HYPOGENE PROCESSES IN THE EVOLUTION OF THE KARSTIC EDWARDS PLATEAU AQUIFER, TEXAS

Hypogene caves occur throughout much of the Edwards Plateau. Most, and the best examples, are located in the Central Edwards Plateau Karst Region and occur primarily over the Canyon Sandstones natural gas fields situated mostly beneath Crockett and Sutton counties, Texas. Mineralogical evidence in Caverns of Sonora suggests that rising gases or fluids from the Canyon Sandstones may have contributed to the development of at least that cave and possibly others. Additionally, this region coincides with the area where the Del Rio Clay was not deposited. The absence of the Del Rio allowed upward leakage of hypogenic water, creating ground­water circulation patterns that allowed the development of hypogenic caves by water recharged in the western portion of the Edwards Plateau. While the Del Rio Clay is present south of the Sonora area around the Carta Valley Fault Zone, that area’s deep and extensive faulting has promoted hypogenic cave development, including the three most complicated maze caves in Texas.

Karst aquifer development of the Edwards Plateau oc­curred in three main stages:

1. Stage 1: pre-Miocene to Early Miocene. A period prior to Balcones Faulting when the Edwards Limestone was bur­ied under confining units and with minor development of secondary permeability, primarily along its western margin where the Edwards was first exposed.
2. Stage 2: Middle Miocene to Late Miocene. Following Balcones Faulting, groundwater circulation and cave de­velopment began along the Balcones Escarpment, in the western part of the plateau, and by hypogene processes in the central part of the plateau.
3. Stage 3: Late Miocene to the present. Change from hypo­gene to epigene speleogenesis, accompanied by collapse and speleothem development.

While some parts of the plateau show clear evidence of hypogene development, others that show phreatic morpholo­gies are not conclusively hypogenic. Additional work is needed to better relate morphologic features to mode of origin, and to refine the definition of hypogene speleogenesis.