Dating of late Holocene beach shoreline positions by regional correlation of coseismic retreat events in the Columbia River littoral cell, USA

The seven regional scarps (A-G) are directly correlated to seven coseismic subsidence events, reported to date between 0.3 and ∼ 3.2 ka. The correlation of scarps to earthquake events provides high-resolution proxy ages for shoreline positions in late Holocene time. The proxy shoreline positions are dated at 0.3, 1.1, 1.3, ∼ 1.7, ∼ 2.5, ∼ 2.8, and ∼ 3.2 ka, based on buried wetland sequences from adjacent tidal basins. The paleoshoreline dating demonstrates that the onset of net shoreface progradation (between 4.7 and 0.3 ka) was delayed with increasing distance (0-100 km) from the Columbia River sand source. The history of beach progradation reflects the sequential filling of bay and shoreface accommodation spaces located down-drift of the Columbia River mouth. These late Holocene shoreline changes portend future redistributions of existing sand within the littoral system as the limited sand reserves continue to migrate away from the Columbia River mouth source.