NEW COASTAL EVIDENCE FOR CRUSTAL FAULTING ~1,000 YEARS AGO, SOUTHWESTERN PUGET LOWLAND, WASHINGTON

One or more earthquakes about 1,000 years ago produced a tsunami near Bremerton and liquefaction beside two parts of Hood Canal. These findings, from new studies of intertidal outcrops at Gorst, Lynch Cove, and the Skokomish Delta, are supported by data from topographic profiles, particle-size analysis, and identification of fossil mollusks and diatoms. Detailed stratigraphy and sedimentology from salt marshes in Washington state reveal widespread evidence for uplift of more than 2 meters associated with sand layers interpreted to be tsunami and liquefaction deposits. Uplift is constrained by the presence of mid to lower intertidal bivalve shells in the basal mud and fresh water swamp seeds and leaves in the lower peat. In contrast to previous publications, no conclusive evidence for tsunamis was found along Hood Canal. At Gorst, on Sinclair Inlet and the hanging wall of the Seattle fault, a sand sheet 5 to 12 cm thick containing pebbles, and an overlying thicker, silty deposit of uncertain origin split the contact between shelly tideflat mud and cedar forest peat. The coarser, fining upward layer is interpreted as a tsunami deposit. In Lynch Cove, on the hanging wall of the Tacoma fault, multiple domal liquefaction features occur over an area 0.9 km wide and 1.4 km long. Commonly, but not everywhere, there is a sand layer up to 80 cm thick between the tide-flat mud and forest peat associated with sand dikes. On the Skokomish Delta only three liquefaction features are found. The sand layers extend as much as 10 meters. In the three localities the grain size ranges from fine sand to pebble and the deposits contain flame structures, rip-up clasts, fluid escape structures, and grading. These data will be used to help constrain the relationships of crustal faults in the Puget Lowland.