Scale and Timing of Focused Fluid Flow along Faults in the Miocene Monterey Formation, California

Peter Eichhubl and James R. Boles

Fluid flow along faults in the Miocene Monterey Formation is indicated by extensive carbonate and quartz cementation along fault surfaces and spatially related veins. Faults exposed at coastal outcrops along the uplifted margins of the Santa Barbara and Santa Maria basins have estimated total displacements in the order of 100 m. Large volumes of fluids are indicated by up to 6 m cumulative thickness of cement across individual faults and accompanying veins. Oxygen isotopic and fluid inclusion data indicate temperatures of vein fill of up to 130°C. Based on cross-cutting relations with folds, faulting and vein cementation occurs during uplift of the basin flanks. Abundant hydrocarbon inclusions indicate that vein and fault cementation is concurrent with hydrocarbon migration. High rates of fluid flow in the order of centimeter to meter per second are inferred based on graded geopetal infill of breccia cavities indicative of fragment transport in suspension.

Large volumes of fluid, high temperatures, and high flow rates would be suggestive of a deep basinal source for the fluids. Yet, strontium isotopic ratios of fault related carbonate cements are only slightly lower than the sea water value corresponding to the stratigraphic age of the exposed sections, with a difference equal to a cross-stratigraphic transport distance of less than 100 m. Thus, the strontium isotopic composition indicates a local source of the carbonate rather than a distant source within deeper parts of the basin. We interpret this apparent discrepancy as a result of extreme focusing of fluid flow along faults and interaction with the wall rock.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California