ICHNOLOGIC DELINEATION OF THE FLUVIAL-TIDAL TRANSITION AND ITS SIGNIFICANCE FOR FACIES MODELLING OF MARGINAL-MARINE ENVIRONMENTS

Marginal-marine environments comprise a wide variety of coastal settings characterized by rapid environmental perturbations, such as salinity changes, increased sediment discharge and extreme clay flocculation. The brackish-water model is central to our understanding of the ichnologic signature or marginal-marine deposits. However, the importance of freshwater ichnofaunas to delineate the fluvial-tidal transition in both estuarine and deltaic systems has been commonly overlooked. Analysis of a several case studies (e.g. Carboniferous Tonganoxie Sandstone of Kansas, Cretaceous Tremp Formation of Spain, Miocene Oficina Formation of Venezuela) indicates that freshwater trace-fossil assemblages are widespread in the fluvial-tidal setting. In terms of ichnofacies, the fluvial-tidal transition is characterized by a mixed Mermia-Scoyenia ichnofacies. This ichnofauna is characterized by moderate to relatively high diversity of surface trails and trackways in Paleozoic examples, whereas meniscate trace fossils dominate in Mesozoic-Cenozoic examples. Paleozoic ichnofaunas are commonly associated with a wide variety of bedding-surface structures, including tool marks, drainage or seepage rill marks, runnel marks, runoff washouts, foam marks, raindrop impressions, gas escape structures, falling-water marks, and wrinkle marks. In contrast, post-Paleozoic occurrences are characterized by more penetrative and intense bioturbation, with assemblages being typically of low ichnodiversity and dominated by Taenidium or Beaconites. As a result of an increase in depth and extent of bioturbation by the continental infauna through geologic time, the preservation of delicate bedding-plain structures was hampered. Also, overprinting of previously emplaced brackish-water suites became increasingly common, resulting in the formation of composite ichnofabrics that sometimes have defied recognition. Freshwater ichnofacies should be incorporated as a fundamental element of estuarine and deltaic ichnofacies models.