CONCURRENT TRANSGRESSION AND REGRESSION AT WILLAPA BAY, WS

Estuarine valley fill researchers persistently recognize a high degree of complexity in estuarine these systems. Development of higher resolution models depends upon the thorough integration of sedimentological, ichnological, and stratigraphical data. Consistently, better models are derived using modern and ancient datasets. Willapa Bay is ideal for this purpose because: (1) the bay is margined by well exposed, terraced outcroppings, each demarcating a distinct phase of Pleistocene deposition: (2) a sedimentologically-based stratigraphic model, established by H.E. Clifton and coworkers, equating the modern to these outcroppings, already exists: (3) the bay's uncommonly pristine state. The primary focus of this paper is to discuss the mechanisms of estuary widening associated with wave- and tidal-erosion of Pleistocene bluffs that rim the bay. The rate of erosion, which is locally very high (approximately 1 m per year), is largely dependant on the sand proportion present in the outcrop, the age of the outcropping strata, and the degree to which the bluffs are exposed to incident storm waves. Erosion along the margin of the bay represents trangression of the bay margin that is concurrent with bay infilling (high stand). Notably, wave-cut platforms anchor the modern tidal flats and large areas of the tidal flats are underlain by wave-eroded Pleistocene sediment (firm to hard substratum). The morphology of the substratum is stepped and reveals a complex history of wave and tidal erosion. Also, the surface is demarcated by a Glossifungites-demarcated surface that indicates the emplacement of this surface was sporadic and occurred in a step-wise fashion. Of further interest to this study are the complex stratigraphical relationships between the ‘old’ Pleistocene, the ‘young’ Pleistocene, and the modern bay. Comparison of these strata reveals an extremely complex stratigraphy. Furthermore, the internal geometry, sedimentology, and ichnology of these strata show that each of these units accumulated under different depositional parameters (fluvial input, tidal-influence, sediment supply, and accommodation space).