DETRITAL ZIRCON ANALYSIS OF LATE JURASSIC-EARLY CRETACEOUS GREAT VALLEY GROUP FOREARC BASIN STRATA IN THE NEWVILLE AND WILBUR SPRINGS AREA, SACRAMENTO VALLEY, CALIFORNIA

Detrital zircon analysis of Northern California's Great Valley Group (GVG) forearc basin strata is a powerful tool for delineating stratigraphic age and timing of basin development, especially where fossil data are scarce, as well as interpreting the dissection of the Klamath-Sierran magmatic arc. In this study we sampled Jurassic sandstones of the GVG that crop out on the western edge of the Sacramento Valley for 50 km along strike between the towns of Newville and Wilbur Springs. Detrital zircon analysis using the Stanford-USGS SHRIMP-RG places tighter constraints on stratigraphic age because depositional ages of strata in the study area are presently based on biostratigraphy using fossils that are increasingly scarce toward the southern reach of the study area. Some samples analyzed that are assigned to Late Jurassic (Tithonian) Buchia fossil zones revealed detrital zircon grains with Early Cretaceous ages suggesting an Early Cretaceous maximum depositional age. Three of the five samples contain at least one detrital zircon grain with a U-Pb age younger than 145 Ma, suggesting that the Jurassic-Cretaceous boundary may be lower in the section than previously mapped. In addition, the age distributions of detrital zircon contain a major Late Jurassic peak, reflecting a pulse in magmatism in the source regions at about 150 Ma that is not well-preserved in the Klamath-Sierran arc itself. The age distributions also contain a large number of pre-Mesozoic grains (23% of all grains analyzed), indicating that a significant portion of Great Valley basin sediment was derived from older, cratonic sources on the eastern side of the magmatic arc during the Late Jurassic and Early Cretaceous. Together, these detrital zircon age data imply that (1) the Jurassic-Cretaceous boundary occurs lower in the GVG stratigraphy than previously thought, (2) there was a significant pulse of Late Jurassic magmatism that is not evident in the Klamath-Sierran arc today, and (3) a significant percentage of the sediment deposited in the GVG was derived from the east of the magmatic arc, suggesting that drainage systems originated from cratonic sources and crossed the magmatic arc early in the depositional history of the GVG.