DETRITAL ZIRCON DATING OF JURASSIC GREAT VALLEY GROUP STRATA: IMPLICATIONS FOR THE EARLY EVOLUTION OF THE GREAT VALLEY FOREARC BASIN

The Great Valley Group (GVG) contains over 15 km of strata deposited from the Late Jurassic through the Cretaceous in a forearc basin west of the Sierra Nevada-Klamath magmatic arc. The depositional age designation of the strata in the lower GVG is based largely on macrofossils, including the pelecypod Buchia and ammonites, corroborated by microfossils and ash layers in the lower Cretaceous section. In order to more precisely delineate stratigraphic age in the Jurassic section and constrain the position of the Jurassic/Cretaceous boundary within the stratigraphy, we took sandstone and Buchia fossil samples from the Jurassic stratigraphy exposed in the Paskenta area of the northwestern Sacramento Valley. Samples were taken from previously identified Buchia fossil locations whenever possible, and we analyzed detrital zircon from the sandstone samples using the Stanford-USGS SHRIMP-RG. Preliminary results show a major Late Jurassic detrital zircon age peak, indicating a significant pulse of Late Jurassic magmatism in the source region that is not well documented in either the Klamath or the Sierra Nevada Mountains. In addition, several grains of Cretaceous age occur in strata previously mapped as Jurassic. If the ages of the youngest cluster of detrital zircon grains delimits the maximum depositional age, our results indicate that the Jurassic/Cretaceous boundary may be significantly lower in the stratigraphic section than was previously thought. Our data indicate that the position of the Jurassic/Cretaceous boundary in the Paskenta area is approximately 4 km to the west of where it is currently mapped, suggesting that about 3 km of strata were deposited in the Cretaceous and not the Jurassic. These data imply that Late Jurassic sedimentation rates in the GVG were much lower than previously thought, and Cretaceous sedimentation rates much higher. This, in turn, suggests that the uplift and dissection of the SNB-Klamath magmatic arc was more rapid in the Cretaceous than in the Jurassic. Both of these conclusions have significant implications for the early development and evolution of the Great Valley forearc basin and the dissection of the Klamath-Sierran magmatic arc.