IMPLICATIONS FROM THE DISTRIBUTION OF INTRUSIVE IGNEOUS ROCKS FOR GROUND WATER DEVELOPMENT AND FLOW IN THE CENTRAL AND WESTERN EDWARDS AQUIFER, TEXAS

High-resolution airborne magnetic (HRAM) data flown over Medina and Uvalde counties, Texas, have been processed by spatial filtering methods and automated modeling techniques. The area encompasses parts of the central and western storage units of the Edwards Aquifer, where ground water flow reverses from westerly to easterly. The HRAM processing has defined the location of near surface volcanic rocks, which are important in the placement, and development of local water wells. Ground gravity and electrical surveys have been used to provide additional subsurface detail of selected shallow intrusives. Previous K-Ar age determinations on these shallow volcanic rocks provided ages between 60 and 90 Ma (Late Cretaceous). Preliminary ⁴⁰Ar/³⁹Ar ages provided a more precise timing of 74-73 Ma (phonolites) and 80 Ma (alkali basalts and olivine nephelinites). Highly accurate dates help time the emplacement of the instrusives relative to the development of the Edwards Aquifer system. Currently, nepheline, analcite and alunite (all K-bearing minerals) are being separated for ⁴⁰Ar/³⁹Ar geochronology to provide the overall timing of these intrusive volcanic rocks from the entire Medina and Uvalde counties. Spatial filtering of the HRAM has identified areas of deep volcanic rocks that are undoubtedly the source of the shallow pipes that constitute the near surface igneous swarm. The great number of pipes probably reduces the transmissivity of the Edwards Aquifer on a regional scale. Interpretive two- and three-dimensional visualizations of the shallow intrusives help to relate the importance of their distribution on groundwater flow.