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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 945

Last Page: 946

Title: Evolution of Late Holocene Beach Accretion Plain on Pacific Coast: Grayland, Southwestern Washington: ABSTRACT

Author(s): John C. Kraft, C. Barnosky, R. Nickmann

Article Type: Meeting abstract


An extensive (140 km+) beach-accretion plain and lagoon-barrier coast has evolved over the last thousand years in southwestern Washington and northern Oregon. Before 1,100 years, a highly indented shoreline with a steep straight wave-cut cliff and terrace faced the Pacific much like the remainder of the American west coast. In the Grayland area, the first beach-accretion ridge formed at distances varying up to 7 to 10 km seaward from the former sea cliff, followed by two additional beach-accretion ridges with a maximum 2.5 km width of the plain at its southern end. A boring in the bog between the first ridge and older wave-cut cliff included a 1.2-m thick peat underlain by beach sands. Bog pollen is dominated by Picea and Tsuga heterophylla, with significant Pinus, Cupres aceae, and Alnus, essentially a modern flora. A basal peat RC14 date indicates the earliest barrier formed approximately 1,100 years ago. South of the Grayland accretion plain between Cape

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Shoalwater-Cove Point and the North Beach Peninsula barrier-accretion plain, occurs a 9-km tidal shoal and channel area. A narrow tidal channel, over 20 m deep, with strong tidal currents, has eroded northward over 3 km in this century. Accompanying this erosion, the flood-tide Graveyard-Empire spit is prograding into Willapa Bay, overlapping two older spits at Tokeland and Kindred Island. The deep tidal channel between Willapa Bay and the Pacific Ocean has migrated from south to north at least three times since the initiation of the Grayland beach-accretion plain, correlating the three flood-tidal spits with the three major beach-accretion ridges of the Grayland plain. A secondary, southerly deep tidal channel is forming and may again erode northward, driven by tidal and littoral pro esses, initiating another beach-accretion ridge on the Grayland plain and another southeasterly trending flood-tidal spit in the near future. The geologic events causing this large late Holocene Epoch coastal plain format, unique to the Pacific coast, remain obscure.

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