CONTRASTING STYLES OF FAULT ZONE DEFORMATION IN LIMESTONES OF THE GLEN ROSE FORMATION, EDWARDS GROUP, AND BUDA LIMESTONE IN THE BALCONES FAULT SYSTEM, SOUTH-CENTRAL TEXAS

Normal faults in Cretaceous limestones along the Balcones fault system, south-central Texas, show contrasting styles of fault zone deformation. These styles are controlled by lithology, fluid content, and stress conditions at the time of faulting - all fundamental constraints on carbonate fault zones. Large planar faults with low displacement gradients are developed in massive, strong (clay-poor) Edwards Group limestones (Edwards aquifer). In the thinner-bedded and lithologically variable Glen Rose Formation limestones (Trinity aquifer), weak (clay-rich) beds impede fault propagation, resulting in fault-related folding, and steep bedding dips in fault damage zones. Smear of clay beds in fault zones decreases across-fault permeability.

Faults in clay-poor massive limestones tend to be steep (70°+) whereas weaker, clay-rich limestones develop less steep faults (°Ü60° dips). Faults cutting interlayered strong and weak limestones tend to have refracted profiles and variable fault zone thickness. Refracted fault profiles commonly form where low differential stress results in changes in failure mode (and angle) through the layered sequence. The Buda Limestone, which lies between shales of the Eagle Ford Formation above, and the Del Rio Formation below, exhibits fault-related folding accompanied by distributed faulting and fracturing before breakthrough of large (10's meters throw) faults.

Fault zones in Edwards limestone show evidence of cataclasis, cementation, deformation of cement by mechanical twinning and pressure solution, and multiple generations of cement with differing degrees of deformation. Evidence of synchronous fault-zone cementation and fault slip implies that these faults may have behaved as conduits for fluid movement after a slip event and prior to complete cementation, and barriers to fluid movement as cementation progressed. The Hidden Valley fault zone (10's meters of displacement in the Glen Rose Formation), exposed in the Canyon Lake Spillway gorge, is a locus of groundwater discharge, surface flow, and groundwater recharge at different locations on the fault zone. This is a well-exposed illustration of the complex hydrologic role of faults that cut mechanically heterogeneous stratigraphy.