in fault rocks is a useful indicator in determining: Fault rock composition and mechanical and hydrologic fault zone properties Paleoflow migration in the Moab Fault Fault seal prediction Mechanical and hydrologic properties The presence of clays in fault rock influences both the mechanical and hydrologic properties of clay-bearing faults. Field mapping shows that layers of clay gouge and shale smear are common along the Moab Fault. Solum et al. describe the occurrence of clay-rich fault along four fault zones: R191, Corral Canyon, Courthouse Canyon, and the Bartlett Wash exposure. While fault rocks along the R191 location and Bartlett Wash are enriched in clays relative to protolith, those at Corral Canyon and Courthouse Canyon show little difference in composition relative to protolith, suggesting that fault rock formation at the latter locations is governed by mechanical as opposed to authigenic processes. Paleoflow migration Many studies have documented multiple episodes of fluid flow along the Moab Fault since its formation in the Permo-Triassic. In one study, Eichhubl et al. use the distribution of calcite cement as an indicator of paleofluid migration. They infer that fault-parallel fluid flow was focused along fault segments overprinted by joints and sheared joints. These conclusions support Chan et al.’s hydrogeologic model, which proposes that hydrocarbon and basin brines from Pennsylvanian source rocks migrated along the Moab fault, moving into the porous sandstone units where they interacted with oxygenated meteoric water. Chan et al. use Ar-Ar dating to define the age of this fluid movement. They determine a mineralization age of c. 25-20 Ma, which coincides with episodic uplift of the Colorado Plateau or with La Sal Mountains volcanism. The Moab Fault is characterized by patterns of iron oxide reduction that also suggest the fault acted as a conduit for mineralizing fluids. It is suggested that red sandstones adjacent to the Moab Fault were bleached by reducing fluids. Iron oxide reduction is concentrated within high permeability, well-connected sandstones and spatially associated with cemented veins – indicating that the reduction event coincided with vein formation, and thus with the final stages of faulting. Permeability along the fault may have been promoted by the highly anisotropic shaley gouge fabrics or by fault zone fractures. After analyzing geochemical data of carbonate cements and iron-oxide reduced sandstones from the Moab Anticline, Garden et al. suggest that following fault movement, the fault zone was the locus of vertical migration of over-pressured hydrocarbon and aqueous,