Ichnology of late Paleozoic postglacial transgressive deposits in Gondwana: Reconstructing salinity conditions in coastal ecosystems affected by strong meltwater discharge

Late Paleozoic ichnofaunas from eight different Gondwanic basins (Paganzo, San Rafael, Tarija, Paraná, Karoo, Falkland, Transantarctic, and Sydney) provide valuable evidence for reconstructing the environmental conditions of postglacial transgressions. The depositional environment of most of these transgressive fine-grained deposits historically has been controversial, with interpretations ranging from freshwater lacustrine to brackish-water estuarine, and even normal-salinity, open-marine platforms. Although the various units differ in the degree of marine connection, the common theme in all is the presence of freshwater ichnofaunas in direct association with glacially influenced coasts affected by strong discharges of meltwater. Ichnofaunas are typically dominated by nonspecialized grazing trails (Mermia, Helminthopsis, Helminthoidichnites), simple feeding traces (Treptichnus), arthropod trackways (Diplichnites, Umfolozia), and fish trails (Undichna ), representing examples of the Mermia and, to a lesser extent, the Scoyenia ichnofacies. A complex paleogeography of fjords and deep, large coastal lakes is suggested. Freshwater conditions were prevalent during most of the time because these areas were affected by a strong discharge of fresh water due to melting of the ice masses during deglaciation. The simple dichotomy between marine and nonmarine settings is misleading because these peculiar assemblages should first be understood in terms of their paleoecologic significance, and subsequently placed within a larger paleoenvironmental context. Laterally persistent, albeit diachronous, peri-Gondwanan ichnofaunas characterize melting of the late Paleozoic ice caps. Temporal recurrence of these ichnofaunas through the Late Carboniferous–Middle Permian indicates a common response of benthic faunas under similar ecological conditions during deglaciation events.