Abstract

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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