Potential Cascadia Tsunami Deposits From a Tidal Marsh at Hood Canal, Puget Sound, Washington

Abstract:

Two candidate Cascadia tsunami deposits are found in tidal marsh sediments near Lynch Cove, at the head of Hood Canal. The deposits are suggestive of tsunami deposits in their tabular form, distribution in the marsh, sediment type, and microfossil content. The deposits are traceable in channel bank exposures, dug pits, and sediment cores on the outer edges of the marsh, and thin landward to an inland extent of at least 200 m. The deposits are made up of mud to fine sand, and are similar in appearance and grain size to the broad adjacent tidal flat. The deposits also contain tidal flat diatoms, and are notably different from the diatoms in the marsh sediments below and above the deposits. Radiocarbon ages of plant fossils show that the younger deposit (Layer A) postdates A.D. 1680, and may therefore represent the A.D. 1700 Cascadia tsunami. The older deposit (Layer B) has a two sigma age of A.D. 1170-1220, which overlaps in time with coseismic subsidence at the mouth of the Columbia River (subsidence of Soil W, A.D. 1000-1190), and with adjusted age estimates for deep-sea turbidites (turbidite T3, A.D. 960-1180). It is unknown whether Cascadia subduction zone tsunamis have left a geological record in Puget Sound. A recent unpublished simulation by the Pacific Marine Environmental Laboratory, NOAA, of a Cascadia tsunami propagating into Puget Sound predicts maximum water levels of over 4 m for some high water “hot spots” at the heads of narrow waterways. The marsh near Lynch Cove is a simulation “hot spot” with predicted water levels over 3 m. Narrow, wave-amplifying waterways, together with an available sediment supply, and predicted high water levels suggest that Cascadia tsunamis could leave behind a geologic record in Puget Sound, given conditions favorable for deposit preservation. Alternatively, one or both of the deposits may represent tsunamis generated by slides or fault displacement in Hood Canal.