Fluvio-tidal transition zone: Terminology, sedimentological and ichnological characteristics, and significance

The fluvio-tidal transition zone (FTZ) is sedimentologically and ichnologically complex, encompassing variable sedimentary facies. Published research on the FTZ implies a wide range of physical and chemical conditions as well as resultant sedimentary structures. Variable definitions and terms have been applied to this transition zone for decades. This paper aims to refine the existing models of fluvio-tidal sedimentation, provide an inventory of sedimentological and ichnological characteristics collected from modern and ancient datasets, and produce useful criteria to facilitate its paleogeographic recognition. The usage of the descriptive term fluvio-tidal transition zone (abbreviated FTZ) is recommended. The zone is defined by the effective tidal limit at its upper end and the salinity limit at its lower end. The length and position of the FTZ can change with time depending on the fluvial flux and tidal force. However, the averaged position of the FTZ should be determined for the rock record based on a number of physical and biogenic sedimentary structures. Lithologically, the FTZ is represented by medium-grained sandstone to pebble conglomerate in smaller systems, and very fine- to fine-grained sandstone in middle- to large-scale depositional systems. The typical sedimentary structures constitute: unidirectional and bidirectional trough cross-stratification and ripple cross-lamination, abundant carbonaceous debris, coal beds, mudstone and peat rip-up clasts, massive bedding, tidal bundles and rhythmites, inclined heterolithic stratification (IHS), soft-sediment deformation, herringbone bedding, sigmoidal cross-bedding, and reactivation surfaces. Ichnologically, the FTZ is subdivided into the lower or tidal-fluvial part characterized by the Skolithos and depauperate Cruziana ichnofacies passing upstream into the upper or fluvio-tidal part, represented by the Scoyenia and Mertnia ichnofacies.