Using hydrocarbon seep limestone to study the evolution of spatially and temporally linked hydrocarbons in Northern California’s Mesozoic Great Valley Group forearc strata

Kristin Hepper, 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 variably fossiliferous hydrocarbon seep outcrops, distributed over 700 km and 70 my of subduction history. 

Northern California’s GVG forearc strata offers the first opportunity to study long-term evolution of hydrocarbons using hydrocarbon seep limestone.  Fueled by hydrocarbons venting at the seafloor, seeps are generally found in small numbers spanning short periods of time, making them ineffective tools for studying the evolution of hydrocarbons through space and time.  A study which examines the evolution of hydrocarbons using temporally and spatially linked seeps has not been performed because a locality with the necessary features has not been found.  Since the GVG seep localities are now known to occur in great numbers (at least 30+ occurrences over >700 km) which have an extensive temporal record of fluid flow (~70 m.y., Tithonian-Campanian), a project that examines the evolution of hydrocarbons using hydrocarbon seep limestone is possible for the first time. 

Cumulatively investigating the life histories of these largely unstudied and uncharacterized limestone outcrops offers the unique opportunity to document the evolution and variability of the hydrocarbons among spatially and temporally disparate seeps and to test the hypothesis that there is a recognizable spatial and temporal pattern to this variability.