Regional Distribution of Fractures in the Southern Edwards Plateau and Their Relationship to Tectonics and Caves

In order to better understand the control exerted by fractures on the recharge and production zones of limestone aquifers, the authors delineated surficial fracture zones in the southern Edwards Plateau, Texas. Lineations were marked on nearly 200 mosaics scale of 1:24,000. Approximately 400 fracture zones were identified on each mosaic. Moving averages of the density and orientation of fracture zones were computed in order to map (1) number of fractures, (2) length of fracturing, (3) distribution of fracture intersections, (4) weighted arithmetic mean of fracture orientation in northeast and northwest quadrants, and (5) standard deviation of the means. All were calculated per unit area. In addition, rose diagrams were computed for selected areas, generally a standard 7.5-minute quadrangle. The method introduced bias in that (1) fracture zones were identified with greater difficulty at the margins of mosaics, and (2) more north-south and east-west fracture zones were observed in field measurements than in aerial photographic interpretations. The fractures appear controlled both by an old system of basement fracturing and by the trend of the relatively young Balcones fault system. The basement system contains prominent northeastward and northwestward-trending sets, and north-south and east-west-trending subsets. Fractures related to the Balcones fault zone trend east-west in the western Edwards Plateau and generally approach north in the eastern plateau. Cave passageways in the western plateau appear to parallel fracture zones trending northwestward, northeastward, and westward, whereas abundant cave corridors of eastern caves commonly align with the principal faults of the Balcones fault zone.