TSUNAMI DEPOSITS BENEATH TIDAL MARSHES ON NORTHWESTERN VANCOUVER-ISLAND, BRITISH-COLUMBIA

Two sand sheets underlying tidal marshes at Fair Harbour, Neroutsos In let, and Koprino Harbour on the northwestern coast of Vancouver Island , British Columbia, were probably deposited by tsunamis. The sand shee ts become thinner and finer-grained landward, drape former land surfac es, contain marine microfossils, are locally graded or internally stra tified, and can be correlated with earthquakes that generated tsunamis in the region. Cs-137 dating and historical accounts indicate that th e upper sand sheet was deposited by the tsunami from the great Alaska earthquake in 1964. Radiocarbon ages on plant fossils within and on to p of the lower sand sheet show that it was deposited sometime after ab out A.D. 1660. We attribute the lower sand sheet to a tsunami from the most recent plate-boundary earthquake on the Cascadia subduction zone about 300 yr ago, extending the documented effects of this earthquake north of the Nootka fault zone. The 1964 tsunami deposits differ litt le in thickness and continuity among the three marshes. In contrast, t he lower sand sheet becomes thinner and less continuous to the north, implying a tsunami source south of the study area. (C) 1997 University of Washington.