Abstract: Geology of the Balcones Fault Zone Along the Growth Corridor of San Antonio, Boerne, Wimberley, and New Braunfels, South-Central Texas
Edward W. Collins
Geologic maps, scale 1:24,000, were prepared for a 1250-mi2 region that is within a complex part of the Balcones fault zone and is part of the recharge zone of the Edwards aquifer. This area is undergoing rapid urban growth; therefore, the geology of the region is important to geologists and other professionals involved in planning land use, designing construction projects, and studying the Edwards aquifer.
The Balcones fault zone marks the northwest edge of the Texas coastal plain and is at the southeast limit of Cretaceous outcrops that are part of the recharge zone of the karstic Edwards aquifer. The >20-mi-wide normal fault zone is composed of en echelon fault strands that mostly strike N4-70°E and dip southeastward. Fewer faults dip northwestward. Subsidiary faults strike northwestward, northward, and eastward. Large individual fault strands are commonly between 6 and 16
mi long. Maximum fault throw ranges between ~150 and 600 ft, although most faults have between 10 and 150 ft of throw. Composite stratigraphic displacement across the fault zone is 1600 to 1800 ft. Local areas within the zone are intensely fractured by well-interconnected small faults (<6 ft of throw) and joints. Faults and joints have at least partly influenced the development of karst features.
Cretaceous limestone, marl, and shale crop out along the fault zone and represent >2000 ft of shelf deposition. Northwest of the Balcones escarpment, a prominent fault-line scarp of the fault zone, the outcrop belt mostly consists of cyclic, shallow subtidal to tidal flat limestones of the 650-ft-thick Glen Rose Limestone and the ~550-ft-thick Edwards Group. Southeast of the escarpment, rarely exposed shelf marls, argillaceous limestones, and shale/clay of the Taylor Group make up much of the Cretaceous outcrop belt that is locally covered by Quaternary sand and gravel of the Leona Formation and terraces of main drainageways.
AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994