PROVENANCE AND DEPOSITIONAL AGE ANALYSIS OF THE BASAL GREAT VALLEY FOREARC IN NORTHERN CALIFORNIA USING DETRITAL AND IGNEOUS ZIRCON U-PB GEOCHRONOLOGY

The Great Valley Forearc in northern California is an example of a composite basin that began to form during Late Jurassic to Early Cretaceous time and continued to accumulate sediment into the Early Cenozoic. Although many geologic studies exist on this region, there are still uncertainties regarding the formation of the Great Valley forearc and the age relationship between the underlying basement and overlying stratigraphy. For this study, 2100 single-grain analyses from seven detrital samples from the forearc and 150 single-grain analyses from three igneous samples from the underlying Coast Range Ophiolite from the Paskenta region were analyzed and compared to existing geochronologic data from neighboring areas to determine crystallization ages and maximum depositional ages (MDA), as well as provenance. Previous provenance studies have interpreted detritus deposited in the Elder Creek region (north of Paskenta) and the Grindstone Creek region (south of Paskenta) to be sourced from the erosion of sediment of the Sierra Nevada magmatic arc and the continental interior by arc-retroarc river systems based on major U-Pb age populations of ~140-250 Ma, ~300-450 Ma, ~550-700 Ma, ~950-1300 Ma, ~1400-1500 Ma, ~1600-1800 Ma and >2500 Ma. Preliminary results from this work support these interpretations and document changes in the proportions of magmatic arc versus retroarc material based on the depositional age. Previous findings report MDAs around Elder Creek to be 140-153 Ma and Grindstone Creek to be 142-151 Ma, indicating that these rocks were not deposited prior to the latest Jurassic and some areas began accumulating sediment in earliest Cretaceous time. Analyses on the crystallization age of zircons derived from the Coast Range Ophiolite and overlying breccia are ongoing and will be compared to MDAs from the forearc stratigraphy near Paskenta.