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The Evolution of Great Valley Group Hydrocarbons and their Relationship to Temporally and Spatially Disparate Hydrocarbon Seep Localities, northern California

Kristin Hepper

The University of California, Riverside, Department of Earth Sciences

Riverside, California

[email protected]

During the Late Mesozoic, the western North American continental margin consisted of an eastward migrating volcanic arc, a westward migrating subduction complex, and a forearc basin in which hydrocarbons were active. Today, these forearc sediments, known as the Great Valley Group (GVG) forearc strata, have an uncommonly high concentration and temporal distribution of petroliferous and fossiliferous hydrocarbon seep outcrops, distributed over 700 km and 70 my of subduction history.

The GVG seeps are variable in size, physical expression, faunal composition, ecological maturity, and geochemical signature. Initial petrographic observations of these localities have revealed the presence of typical hydrocarbon seep components, including clotted microbial and detrital micrite, pyritic corrosion surfaces, peloids, and/or foraminifers, as well as fibrous, yellow, and sparry calcite. Many of the localities have prolific faunal assemblages including bivalves (Buchia), brachiopods (Peregrinella whitneyi), tube worms, and gastropods (Lithomphalus enderlini), all of which are recognized as common hydrocarbon seep constituents. Additionally, preliminary carbon and oxygen isotope valuesfall into the known range for hydrocarbon seeps found within the GVG and worldwide.

Hydrocarbon seeps are usually found in small numbers spanning short periods of time, making the extensive temporal and spatial distribution of the GVG seeps unusual. Cumulatively investigating the geochemical and faunal histories of these largely unstudied and uncharacterized limestone outcrops therefore offers the opportunity to document the evolution and potential variability of the GVG hydrocarbons through space and time using disparate seeps to test the hypothesis that there is a recognizable spatial and temporal pattern to their variability.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid