Factors Controlling the Development and Maintenance of Fault Seals in Heterogeneous Sedimentary Rocks: A Case Study from the Moab Fault, Utah

Solum, John G.¹, Nicholas C. Davatzes², David A. Lockner² (1) Sam Houston State University, Huntsville, TX (2) U.S. Geological Survey, Menlo Park, CA

The Moab normal fault, located in central eastern Utah, cuts a heterogeneous sequence of siliciclastics. This study examines the lithologic, mineralogic, and mechanical properties at the point of maximum throw to quantify the impact of authigenesis on fault zone evolution. The fault consists of two zones of clay gouge, with 60 and 900 m of throw respectively, bounded by shale smear and fractured sandstone grading into undeformed protolith. The heterogeneous protolith has coefficient of friction, µ, ~0.4-0.6 (all values of µ are with distilled water as a pore fluid). Shale smear is dominated by protolithic clays but also contains faulting-related illite-smectite resulting in µ ~0.4-0.5. Gouge from the minor fault contains significant neoformed smectitic clays resulting in µ ~0.25-0.43. Gouge in the major zone is largely composed of neoformed illite resulting in µ ~0.35-0.4. Clay neoformation suggests significant fluid-rock interaction throughout the fault zone. However, bleaching patterns indicate that reducing fluids (related to hydrocarbons) were largely confined to fractured sandstone between the two gouge zones. These results indicate spatiotemporal variations in fluid chemistry and fault zone permeability structure. In addition to reductions in mu and permeability resulting from clay growth, the slip behavior of smectites is sensitive to fluid chemistry. The coefficient of friction varies by a factor of ~2 between a K-rich brine and distilled water and the rate dependence varies from neutral to slightly negative suggesting a transition from creep in fresher water to stick slip in brine. Persistent seals in fault zones depend on clays and these results demonstrate these clays evolve due to strain, neomineralization, permeability structure, and fluid chemistry.