Fault-Zone Deformation Mechanisms in the Cretaceous Limestones of South Texas

David A. Ferrill¹ and Alan P. Morris^2
1 CNWRA, Southwest Research Institute, San Antonio, TX
² University of Texas San Antonio, San Antonio, TX

Normal faults exposed in Cretaceous (Glen Rose, Edwards, and Buda) limestones along the southeastern margin of the Edwards Plateau and in the Tertiary extensional province of West Texas, provide important analogs for fault zone architecture and fault zone deformation characteristics in carbonate reservoirs around the world. Mechanical layering, clay content, rock strength characteristics, and depth at the time of faulting are 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. Faults in clay-poor massive limestones tend to be steep (70° or steeper) whereas weaker, clay-rich limestones develop faults with shallower (60° or gentler) 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 contempoaneous with fault slip. Because the fault-zone cementation occurred contemporaneously with fault slip, the estimated "minus-cement" porosity does not reflect actual porosity of the fault zone at any stage in development. This implies that while active, these faults may have alternately behaved as (i) conduits for fluid movement after a slip event during cementation, and (ii) barriers to fluid movement after cementation was complete or nearly complete, prior to the next slip event.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005