LATE-HOLOCENE SEDIMENTATION IN THE BAYS AND ESTUARIES OF THE COLUMBIA RIVER LITTORAL CELL, WASHINGTON AND OREGON, USA

Late-Holocene sediments in Willapa Bay, Grays Harbour, and the Columbia River estuary are preserved in a variety of tidally dominated depositional environments. To fully understand net-sedimentation rates and late prehistoric, early historic and historic sedimentation, a total of 103 vibracores were collected as part of the Southwest Washington Coastal Erosion Study (21 in Grays Harbour, 35 in Willapa Bay and 47 in the lower Columbia River estuary). Penetration depths generally ranged from 2-7 m and core recovery was typically greater than 90%. Coring was accomplished using oyster dredges and a boat-mounted drill platform while a quad with trailer was used for shoreline sand and mud flats. All cores were logged in the field and sampled for lithology, sedimentary structures, sand grain size, wood and shell fragments, and sedimentary structures.

Preliminary results show that modern sand dominated tidal-flats along bay shorelines are anomalous in the geologic record of Grays Harbour and Willapa Bay. Also, episodic tidal flat subsidence in the bays and estuaries is recorded in buried root horizons, as up-core reductions in sand laminae of sand/mud couplets, or as fining-upward trends. Additional coseismic features include possible tsunami pebble layers, fluidized dike and sill intrusions, and slope failure deposits. In the Columbia River estuary a net increase in Holocene sedimentation rates was observed, in addition to a distinctive facies transition in a seaward prograding bay-fill delta.