Late Holocene Paleoseismicity, Tsunamis, and Relative Sea-Level Changes in Yaquina Bay, Central Coastal Oregon

Wetland sediments that border Yaquina Bay and the lower Yaquina River on the central Oregon coast record paleoearthquakes on the Cascadia Subduction Zone (CSZ). We used gouge corers, Russian peat augers and test pits to examine shallow (3-7 m deep) stratigraphy beneath intertidal and freshwater marshes surrounding the estuary. We focused eight weeks of field investigations at Sally’s Slough because of the presence of a long and stratigraphically contiguous record of buried peat layers overlain by sheets of silty sand or tidal mud. Great earthquakes on the CSZ dropped Yaquina Bay and surrounding tidal marshes and freshwater spruce bogs to low marsh or tidal flat conditions at least 8 times during the late Holocene. Furthermore, several buried marsh deposits were overlain by sheets of silty sand that thinned landward. The sharp upper contacts of buried peat layers were correlated across two transects of closely-spaced gouge cores. Correlations were based on lithostratigraphic characteristics, relative elevation of the sharp upper contacts of peat and lateral continuity. Elevations of core locations were precisely surveyed using a real-time kinematic GPS unit and total station. Relative-sea level changes will be reconstructed from changes in fossil diatom assemblages across sharp contacts between peat and overlying intertidal mud. AMS radiocarbon analyses of fossil spruce needles and other identifiable detritus will provide maximum limiting age estimates for time of burial of peat deposits. We will evaluate whether the chronology of subduction related earthquakes recorded at Sally’s Slough in the Yaquina River estuary is in part or wholly consistent with paleoseismological investigations along other parts of the CSZ. Specifically, we will evaluate whether the central segment of the margin may have at times ruptured independently.