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

Abstract

Beaches and barrier spits of the Columbia River littoral cell (CRLC), 160 km in length, experienced substantial progradation, 0.5–2.5 km in width, during the late Holocene. The accreted beach plains and barriers preserved evidence of episodic catastrophic retreat that was forced by coseismic subsidence. The abrupt subsidence events (0.5–1.5 m submergence) are associated with slip on the underlying Cascadia megathrust, with a mean recurrence interval of ∼ 500 yr. Ground penetrating radar (GPR) profiles taken across the barriers and beach plains show high-angle beach retreat scarps (3–15 m vertical relief) and foredune-ridge slump scarps (5–15 m vertical relief). The upper-shoreface retreat features are correlated throughout the littoral cell by radiocarbon age, position relative to abandoned foredune ridges, and distinct sequences of large and small scarps. Particularly large retreat scarps (10–15 m in height) are associated with the first oldest and fourth oldest prehistoric scarps, ‘A’ and ‘D’, respectively. A maximum of 10 retreat scarps in the Long Beach subcell date from 216 ± 211 calRCYBP to 4756 ± 114 calRCYBP. The youngest seven scarps (A–G) are correlated between all four subcells, showing a regional response to the coseismic strain cycles. Backshore disturbance features (1–3 m vertical relief) are observed in the GPR profiles, but they are not identified here due to their discontinuities between adjacent profiles at ∼ 5 km longshore spacing. 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.