TECTONOSTRATIGRAPHIC EVOLUTION OF SW OREGON FROM EARLY EOCENE COLLISION OF SILETZIA TO THE MODERN CASCADIA CONVERGENT MARGIN

Early to middle Cenozoic sedimentary and volcanic rocks in SW Oregon record a complex history of terrane accretion, subsidence, basin filling, crustal shortening, rotation, post-collisional subduction, and transition to the modern Cascadia convergent margin. Integration of legacy data sets with recent and ongoing geochronologic, stratigraphic and geochemical studies provides new insights into tectonic events and processes that have shaped the Cascadia forearc region through time. The regional stratigraphy can be divided into 4 major groups, some informally defined, that record distinct stages of tectonic evolution and basinal response. (1) The Umpqua Group (~ 54 to 48 Ma) includes conglomerate, sandstone, turbidites, shale and deltaic deposits that thicken and coarsen to the SE toward the Klamath Mountains, and thin to the NW over the NE-trending Umpqua Arch (flexural forebulge). It filled a large syn-collisional foredeep basin (Umpqua basin) and contains lithic-rich detritus shed from the Klamath Mountains. (2) The Tyee group (~ 47 to 40 Ma: Tyee, Elkton, Bateman, Coaledo, & Spencer formations) unconformably overlies the Umpqua Group and represents a thick clinoformal package of micaceous fluvial to deltaic clastic deposits and slope to distal marine turbidites that prograded NW (in modern coordinates) over the Umpqua Arch. Paleocurrent data, paleogeography, sandstone petrography, detrital zircon ages, and other data suggest that the Tyee group was derived from the exhuming Klamath Mountains, not Idaho. The Tyee group formed after the end of Siletzia accretion and prior to the onset of magmatism in the ancestral Cascades arc (ACA). (3) The late Eocene to early Oligocene Fisher and Eugene formations and equivalent units in the Coos Bay area (~ 40 to 30 Ma) record sedimentation and input of arc-derived tuffs in a regional forearc basin situated between the early ACA in the east and the newly integrated Cascadia trench and subduction zone in the west. (4) Starting ~ 30–20 Ma, the forearc basin was uplifted, inverted, and folded to form at least three angular unconformities that bound the Miocene Tarheel and Empire formations 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.