Controls on Paleochannel Morphology on the Continental Shelf Offshore Central Oregon, USA
Matthew G. Skakun
Advisor: Dr. Jillian M. Maloney
Monday Dec. 17th, 2018
CSL 422 – 10:30 am
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The continental shelf offshore central Oregon provides a glimpse of how eustatic sea level has affected coastal rivers on a convergent margin since the Last Glacial Maximum (LGM, ~20 ka). Using high-resolution seismic reflection data, we characterized three adjacent submerged paleodrainages and assessed the factors controlling the evolution of drainage morphology over a sea-level cycle. During the LGM, with eustatic sea level ~120 m below present, the shoreline was located near the shelf edge. During that time, the Yaquina, Siuslaw, and Umpqua Rivers drained across the shelf as they adjusted to, and extended towards, a lower base-level. We observed variations in acoustic properties and morphology of projected paleodrainages offshore from each of the modern rivers. All three rivers are influenced by the same regional climate and convergent margin tectonics, and provide an opportunity to compare other small-scale factors that could explain the observed variability of incision along the shelf. We examined several of these characteristics, including drainage basin area, discharge, and continental shelf morphology. The Umpqua River and the Yaquina River, the largest and smallest of the three drainage basins, respectively, were imaged as incised valleys. However, the Siuslaw River, of intermediate size and discharge, does not indicate a well-developed signature of incision. This difference in morphology is best explained by the shape of shelf profiles from the modern mouth of each river to the LGM shoreline. Where convex morphology is observed seaward of the coastal prism, the Umpqua and Yaquina Rivers incised into the shelf, but offshore the Siuslaw, a lack of convexity appears to have resulted in bypass, rather than incision. These results illustrate that even adjacent rivers along the same margin can have a different response to sea level change if shelf morphology is variable. This has implications for lowstand sediment transport from small mountainous rivers, sequence stratigraphic models of paleochannels and associated facies, hydrocarbon exploration studies, and reconstructions of paleoenvironments.