TSUNAMI ORIGIN FOR AN 1,100 YEAR OLD ENIGMATIC SAND SHEET IN LYNCH COVE, PUGET SOUND, WASHINGTON, U.S.A

Grain size trends and field observations of an ~1,100-year-old sand sheet at a salt marsh at the southern end of the Hood Canal suggest deposition via tsunami that correlates to seismic activity (M > 7.0) along the Tacoma Fault.

The presence of saltwater tidal, capped by an abrupt and anomalous sand layer, followed by freshwater peat and modern day saltwater peat suggests a co-seismic event. Further field observations reveal a massive sand unit (average thickness 0.24 m), erosional basal contact, and loading structures.

Approximately 400 core-samples gathered using a hand-push auger show that the sand sheet is continuous over an area of at least 0.5 km2. To process the sand unit collected from each core for grain size analysis, we used a laser diffraction particle-analyzer. Grain size analysis reveals that the sand sheet fines landward and that the sand is poorly sorted. Three box cores collected in the field, and later sampled at 2 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.

Before this study, researchers suspected that the sand sheet at Lynch Cove resulted from a tsunami generated by seismic activity on the Seattle Fault. However, inherent problems with this argument including that the Seattle Fault does not intersect the Hood Canal, and the Seattle Fault terminates ~20 km away from Lynch Cove. There is now evidence to show that the Tacoma Fault intersects the Hood Canal only 4km south of Lynch Cove and that it was seismically active ~1,100 yrs B.P.

Other hypotheses also explored as part of this research: landslide generation of tsunami, sand deposition via liquefaction, sand deposition via a large storm in the Hood Canal, burial of an ancient beach ridge complex.