CRUSTAL DEFORMATION, BASIN SUBSIDENCE, AND PALEO-RIVER RESPONSE TO EOCENE ACCRETION OF SILETZIA, SW OREGON (Invited Presentation)

Sedimentary deposits in western Oregon provide a record of tectonic events that resulted from early Eocene accretion of Siletzia to North America. Paul Heller’s work in this region generated many insights that continue to shape our understanding of feedbacks among tectonic, sedimentary, and earth-surface processes. The oldest rocks, 56-49 Ma tholeiitic and alkalic basalts of the Siletz terrane, formed in a mantle-plume fed oceanic basalt plateau. Near Roseburg, OR, Siletz basalts are deformed in a belt of NE-striking, SE-dipping imbricate thrust faults bounded on the SE by a crustal suture with metamorphic and plutonic rocks of the northern Klamath Mts. The 54-49 Ma Umpqua Group (UG) is a succession of syn-orogenic marine and deltaic deposits that overlie Siletzia and Klamath Mts. rocks, and thicken/coarsen toward the suture. The UG is overlain by 49-46 Ma turbidites of the Tyee Fm. along a regional contact that records the end of collisional deformation and initiation of a new subduction zone to the west (modern Cascadia S.Z.). The UG and Tyee Fm. accumulated in a single phase of rapid subsidence that produced ~6 km of sediment in ~8 Myr (Ryu and Niem, 1999). Heller et al. (1985, 1992) proposed that 55-90 Ma detrital micas in the Tyee Fm. were derived from the Idaho Batholith in Idaho. This idea is supported by a 49-Ma age peak in detrital zircons inferred to be sourced in Challis volcanics (Dumitru et al., 2013), though zircon could be derived from the Clarno Fm. in Oregon. Detrital muscovite and biotite are present in both the UG and Tyee Fm. Petrographic data reveal a continuous trend of increasing [Qm + F] and decreasing Lt, from lithic-rich sandstones in the lower UG to arkosic litharenites in the Tyee Fm. This unroofing trend appears to record progressive erosion into deeper levels of the collisional orogen through time, consistent with early interpretations that Tyee and similar-aged units in northern California were derived from the Klamath Mts. (e.g., Snavely et al., 1964). If Tyee sands were eroded from the Klamath Mts., as suggested by petrographic trends, subsidence curve, and new paleocurrent data (Santra et al., 2013), it would imply a now-buried source in the east-central Klamath Mts. for the detrital micas. New ⁴⁰Ar-³⁹Ar dating of detrital micas from the Umpqua Group, in progress, will help us test these alternate hypotheses.