Grain size trends and field observations of an approximately 1100-year-old sand sheet within a salt marsh in Hood Canal at Lynch Cove, Puget Sound, Washington, suggest deposition by multiple mechanisms, including liquefaction and tsunami. Because a tsunami has not occurred within Puget Sound during modern times, it is important to study past sedimentary evidence of tsunamis in the geological record to understand where, when, and how often these events may occur at this location. The sand sheet is about 0.24 m thick, overlies tidal-flat mud, and is abruptly capped by freshwater peat. To determine extent, thickness, and grain size trends within the deposit, we gathered 350 core samples using a hand-push auger. A laser diffraction particle analyzer was used to determine grain size from each sand unit collected via coring. Grain size analysis reveals that the sand sheet fines landward and is poorly sorted. Three box cores, sampled at 2.0-cm intervals, reveal upward-fining grain size trends. Sedimentary structures suggesting bedload transport (such as cross-bedding and ripple marks) were not visible either in the field or in X-ray radiographs. Despite local evidence of liquefaction, the Lynch Cove sand sheet is largely consistent with sedimentation patterns observed following modern tsunami events. The Tacoma Fault was seismically active around the time of deposition. The Sunset Beach Fault and landslide, some 4 km south of Lynch Cove, was likely created by movement from the Tacoma Fault. This is the most likely source of tsunami generation at Lynch Cove around 1100 years ago; however, the possibility of submarine landslide-generated tsunami has not been ruled out entirely. Beyond earthquake and submarine landslide activity, other source mechanisms are less likely to produce a wave that could deposit significant amounts of sand after navigating the entire 70 km length of Hood Canal.
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Vol. 2009 • No. 252