Firmness Profiles Associated with Tidal-Creek Deposits: The Temporal Significance of Glossifungites Assemblages

Abstract

Indentation tests are used to detail firmness profiles from intertidal creek deposits and wave-exhumed substrates at Willapa Bay. Both of these sedimentological settings are characterized by occur- rences of modern Glossifungites assemblages. Firm substrates associ- ated with the intertidal-creek deposits are derived from dewatered modern sediments, whereas firmgrounds associated with wave erosion consist of dewatered and compacted Pleistocene strata. The Pleistocene firmgrounds are notably firmer than those derived from modern de- posits. A strong correlation between sediment firmness and burrowing behavior is evident in these deposits. In tidal-creek systems, the comparatively firm cutbank is character- ized by unlined, large-diameter, open burrows that form a Glossifun- gites assemblage. Intertidal point-bar deposits contain a softground suite consisting of mucous-lined, small-diameter, dominantly vertical traces. Finally, a softground suite of robust, mucous- or mud-lined, vertical and horizontal traces are observed in intertidal-flat deposits. In contrast, Pleistocene firmgrounds generally contain large- and small-diameter traces, with dominantly vertical architectures (Thalas- sinoides-, Gastrochaenolites-, Diplocraterion-, or Arenicolites-like bur- rows), depending upon the firmness of the substrate. Glossifungites occurrences on modern firmgrounds are temporally insignificant, whereas similar occurrences in Pleistocene substrates are temporally significant. Contrasting these two databases suggests that the stratigraphic significance of a Glossifungites-demarcated disconti- nuity can be assessed in the rock record. Several criteria that are useful for identifying temporally significant surfaces are suggested, including: absence of compaction of the Glossifungites assemblage; presence of well-preserved burrow sculptings; and planar to gently undulatory ero- sional surfaces as opposed to surfaces with notable topographic relief. Conceptual models demonstrate that muddy substrates potentially re- quire several millennia to compact and dewater. Sandy deposits, on the other hand, have indeterminate significance.