Eocene sedimentary rocks in southwest Oregon preserve an unparalleled archive of the surface response to collisional mountain building and post-collision tectonic reorganization. The syn-collision Umpqua Group (~ 54–49 Ma) rests on basalts of the Siletz terrane and filled a syn-orogenic basin on the north margin of the Klamath Mountains orogen. The post-collision Tyee Group (~ 47–45 Ma) is a thick succession of fluvial, deltaic, and marine turbidite deposits that record rapid progradation of an integrated fluvial-delta-shelf-slope-basin clinoform system during initiation of the modern Cascadia subduction zone.
Hypothesis 1 postulates that the Tyee paleoriver originated in western Idaho, traversed a large low-gradient continent-interior drainage, and flowed through the former collisional orogen to a prograding fluvial-deltaic to offshore marine turbidite system. According to Hypothesis 2, the Tyee paleoriver was sourced in the Klamath Mountains, and changes in sand composition record bedrock exhumation, recycling of older sediments, and/or catchment growth.
Because paleocurrent data show unequivocally that the Tyee paleoriver flowed directly out of – not around – the Klamath Mountains, evidence of a large continent-interior catchment for the Tyee paleoriver would imply major post-collision reorganization of the drainage system by headward erosion and stream capture and/or extensional collapse of the former collisional orogen. These hypotheses will be tested through integration of modern provenance tools, detrital thermochronology, geologic mapping, and detailed field observations.