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 stratigraphic significance. Unfortunately, their stratigraphic utility commonly eclipses the palaeoecological information derived from such surfaces. This paper attempts to identify factors that influence the nature of trace assemblages observed in modern firmgrounds. In particular, it focuses on the causes of lateral variability and erosional topography inherent to Glossifungites surfaces. The extensive Pleistocene database at Willapa Bay is used to test the interpretations.Striking heterogeneity in Glossifungites assemblages is documented from modern firmgrounds at Willapa Bay. This variability is related to intertidal zonation, sediment texture, the absolute firmness of the firmground, and the presence or absence of a sediment veneer. Generally, subtidal and lower- to middle-intertidal firmgrounds are colonized by crustaceans (the Upogebia Association) or the bivalve Petricola (the Petricola Association). These surfaces are characteristically burrowed with Thalassinoides- and Gastrochaenolites-like traces. The upper intertidal is dominated by the Polydora Association, the burrows of which include diminutive Diplocraterion and Arenicolites-like traces. Sandy and heterolithic substrates are avoided by burrowing 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 burrows produced by these bivalves. The absolute firmness of exhumed firmgrounds is quite variable. The Upogebia Association is absent in substrates that exceed 10 super(9) Pa. Polydora-dominated assemblages are poorly developed in similar firmgrounds. The Petricola Association, however, is well-developed in relatively firm substrates. Finally, the morphologic characteristics of exhumed (modern) firmgrounds show that the history of a Glossifungites- demarcated surface can be extremely dynamic. Firmground surfaces are exposed to an array of physico-chemical conditions as they are exhumed, colonized, and pass into the geological record. These surfaces are commonly re-exhumed due to auto- and allocyclic processes.