CAN THE STRATIGRAPHY OF CRESCENT HARBOR MARSH BE EXPLAINED WITHOUT LAND-LEVEL CHANGES?

Active faults cross-cutting Whidbey Island, Washington may pose significant seismic and tsunami hazard yet to date we know little about the risk of potential damage from events on these faults. Fieldwork was conducted in Crescent Harbor, northern Whidbey Island that aimed to identify evidence of past earthquakes and tsunamis in the stratigraphic record of the tidal marsh. Detailed coring and trenching revealed a stratigraphy of beach gravel overlain by a pervasive intertidal mud unit with a spatially restricted peaty soil between those two units, indicating an isolated region that once existed above high tide before the entire region became subtidal. Above the intertidal mud was a second peat that included sediment indicative of laterally adjacent wetland forest and marsh environments. Today the upper surface at Crescent Harbor is intertidal. Preliminary analysis results in competing hypotheses that facies changes at Crescent Harbor can be explained by land level changes from earthquakes on the Utsalady Point fault, breaching or healing of the beach berm, or a combination of the two. Two previous earthquakes have been identified on the Utsalady Point fault dating to 100 to 400 BP and 1100 to 2200 BP (Johnson et. al., 2004). Future work including radiocarbon dating will help resolve the depositional history of Crescent Harbor, including whether land-level change is required to produce observed stratigraphy. If so, this work could refine the date of these two known ruptures.