Geologic controls on paleodrainage incision and morphology during sea level lowstands on the Cascadia shelf in Oregon, USA

The lowstand extensions of two fluvial systems from the Cascadia subduction margin are investigated using high-resolution seismic reflection data to determine relative controls on paleodrainage morphology. We document distinct differences between systems, located ~40 km apart, attributed underlying structure and lithology, shelf gradient, drainage basin characteristics. A cross-shelf valley exists for only one Umpqua River. outer part is a syncline, which formed natural structural low drainage, while inner exhibits incision associated with mid-shelf convex change slope. other system, Siuslaw River, did not develop an across-shelf denoted in by erosional subaerial unconformity. Structural trends perpendicular supporting more anastomosing stream morphology paleo-Siuslaw. edge this system was exposed long period time during last lowstand, preventing substantial incision. Differences elevation, discharge, hinterland geology, also played role morphologies paleo-Umpqua These results highlight how can over short distances active margin.