CHARACTERIZATION OF SHALLOW AND DEEP-SEATED LANDSLIDE-DERIVED SEDIMENT IN THE QUINAULT RIVER CATCHMENT, OLYMPIC PENINSULA, WASHINGTON

Lake Quinault is located at the western base of the Olympic Mountains in Washington State. A natural lake, its dam is composed of terminal moraine from Late Pleistocene glaciers that carved out its present-day catchment. As a result of the area’s geologic structure and high-relief setting, denudation within the catchment is dominated by landsliding. We hypothesize that the flux of landslide-derived sediment into the Upper Quinault River and Lake Quinault may be modulated by the triggering of mass wasting events during great Cascadia earthquakes which are estimated to occur every 300-500 years. Suites of sediment samples were obtained at shallow and deep-seated landslide locations as well as palaeolacustrine outcrops. Organic and inorganic geochemical data are presented in order to determine signatures of each landslide type that can be distinguished in lacustrine deposits. We hypothesize that shallow landslides will contribute sediment that contains evidence of advanced weathering as well as young organic matter derived from recent biomass. Deep-seated landslides, however, should deliver sediment that is less altered with a much older organic contribution. We hypothesize that background sediment in lacustrine deposits will consist primarily of shallow landslide-derived material and that pulses of sedimentation following great earthquakes will exhibit a deep-seated landslide signature. The results will help to elucidate the response of fluvial and geomorphic systems along the Cascadia margin to severe seismic-induced ground shaking.