From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

39th Annual Meeting, South-Central Section of the Geological Society of America, San Antonio, Texas, April 1-2, 2005

Structural Characteristics of the Balcones Fault Zone and Edwards Aquifer within the Austin-San Antonio-Del Rio Region of Central Texas

Edward Collins

Abstract

Maps, cross sections, and geologic data of the Balcones Fault Zone will be presented to demonstrate similarities and differences in the structural characteristics affecting strata composing the Edwards aquifer segments. Between Uvalde and Austin, large normal faults (throws of 300 to 800 ft) are main structural features that control the aquifer’s structural position. The prolific San Antonio segment contains a network of large and small faults and fault blocks and structural relay ramps between en echelon fault tips. Between 1,000 and 1,900 ft of composite structural relief exist across the San Antonio aquifer. In contrast, the prominent structural element of the northern aquifer segment is the large Mount Bonnell fault. The Mount Bonnell fault bounds the western edge of the aquifer near the Colorado River, but northward this fault cuts across the aquifer outcrop belt and marks the recharge zone’s eastern margin. Fault intensity and the composite structural relief, 1,600 to 600 ft, decreases northward across the northern segment. South of the Colorado River, the Mount Bonnell fault also marks the western edge of the Barton Springs aquifer segment that contains numerous large- to small-scale fault blocks. Balcones faults cutting Edwards Group strata within the southern Edwards Plateau aquifer near Del Rio display relatively small displacements and do not significantly control the aquifer’s structural position. Throughout the study area, faults are commonly surrounded by zones of breccia and highly fractured strata. These zones are wider for faults with larger throws. Within some zones dissolution has caused open fractures, vuggy areas, and small caves. However, at some locations breccia and highly fractured strata contain clay-rich matrix and fracture-fillings. Fault patterns throughout the Balcones Zone suggest that fault attributes, such as intensity and geometry, differ within different strata intervals, probably because of rock property differences. Steeply dipping clay-rich strata adjacent to limestone fault breccia associated with a large fault in the northern aquifer present a possible example of the potential sealing and smearing effects of faulted clay-rich Upper Cretaceous deposits.