Scale, rates, and timing of fracture-related fluid flow in the Miocene Monterey Formation, coastal California

tructural characteristics and the mineralogic and isotopic composition of veins provide insight into the evolution of basinal fluid flow within coastal sections of the Miocene Monterey Formation. Based on cement composition and structural context, up to four vein generations can be distinguished. The heavy carbon isotopic composition of dolomicrite veins suggests formation at shallow burial during microbial methanogenesis. Subsequent quartz and baroque dolomite veins seem to correlate with opal CT to quartz transformation and dolomite recrystallization within the country rock at deep burial. The last generation of veins is most extensively developed adjacent to mesoscale faults. Unlike earlier veins, these veins formed during and after regional folding and contain abundant hydrocarbon inclusions, suggesting formation during hydrocarbon migration. The timing of extensive regional folding limits the formation of fault-related veins to within the last 1 m.y. The oxygen isotopic and fluid inclusion composition of vein cement, in combination with the composition of analyzed, present-day, formation fluid, indicates that fault-related veins precipitated from diagenetically altered connate water expelled during sediment compaction and hydrocarbon maturation.;Fault-related veins contain large volumes of carbonate cement, calcite at Arroyo Burro Beach and dolomite at Jalama Beach. The strontium isotopic composition of vein carbonate at both locations is consistently less than the surrounding host rock and suggests upward fluid movement by as much as 500-700 m, with a source still within the formation. Estimates of fluid volume necessary to precipitate the observed cement volume give a minimum distance of formation-parallel fluid flow of 3-10 km for Jalama Beach. Fluid flow is thus effectively focused toward faults in the Monterey Formation which provide effective pathways for upward fluid flow.;Rates of fluid flow within fault-related fractures at Jalama Beach can be estimated based on (1) the size of rock fragments apparently transported in moving fluid, (2) bifurcation of veins splaying off the fault, and (3) a thermal anomaly adjacent to the fault, inferred from the oxygen isotopic composition of dolomite vein cement and fluid inclusion micro-thermometry. The size of entrained rock fragments and vein bifurcation provide minimum estimates of peak flow velocity on the order of 0.1-10 m/s. The thermal anomaly, on the other hand, suggests a maximum average flow velocity of...