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Fault zone deformation controlled by carbonate mechanical stratigraphy, Balcones fault system, Texas

¹ Geosciences and Engineering Division, Department of Earth, Material, and Planetary Sciences, Southwest Research Institute®, 6220 Culebra Road, San Antonio, Texas 78238; dferrill{at}swri.org
² Geosciences and Engineering Division, Department of Earth, Material, and Planetary Sciences, Southwest Research Institute®, 6220 Culebra Road, San Antonio, Texas 78238; amorris{at}swri.org

David Ferrill received his B.S. degree in geology from Georgia State University in 1984, his M.S. degree in geology from West Virginia University in 1987, and his Ph.D. in geology from the University of Alabama in 1991. He is a licensed professional geoscientist (geology) in the state of Texas. Before joining Southwest Research Institute in 1993, he was an exploration geologist at Shell Offshore Incorporated. David is now a director at Southwest Research Institute and performs analyses of faulting and fracturing and reservoir deformation and structural geological training and contract consulting for the oil and gas industry.

Alan Morris received his B.Sc. degree (honors) in geology from the Imperial College of Science and Technology in 1973 and his Ph.D. in geology from the University of Cambridge in 1980. He is a licensed professional geoscientist (geology) in the state of Texas. Before joining Southwest Research Institute in 2005, Alan was a full professor at the University of Texas at San Antonio, having been on the faculty for 22 years. Alan is now a staff scientist at Southwest Research Institute and focuses on quantitative analysis of deformation processes and stress in diverse tectonic regimes and conducts research and technical assistance projects for the oil industry.

Normal faults in Cretaceous carbonates in the Balcones fault system provide important analogs for fault zone architecture and deformation in carbonate reservoirs worldwide. Mechanical layering is a fundamental control on carbonate fault zones. Relatively planar faults with low-displacement gradients develop in massive, strong, clay-poor limestones and dolomites. In less competent clay-rich strata, shale beds impede fault propagation, resulting in fault-related folding, and locally steep bedding dips. Faults in clay-poor massive limestones and dolomites tend to be steep (70° or more), whereas weaker, clay-rich limestones develop faults with shallower dips (60° or less). Fault zone rocks 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 contemporaneous cementation and fault slip. In stratigraphic sequences consisting of both competent and incompetent strata, the ratio of incompetent to competent strata by thickness is a useful guide for inferring the relative rates of fault displacement and propagation. Low displacement-to-propagation ratios associated with competent strata generate low-displacement gradients, inhibiting fault-related folding. Conversely, high displacement-to-propagation ratios associated with incompetent strata promote high-displacement gradients and fault-related folding.

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