Bathymetry, sediment texture, and substrate cohesiveness; their impact on modern Glossifungites trace assemblages at Willapa Bay, Washington

Glossifungites-demarcated discontinuities have been demonstrated to have st ratigraphic significance. Unfortunately, their stratigraphic utility common ly eclipses the palaeoecological information derived from such surfaces. Th is paper attempts to identify factors that influence the nature of trace as semblages observed in modem firmgrounds. In particular, it focuses on the c auses of lateral variability and erosional topography inherent to Glossifun gites surfaces. The extensive Pleistocene database at Willapa Bay is used t o test the interpretations. Striking heterogeneity in Gloosifungites assemblages is documented from mod ern firmgrounds at Willapa Bay. This variability is related to intertidal z onation, sediment texture, the absolute firmness of the firmground, and the presence or absence of a sediment veneer. Generally, subtidal and lower- t o middle-intertidal firmgrounds are colonized by crustaceans (the Upogebia Association) or the bivalve Petricola (the Petricola Association). These su rfaces are characteristically burrowed with Thalas-sinoides- and Gastrochae nolites-like traces. The upper intertidal is dominated by the Polydora Asso ciation, the burrows of which include diminutive Diplocraterion and Arenico lites-like traces. Sandy and heterolithic substrates are avoided by burrowi ng crustaceans and polychaetes alike. In contrast, Petricola is observed in muddy, sandy-mud, and heterolithic substrates. Therefore, where colonized, sandier substrates are dominated by the clavate, Gastrochaenolites-like bu rrows produced by these bivalves. The absolute firmness of exhumed firmgrou nds is quite variable. The Upogebia Association is absent in substrates tha t exceed 10(9) Pa. Polydora-dominated assemblages are poorly developed in s imilar firmgrounds. The Petricola Association, however, is well-developed i n relatively firm substrates. Finally, the morphologic characteristics of exhumed (modern) firmgrounds sh ow that the history of a Glossifungites-demarcated surface can be extremely dynamic. Firmground surfaces are exposed to an array of physico-chemical c onditions as they are exhumed, colonized, and pass into the geological reco rd. These surfaces are commonly re-exhumed due to auto- and allocyclic proc esses. (C) 2001 Elsevier Science B.V. All rights reserved.