Vein Formation in Relation to Burial Diagenesis in the Miocene Monterey Formation, Arroyo Burro Beach, Santa Barbara, California. Peter Eichhubl1 and James R. Boles Department of Geological Sciences, University of California, Santa Barbara, CA 93106 1 presently at: Department of Geological and Environmental Sciences, Stanford University, CA 94305 ABSTRACT Four distinct generations of veins are observed at Arroyo Burro Beach, each representing repeated episodes of fracture opening and cementation. The first set of dolomicrite veins formed during early burial in the zone of microbial methanogenesis, based on the heavy d13C composition of dolomite cement. The second vein set, cemented by quartz, apparently accompanied opal CT dissolution and quartz precipitation in the siliceous dolostone host rock. The third vein generation, forming dolomite cemented breccias, correlates with recrystallization of dolomite pore cement in the host rock. The fourth set of calcite veins partly reactivates earlier vein generations, but is most extensively developed adjacent to mesoscale faults. Calcite veins are characterized by their clustered occurrence, with the formation of subparallel veins ultimately leading to host rock fragmentation and to breccia-filled veins. Carbonate dissolution in the host rock and the precipitation of carbonate vein cement may be related to organic matter diagenesis and incipient catagenesis leading to release of organic acids into the pore fluid. Enhanced diagenetic alteration of host rock fragments in all vein generations suggests that fractures served as catalysts for diagenetic reactions, presumably by opening the system to mass transfer and by increasing the water-rock ratio.