STRAIN AND FRACTURES IN AN EXTENSIONAL RELAY RAMP: IMPLICATIONS FOR DETERMINING STRUCTURAL FRAMEWORK CONTROLS ON GROUNDWATER FLOW PATHWAYS IN THE EDWARDS AQUIFER, SOUTH-CENTRAL TEXAS

The Hallie Stillwell relay ramp links two NW trending normal faults in the Cretaceous Santa Elena Formation along the eastern margin of Big Bend National Park, Texas. The ramp was mapped using a Global Positioning System (GPS) to collect cutoff location data along the bounding faults and fracture data within the ramp itself. Analysis of the fault cutoffs and fracture systems enables computation of both cutoff-parallel and cutoff-perpendicular extensions within the relay ramp, for comparison with the orientations of extensional fractures within the ramp. This particular relay ramp accommodates all of the displacement transfer and lateral displacement-gradient deformation, with all strata outside the ramp displaced but remaining horizontal and not internally deformed by the lateral fault displacement gradients. Fractures within the relay ramp represent the extension accommodated by the ramp in response to the deformation gradients in the bounding faults. Extensional fractures indicate extension both parallel and perpendicular to the bounding faults. The resultant fracture porosity within the ramp is considerable and provides fracture permeability both parallel and perpendicular to the regional fault trend. The Santa Elena limestone is equivalent to the principal limestone members of the Edwards Aquifer in south-central Texas. One of the most productive carbonate aquifers in North America, the Edwards is the sole source of water for many communities including the city of San Antonio. Groundwater flow within the Edwards is primarily parallel to the regional fault trend (ENE), but locally crosses the regional trend at high angles. The en echelon Balcones fault system contains many relay ramp structures, but these are not as well exposed as those within the Santa Elena limestone of west Texas. Although the Edwards regional groundwater flow is ENE, parallel to the major fault trend, local variations in groundwater flow patterns are likely caused by fracture permeability analogous to that developed in the Hallie Stillwell ramp. Analysis of structures within the Hallie Stillwell ramp provides a conceptual and semi-quantitative model that can be applied to the less well-exposed Edwards aquifer.