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Abstract: Sedimentary Facies and High-Resolution Sequence Stratigraphy of Pleistocene Valley-Fill/Estuarine Deposits, Willapa Bay, Washington

H. Edward Clifton

Willapa Bay, a large estuary on the coast of southwestern Washington, provides excellent examples of estuarine and related valley-fill deposits and the complex stratigraphic relationships that can exist therein. Prominent marine terraces around the margins of the bay result from recent tectonic uplift. The assemblage of stacked estuary deposits beneath the terrace surfaces indicates a complex relative sea level history. Many of the facies in these deposits resemble those of the present estuary, as identified in several hundred box cores, mostly taken using SCUBA in the subtidal environments of the modern bay. Most of the Pleistocene sediment accumulated in migrating tidal channels, although muddy intertidal, supratidal, and alluvial facies also occur.

Five depositional units beneath a well-developed terrace surface about 13 m above present sea level demonstrate the complexity inherent in estuary/valley-fill accumulations. Dating using amino acid racemization techniques indicates that the stratigraphic succession reflects the superposition of fifth-order fluctuations in relative sea level onto the latest fourth-order glacioeustatic transgressions and highstands. Lateral shifting of tidal channels during sea level stillstands produced extensive erosional surfaces in the central bay region. Preservation of the different units depends on a combination of their location within the bay, their lithology as it relates to resistance to erosion, and the subsequent history of relative sea level change. Interchanneling of the units (and therefore stratigraphic complexity within the set) increases dramatically in an up-bay direction over a distance of about 10 km.

AAPG Search and Discovery Article #90953©1995-1996 AAPG Distinguished Lecturers