Palaeoenvironmental Framework of the Glacial-Postglacial

PAZOS, P. J.

GONDWANA RESEARCH

Elsevier

Año: 2002 p. 619 - 640

1342-937X

Sedimentological, palaeontological and geological data from the glacial to postglacial transition in the late Paleozoicsuccessions 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 areaspreviously considered to be nonmarine based upon ichnological interpretations. Three stratigraphic intervals have beendefined based on changes in sedimentary facies and trace fossils association: The glacial interval (GI), early postglacialinterval (EPI) and late postglacial interval (LPI). The GI and EPI contain a dominance of arthropod trackways, fish trailswith and subordinate grazing and feeding traces. The EPI in the PC Basin comprises both nonmarine and brackishmarineichnocoenoses without significant differences in ichnological composition. >ace fossils are preserved in underflowand turbidite beds of deltaic deposits. Opportunistic grazing traces constitute a post-event ichnocoenosis, while a preeventichnocoenosis is preserved at the base of turbidite beds. In the GKK Basin ichnofossils were documented inturbidite fans. The LPI in the GKK Basin contains the first evidence of shallow water deltaic infauna and subordinategrazing 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 andlong-lived stable marine basins which received enormous inputs of melt-water that markedly reduced the salinity. Theyare not ichnologically comparable with brackish marginal marine environments that are affected by frequent, sometimesdiurnal, fluctuations in physical parameters such as salinity and temperature. The LPI in the GKK Basin constitutes atypical record of the classical marginal marine ichnofaunas, but in the PC Basin the ichnofacies assignment is problematic.Grazing traces of the Mermm ichnofacies appear in middle estuarine deposits where both palynomorphs and the presenceof tidal features suggest brackishness. The Paganzo Basin ichnology, therefore shows some difficulties in differentiatingnonmarine from subtle brackish environments without the full integration of sedimentological information. Ichnocoenosesdominated by trackways of arthropods are particularly inappropnate to distinguish between these types of depositionalsettings.