BASINAL AND PALEORIVER RESPONSE TO EOCENE ACCRETION OF SILETZIA IN SW OREGON: CONFLICTING DATA AND UNRESOLVED QUESTIONS

Ray Wells’ research in western Oregon has generated insights that continue to shape our understanding of tectonic events that resulted from early Eocene accretion of Siletzia to North America. The Siletz terrane is a thick, ~56-49 Ma sequence of tholeiitic to alkalic basalts that formed in a mantle-plume-fed oceanic basaltic plateau. Near Roseburg, OR, Siletz basalts are deformed in a belt of NE-striking, SE-dipping imbricate thrust faults that are bounded on the SE by a major suture with metamorphic and plutonic rocks of the northern Klamath Mts (Wells et al., 2014). The 54-49 Ma Umpqua Group (UG) is a succession of syn-orogenic marine and deltaic deposits that thicken and coarsen toward the suture, and accumulated in a flexural foredeep basin during collision and accretion of Siletzia to North America. The UG is overlain by the 49-46 Ma Tyee Formation along a conformable to unconformable contact that records the end of collision-related deformation and initiation of the modern Cascadia subduction zone to the west. 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), suggesting a single subsidence driver (thrust load) for both formations. 55-90 Ma detrital micas in the Tyee Fm are widely believed to be derived from western Idaho (Heller et al., 1985, 1992). An Idaho source is supported by a 49-Ma age peak in detrital zircons inferred to be sourced in Challis volcanics (Dumitru et al., 2013), though those zircon could be derived from Clarno volcanics in Oregon. Petrographic data reveal a linear unroofing trend of increasing [Qm + F] and decreasing Lt, from lithic-rich sandstones in the lower UG to arkosic litharenites in the Tyee Fm. This trend appears to record erosion into deeper levels of the collisional orogen through time, consistent with early interpretations that the Tyee Fm and similar-aged units in northern California were derived from the Klamath Mts. (e.g., Snavely et al., 1964; Aalto, 1988). If Tyee sands were eroded from the Klamath Mts, as suggested by sst petrography, basin architecture, subsidence curves, and paleocurrent data (Santra et al., 2013), it would imply a now-buried source of sediment in the east-central Klamath Mts. New isotopic data and crystallization ages from UG and Tyee Fm micas are needed to help test these alternative hypotheses.