Tidal Ichnology of Shallow-Water Clastic Settings

This chapter explores using ichnological data as an indicator of tidal influence on sedimentation. Presently, the most commonly used method for establishing tidal influence with trace-fossil datasets is the presence of a brackish-water suite of trace fossils. Brackish-water trace-fossil suites generally comprise low-diversity, comparably diminutive trace fossils including forms such as Cylindrichnus, Planolites, Thalassinoides, Teichichnus, Arenicolites, Skolithos and cryptobioturbation. A problem with this method is that tidal currents and brackish-water do not always accompany one another. We attempt to build on previous brackish-water studies by examining the influence of tides on trace-fossil assemblages by considering 5 major impacts of tidal sedimentation. (1) The impact of rhythmic changes in current velocity on (passive) burrow infill, which can generate rhythmically infilled, large-diameter trace fossils (e.g. Thalassinoides and Psilonichnus) that correspond to tubular tidalites. (2) The influence of longer periodicity tidal rhythms on bed colonization, which produce regular waxing and waning of bioturbation intensity at the bed and/or bed-set scale. (3) The effect of tidal currents on burrow distributions, which produces a characteristic bioturbation-increasing-landwards trend into the middle part of bays and estuaries. (4) The impact of differing sedimentation rates between subtidal and intertidal settings, which contributes to a characteristic bioturbation-increasing-upwards profile on tidal bars. (5) And, the influence of variable resource distribution due to variable current energy, which results in a preponderance of interface deposit-feeding structures, stellate feeding traces and the presence of systematic deposit feeding trace fossils. Notably, the potential of using trace fossils to identify tidal influence on sedimentation and faunal colonization is at its inception. Substantial data is still needed to refine the ichnological model proposed herein.