UPLIFTED PLEISTOCENE TERRACES, WILLAPA BAY AND GRAYS HARBOR, SOUTHWESTERN WASHINGTON COAST

Uplifted estuarine deposits inform the permanent, late Pleistocene vertical displacement rates in the upper plate of the Cascadia subduction zone at the latitude of southwestern Washington. The Holocene geologic record indicates coastal co-seismic subsidence associated with Cascadia megathrust earthquakes; yet, long-term coastal uplift is observed at many locations along the Cascadia margin, including near Grays Harbor and Willapa bays, Washington where estuarine deposits are preserved as uplifted terraces. Our new detailed mapping includes three late Pleistocene estuarine units at up to 60 m elevation and one terraced, early-to-mid Pleistocene estuarine unit underlying the younger estuarine units. We use new infrared stimulated luminescence dates, coupled with other recent optically stimulated luminescence dates and previously published age estimates based on fossil bivalves, to constrain ages for these deposits. Within error, the late Pleistocene sediments were likely deposited during marine isotope stages 5a, 5c, and 5e (82, 96, and 123 ka respectively). Using elevation ranges for the surface of the terraces as well as estimates of terrace back-edge elevation, we calculate surface uplift for the estuarine units. Uplift rates, accounting for the range of potential ages and marine isotope stage sea-level heights, range from -0.1 to 1.4 mm/yr with the most likely uplift rates between -0.1 to 0.8 mm/yr. Although southwestern Washington shows low relief compared to much of coastal Cascadia, the late Pleistocene uplift rates are consistent with previously published uplift and incision rates along the Cascadia margin. In other segments of the Cascadia coastline, permanent uplift of terraces can be tied to active local structures in the upper plate, and ongoing mapping and potential field investigations in the study area are addressing such a possibility.