CONTRASTING STYLES OF FAULT ZONE DEFORMATION IN LIMESTONES OF THE BALCONES FAULT SYSTEM, SOUTH-CENTRAL, TEXAS

Normal faults exposed in Cretaceous (Glen Rose, Edwards, and Buda) limestones along the Balcones fault system at the southeastern margin of the Edwards Plateau, south-central Texas, show contrasting styles of fault zone deformation. These styles are controlled by lithologic (mechanical) characteristics, fluid content, and stress conditions at the time of faulting - factors such as mechanical layering, clay content, rock strength characteristics, and depth at the time of faulting are all fundamental constraints on carbonate fault zones. Large planar faults with low displacement gradients are developed in massive, strong (clay-poor) Edwards Group limestones. In the more thinly bedded, lithologically variable, Glen Rose, weak (clay-rich) beds impede fault propagation, resulting in fault-related folding, and locally steep bedding dips in fault damage zones. Smear of clay beds within fault zones results in reduced across-fault permeability. Faults in clay-poor massive limestones tend to be steep (70° or steeper) whereas weaker, clay-rich limestones develop faults with shallower (£60°) dips. Faults cutting interlayered strong and weak limestones tend to have refracted profiles and substantial vertical variability in fault zone thickness. Refracted fault profiles have commonly formed at shallow depths where low differential stress results in variable failure angles due to changes in failure modes through the mechanically layered sequence.

Thin sections from a fault zone in the 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, indicating cementation was contemporaneous with fault slip. Because fault-zone cementation and fault slip were synchronous, the estimated "minus-cement" porosity does not reflect actual porosity of the fault zone at any stage in development. This implies that when active, these faults may have alternately behaved as (i) conduits for fluid movement after a slip event during and prior to complete cementation, and (ii) barriers to fluid movement after cementation was complete or nearly complete.