SEDIMENT AND MICROFOSSIL MARKERS OF TSUNAMI INUNDATION EXTENT FOLLOWING THE GREAT CA. 1700 CE EARTHQUAKE AT THE SALMON RIVER ESTUARY, OREGON, USA (Invited Presentation)

Coastal marshes along the Cascadia Subduction Zone (CSZ) preserve widespread stratigraphic evidence from the most recent great earthquake circa (ca.) 1700 CE, including sandy tsunami deposits. To better understand the magnitude and distribution of slip in the ca. 1700 event, earthquake source models are commonly evaluated on their ability to generate simulated tsunamis that can match or exceed the minimum inundation marked by ca. 1700 sandy tsunami deposits. However, observations of modern tsunamis demonstrate that the true inundation extent can exceed the inland limit of sandy sediment deposition, while mud-sized tsunami sediments can be deposited up to the full extent of wave inundation. Defining the full extent of tsunami inundation with fine-grained tsunami deposits from the ca. 1700 event would provide a valuable constraint on earthquake and tsunami models. At the Salmon River estuary, Oregon, distinct evidence of earthquake subsidence and tsunami inundation ca. 1700 is preserved in the stratigraphy as intertidal, organic-rich, wetland soils sharply overlain by commonly normally graded sand which then transitions to tidal flat silts. Here, we use grain size, microfossils (diatoms), and computed tomography (CT) scans, to identify evidence of tsunami inundation in cores beyond the previously mapped inland extent of ca. 1700 CE tsunami-deposited sand. New grain size analyses reveal silty sand (72% sand) and sandy silt (35% sand) above the sharp ca. 1700 CE subsidence contact up to ~1 km beyond where sand was mapped in earlier studies. Preliminary diatom results show that taxa assemblage characteristics observed within the sandy portion of the mapped tsunami deposits have also been identified in more fine-grained sediments above the ca. 1700 CE earthquake subsidence contact at core sites throughout the estuary, and more importantly, beyond the mapped inland extent of sandy tsunami deposits. Extending these methods to other sites along the CSZ to define the full extent of tsunami inundation will improve understanding of potential impacts and hazards of future megathrust events.