ANALYSIS OF A NEW MESOZOIC COLD-SEEP LOCALITY, GREAT VALLEY GROUP, GUENOC RANCH, NORTHERN CALIFORNIA

A new Upper Cretaceous cold-seep limestone locality has been discovered in Great Valley Group (GVG) forearc strata of the Guenoc Ranch, bordering southeast Lake and northern Napa counties, northern California. Numerous boulder-strewn outcrops (representing ~6 centers of authigenic carbonate formation) occur over ~4 km, of which one petroliferous deposit (scattered across ~1.8 km) has been analyzed in detail. Initial petrographic observations show clotted microbial and detrital micrite, pyritic corrosion surfaces, burrows, foraminifers, wood fragments, as well as fibrous, yellow, and sparry calcite; all are components typical of hydrocarbon seep deposits in the GVG. Lucinid and solemyid bivalves also are present, as are both large and small worm tubes, gastropods, and a Campanian(?) ammonite. Carbon and oxygen isotopic analyses are underway to confirm a hydrocarbon-seep origin for the carbonates.

The hydrocarbon seeps of California’s GVG are now known to occur in great numbers (at least 17 major geographic occurrences over >700 km), and have an extensive temporal record (~70 m.y., Tithonian-Campanian) of fluid flow associated with subduction along the western North American convergent margin of the Eastern Pacific. The Guenoc Ranch, with its multiple individual seep outcrops over a small geographic area, indicates that there are likely hundred(s) of ancient seeps to be discovered in the GVG. Hence, with continued recognition of new localities like the Guenoc example, GVG cold seeps can be viewed collectively as a quasi-continuous, dynamic fluid-sediment-biotic system, with the potential to reveal larger evolutionary, biogeographic, geotectonic, and geochemical patterns of seep processes and associated faunas through geologic time.