STRUCTURAL GEOLOGY AND TECTONIC ANALYSIS OF THE BALCONES FAULT ZONE IN THE UPPER SECO CREEK AREA, EDWARDS AQUIFER, SOUTH-CENTRAL TEXAS

The Edwards aquifer of south-central Texas lies within and adjacent to the Balcones fault zone (BFZ) and is considered one of the most productive carbonate aquifers in the U.S. The Edwards aquifer in the upper Seco Creek area is largely contained within the Lower Cretaceous Devils River Formation which consists of approximately 600 feet of highly porous and permeable limestone and dolomite. Geologic mapping in the Seco Creek area in northern Uvalde and Medina Counties revealed a number of major faults that parallel the NE-SW trend of the Balcones fault system. Recent geologic mapping efforts and a helicopter electromagnetic (HEM) survey for parts of the Seco Creek area revealed several unmapped faults that also trend NE-SW in addition to several major cross-faults.

Miocene age faults within the BFZ are en echelon, exhibit primarily normal displacement, and trend northeast with downthrow to the southeast. Numerous smaller cross-faults trending to the NW-SE connect the larger primary faults creating a sizeable, highly fragmented, SW-dipping relay ramp. Continued displacement and subsequent deformation of the ramp-bounding hanging wall and footwall cutoffs generate cross-faults trending at high angles to the overlapping faults, which breach the relay ramp and link the major faults. Because intact relay ramps can provide ground-water flow pathways between faults, the intrablock deformation and faulting associated with these ramps in a carbonate aquifer, such as the Edwards, can result in increased fracture porosity and directional permeability within the relay ramp and may provide zones of enhanced flow across the regional NE-SW trend.