DYNAMIC TOPOGRAPHY OF SOUTHERN CASCADIA RECORDED IN PROVENANCE OF PLIO-PLEISTOCENE FOREARC DEPOSITS

The Southern Cascadia forearc of northern California records a complex history of subduction accretion, spatial variations in trench margin location, tectonic submergence and emergence, and accumulation of a late Miocene-Quaternary clastic sedimentary package on its narrow continental margin. What is preserved of this record is overprinted by a propagating wave of mid-Pleistocene to present contractual deformation due to the northward migrating Mendocino Triple Junction (MTJ), which uplifted, faulted and folded forearc deposits. Two continuous, well-preserved, and exposed sections of these deposits are the Miocene-Pleistocene marine-fluvial Wildcat Group in the modern Eel River basin, and the >550m thick fluvial conglomerate Plio-Pleistocene Prairie Creek Formation, near the mouth of the modern Klamath River. If the Eel and Klamath drainages were reorganized by a regional paleogeographic change, such as the migration of the MTJ, during this time frame, changing provenance could be potentially recorded in fluvial deposits.

We determine detrital zircon U-Pb age populations from Plio-Pleistocene fluvial sands and examine clast composition of the gravels. In the Prairie Creek Formation, clast composition indicates near-source Franciscan and Klamath plutonic sources, with an appearance of, and increase in proportion of plutonic clasts up section. Glaucophane and hornblende in heavy mineral fractions also supports near-source Franciscan and Klamath sources. Detrital zircon U-Pb geochronology indicates a strong Klamath plutonic fingerprint (130-170Ma), but the middle fluvial section of the Prairie Creek Formation contains a 65-100Ma detrital zircon age population, which could be a Sierran or Idaho-Challis source, but more closely matches Idaho. This far-source population most likely suggests either punctuated reworking of Eocene Tyee or Yager Formations from proximal coastal sources, or erosion of an early-mid Tertiary sedimentary cover from the Klamath Mountains coeval with regional forearc uplift in Plio-Pleistocene time.