Resistivity Imaging and Natural Potential Applications to the Antioch Fault Zone in the Onion Creek/Barton Springs Segment of the Edwards Aquifer, Buda, Texas
Resistivity imaging and natural potential (NP) surveys were conducted in the vicin-ity of Antioch Cave, the largest recharge feature in the bed of Onion Creek within the Barton Springs segment of the Edwards Aquifer. Site-scale geologic mapping of the study area indicates a fault zone bound by two faults crossing Onion Creek: 1) the west-ern fault occurs within the Georgetown Formation, and 2) the eastern fault occurs within the Buda Limestone. Both faults show tilted bedding across a total fault-bounded zone with a width about 500 ft and about 100 ft of estimated throw. This geophysical work characterizes the geometry of the geologic units, the throw of the individual fault (s), and identifies possible karstic features within the fault zone. Two 1100 ft long resistivity and NP transects were run parallel to the north and south banks of Onion Creek and across the fault zone. Resistivity results show chaotic fault zone deformation in the vicinity of the western portion of the fault zone where the Georgetown Formation is juxtaposed against the Del Rio Clay with about 50 ft of throw. Resistivity results on the eastern fault indicate a discrete fault where the Del Rio Forma-tion is juxtaposed, with the Buda Limestone and about 40 ft of throw. The Del Rio is about 25 ft thick on the upthrown side and about 100 ft of apparent thickness on the downthrown side. The resistivity results from the southern bank of the creek also ap-pear to indicate that the Del Rio Formation terminates at about station 960 ft along the profile and at a depth of about 65 ft. Additional surface geological observations along Onion Creek also suggest the existence of an unmapped fault in that vicinity. NP results confirm karstic anomalies across Antioch Cave along the northern bank, where Antioch Cave is known to extend, and additional high and low NP anomalies across the Edwards units that may indicate additional karstic features at depth.