Abstract: Structural Framework of the Edwards Aquifer, Balcones Fault Zone, Central Texas

Edward W. Collins

The 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 groundwater flow because faults control the aquifer's structural position. Some large faults within the aquifer probably act as barriers to groundwater flow, and faults and joints associated with the fault zone form local and regional groundwater conduits. The study area extends from San Marcos, Texas, ~140 mi west-southwest to Bracketville, Texas.

Aquifer strata mostly range between ~500 and ~800 ft in thickness. Faults have caused as much as 1,850 ft of cumulative structural relief across the aquifer where its maximum width is ~30 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 major 2- to 7-mi-wide fault blocks bounded by long series of southeast-dipping, closely overlapping, en echelon large normal faults with throws between ~100 and 850 ft. Smaller fault blocks lie within larger fault blocks, and many smaller faults with throws between <1 ft and 100 ft cut strata across the fault zone. Relay ramps exist between en echelon faults, and graben with widths <4,000 ft are common. The few outcrops containing larger faults indicate that fault surfaces are curved and rough, 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 are similar to those in the outcrop belt. Most mapped subsurface faults have throws <300 ft, although some have throws <500 ft.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana