Trace fossil distribution in lacustrine deltas: examples from the Triassic rift lakes of the Ischigualasto-Villa Unión basin, Argentina

This paper reports six trace fossil assemblages from lacustrine deltas of the Triassic Ischigualasto—Villa Unión rift basin, northwest Argentina. They were recognized in three correlated stratigraphic sections separated by about 100 km, and come from river- and wave-influenced deltas developed in low- and high-accommodation lacustrine basins. Trace fossil assemblages correspond to delta front (six), delta plain (two) and marginal lacustrine (one) facies associations. Each trace fossil assemblage is described, together with a detailed lithofacies characterization of the trace fossil-bearing interval. They are analysed in conjunction with previously described ichnological assemblages from partially correlative sections of the same basin. Delta front facies contains a high-diversity assemblage (22 ichnotaxa), including Cochlichnus (a ubiquitous form), Gordia, Helminthoidichnites, Helminthopsis, Didymaulichnus, Diplichnites, Stiaria, Cruziana, Bifurculapes, Protichnites, Diplopodichnus, Archaeonassa, Palaeophycus, Treptichnus, Rusophycus, Avolatichnium, ‘rhomboidal traces’, ‘fusiform structures’ and ‘millimetre burrows’. Trace fossil assemblages from delta plain facies are much less diverse (seven ichnotaxa), but display representatives of a greater variety of ethological categories, including Rhynchosauroides, Skolithos, Palaeophycus, ‘horseshoeshaped structures’, escape trace and drab-haloed root traces. Marginal lacustrine deposits of a river-dominated delta yielded a monospecific assemblage consisting of Cochlichnus anguineus. Comparison of trace fossil assemblages in wave- and river-dominated lacustrine deltas from the basin (mainly those of the delta front facies) revealed important differences in ichnodiversity that can be useful in the discrimination between these lacustrine delta types. Lacustrine delta deposits contain trace fossils that can be ascribed to three different ichnofacies: a high-diversity occurrence of the Mermia ichnofacies in subaqueous delta front sediments, a low-diversity occurrence of Mermia ichnofacies in subaqueous marginal lacustrine facies, the Skolithos ichnofacies in high-energy upper delta front/lower delta plain facies, and the Scoyenia ichnofacies in intermittently exposed upper delta plain facies. The analysed trace fossil assemblages from delta front and marginal lacustrine settings suggest environmental gradients within the Mermia ichnofacies.