FAULT GEOMETRY AND LITHOFACIE CONTROLS ON THE HYDROSTRATIGRAPHIC FRAMEWORK OF THE EDWARDS AQUIFER, SOUTH-CENTRAL TEXAS

The Edwards aquifer of south-central Texas, considered one of the most productive carbonate aquifers in the United States, lies within and adjacent to the Balcones fault zone. The lithostratigraphy of the aquifer varies from northeast to southwest because of three fluctuating depositional environments: the San Marcos Platform, the Devils River Trend, and the Maverick Basin. The Edwards Group rocks in the San Marcos Platform facies are characterized by the Kainer and Person Formations, which are subdivided into seven informal hydrostratigraphic units. The undivided Devils River Formation is comprised of reefal facies lithologies of the Devils River Trend facies, while to the west in the Maverick Basin, the water-bearing units of the Edwards aquifer are identified as the West Nueces, McKnight, and Salmon Peak Formations, which have also been subdivided into seven informal hydrostratigraphic units.

The Balcones fault zone, an en echelon network of Miocene-age normal faults, interacts with the variable lithologies of the region to compartmentalize the Edwards aquifer and influence the generation of preferential flow pathways in the ground-water system. Because of the crystalline nature of the host rock and the susceptibility of the carbonate strata to dissolution, the enhancement of secondary porosity and permeability along faults/fractures is common. The hydrologic significance of these faults is found in the eastern San Marcos platform where groundwater flows rapidly to the northeast, parallel to primary faults. Conversely, to the west in the Devils River Trend and Maverick Basin, flow pathways are quite circuitous and much slower, but eventually they align with the regional northeast ground-water flow. Recent ground-water tracer studies have demonstrated these contrasts in dominant flow pathways in the Edwards aquifer on a sub-regional scale, which are constrained by the litho-facies distribution and location within the Balcones fault zone. An improved understanding of the lithologic and stratigraphic controls on fault geometry and fault plane deformation provides the critical data needed to determine the multi-dimensional influence of complex faulting on preferential flow pathway development in the Edwards aquifer.