Heterogeneous Deformation in an Extensional Fault-Tip Monocline: Insights from Field Analysis

David A. Ferrill, Alan P. Morris, and Kevin J. Smart
Southwest Research Institute, San Antonio, TX

Analog modeling and field studies have shown that mechanical stratigraphy strongly influences the style of deformation associated with normal-fault propagation. In a heterolithic stratigraphic sequence, fault propagation is inhibited through weak layers causing monoclinal folding beyond fault tips. Understanding deformation associated with extensional monoclines is important to interpreting or predicting sub-seismic scale deformation associated with intact and faulted monoclines in reservoir strata. In this study, we analyze a fault-tip monocline developed in Cretaceous limestone and shale above a steep (80°) normal fault with 30 m displacement exposed at Big Brushy Canyon in the Sierra Del Carmen of west Texas. Bedding dip within the monocline reaches a maximum of 59°. In this structure, fault offset of a thick strong limestone is overlain by a monocline developed in weak clay-rich shale (~36 m thick but locally structurally attenuated to 1 m), and a limestone unit (~30 m thick) that contains two relatively strong limestone interlayers 6 m and 2.7 m thick separated by nodular argillaceous limestone. Brittle deformation of the two thin limestone units is concentrated in the monocline limb, and consists of bedding-perpendicular extension fractures partially filled by 0.5 - 15 cm-thick calcite veins, and bedding-parallel slip surfaces that offset these veins with a consistent up-dip sense of shear. Fractures accommodated 2.5% layer-parallel extension. Shear offset and bed-parallel extension increase with bedding dip. Deformation is concentrated within the monocline limb and in the high-curvature hinge regions, indicating that curvature analysis alone is insufficient to identify areas of increased fracturing. Finite element modeling using mechanical stratigraphy, fault architecture, and boundary conditions simulating the Brushy Canyon monocline confirm the correlation of increased layer extension and up-dip bedding plane slip with increasing limb dip.