Tsunami origin for an ∼1,100 year old enigmatic sand sheet in Lynch Cove, Puget Sound, Washington

Grain size trends and field observations of a ∼1,100-year-old sand sheet within a salt marsh in the flood Canal at Lynch Cove suggest deposition by multiple mechanisms including liquefaction and tsunami. Because a tsunami has not occurred within the Puget Sound during modern time, it is important to study past sedimentary evidence of tsunamis in the geologic record to understand where, when, and how often these events may occur at this location.; The sand sheet is ∼0.24 m thick, overlies tidal flat mud, and is abruptly capped by a freshwater peat. To determine thickness and grain size trends within the deposit, I gathered 350 core-samples using a hand-push auger. The collection of core samples demonstrates that the sand sheet is continuous over an area of at least 0.5 km2. To conduct grain size analysis of the sample of the sand unit collected from each core, I used a Malvern Mastersizer 2000 Particle Analyzer that measures via laser diffraction. Grain size analysis reveals that the sand sheet fines landward from about 0.13 mm to 0.05 mm and that the sand is poorly sorted. Three box cores collected in the field, and later sampled at 2.0 cm intervals, revealed upward fining grain size trends. No sedimentary structures were visible either in the field or in x-ray radiographs of lacquer-acetate peels gathered from the box cores. Although feeder dikes found in Lynch Cove provided evidence of liquefaction. The Lynch Cove sand sheet is thus largely consistent with sedimentation patterns observed following modern tsunami events.; Movement along the Tacoma Fault, which intersects Hood Canal only 9 km south of Lynch Cove and was seismically active around the time of deposition, is the most likely source of tsunami generation at Lynch Cove ∼1,100 years ago. Other source mechanisms, such as movement on the Seattle Fault, or a landslide into Hood Canal, are less likely to have produced a wave large enough to leave 24 cm of deposit after navigating the entire 70 km, fishhook shaped length of Hood Canal.