STRATIGRAPHY AND PROVENANCE IMPLICATIONS FROM DETRITAL ZIRCON ANALYSIS OF THE UPPER JURASSIC-LOWER CRETACEOUS PORTION OF THE GREAT VALLEY FOREARC BASIN, NORTHERN CALIFORNIA

Detrital zircon provenance analysis of the Upper Jurassic-Lower Cretaceous Great Valley Group (1) reveals that the lowermost Great Valley Group is actually Lower Cretaceous, rather than Upper Jurassic, and (2) indicates that the Great Valley forearc basin received cratonal North American detritus early in its depositional history. For this study, seven sandstone samples were collected from outcrops along the western Sacramento Valley. SHRIMP U-Pb ages of detrital zircon exhibit both unimodal (155.8-125.2 Ma) and polymodal (2800-139.5 Ma) age distributions.

Buchia fossils traditionally define the Late Jurassic depositional age of the lowermost several km of the Great Valley Group. However, Early Cretaceous (144-125.2 Ma) detrital zircon from purported Tithonian Buchia–bearing strata demonstrate that the base of the Great Valley Group is in fact Lower Cretaceous in our study area. In addition, polymodal age distributions with grains as old as 2800 Ma indicate that the Great Valley forearc basin received detritus ultimately derived from North American cratonal sources even very early in its history. These discoveries demonstrate that the early history of the Great Valley forearc basin is not as well understood as previously thought.