The ichnology of the fluvial–tidal transition

Ichnologic data reveal the complexity of the fluvial–tidal transition in terms of the interaction of riverine and marine processes and changes in the salinity conditions. The ichnofaunas of the fluvial–tidal transition are mainly controlled by the salinity limit that separates two environmental zones, seaward of this limit brackish-water ichnofaunas occur, whereas landward terrestrial/freshwater ichnofaunas are present. The ichnology of the fluvial–tidal transition is summarized based on a series of case studies, spanning the Carboniferous to the Miocene. Freshwater trace-fossil assemblages are widespread in the fluvial–tidal setting and are characterized by moderate to relatively high diversity of surface trails and meniscate trace fossils typically present in continental environments, whereas brackish-water trace-fossil assemblages are typified by common monospecific to low-diversity suites of diminutive simple marine trace fossils. In addition, a comparison of freshwater ichnofaunas at the fluvial–tidal transition through time reveals a remarkable evolutionary control. Whereas Paleozoic ichnofaunas are dominated by shallow-tier grazing trails and arthropod trackways, post-Paleozoic examples are dominated by deeper-tier meniscate trace fossils, which significantly contributed to a remarkable increase in intensity of bioturbation of deposits formed in fluvial–tidal transition. Based on information from the stratigraphic record of freshwater ichnofaunas, these changes could be linked to the Mesozoic Lacustrine revolution, an evolutionary event representing a breakthrough for freshwater biotas. The increase in depth and extent of bioturbation obliterated shallow-tier traces and allowed overprinting of previously emplaced brackish-water suites, resulting in the formation of composite ichnofabrics.