MAGNETIC GEOPHYSICAL APPLICATIONS REVEAL IGNEOUS ROCKS AND GEOLOGIC STRUCTURES IN THE EDWARDS AQUIFER, TEXAS

High-resolution aeromagnetic surveys were completed over the Edwards aquifer in Uvalde and Medina Counties west of San Antonio, Texas. These surveys have provided new information on the geology and structure of one of the most productive and permeable carbonate aquifers in the United States. A regional scale fixed-wing survey, flown in 2001, revealed the widespread occurance of shallow igneous rocks. Geophysical interpretations show many of the magnetic anomalies to be vertical or subvertical volcanic pipes. Other shallow anomalies are interpreted as sills, lava lakes and pyroclastic flows. The absence of dikes and dike-like structures leads to the hypothesis that the emplaced volcanic rocks affect ground water flow locally, but not significantly on a regional scale. The interpreted intrusive boundaries and geometry can be used in regional hydrologic models to evaluate their influence on ground water flow. Deeper seated anomalies are interpreted as magmatic reservoirs that perhaps served as sources for the late-Cretaceous volcanism. A small scale very high resolution magnetic data set was acquired in 2003 as part of a helicopter electromagnetic survey of the North Seco Creek study area, which is outside the main Uvalde volcanic field. In addition to a single small volcanic pipe, this data set reveals the trace of the Woodard Cave fault, a major normal fault juxtaposing the rocks of the Trinity Group, comprising the upper Trinity aquifer to the north, with the Devils River Formation, constituting the Edwards aquifer to the south. This important finding, that a fault between adjoining limestone units is associated with a linear magnetic low, led to a re-examination of the fixed-wing aeromagnetic data. Through careful microleveling, filtering and image enhancement techniques, we see that major faults of the Balcones fault zone are associated with vestigial magnetic lineaments on a regional scale.