Accommodation space in a high-wave-energy inner-shelf during the Holocene marine transgression: Correlation of onshore and offshore inner-shelf deposits (0–12 ka) in the Columbia River littoral cell system, Washington and Oregon, USA

The Columbia River Littoral Cell (CRLC), a high-wave-energy littoral system, extends 160km alongshore, generally north of the large Columbia River, and 10–15km in across-shelf distance from paleo-beach backshores to about 50m present water depths. Onshore drill holes (19 in number and 5–35m in subsurface depth) and offshore vibracores (33 in number and 1–5m in subsurface depth) constrain inner-shelf sand grain sizes (sample means 0.13–0.25mm) and heavy mineral source indicators (>90% Holocene Columbia River sand) of the inner-shelf facies (≥90% fine sand). Stratigraphic correlation of the transgressive ravinement surface in onshore drill holes and in offshore seismic reflection profiles provide age constraints (0–12ka) on post-ravinement inner-shelf deposits, using paleo-sea level curves and radiocarbon dates. Post-ravinement deposit thickness (1–50m) and long-term sedimentation rates (0.4–4.4mka−1) are positively correlated to the cross-shelf gradients (0.36–0.63%) of the transgressive ravinement surface. The total post-ravinement fill volume of fine littoral sand (2.48×1010m3) in the inner-shelf represents about 2.07×106m3year−1 fine sand accumulation rate during the last 12ka, or about one third of the estimated middle- to late-Holocene Columbia River bedload or sand discharge (5–6×106m3year−1) to the littoral zone. The fine sand accumulation in the inner-shelf represents post-ravinement accommodation space resulting from 1) geometry and depth of the transgressive ravinement surface, 2) post-ravinement sea-level rise, and 3) fine sand dispersal in the inner-shelf by combined high-wave-energy and geostrophic flow/down-welling drift currents during major winter storms.