--> ABSTRACT: (Paleo)ecologic and Stratigraphic Applications of Trace Fossils in Estuarine Valley Fill Deposits: Examples from Willapa Bay, Washington, by M. K. Gingras, S. G. Pemberton, and T. D. A. Saunders; #91021 (2010)

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(Paleo)ecologic and Stratigraphic Applications of Trace Fossils in Estuarine Valley Fill Deposits: Examples from Willapa Bay, Washington

GINGRAS, Murray K., S. G. PEMBERTON, and T. D. A. SAUNDERS

Estuarine valley fill researchers are only beginning to recognize the degree of potential complexities inherent to these systems. Development of higher resolution models could further benefit from a more thorough integration of sedimentology and ichnology. Willapa Bay was selected for this purpose because: (1) the bay is margined by well exposed, terraced outcroppings, each demarcating a distinct phase of stillstand 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.

Building on Clifton's work, this study focuses on the bay's ichnology. Employing a variety of techniques (e.g. resin casting, radiography, core and trench excavation, SCUBA, and aquarium studies), ichnologic characteristics of macrobenthic communities were studied and mapped from a wide spectrum of subtidal and intertidal environments. Most communities were found to include characteristic species whose burrows impart a distinct signature to the ichnocoenoese.

Comparing modem ichnocoenoeses to preserved Pleistocene ichnofacies reveals both similarities and differences in analogous facies. Pleistocene deposition appears to have taken place under physico-chemical conditions different to the modern estuary. This is evidenced in the absence of certain key ichnofossils, reduced bioturbation, and typically small trace fossils. Salinity and sedimentation stresses, may account for some of these differences. Sporadic outcrop occurrence of certain ichnofacies are undoubtably tied to the dynamic nature of this estuary. In the development of any actualistic model, understanding man's long-term effects on the biotope (e.g. timing of deforestation, introduction of exotic species, and damming of the Columbia River and the cyclicity of marine processes (e.g. El Nino) is critical.

Finally, modem developments of the Glossifungites ichnofacies (burrows of Upogebia pugettensis) served as a remarkable analog to a Pleistocene surface (nearly identical, large diameter, Ophiomoipha) developed locally along an internal stratigraphic discontinuity. This and other surfaces were examined for their stratigraphic and hydrogeologic significance. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.