Estuaries

Estuaries are characterized by the mixing of fluvial and marine waters and the presence of energy gradients associated with wave, tide, and river processes. This chapter explores the influence that the juxtaposition of unsteady through reduced salinities and variable energy distributions has on the distribution and composition of trace fossils in estuary settings. Due to the wide physiographic range of estuary occurrences, a simple summary model for the identification of estuaries is not presented. However, some common ichnological and sedimentological elements of estuaries are explored: (1) the presence of brackish-water bioturbation, (2) the observance of bioturbated intertidal flat deposits, (3) the presence of burrowed inclined heterolithic stratification, and (4) the identification of Glossifungites-demarcated omission surfaces, which may be associated with transgressive erosion in marginal-marine settings. More generally, the distribution of food resources and brackish water—a result of tidal mixing and energy distributions—dictates the longitudinal distributions of tracemaking organisms and their biogenic structures. Considering the above ichnological characteristics, the identification of estuaries from ichnological datasets very much depends on a large number of observations from a spatially significant dataset.