Trace fossils from Carboniferous floodplain deposits in western Argentina: implications for ichnofacies models of continental environments

Abstract

Floodplain deposits from the Carboniferous Tupe Formation of western Argentina contain a low-diversity ichnofauna of invertebrates and plants. The assemblage consists of Archaeonassa fossulata, Didymaulichnus lyelli, Helminthoidichnites tenuis, Palaeophycus tubularis, Planolites isp., and root traces. This ichnofauna is characterized by the dominance of very simple forms, superficial or very shallow trace fossils, combination of locomotion, grazing, and dwelling structures, and production by arthropods and vermiform organisms. Primary sedimentary fabrics are disturbed only by plant trace fossils while animal trace fossils are mostly restricted to bedding surfaces, resulting in the virtual absence of bioturbation. The poorly-defined morphology of the ichnofossils, and the absence of backfilled striated trace fossils and structures indicative of subaerial exposure suggest subaqueous production. The envisaged depositional environment is a freshwater body developed in a floodplain and overfilled by overbank deposits. A review of floodplain ichnofaunas documented from the stratigraphic record indicates that two recurrent assemblages may be recognized. The first assemblage is characterized by low to rarely moderate diversity of invertebrate trace fossils, moderate to high diversity of vertebrate structures, and common presence of meniscate backfilled structures, bilobate trace fossils with scratch marks, arthropod trackways and tetrapod trackways. It is regarded as an example of the Scoyenia ichnofacies developed in desiccated floodplains. The second type of floodplain assemblage, illustrated by the example documented in this paper, is characterized by grazing trails, locomotion trails and dwelling burrows, low to rarely moderate ichnodiversity, simple trace fossils, and superficial to very shallow structures. Although formed in floodplain environments also, this type of assemblage is remarkably different from the Scoyenia ichnofacies. It is here regarded as an example of the Mermia ichnofacies. Basic features of these ichnofaunas reflect subaqueous production of the assemblage. The lower ichnodiversity of these floodplain assemblages in comparison with their equivalents from lacustrine basins is probably an expression of the less stable conditions and temporary nature of floodplain water bodies. Our study supports the view that ichnofacies are not indicators of sedimentary environments but reflect sets of environmental factors.