Isotopic Analyses of a new Cold Seep locality, Guenoc Ranch, Great Valley Group, Northern California

A new Mesozoic 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 scattered outcrops occur over 4 km, of which one petroliferous deposit (extending across 1.8 km) has been analyzed in detail. Petrographic and paleontologic observations indicate that this limestone unit formed via cold seep related processes during deposition of the GVG. Petrographic observations show clotted microbial and detrital micrite, pyritic corrosion surfaces, burrows, foraminifers, wood fragments, as well as fibrous, yellow, and sparry calcite; all of which are components typical of hydrocarbon seep deposits in the GVG. Lucinid and solemyid bivalves are also present, as are both large and small worm tubes, gastropods, and a Campanian (?) ammonite. The petrographic and paleontologic analyses strongly suggest that the Guenoc material resulted from venting methane. Carbon and oxygen isotope analyses are also being performed on the various micrites and cements seen in the Guenoc thin sections to add credence to the cold seep origin. Preliminary isotopic values for carbon and oxygen fall into the known range for cold seeps found within the GVG and worldwide. Generally, δ ¹³C values have been found to vary from seep to seep, ranging anywhere from -110 per mil to -20 per mil. Assuming no post-depositional alteration has occurred, values heavier than -20 per mil can still indicate a partial influence of venting methane on the carbon being incorporated into the carbonate. If post-depositional alteration has occurred however, the results of the isotopic data are inconclusive until further analyses are performed to ascertain the degree of alteration.