CRETACEOUS VOLCANIC INTRUSIVES IN THE EDWARDS AQUIFER, TEXAS AS IDENTIFIED FROM A HIGH-RESOLUTION AEROMAGNETIC SURVEY

The U.S. Geological Survey and the San Antonio Water System contracted a high-resolution aeromagnetic survey in March 2001 over Uvalde and Medina counties, Texas, to map geologic features and delineate igneous intrusives. The survey was flown approximately 167 m above terrain, along east-west flight lines spaced 400 m apart with north-south tie lines at 4000 m, encompassing approximately 5600 square kilometers. The survey covers the Edwards-Trinity aquifer on the southern edge of the Edwards Plateau. This aquifer serves the domestic, industrial, and agricultural needs of 2 million people and is the sole source for the city of San Antonio. Previous geologic mapping has identified Cretaceous intrusive rocks in the southwestern portion of the Edwards Group limestone aquifer in Uvalde county. The magnetic data indicate extensive volcanic swarms that have not been previously mapped and were not seen in widely-spaced, vintage (c. 1976) National Uranium Resource Evaluation (NURE) aeromagnetic data. These volcanic rocks, in addition to extensional block faulting, may influence regional ground water flow paths in the aquifer system. Preliminary reduced-to-pole total field intensity maps are shown and discussed, along with audio-magnetotelluric (AMT) data collected in follow-up ground surveys.