THE EVOLUTION OFTHE BIG DUNE IN THE GRAYLAND SEGMENT OF THE COLUMBIA RIVER LITTORAL CELL

In most of the Columbia River Littoral Cell(CRLC)the dunes are linear paleo-foredunes formed by the vegetation trapping eolian sand on a prograding shoreline. However in the Washington portion of the CRLC there is a single extraordinarily high dune that has a more complex history. The dune,called the "Big Dune" by the locals, has been eroded by the Willapa Harbor Channel producing an excellent exposure in the Grayland segment of the CRLC. This exposure shows a buried soil horizon that once supported a spruce forest. The fossil trees from this forest,along with the bedding and some heavy mineral ratios were used to resolve the evolution of this dune.

Beach sands with abundant heavy minerals mark the oldest part of the exposure. A thin layer of eolian sand upon which the soil and fossil forest developed overlies these beach sands. Above the soil lies a second set marine sands, the uppermost of which exhibit convolute bedding caused by liquefaction. These deposits are associated with the coseismic subsidence and the tsunami that occurred during the last great subduction zone earthquake(1700 AD). These deposits partially buried the spruce forest which was subsequently completely buried by eolian sand. This dune reactivation sequence is responsible for the dune's unique height and its persistence across all three segments of the Washington CRLC.