PALEOECOLOGICAL ANALYSIS OF NEW GREAT VALLEY GROUP HYDROCARBON SEEP LOCALITIES

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 85 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.

The large number of hydrocarbon seep limestone outcrops in California's Mesozoic GVG is unparalleled worldwide since to date, a locality with a large quantity of temporally and spatially linked seeps has not been found. The GVG is the first area addressing these topics is possible, therefore, questions concerning the evolution of hydrocarbon seep paleocommunities and their paleoecologic signatures can be addressed for the first time, including large-scale spatial and temporal patterns of species composition, species dispersal, species richness, and species dominance, in a long-lived and extensive hydrocarbon seep paleocommunity.

As the abundance of recognized GVG hydrocarbon seeps continues to increase, they 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, both in the Great Valley as well as worldwide. Additionally, studies of ancient hydrocarbon seeps have the potential to provide insight into the evolution of life in extreme environments. Most modern organisms are found in oxygen- and sunlight-rich environments, but life likely arose under more extreme conditions of low oxygen and high temperature that predominated early in Earth's history.