STRATIGRAPHIC AND GEOMORPHIC EVOLUTION OF THE GLACIATED AND TECTONICALLY ACTIVE COASTAL ZONE OF THE OLYMPIC PENINSULA, WASHINGTON, USA

The Olympic coastal zone spans the outlets of major glaciated valleys and lies atop the Cascadia Subduction Zone. Its Middle and Late Pleistocene stratigraphic and geomorphic evolution reflects interplay of those factors, modulated by sea level fluctuations. New LiDAR topographic data and luminescence dating refine understanding of the evolution of this dynamic coastal zone.

LiDAR topographic data reveal new details of glacial events in the lower valleys. In the Hoh valley, newly recognized Late Pleistocene ice limits lie within several km of the modern coastline and include small-scale push moraines and fluted moraines. In the Queets valley, similarly well-preserved ice-marginal features suggest that moraine systems there are much younger than previously inferred.

Luminescence dating of sand lenses also compresses the timeframe of coastal evolution. Pleistocene stratigraphy in the coastal zone includes glacial-fluvial outwash, beach and nearshore marine sediments, and locally derived non-glacial sediments. A prominent wave-cut surface separates older and younger stratigraphic sequences. Beach sands associated with the wave-cut surface yield multiple OSL ages indicating MIS 5a origin (previously MIS 5e). This revised correlation suggests significantly higher coastal uplift rates. Sediments in the older sequence appear to be MIS 6 and younger, contrasting with previous age inferences spanning Early-Middle Pleistocene. OSL and IRSL ages in younger outwash units confirm previous inferences of multiple MIS 4-2 glacial events.

These new geomorphic, stratigraphic, and geochronologic data indicate a dynamic interplay of MIS 6-2 glaciation, coastal uplift, and sea-level variation. While previous findings had suggested alternation between coastal uplift and subsidence through a relatively long Pleistocene geologic history, our new findings indicate that the coastal zone geomorphology and stratigraphy reveal more dynamic processes over shorter time scales.