INVESTIGATION OF A 12,000 YEAR RECORD OF CASCADIA TSUNAMIS AND COASTAL SUBSIDENCE IN OREGON

Evidence of great plate-boundary earthquakes and accompanying coseismic subsidence events is revealed at numerous land-sea interfaces along the Pacific Northwest coast. We recently extracted a 27 meter-deep core at a site near the Sixes River on the southern Oregon coast, approximately two kilometers from the Pacific Ocean. A basal date of ~12,300 cal. yr B.P. implies the core has the potential to yield a continuous terrestrial stratigraphic record extending into the late Pleistocene. If correct, this site may provide the first onshore chronology of plate boundary earthquakes along the Cascadia margin on a timescale comparable in length to the turbidite records off shore. However, numerous variables complicate the interpretation of this long sedimentary record, perhaps more so than some of the younger terrestrial records of coastal paleoseismology. These variables include long-term tectonic movement of the landscape on local and regional scales; sedimentation from marine, riverine, and aeolian sources; base-level rise and fall due to tectonic activity and eustatic sea-level changes; and compaction and degradation of the biologic and sedimentary record over time. Landscape reconstruction of the coring locality through time, with reference to relative sea level, depositional status, and position of the site relative to the coastal shoreline, aids in the recognition and interpretation of paleoseismic indicators. These indicators include changes in bio-, litho-, and chrono-stratigraphy. This study will potentially expand the toolkit for the analysis of other late Pleistocene, nearshore terrestrial records on the Cascadia subduction zone.