Latest Pleistocene and Holocene (2–16 ka) sedimentation in the Columbia River Estuary, Oregon, USA

A deep borehole drilled at Warrenton, Oregon in the ancestral valley of the Columbia River represents a geologic record that extends from 2 ka to 16 ka. Prior to the onset of the Holocene marine transgression at 16 ka, the incised Columbia River valley was cut to 112 m below present sea level at this location. The onset of estuarine circulation in the Columbia River estuary occurred at 11.5 ka as determined by the first appearance of brackish water diatoms in sediments from borehole cores at a depth of 70 m. Grain size and heavy mineral analyses indicate that the Columbia River tidal basin served initially (16–11 ka) as a bedload-bypassing conduit to the continental shelf, and/or the Astoria Canyon. With the ongoing Holocene transgression, the Columbia River tidal basin became more efficient as a river sediment sink between 11.5 and 9.0 ka. After 9.0 ka, the filling tidal basin again served as a sediment-bypassing conduit (source) of sand to the coastline and continental shelf. Heavy mineral analyses indicate that between 16 and 11 ka the Columbia River tidal basin was dominated by sediment from the metamorphic interior basins of the Columbia drainage basin. The dominant sediment source changed from the metamorphic interior basins of the Columbia drainage basin to the Cascade volcanic arc between 11.5 and 9.0 ka. After 9.0 ka, the tidal basin was dominated by Cascade volcanic arc derived sediments. The total volume of Holocene sediment, primarily bedload, which has accumulated in the lower Columbia River valley, is 73 km 3. This compares to only 13-km 3 accumulation during the last 5 ka. The tidal basin accumulation rate gradually increased from 0.6 million m 3/yr to slightly over 18 million m 3/yr between the depths of 112 m and 30 m (16 to 9 ka). Above the 30 m depth (corresponding to 8.2 ka), the rate of basin volume fill dramatically decreased to just over 4 million m 3/yr. Such a large decrease in sediment accumulation rate suggests that after 9 ka sediments were bypassing the nearly full tidal basin to the beaches and inner shelf. The rates of bedload bypassing the lower Columbia River valley, substantially greater than 2.4 million m 3/yr, supported the shoreface progradation in the littoral cell during the late Holocene.