QUATERNARY DEPOSITS NEAR GRAYS HARBOR AND WILLAPA BAY, COASTAL SOUTHWESTERN WASHINGTON, MAPPED AT 1:24,000 SCALE

Southwestern Washington geology encompasses a broad range of Cenozoic lithologies and a complex structural and tectonic history. Modern estuarine systems are found in Grays Harbor and Willapa Bay, with highlands east of Willapa Bay. The coastal region adjacent to these modern estuaries was previously mapped as a single unit, with Quaternary sediments at elevations up to 160m a.s.l. Previous workers described these as late Pleistocene marine deposits exhibited as a flight of terraces. These sediments are thus a record of long-term coastal uplift. In order to better constrain lithologies and uplift rates I revisited regions previously mapped at 1:62,000 scale, with my mapping at a scale of ~1:24,000. Observations made at outcrops or in hand-dug pits in the field area support my unit descriptions, geologic maps, and stratigraphic sections. These included hand sample and stratigraphic descriptions, gravel point counts, and where possible, depth measurements to the oxidated C-horizon (Cox horizon) in soil profiles. Geomorphic analyses of slope angle and topographic smoothness on 1 m and 10 m resolution digital elevation models (DEM) aided field identification of terraces. My new mapping recognizes nine new units, including estuarine and fluvial sediments. Units range from shallow marine or estuarine sands and silts, to tidal channel sands and gravels, to cross-bedded fluvial gravels and sands with occasional silt interbeds. The marine/estuarine units are found along the southern margin of Grays Harbor and along Willapa Bay. The detailed mapping in conjunction with dating, supports late Pleistocene ages for deposits, with estuarine deposits at elevations up to ~60 m a.s.l. evidence of long-term strain accumulation.