SEDIMENTARY GEOCHEMISTRY OF THE GREAT VALLEY GROUP, CALIFORNIA: RECORDS OF THE LOST ARC

Where magmatic arcs have been deeply eroded, associated forearc basins may contain more complete records of arc evolution than the remnant arcs themselves. Geochemical analysis of fine-grained sedimentary rocks in the Great Valley Group (GVG) complement previous provenance studies to generate a more robust characterization of sediment sources. Although major-element sedimentary geochemistry is limited by the potential mobility of these elements, northern GVG (Sacramento Valley) samples consistently record more mafic signatures in the less-mobile Fe-, Mg-, and Ti-oxides than southern GVG (San Joaquin Valley) samples. Trace elements Cr, Ni, and V are also elevated in the northern relative to the southern GVG, reflecting more mafic, including ophiolitic, components in northern sources. The Sacramento Valley geochemical signature is remarkably consistent throughout deposition and between samples collected from the northeast, north, northwest and southwest sections of the valley. Samples from the Sacramento Valley and the basal, Lower Cretaceous San Joaquin strata fall between Mid-Ocean Ridge Basalt and Continental Arc andesite values on plots of compatible and incompatible trace elements, with basal San Joaquin samples showing the most juvenile signatures and high V/Sc ratios, suggesting increased mafic and volcanic detritus. In contrast, the Upper Cretaceous strata in the southern GVG are the most evolved in their trace-element signatures and have low V/Sc ratios indicative of limited volcanic input. The Upper Cretaceous San Joaquin samples plot closest to the North American Shale Composite and Upper Continental Crust values in compatible versus incompatible trace element plots. Northern GVG geochemical results are consistent with provenance limited to the accreted arc terranes of the Klamath Mountains and northern Sierra Nevada Mountains. The mafic provenance signal in the Early Cretaceous San Joaquin Valley may result from erosion of mafic rocks of the Early Cretaceous Sierran arc to the east, perhaps combined with removal of extensive volcanic cover from the southern Sierra Nevadan arc. Late Cretaceous exhumation of the southern Sierran arc provided a more felsic and evolved source for the southern GVG.