A PROBABLE IDAHO SOURCE FOR DETRITAL ZIRCONS IN A LATE CRETACEOUS-EOCENE SUBMARINE FAN SEQUENCE OVERLYING THE CENTRAL BELT OF THE FRANCISCAN COMPLEX, NORTHERN CALIFORNIA

Recent tectonic models propose that abundant Proterozoic detrital zircons (1800-1300 Ma U-Pb ages) in certain Late Cretaceous to Paleocene deposits of Vancouver Island and western Washington are derived from the Pelona, Orocopia, and Rand schists or their sources in southeastern California, Arizona, or further south. Sediment with these Proterozoic zircons is considered to have been displaced northward via undefined faults and paleodrainages. Here, we present new detrital zircon U-Pb ages supporting a model in which abundant 1800-1300 Ma zircons in Late Cretaceous to Paleogene strata overlying the Central Belt of the Franciscan Complex are instead sourced from Proterozoic country rocks surrounding the Idaho batholith.

West of Ukiah, in northern California, a composite unit of cavernous-weathering massive sandstone incised into marine turbidites, referred to herein as the “Ukiah sequence” (US), is deposited on mélange of the Central Belt. The US is petrographically similar to the Coastal Belt of the Franciscan Complex (considered to be sourced mainly from the Idaho batholith region), with 3-20% K-feldspar, >2% grains of biotite locally intergrown with muscovite, and angular first cycle quartz and feldspar.

The depositional age of the US is Late Cretaceous (Santonian to Campanian) and early Paleogene based on a locality with Sphenoceramus schmidti, and on local co-occurrence of redeposited Cretaceous? oysters with Paleogene Discocyclinid foraminifers. U-Pb ages of 1332 detrital zircons from 9 US localities yield maximum depositional ages (MDAs) ranging from ~86-64 Ma, older than ~53-32 Ma MDAs from the Coastal Belt. Some of the US zircons have ca. 1370 Ma or 1650-1800 Ma cores with <86 Ma low Th/U rims, consistent with ages of magmatism and high-T metamorphism in the Idaho batholith region. We interpret the US as remnants of submarine fans deposited on top of the Central Belt that linked a Late Cretaceous to Paleogene subduction trench on the west with paleoriver systems originating farther east and north in Idaho. Uplift of the northern Coast Ranges after ~64 Ma rerouted sediment transport southeastward to the Princeton submarine canyon in the Great Valley forearc basin, delivering ~53 Ma and younger Coastal Belt sediment to the trench farther south.