Focused fluid flow along faults in the Monterey Formation, coastal California

Fluid how in fractured siliceous mudstone of the Miocene Monterey Formation of California is inferred to be highly focused toward map-scale faults tha t locally contain extensive amounts of carbonate and minor silica cement. T he distance of cross-stratigraphic how, as inferred based on the strontium isotopic composition of carbonate fault cement, is close to the thickness o f the Monterey Formation of 700 m in one of two study locations, Jalama Bea ch, and less than the formation thickness at another location, Arroyo Burro Beach. Fluid is thus derived from within the Monterey Formation rather tha n from underlying older units. Based on mass-balance estimates of the fluid volume required for fault cementation at Jalama Beach, the minimum distanc e of formation-parallel flow into the fault zone is 4 km and possibly >12 k m, The inferred distance of flow parallel to the formation into this fault thus exceeds the distance of cross-formational upward how along the fault b y at least a factor of six, The mass-balance estimate requires that fluid f low along the fault is channeled into a pipe-shaped conduit rather than dis tributed along fault strike. Fluid flow from the surrounding formation into fault pipes is inferred to follow a radial rather than uni-or bilateral fl ow symmetry, using bedding-confined sets of extension fractures and stratab ound breccia bodies. Radial fluid flow toward fault pipes requires fairly i sotropic fracture permeability for flow along bedding and a low permeabilit y across bedding. The inferred flow geometry illustrates the combined effec t of fault permeability structure, permeability anisotropy of the surroundi ng formation, and hydraulic head distribution in controlling basinal fluid flow in faulted sequences.