PALEOECOLOGICAL ANALYSIS OF MESOZOIC HYDROCARBON SEEP FAUNAL COMMUNITIES, GREAT VALLEY GROUP, NORTHERN CALIFORNIA, USA

The western North American continental margin evolved from the Late Jurassic through the Latest Cretaceous and consisted of an eastward migrating volcanic arc, a westward migrating subduction complex, and a forearc basin which opened in between the two. During this time, the forearc basin was the recipient of organic-rich sediment which is distributed today over 700 km and throughout 70 Ma of subduction history and is known as the Great Valley Group (GVG) forearc strata. The GVG forearc strata contains, at a minimum, ~30 individual outcrops of petroliferous and variably fossiliferous white limestone, some of which has already been identified as hydrocarbon seep limestone, although most remains unstudied.

Initial petrographic observations of the limestones show clotted microbial and detrital-rich micrite, pyritic corrosion surfaces, peloids, and foraminifers, as well as fibrous, yellow, and sparry calcite, all of which are typical components of GVG hydrocarbon seep deposits. Preliminary carbon and oxygen isotopic values fall into the known range for ancient hydrocarbon seeps. The cumulative diversity list for these localities include inoceramid and buchiid bivalves (Buchia okensis), rhynochonellide brachiopods (Peregrinella whitneyi), tube worms, and gastropods (Lithomphalus enderlini), all of which are characteristic seep taxa. Each locality is dominated by a single taxa, almost to the complete exclusion of all others, although there are no patterns explaining why one taxa dominates over another at any one locality.

To date, diversity lists form the bulk of the data on hydrocarbon seep paleocommunities, but diversity lists alone cannot reveal the ecological structure of the communities they describe. By cumulatively investigating the life histories of these largely unstudied and uncharacterized limestone outcrops in the GVG, hydrocarbon seep ecological structure can be examined, allowing the evolutionary and ecological processes that underlie community composition and dominance to be more clearly defined and understood. Examining all of the known GVG hydrocarbon seeps will ultimately place them into the global spatial, temporal, and stratigraphic seep system and will allow larger questions concerning their phylogenetic, biogeographic, and paleoecologic origins to be addressed.