a series of pits and passages may develop. Conduit caves in the upper Glen Rose Formation appear to be localized within the more calcareous strata because of the high solubility of these layers in contrast to the shales, marls, and dolomites composing the major part of the section. Secondary collapse of the overlying less soluble strata may provide access for additional inflow, but in most cases this can be eliminated as the primary cause of cavern development. Vadose inflow and the mixing effect which Thrailkill (1968) has postulated as causes of increased solution do not seem responsible either. All the conduit caves examined in detail, however, occur in areas of ground-water convergence. It is hypothesized that the abnormally high flow rate and volume due to this convergence is responsible for their development, thus supporting the mechanism proposed by Swinnerton (1932) and Davies (1957). A cursory examination of caves in the Edwards Limestone reveals that effluent caves may also be caused by the overflowing of a perched aquifer into surface valleys. In the Edwards Limestone it also appears that conduit caves may be localized in a horizon which has high permeability, even within a compositionally homogeneous section.