EVIDENCE OF EARLY CRETACEOUS TECTONISM IN THE GREAT VALLEY FOREARC BASIN FROM DETRITAL ZIRCON PROVENANCE ANALYSIS, SACRAMENTO VALLEY, CALIFORNIA

The Lower Cretaceous Great Valley Group stratigraphy records multiple provenance shifts related to episodic tectonism within the arc-forearc system. Detrital zircon age spectra of six samples spanning the Lower Cretaceous stratigraphy fall into two types: 1) polymodal age distributions, which occur at the base and top of the Upper Jurassic-Lower Cretaceous strata (Stony Creek petrofacies) and in the middle and top of the Lower Cretaceous strata (Lodoga petrofacies); and 2), unimodal age distributions, which occur in the middle of the Stony Creek petrofacies, at the base of the Lodoga petrofacies, and at the base of the Upper Cretaceous strata. The provenance of the polymodal age distributions was likely in both the Klamath Mountains and northern Sierran terranes, consistent with previous interpretations that the Lower Cretaceous strata were deposited by longitudinal (south-directed) drainage from the Klamaths fed by transverse (west-directed) drainage off the northern Sierran terranes. The most probable source for the samples with unimodal age distributions are the plutons within the northern Sierran terranes and now-covered plutons buried by Upper Cretaceous Great Valley strata. Therefore, the multiple shifts from polymodal age distributions with widespread provenance to unimodal age distributions with more localized provenance suggest episodic tectonism in the young arc-forearc system that temporarily localized sediment sources. Early Cretaceous tectonism, perhaps related to syndepositional sinistral normal faulting in the northern Great Valley, may have blocked longitudinal drainage within the Upper Jurassic-Lower Cretaceous basin such that the only sediment source was the transverse drainage off of the northern Sierran terranes, resulting in unimodal Late Jurassic-Early Cretaceous age distributions. Longitudinal transport was restored by deposition of the upper Stony Creek petrofacies, but was interrupted by tectonism at the beginning of Lodoga petrofacies deposition. Longitudinal transport again dominated by the time of deposition of the middle Lodoga petrofacies, and continued until interrupted at the beginning of the Late Cretaceous.