Prehistoric Tsunami Deposits in Floras Lake, Oregon: Using Vibracore Data to Identify Tsunami Events and Constrain Rupture Recurrence Intervals Along the Southern Cascadia Subduction Zone

Although the geologic record of rupture on the Cascadia megathrust has been documented at numerous coastal and offshore sites, many questions remain on how earthquakes and tsunamis have varied in magnitude, extent, and recurrence. Since the onshore record of tsunamis and earthquakes may be the most complete in coastal lakes, we identified Floras Lake on the southern coast of Oregon as an optimal location for the purpose of this study. 38 vibracores were collected from Floras Lake that highlight at least 7 sandy event deposits within the last 7,000 years. We investigate the evidence that supports whether the sandy deposits observed in lake muds are caused by tsunamis, the frequency of tsunami inundation, and whether the ages of sandy tsunami deposits found in Floras Lake can be correlated with nearby coastal lake records such as Bradley and Garrison Lakes. X-Ray Computed Tomographic (CT) scanning, high resolution line scan photography, and detailed, sedimentological core analysis were used to describe the different sediment facies observed and to correlate event layers between cores. Grainsize analyses of sandy beds indicate inland fining and suspension graded (fining upward) sequences that are indicative of deposition from a tsunami. Diatom analyses indicate that Floras Lake typically contains freshwater species, and that marine diatom assemblages are observed following the deposition of sandy tsunami deposits. Erosional bases, mud rip-up clasts, and organic-rich mud caps also point towards the tsunami origin of sandy deposits. Plant macrofossils were sampled for 14C dating and tsunami deposit ages were estimated using Bayesian age models. A preliminary comparison of tsunami events between Bradley and Floras lake indicates that the largest (thickest and most extensive) tsunami sand in Floras Lake dates to 100-1040 cal yr BP and is approximately 550 years younger than the largest event in Bradley Lake (Disturbance Event 5/6). Between 2000 and 4500 years B.P., 6 sandy event beds that are observed in Bradley Lake are not found in Floras Lake. The significant differences in tsunami records between these coastal lakes less than 30 km apart could be due to differences in beach, dune, and lake morphologies that act as thresholds to tsunami inundation, or local variability in offshore tsunami sources.