Geomorphology and hydrogeology of the Edwards Plateau karst, central Texas




Kastning, Ernst H.

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The Edwards Plateau is one of the largest continuous karstic regions of the United States; yet, its geomorphic evolution has previously received little systematic study. The objectives of this investigation are to (1) describe the physical characteristics of karstic features, (2) determine which geomorphic and hydrogeologic controls and processes have governed their development, and (3) relate genesis of karst spatially and chronologically to the geomorphic evolution of the Edwards Plateau. Caves provide a record of the evolution of karst and the development of major carbonate aquifers. Some controls on speleogenesis are pervasive; nevertheless, many caves exhibit characteristics that suggest a strong influence of local factors. Lithostratigraphic factors have influenced the development of chambers and passages. Solutional conduits were guided by variations in calcitic and dolomitic content, thickness of strata, and frequency of bedding-plane partings. This is particularly true of the Glen Rose Formation, limestone beds of the Edwards Group, and the Gorman Formation, the three principal cave-forming units. Caves were developed preferentially along fractures associated with regional structural elements, including the Devils River Uplift, Llano Uplift, San Marcos Arch, and Balcones Fault Zone. Locally, faults have modified passages. Evolution of flowpaths within major aquifers has governed the morphology, distribution, and orientation of caves. Patterns of eaves vary markedly and include isolated cavities, interconnected cavities, single conduits, branching conduits, and network, anastomotic, and spongework varieties of mazes. These suggest a diversity of hydrogeologic controls and, in many places, a strong influence of the evolving topography. Most caves of the plateau are relict features, abandoned by groundwater as dissection of the area progressed. They are situated well above floors of valleys and represent early stages of speleogenesis. However, where streams have incised deeply along the Balcones Escarpment and at some places in the interior of the plateau, caves are well integrated, at grade with surficial streams at baselevel, and characteristic of later stages of development. Climatic changes in central Texas during the Quaternary are reflected in siliclastic sediments and speleothems in caves. Ample evidence suggests that caves were severely flooded during periods of colder and wetter climates