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

Peter Eichhubl* and James R. Boles Department of Geological Sciences, University of California, Santa Barbara, CA 93106
* Presently at: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, and Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039

Fluid flow in fractured siliceous mudstone of the Miocene Monterey Formation is inferred to be highly focused toward map-scale faults that locally contain extensive amounts of carbonate and minor silica cement. The distance of cross-stratigraphic flow, as inferred based on the strontium isotopic composition of carbonate fault cement, is close to the thickness of the Monterey Formation of 700 m in one of two study locations, Jalama Beach, and less than the formation thickness at another location, Arroyo Burro Beach. Fluid is thus derived from within the Monterey Formation rather than from underlying older units. Based on mass balance estimates of the fluid volume required for fault cementation at Jalama Beach, the minimum distance of formation-parallel flow into the fault zone is 4 km and possibly >12 km. The inferred distance of flow parallel to the formation into this fault thus exceeds the distance of cross-formational upward flow along the fault by at least a factor of 6. The mass balance estimate requires that fluid flow along the fault is channeled into a pipe-shaped conduit rather than distributed along fault strike. Fluid flow from the surrounding formation into fault 'pipes' is inferred to follow a radial rather than (bi)lateral flow symmetry, using bedding-confined sets of extension fractures and stratabound breccia bodies. Radial fluid flow toward fault 'pipes' requires a fairly isotropic fracture permeability for flow along bedding and a low permeability across bedding. The inferred flow geometry illustrates the combined effect of fault permeability structure, permeability anisotropy of the surrounding formation, and hydraulic head distribution in controlling basinal fluid flow in faulted sequences.