DID THE EOCENE TYEE RIVER TRANSPORT DETRITUS FROM EXTENSIONAL HIGHLANDS AROUND THE IDAHO BATHOLITH AND BITTERROOT CORE COMPLEX TO THE FRANCISCAN TRENCH IN NORTHERN CALIFORNIA?

The Coastal belt (CB) of the Franciscan subduction complex preserves a huge surge of trench and related sediments deposited mainly in Eocene time, but depositional ages are poorly known. U-Pb ages of detrital zircons from 6 out of 7 CB samples exhibit these features: (1) most zircons are 50 to 85 Ma; (2) only lesser numbers of zircons yield the ≈85 to 200 Ma ages expected from Sierra Nevada (SN) sources; (3) the youngest zircon population in a given sample ranges from ≈47 to ≈58 Ma. These points support Underwood and Bachman’s (1986) model that Coastal belt sediments were derived mainly from the Idaho Batholith (IB) region, rather than from the SN, and further suggest that much of the CB was deposited over a surprisingly short interval of time within the Eocene. Two Eocene samples from the Great Valley forearc basin (Markley Fm. near Mt. Diablo) contain mostly SN-age zircons, but also contain smaller numbers of 85-45 Ma zircons, plus considerable detrital muscovite. IB plutonism spanned 98 to 53 Ma and included muscovite-rich, S-type granitoids; it was quickly followed by major extensional exhumation in the Bitterroot, Anaconda, Clearwater, and Priest River metamorphic core complexes (53-40 Ma) and major volcanism in the Challis volcanic field (50-46 Ma) (e.g., Foster et al., 2007). This extensional tectonism apparently generated the surge of Coastal belt sedimentation. Apparent IB–Challis material in the Markley and in Coastal belt trench slope deposits suggest that the river transporting this material reached the sea at the north end of the Great Valley forearc basin, rather than debouching in Oregon followed by turbidite sediment transport 200-800 km farther south down the trench axis to the Coastal belt basin. The same highland region in Idaho also sourced volcanoclastic sediments deposited in the Green River Basin lake system of Utah between 49.6 and 47.0 Ma (Chetel et al., 2011). In coastal Oregon, Tyee Fm. forearc(?) basin deposits preserve a huge surge of muscovite-rich sediment deposited between ≤48.9 and ≥46.6 Ma. Extensive age and isotopic data indicate sources in the IB (Heller et al., 1985). Speculatively, the Tyee Fm. sediment surge may post-date the Coastal belt surge and record the shift of the mouth of an Idaho-sourced, Eocene Tyee River from the south (into northern California) to the north (into southern Oregon).