Paleotsunami records in two localities of the central Cascadia margin, Neskowin and Beaver (West Coast, U.S.A., Northeast Pacific Ocean coast), are extended landward to distal flood plain settings. Three paleotsunami sand sheets are correlated to Cascadia subduction zone earthquakes, between 0.3 and ∼1.3 ka in age. One older paleotsunami layer (2960–3220 cal YBP) is apparent in some deeper core sites from the Beaver Creek locality. Marine sand (22%–100%) and marine diatoms (40%–100%) from the distal sand sheets distinguish the catastrophic marine inundations from creek floods. The greatest inundations are correlated to two Cascadia paleotsunami events, #3 at ∼1.3 ka and an older event between ∼2.6 and ∼3.2 ka, based on radiocarbon dating and great earthquake sequence. The best-preserved records are from paleotsunami #3, which reached 4.1 km in overland inundation up the North Beaver flood plain (3 m elevation North American Vertical Datum). At the Neskowin locality, a sand sheet from the #3 paleotsunami was traced to 8.3 m elevation in the Hawk flood plain. Adjusting for paleosea level at 1.3 ka, we estimate that the #3 paleotsunami run-up height reached 9 m at a landward distance of 1.0 km in Neskowin. The paleotsunami sand sheets in Neskowin and Beaver represent the maximum recorded distal run-up for Cascadia paleotsunami reported to date. The potential for preservation of marine surge deposits in alluvial flood plains should greatly extend the geologic record of prehistoric inundations in other susceptible coastlines.
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Vol. 2010 • No. 264