Structural Framework of the Edwards Aquifer, Balcones Fault Zone, Central Texas
Edwards limestone aquifer and recharge zone lies within the complex, regionally extensive Balcones Fault Zone. Knowledge of the aquifer's structural framework is important to studies of aquifer recharge and ground-water flow because faults control the aquifer's structural position. Some large faults within the aquifer probably act as barriers to ground-water flow while other faults and joints associated with the fault zone form local and regional ground-water conduits. The aquifer study area extends from San Marcos, Texas, 225 km (^sim140 mi) west-south-westward to Bracketville, Texas. Aquifer strata range mostly between ^sim150 and ^sim240 m (^sim500 and ^sim800 ft) in thickness. Faults have caused as much as 565 m (1,850 ft) of cumulative structural relief across the aquifer where its maximum width is ^sim48 km (^sim30 mi). Most faults strike N40°-70°E and dip southeast, although some dip northwest. Subsidiary faults strike northwest, north, and east. Field mapping in the Edwards outcrop belt in Comal, Bexar, and eastern Medina Counties indicates that the fault zone consists of multiple large 3- to 11-km-wide (2- to 7-mi) fault blocks bounded by a long series of southeast-dipping, closely overlapping, en echelon large normal faults that have throws ranging between ^sim30 and 260 m (^sim100 and 850 ft). Smaller fault blocks lie within larger fault blocks, and many smaller faults having throws between <0.3 and 30 m (<1 and 100 ft) cut strata across the fault zone. Relay ramps exist between en echelon faults. Grabens having widths <1,220 m (<4,000 ft) are common. The few outcrops containing larger faults indicate that the curved and rough fault surfaces have dips between 60° and 85° and display striations parallel to subparallel to the fault dip. Map patterns of faults interpreted to be in the subsurface resemble those in the outcrop belt. Most of the mapped subsurface faults have throws <90 m (<300 ft), although some have throws >150 m (>500 ft).