Overprinting faulting mechanisms in high porosity sandstones of SE Utah

Normal faults in sandstone of the Jurassic Entrada Formation, South-East Utah formed by two mechanisms: (1) deformation band faulting overprinted by (2) jointing and subsequent shearing along joints. Fundamental structural elements of deformation band faults are single deformation bands, zones of deformation bands, and slip surfaces. Joint-based faults are composed of joints, sheared joints, splay fractures, fragmentation zones, breccia, and fine-grained fault rock. We demonstrate that both mechanisms contribute to slip in a single fault zone with joint-based faulting consistently postdating deformation band faulting at a given location along a fault. The occurrence, distribution, and geometric arrangement of structures formed by the two mechanisms resulted in faults with distinct fault architecture. This fault architecture is related to the relative contributions of each deformation mechanism to the total offset and to their relative timing. Overprinting of a deformation band-based fault by a joint-based mechanism introduces extensive localized structural heterogeneity with a distinct hydraulic signature. Whereas deformation bands tend to act as fluid baffles, joints may act as preferred fluid conduits. Therefore, fluid flow properties such as the permeability of the faults with overlapping mechanisms are expected to change over time accompanying the overprinting process.