Structural Controls on Paleodrainage Morphology on the Oregon Continental Shelf During the Last Glacial Maximum

High-resolution Chirp and sidescan sonar data, collected on the continental shelf offshore from southern Oregon, image paleodrainages, extensive faulting and deformation, and submerged paleolandscapes that might contain important archaeological resources. During the last glacial maximum, eustatic sea level was ~120 m below present and the coastline was located near the continental shelf edge. During this time, rivers flowed across the shelf creating seaward extensions of the modern fluvial systems in the study area, the Umpqua and Siuslaw rivers. We observe major variations in the acoustic character and morphology of projected paleodrainages offshore from each of the modern rivers. There is little to no evidence of the paleo-Siuslaw River, which is observed only as a surface with high amplitude acoustic character. In contrast, the Umpqua River is observed as an incised valley with onlapping infill. Furthermore, the Umpqua River does not flow directly east-west, but appears to deflect southward around substantial structure mapped on the mid to outer shelf. The study area is part of the Cascadia subduction margin, which is characterized by wide-spread compression and uplift. The structure on the shelf is likely a result of compression along the margin, expressed as faulting and folding, and possible fluid seeps, which control the fluvial drainage pathways. The mapped paleochannels are good indicators of potential archaeological sites, as many Holocene-aged sites have been documented along the Umpqua and Siuslaw rivers onshore. Structural controls on shelf morphology appear to play an important role in fluvial pathways. Detailed mapping of these structures could aid future archaeological investigations and paleochannel modeling in the region.