Ancient hydrocarbon seeps from the Mesozoic convergent margin of California: carbonate geochemistry, fluids and palaeoenvironments

Abstract

More than a dozen hydrocarbon seep-carbonate occurrences in late Jurassic to late Cretaceous forearc and accretionary prism strata, western California, accumulated in turbidite/fault-hosted or serpentine diapir-related settings. Three sites, Paskenta, Cold Fork of Cottonwood Creek and Wilbur Springs, were analyzed for their petrographic, geochemical and palaeoecological attributes, and each showed a three-stage development that recorded the evolution of fluids. through reducing-oxidizing-reducing conditions. The first stage constituted diffusive, reduced fluid seepage (CH4, H2S) through seafloor sediments, as indicated by Fe-rich cletrital micrite, corroded surfaces encrusted with framboidal pyrite, anhedral yellow calcite and negative cement stable isotopic signatures (delta(13)C as low as -35.5parts per thousand PDB; delta(18)O as low as -10.8parts per thousand. PDB). Mega-invertebrates, adapted to reduced conditions and/or bacterial chemosymbiosis, colonized the sites during this earliest period of fluid seepage. A second, early stage of centralized venting at the seafloor followed, which was coincident with hydrocarbon migration, as evidenced by nonluminescent fibrous cements with delta(13)C values as low as -43.7%. PDB, elevated delta(18)O (up to +2.3%. PDB), petroleum inclusions, marine borings and lack of pyrite. Throughout these early phases of hydrocarbon seepage, microbial sediments were preserved as layered and clotted, nondetrital micrites. A final late-stage of development marked a return to reducing conditions during burial diagenesis, as implied by pore-associated Mn-rich cement phases with bright cathodoluminescent patterns, and negative delta(18)O signatures (as low as -14parts per thousand PDB). These recurring patterns among sites highlight similarities in the hydrogeological evolution of the Mesozoic convergent margin of California, which influenced local geochemical conditions and organism responses. A comparison of stable carbon and oxygen isotopic data for 33 globally distributed seep-carbonates, ranging in age from Devonian to Recent, delineated three groupings that reflect variable fluid input, different tectono-sedimentary regimes and time-temperature-dependent burial diagenesis.