Geologic Records of Net Littoral Drift, Beach Plain Development, and Paleotsunami Runup, North Sand Point, Olympic Peninsula, Washington, USA

Abstract

A geologic cross-section was constructed across a narrow late Holocene beach plain in a small southwest-facing pocket beach in North Sand Point. Olympic National Park, Washington, to test hypotheses about net littoral drift, potential tectonic uplift, and paleotsunami runup height. Twenty topographic stations (0–12 m elevation NAVD88) and eight auger core sites (2–5 m depth) were examined to establish the stratigraphic development of the narrow beach plain (120 m width). Existing radiocarbon dates and new optically stimulated luminescence (OSL) analyses were used to establish the onset (∼1.5 ka) and termination (∼0.6 ka) of net beach progradation, confirming net northward littoral drift in latest Holocene time. The anomalous high elevations of the beach plain resulted from an abandoned foredune ridge (9 m elevation) developed above the prograded beach deposits (6 m elevation). No tectonic uplift is required to account for the beach plain elevations. A fine gravel layer (5–30 cm thickness) draped the top of the dune ridge at 7–9 m elevation, but it is not traced to 11–12 m elevation in the adjacent late Pleistocene terrace. The gravel layer is attributed to catastrophic marine surge deposition (10 ± 0.5 m elevation) from a nearfield or locally produced Cascadia paleotsunami at ∼1.3 ka. The short duration of the recorded beach plain progradation (about one thousand years), together with a range of OSL grain bleaching ages (11.1–2.3 ka) that pre-date the beach plain deposition, attest to prior pocket beach instabilities and/or sand recycling in the high wave-energy beaches of the northwest Olympic Peninsula coastline.