Transgressive and regressive deposits forming the barriers and beachplains of the Columbia River Littoral Cell, USA

Holocene transgression of the Columbia River Littoral Cell (CRLC) led to ravinement of the lowstand topography and the subsequent formation of a transgressive sequence of deposits. During the late Holocene, sea-level rise slowed and then stabilized, leading to the deposition of a regressive sequence that was expressed geomorphically as beachplains and barrier spits. The major focus of this study was to describe the architecture of the transgressive and regressive deposits of the CRLC; this was facilitated by the use of drill core, radiocarbon dates, and geomorphic information. ralized stratigraphic sequence for the CRLC sand bodies consists of a ravinement overlain by a coarse transgressive lag, which is overlain by transgressive and regressive beachface and shoreface deposits that are covered, in places, by aeolian sand. Throughout the Holocene, sediment discharge from the Columbia River was so high that accommodation space was rapidly occupied and beach progradation occurred as sea level was still rising. Today, even as areas close to the Columbia River are experiencing highstand regressive conditions, in the most northerly part of the littoral cell transgressive conditions still exist and a ravinement is being cut due to insufficient sediment input to fill available accommodation space. ment under the oldest CRLC barrier spit, near the southern end of Long Beach Peninsula, began at about 7–8 ka, while ravinement of the northern portion of the Clatsop Plains was initiated at approximately 5 ka. The ravinement surface was formed at about 4 ka in the Grayland Plains sub-cell and < 1 ka in the North Beaches sub-cell. The diachronous nature of the ravinement surface is reflected in the generally younger radiocarbon ages toward the northern sub-cells.