ANOTHER LOOK AT ICE RETREAT FROM THE NORTHERN PUGET LOWLAND, WASHINGTON

Cordilleran ice retreat from the northern Puget lowland of Washington in latest Pleistocene time was rapid and occurred mainly along calving margins of the deeply embayed Juan de Fuca and Puget lobes. Reconstructing ice retreat requires ice-contact deposits and those controlled by rapidly changing relative sea level (RSL), which was driven mainly by the balance between rising eustatic sea level and glacioisostatic recovery. Near 48.5° N, RSL fell from an elevation of ~ 100 m to -62 m in latest Pleistocene (?) time, then rose throughout Holocene time. Local RSL records also reflect tectonic activity along this zone of oblique plate convergence. Separating the components of local vertical movement is desirable for many reasons, but difficult in practice, and reservoir effects and dose-rate uncertainties hamper precise dating of events. However, mapping of glacial, glaciomarine and emerged marine deposits during the past two decades and more recent precise DEMs and LIDAR-based topography allow a closer look at the dynamics of ice retreat during the millennium centered at about 13, 000 radiocarbon years BP. Most evidence suggests that Cordilleran ice retreated rapidly, eroding Quaternary deposits subglacially and depositing them in the adjacent marine environment. In a few places on Whidbey Island and in the San Juan Islands and northeastern Puget lowland, ice-contact deposits record where the retreating ice reached local equilibrium. The rate of ice retreat and volume of sediment transported suggest that rapid flow and erosion rates were driven by the warm temperatures and substantial subglacial water flux in the northern Puget lowland.