Tectonic evolution of the Cascadia margin in southwest Oregon during the past 55 million years

Rebecca Dorsey

University of Oregon

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Sedimentary and volcanic rocks in southwest Oregon record a complex history of subduction, accretion, sedimentation, rotation, and uplift at the Cascadia convergent margin over the past ~55 million years. This presentation will integrate knowledge from legacy datasets with ongoing studies to share insights into tectonic events and processes that have shaped the Cascadia forearc region through Cenozoic time. The 54 to 48 Ma Umpqua Group contains conglomerate, sandstone, turbidites, shale and deltaic deposits that filled a large syn-collisional basin with detritus shed from the Klamath Mountains. The Tyee Formation (~47 to 42 Ma) is a thick package of micaceous fluvial to deltaic deposits and marine turbidites that prograded north through western Oregon. Stratigraphic and petrologic studies suggest that the Tyee Formation was derived from the eroding Klamath Mountains, not western Idaho. The late Eocene to early Oligocene Fisher and Eugene formations (~40 to 30 Ma) and equivalent units in the Coos Bay area record deposition of sediments and tuffs in a large forearc basin situated between the early Cascades volcanic arc and the Cascadia subduction zone. Starting ~30 Ma the forearc basin was uplifted, inverted, and folded to form angular unconformities in the Coos Bay area. This marks the change to a structurally fragmented forearc region characterized by regional uplift and erosion in the Oregon Coast Range that continues to the present day.