ABANICOS SUBLACUSTRES, ABANICOS SUBMARINOS O PLATAFORMAS GLACIMARINAS? EVIDENCIAS ICNOLOGICAS PARA UNA INTERPRETACION PALEOAMBIENTAL DEL CARBONÍFERO DE LA CUENCA PAGANZO.

Luis Alberto Buatois, María Gabriela Mángano

Abstract


Thick and widespread Carboniferous (Namurian-Westphalian) sequences of the Guandacol Formation,lower section of the Agua Colorada Formation and equivalents have been indistinctly interpreted as origined in sublacustrine fans, turbidite systems in a deep bay or a glaciomarine shelf. The different proposals are critically reviewed within a paleogeographic framework. The presence of marine elements in the Guandacol depositional area evidences a connection with the sea. Accordingly, the Guandacol Formation is thought to record sedimentation in a fjord setting. Trace fossils previously assessed to Cruziana can be produced by other types of arthropods common in freshwater and brackish water environments. It is considered that both ichnologic and sedimentologic evidences strongly suggest that the remaining depositional areas represent sedimentation in large and deep lakes. Some ichnogenera (e.g. Mermia, the fish trail Undichnia¡ from the Carboniferous of the Paganzo basin are onlyknown from non-marine environments. Additionally, trace fossils from these deposits are smaller than their typical normal marine counterparts. Size reduction (dwarfism) is probably due to freshwater influx. Complex g razing trails and fanning traces (Agrichnia) that typify submarine fans are remarkably absent. Assemblages are dominated by grazing trails (Pascichnia) exhibing non-specialized feeding pattems. These facts suggest less stable conditions than those that characterize deep marine environments. Dominance of surface trails and absence of burrows distinguish this ichnofauna from typical marginal-marine assemblages. The abundance of arthropod trackways also favours comparisons with different gondwanic and modem glaciallakes. Taking into account sedimentologic and ichnologic features, a delta-fed sublacustrine fan model is suggested. Shallow lake deposits are commonly present as a transition between deltaic and turbiditic systems. Similar deposits are relatively common from present-day situations and from the stratigraphic record.

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