EARLY EOCENE TECTONOSTRATIGRAPHIC EVOLUTION OF THE SOUTHERN UMPQUA BASIN DURING COLLISION OF THE SILETZ TERRANE

The sedimentary fill of the Umpqua basin in SW Oregon offers critical insights into the tectonic evolution of the western margin of North America during early Eocene collision and accretion of the oceanic Siletz Terrane. Integration of methods in basin analysis and sequence stratigraphy allows us to interpret controls on shifting patterns of subsidence, sedimentation, and the structural-topographic evolution of source areas through time. The success of these methods relies heavily on establishing a robust geologic and stratigraphic framework. This study presents a new analysis of the southern Umpqua Group (~55–50 Ma) in the Bone Mt. area SW of Camas Valley, Oregon. Using data from prior studies (geologic maps, papers, theses) and new fieldwork for this study, we compiled a detailed geologic map and extracted information about stratigraphic architecture and basin evolution. Previously unrecognized angular unconformities are identified using high-resolution bare-earth LiDAR topography, and stratigraphic thicknesses are calculated using GMDE software (Allmendinger, 2020). Multiple geologic cross-sections provide a refined understanding of this area. The Umpqua Group rests unconformably on pre-Cenozoic metamorphic and sedimentary rocks of the northern Klamath Mountains (KM). North of the dextral-oblique Canyonville fault, the entire Umpqua Group is ca. 3 km thick and comprises four sequences bounded by angular unconformities: (I) Lower member (Mbr) of the Bushnell Rock Formation (Fm); (II) Upper Bushnell Rock and Slater Creek Mbrs, Tenmile Fm, and Berry Creek Mbr of the White Tail Ridge Fm; (III) Remote and Coquille River Mbrs of the White Tail Ridge Fm; and (IV) Camas Valley Fm. The stratigraphic architecture provides a record of punctuated retrogradation of fluvial and deltaic to marine depositional systems despite strong sediment input from the KM that may have been overwhelmed by substantial flexural subsidence. South of the Canyonville fault, the Camas Valley Fm rests directly on pre-Cenozoic basement, marking a regional unconformity that signals the end of Siletzia collision and onset of Cascadia subduction (~50–47 Ma). This study lays the groundwork for further studies using detrital geo/thermochronology to understand the tectono-sedimentary response to Siletzia collision in SW Oregon.