DIFFERENTIATING QUATERNARY ESTUARINE DEPOSITS FROM POTENTIAL PRE-LATE WISCONSIN OUTWASH DEPOSITS SOUTHWESTERN WASHINGTON COAST

Coastal deposits can represent sea level high stands (marine) and low stands (glacial drift/outwash/fluvial). Dense vegetation and limited access in southwestern Washington have limited detailed mapping related to Quaternary sea level change. Sedimentary deposits south of Grays Harbor and east of Willapa Bay were previously mapped as a single unit and inferred to be estuarine terraces, uplifted in some locations to over 120 m a.s.l. This elevation for Quaternary marine sediments implied a rapid uplift rate, inciting further investigation aided by recent lidar data. Characterization of deposits by stratigraphic and lithologic description, geomorphic mapping using GIS analyses, and dating using optically stimulated luminescence (OSL) indicate both estuarine and fluvial/delta deposits in the study region. Estuarine deposits are limited to two terraces at between 13-22m a.s.l., and between 38-48 m a.s.l. These likely represent marine isotope stage (MIS) sea level high stands of 5a and 5c, respectively. Geomorphic analyses indicate higher elevation sediments are not terraced but have gently sloping surfaces that resemble terraces in the field. These consist of cross-bedded sands and gravels with silt interbeds, likely representing fluvial or delta deposits. High volumes of fluvial sediments may be related to ice advances in the Puget Region, with the Chehalis River transporting outwash to the coast. A new OSL date suggests these may represent outwash from the Double Bluff advance during MIS 6. The study region has been uplifted, with uplift rates for the estuarine terraces at approximately half a millimeter per year, in agreement with other regional long-term uplift rates along Cascadia. These findings suggest that Quaternary sediments on the southwestern Washington coast represent both sea level high and low stands, similar to deposits to the north, and that long-term uplift has complicated straightforward correlation of landforms to expected marine isotope stages.