The impact of syn-faulting porosity reduction on damage zone architecture in porous sandstone: an outcrop example from the Moab Fault, Utah

Deformation structures in the Jurassic Moab Member of the Entrada Sandstone have been studied in the Courthouse area where two major fault segments (Segments A and B) of the Moab Fault are connected. Field data show that Segment A developed from an early stage of (thick) deformation band formation and that distinctively thinner deformation bands and fractures were subsequently added to its damage zone at a later stage. Only the second stage is expressed along Segment B. Geometric and kinematic evidence indicates that Segment B linked with Segment A at the time when Segment A (and its thick deformation bands) was already present in the Courthouse area. We attribute the transition from thick deformation bands to thin deformation bands to pore-space reduction caused by syn-faulting quartz dissolution and precipitation that changed the mechanical properties of the rock. In this model, thin deformation bands formed as porosity was reduced during quartz diagenesis. The observations underscore the importance of syn-kinematic diagenetic changes and the variation in small-scale structures along faults that apparently formed during the same faulting event in porous sandstones.