DEPOSITIONAL AGES, PROVENANCE, AND PALEO-DRAINAGE PATTERNS FROM DETRITAL ZIRCON GEOCHRONOLOGY OF EOCENE-OLIGOCENE FLUVIAL SEDIMENTS (“AURIFEROUS GRAVELS”) IN THE NORTHERN SIERRA NEVADA, CALIFORNIA

To gain a better understanding of the Cenozoic tectonic and landscape evolution of the northern Sierra Nevada, we collected samples of Eocene to Oligocene fluvial sediments (‘auriferous gravels’) throughout the Yuba and Feather River drainages for detrital zircon geochronology. Sediments reflect braided stream deposition in basement-incised paleovalleys within a structurally controlled intermontane basin, and age population differences between locations have allowed us to reconstruct source terranes and drainage patterns within the Eocene system. U-Pb ages of zircons in 14 samples were determined using LA-ICPMS. The majority of samples have large populations from one of two distinct Mesozoic sources, as well as 5-30% Proterozoic grains, reflecting provenance from basement terranes of both the Sierra Nevada and central Nevada (i.e., Shoo Fly Complex, Roberts Mountain Allochthon, western Nevada Triassic strata). Samples in the southwestern Yuba River drainage were sourced predominantly from Campanian to Aptian batholithic rocks, whereas eastern Yuba and Feather River drainage samples reflect provenance from Jurassic basement terranes with no Cretaceous grains present. This distinction indicates diachronous sediment deposition across the basin and may reflect differential erosion of batholithic rocks. Eastern samples also contain limited Eocene-early Oligocene populations, ranging from 42.7 to 28.6 Ma, which are interpreted to be youngest single-grain maximum depositional ages (Dickinson and Gehrels, 2009). These results show long-term aggradation of the auriferous gravels as the locus of deposition migrated across the basin, and suggest that age determinations of paleoflora from within the section may require revision. Youngest single-grain detrital zircon ages are similar to the range of ages of Eocene volcanic rocks in central Nevada, and the presence of these grains, along with Proterozoic populations found only in the Roberts Mountain allochthon or western Nevada Triassic strata, suggests that Eocene paleovalleys extended as far east as the Caetano Caldera in central Nevada, as has been suggested by previous studies (i.e., Henry, 2008). This provides support for the existence of a high elevation plateau at the latitude of Nevada (‘Nevadaplano’) from Eocene through Oligocene time.