GEOLOGIC AND HYDROGEOLOGIC CHARACTERIZATION OF GROUNDWATER RESOURCES IN THE EDWARDS GROUP, NORTH NOLAN CREEK REGION, BELL COUNTY, TEXAS

The Edwards and Comanche Peak formations comprise Lower Cretaceous strata of the Comanche Shelf that extends from southwest Texas up through central Texas into Bell County. The dominant reef structure terminates in southern Bell County, though isolated carbonate mounds extend farther to the north into central Bell and Coryell counties. The Edwards Group has been extensively studied in southern and central Texas in association with the Edwards Aquifer system; however, Edwards strata and its outliers to the north are poorly understood. This study focuses on those northern outliers in central Bell County, which range in composition from micritic mudstone to bioclastic, peloidal packstone facies with alternating sequences. Most ridges are capped by coarse-grained packstone facies dominated by rudistid reef complexes, while hill slopes are dominated by mudstone facies.

This northern region of Edwards strata is well known for its extensive karst development with integrated subsurface drainage systems including ephemeral and drought-resistant springs that have maintained flow throughout the drought of 2011. Individual karst features exhibit both epigene and hypogene morphologies that reflect long speleogenic evolutions that are currently being modified by surface denudation associated with entrenchment of Cow House Creek and Leon River. Geochemical mapping of spring discharge and geophysical mapping of shallow features is providing insight to the integrated fluid dynamics of the North Nolan Creek area. Geophysical studies including microgravity coupled with detailed gis analyses are being utilized to further constrain the spatial distribution of shallow karst systems. The geochemistry of individual springs indicates a shallow meteoric, epigene karst system that is currently active with variable flow paths related to differences in strata permeability. Using remote sensing and gis coupled with physical land surveys, the study area is being mapped in detail to better constrain the geology and speleogenesis of the northern Edwards Aquifer. Multidisciplinary studies such as this enable the extrapolation of detailed work to broader regions that have been previously unstudied.