DIATOMS AS AN AID IN IDENTIFYING LATE-HOLOCENE TSUNAMI DEPOSITS

Diatoms (Bacillariophyta) help identify the onshore deposits of tsunam is from earthquakes on the Cascadia subduction zone along the Pacific coast of Oregon, Washington, and British Columbia, and on faults high in the North American plate in the Puget Sound area of Washington. At the Copalis River, Washington, diatom analyses suggest that a tsunami deposit about 300 calendric years old (300 yr BP) originated from sand y shoals of the lower estuary rather than nearby beaches or coastal du nes. At Cultus Bay and West Point, Washington, well-preserved benthic estuarine diatoms in sand sheets overlying tidal-marsh peat indicate t hat the deposits came from intertidal or nearshore areas of Puget Soun d. On an abruptly uplifted mudflat at the landward end of Hood Canal a t Lynch Cove, Washington, tidal-flat diatoms refute the possibility of a terrestrial source for the sand. Diatoms in 300-yr-BP tsunami depos its on the Niawiakum River, Washington, confirm that the sand in these deposits had a marine source, and help to identify the landward exten t of tsunami inundation. Diatom assemblages in deposits of the 300 yr BP and AD 1964 tsunamis at Port Alberni, British Columbia, consist of different dominant taxa, but both indicate that the sand units origina ted from Alberni Inlet. Diatoms add to stratigraphic evidence that tsu namis flooded Bradley Lake, a freshwater lake on the south-central Ore gon coast, three times during the past 1700 years. Planktonic marine d iatoms only found above 1-70-cm-thick sand layers in otherwise clayey lacustrine sediment imply tsunami inundation.