The ichnology of modern and Pleistocene brackish-water deposits at WillapaBay, Washington: Variability in estuarine settings

Detailed analysis of modern and Pleistocene deposits at Willapa Bay, Washin gton concentrated on identifying ichnological and sedimentological characte ristics of five estuarine subenvironments. These include: (1) intertidal fl at deposits, which are typically thoroughly bioturbated by a somewhat dimin utive suite of various vertical and horizontal shafts and burrows that are locally cross-cut by robust crustacean dwellings; (2) unburrowed to rarely burrowed tidal creek point-bar deposits, which are sporadically burrowed by minute Gyrolithes, Skolithos, Cylindrichnus, Planolites, Palaeophycus, and rare bivalve equilibrichnia; (3) fluvially- through tidally-influenced mai n point-bar deposits, which are sporadically bioturbated with vertical trac es superimposed upon inclined heterolithic stratification; (4) well to thor oughly bioturbated bay deposits, burrowed by relatively robust forms of Oph iomorpha, Teichichnus, Skolithos and bivalve equilibrichnia; and (5) locall y bioturbated channel-bar deposits, typically displaying robust bivalve equ ilibrichnia, rare Ophiomorpha, and deep-penetrating Skolithos. Traces from both the modern and ancient deposits display a marked reduction in size and diversity in the upper estuary. Ongoing research at Willapa Bay confirms the validity of the brackish-water model and provides data that potentially improve our understanding of thes e deposits. Our findings suggest that strong textural controls are overprin ted by salinity stresses; commonly vertical deposit-feeding structures, mad e by infauna that are mining and/or stoping organic-rich laminae, are prese nt; and there is a potential for the presence of large diameter crustacean burrows in brackish deposits. This research emphasizes ichnologic variabili ty due to autocyclic changes and climatic variability, and urges caution wh en integrating ichnologic and sedimentologic data into a genetic stratigrap hic model.