Late Holocene relative sea-level changes, Arcata Bay, California : evaluation of freshwater syncline movement using coseismically buried soil horizons

Paleoseismic investigation to identify late Holocene upper crustal movement beneath Arcata Bay has confirmed four buried marsh soils that represent three, possibly four, great southern Cascadia subduction zone earthquakes. Estimated ages of earthquakes are: 250, 1350 to 1190, and 1590 to 1390 cal yr BP, with a possible earthquake at 1290 to 1100 cal yr BP. Coseismically buried soils were identified using lithostratigraphy, estimated diatom biostratigraphy, and radiocarbon age determinations. To test for late Holocene movement of the onshore section of the Freshwater syncline, buried low salt marsh soil horizons were correlated around Arcata Bay. The only continuous buried soil horizon in Arcata Bay is the youngest, which probably subsided coseismically in A.D. 1700 during a regional megathrust event along the Cascadia subduction zone. The youngest buried soil is not warped by the Freshwater syncline and marsh accretion rates are similar at multiple sites, indicating similar depositional histories at the margin of Arcata Bay. Evidence suggests that the Freshwater syncline has not deformed during, or since, the most recent Cascadia subduction zone earthquake.