PROVENANCE ANALYSIS OF THE PLIO-PLEISTOCENE PRAIRIE CREEK FORMATION, HUMBOLDT COUNTY, CA

The Cascadia forearc of northern California marks the southern terminus of the Cascadia subduction zone and the northward migrating Mendocino Triple Junction (MTJ). From east (arc) to west (trench), the forearc includes the high topography of the Proterozoic-Mesozoic Klamath Mountain Province, the northwest trending Cretaceous Franciscan Complex, and structurally bounded basins of Neogene-Quaternary marine and fluvial sediments. Located just south of the modern mouth of the Klamath River on the coastal margin with an area of 150m²is the Plio-Pleistocene Prairie Creek Formation (PCF). The PCF most likely represents an ancestral Klamath River that prograded into the Cascadia forearc, depositing >550m of mostly fluvial sand and rounded fluvial conglomerate. Timing of deposition is thought to be sometime between ~4Ma to ~0.5Ma, and most of the PCF sediments appear to be derived from the Klamath Mountains, which are primarily plutonic and metamorphic lithologies distinct from nearer source Franciscan units (Kelsey and Trexler, 1989). Heavy vegetation and limited land access results in extremely poor exposures. The best exposed section is the Gold Bluffs Beach Unit outcropping along the coastline forming a prominent N-W trending syncline.

During the wet winter of 2016/17 a large landslide created a new exposure on the Gold Bluffs Beach Unit of the Prairie Creek Formation providing an opportunity to describe the stratigraphy, take structural measurements, conduct clast counts and separate zircon minerals for U-Pb detrital geochronology. Clast counts (n=300) from landslide material yield the following proportions: 1) metamorphic: 6%, 2) plutonic: 19%, 3) volcanic: 4%, 4) quartzite and meta-arenite: 21%, 5) chert and argillite: 22% and 6) litharenite/lithwacke: 28%. Medium to fine- grained sand is angular to subrounded with approximate modal abundances of quartz: 50%, feldspar 20% and lithics: 30%. The majority of these lithics appear to have a grey granular or schistose appearance, representing probable source lithologies of quartzite and schist, likely from a Klamath Mountain source. In comparing these results to the lower fluvial section of the PCF, we can identify changing provenance and depositional environments over the timeframe of deposition of this Plio-Pleistocene fluvial section.