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Abstract: Stratigraphic and Diatom Evidence for Storms and Tsunamis on the Coasts of Oregon and Washington, USA

HALEY, EILEEN HEMPHILL, U.S. Geological Survey; ROBERT C. WITTER, University of Oregon

The coasts of Oregon and Washington are subject to severe storms as well as to local tsunamis generated by earthquakes in the Cascadia subduction zone and far-travelled tsunamis from across the Pacific Ocean basin. By reconciling the history of past storms and tsunamis on the northwest coast, the potential for such future hazards may be better understood. In order to differentiate storm deposits from tsunami deposits, data combined from stratigraphic and diatom studies are needed. For tsunami deposits, frequency of deposition, stratigraphic position, and character of deposits are important features, as tsunamis occur much less frequently than coastal storms, may be associated with coseismic subsidence, or may deposit sediment in pulses as succeeding waves come ashore. Diatoms provide valuable insight into sediment sources, and clearly distinguish estuarine and marine from freshwater deposits.

Records of past storms and tsunamis are observed at Euchre Creek and Bradley Lake, south-central Oregon coast, and the Niawiakum River, southwestern Washington. At Euchre Creek, four out of five coarse-grained units interbedded in peat were probably deposited by storms during the past 500 years, while a 10-20-cm thick pulsed, sandy unit is a good candidate for the AD 1700 Cascadia tsunami. At Bradley Lake, relatively undisturbed lacustrine sediment is punctuated by anomalous depositonal events — primarily sandy units overlain by massive mud and organic debris — containing delicate, displaced marine diatoms. These are most likely indicative of tsunami inundation, as the lake is positioned too high above sea level to be easily reached by storm surges. In cutbank exposures along the Niawiakum River, the AD 1700 tsunami deposit is a distinct zone of coarse silt capping a buried upland soil. It shows evidence for pulsed deposition, and contains pristine tidal-flat diatoms. Conversely, a possible storm deposit within the buried soil is recognized only by a slight increase in silt and occurrences of very rare marine planktonic diatoms in otherwise diatom-free peat.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah