Palaeoenvironmental Framework of the Glacial-Postglacial Transition (Late Paleozoic) in the Paganzo-Calingasta Basin (Southern South America) and the Great Karoo-Kalahari Basin (Southern Africa): Ichnological Implications

Abstract

Sedimentological, palaeontological and geological data from the glacial to postglacial transition in the late Paleozoic successions of the Paganzo-Calingasta Basin (PC) in southern South America and the Great Karoo-Kalahari Basin (GKK) in southern Africa are analysed, revised and reinterpreted. A brackish depositional setting is inferred for main areas previously considered to be nonmarine based upon ichnological interpretations. Three stratigraphic intervals have been defined based on changes in sedimentary facies and trace fossils association: The glacial interval (GI), early postglacial interval (EPI) and late postglacial interval (LPI). The GI and EPI contain a dominance of arthropod trackways, fish trails with and subordinate grazing and feeding traces. The EPI in the PC Basin comprises both nonmarine and brackish-marine ichnocoenoses without significant differences in ichnological composition. Trace fossils are preserved in underflow and turbidite beds of deltaic deposits. Opportunistic grazing traces constitute a post-event ichnocoenosis, while a pre-event ichnocoenosis is preserved at the base of turbidite beds. In the GKK Basin ichnofossils were documented in turbidite fans. The LPI in the GKK Basin contains the first evidence of shallow water deltaic infauna and subordinate grazing traces. Conversely, in the PC Basin the infauna is lacking. The GI and particularly the EPI ichnofauna resemble the nonmarine Mermia ichnofacies but they occur in large and long-lived stable marine basins which received enormous inputs of melt-water that markedly reduced the salinity. They are not ichnologically comparable with brackish marginal marine environments that are affected by frequent, sometimes diurnal, fluctuations in physical parameters such as salinity and temperature. The LPI in the GKK Basin constitutes a typical record of the classical marginal marine ichnofaunas, but in the PC Basin the ichnofacies assignment is problematic. Grazing traces of the Mermia ichnofacies appear in middle estuarine deposits where both palynomorphs and the presence of tidal features suggest brackishness. The Paganzo Basin ichnology, therefore shows some difficulties in differentiating nonmarine from subtle brackish environments without the full integration of sedimentological information. Ichnocoenoses dominated by trackways of arthropods are particularly inappropriate to distinguish between these types of depositional settings.