Geophysical and Hydrological Characteristics of the Leona Formation (Pleistocene): East Bexar County, Texas

The Leona Formation in east Bexar County, Texas consists primarily of shallow subsurface fluvial gravels which are Pleistocene in age. Situated along the Balcones Escarpment these gravel deposits are the result erosion of into the Cretaceous carbonates causing debris from the Texas Hill Country to move basin ward off the escarpment into isolated braided stream-like systems. Because of the high permeability and porosity, the Leona Formation is both a local water resource for farmers in the east Bexar County area, and a gravel resource. Using a combination of electromagnetic (conductivity) and resistivity surveys, a 250 acre area on the border of Guadalupe County and Bexar County was mapped to determine both the lateral and vertical extent of the gravel as well as the potential for new water and gravel resources. The electromagnetic survey was used over the entire study area using the instrument's ability to map relatively quickly. Resistivity surveys and excavations of the gravel were used to confirm the electromagnetic data. Gravel, and non-gravel areas were determined from the electromagnetic data and confirmed with resistivity data, while the physical lithological data was used to correlate the geophysical data. With the combination of these three sources of data an accurate map was produced to determine where possible gravel resources are located, as well as the feasibility of using such resources. From the suite of data collected it was determined that the Leona Formation is likely a gravel dominated braided river. The Leona Formation may be part of a larger fluvial fan system.