Hydrocarbon Seeps of the Mesozoic Great Valley Group Forearc Strata, Northern and Central California

Throughout the Late Mesozoic (Late Jurassic to Latest Cretaceous), the western North American continental margin was a dynamically evolving sediment-fluid-tectonic-biotic system that consisted of an eastward migrating volcanic arc, a westward migrating subduction zone, and a forearc basin in between the two. Throughout this 84 m.y. period, clastic detritus from the surrounding Sierra Nevada and Klamath Mountain Ranges was deposited on the basin floor via deepwater turbidites. Today, these sediments are exposed throughout northern and central California and are collectively known as the Great Valley Group (GVG) forearc strata.

The GVG siliciclastics host a relatively great number (twenty-three sites) and temporal distribution of hydrocarbon seep carbonates deposited over an extended time interval that preserve rich chemosynthetic macrofaunal communities. Eleven of the twenty-three localities have been studied in detail. The outcrops range in size from a few to hundreds of meters across and have variable exposure as discrete lenses eroding out of hillsides or creek beds, as small to medium mounds, and as extensive and continuous in situ outcrops. Petrographic analyses show characteristic hydrocarbon seep fabrics and cements, including clotted, microbial and detrital-rich micrite, pyritic corrosion surfaces, and peloids, as well as fibrous, yellow, and sparry calcite. The d13C and d18O values range from -28.6‰ to -9.3‰ PDB and -9.0‰ to 0.12‰ PDB, respectively. Taxon diversity includes rhynochonellide brachiopods (Peregrinella whitneyi), worm tubes, and gastropods (Lithomphalus enderlini, Turbo morganensis), as well as pectinid (Pecten complexicosta), inoceramid, and buchiid bivalves (Buchia okensis), all of which are characteristic seep taxa.

The GVG hydrocarbon seep deposits contain dense aggregations of macrofossils in isolated carbonate exposures in northern and central California. In comparison to other documented seep localities, they are unparalleled worldwide and thus, these exposures provide a singular window into the temporal and spatial evolution of hydrocarbon seep faunas and their associated geochemical signatures.