POTENTIAL APPLICATION OF COSMOGENIC NUCLIDE DATING FOR THE QUANTITATIVE ASSESSMENT OF SHORELINE RETREAT IN THE PUGET SOUND, WA

The long-term retreat rate of Puget Sound shorelines derived from unconsolidated sediment has been difficult to quantify because the geomorphic evidence used to establish prior shoreline positions (i.e., wave-cut platforms) is itself subsequently eroded by submarine processes. A quantitative assessment of the maximum possible Holocene shoreline retreat on southern Whidbey Island, WA has been completed using reconstructed longitudinal profiles of relict post-glacial channels. These channels were graded to the post-glacial marine high-stand (~140-160 feet A.S.L. on south Whidbey Island), and it is possible to infer the original post-glacial shoreline position by extending the longitudinal profile of the remnant channel to a point that intersects the original post-glacial terrace elevation (140 ft). In addition to geomorphic evidence, we are testing whether ¹⁰Be dating can quantify recent erosion rates by dating the exposure age of residual till boulders eroded primarily by wave processes and deposited as a lag on the wave-cut platform. Preliminary data reported from initial test locations are promising. ¹⁰Be dating of a residual till boulder at Maple Point is consistent with the shoreline retreat rate calculated from geomorphic data. Two ages for the Possession Point location indicate that landslide events as young as centuries can be resolved. Preliminary ¹⁰Be results also indicate that prior exposure histories may be problematic in settings where the original till cover is thin. The results will yield important implications, particularly with continuing population growth, for landslide hazard assessment, residential planning, and long-term erosion-sedimentation rates in the Puget Lowland.