Deepwater to fluvio-deltaic stratigraphic evolution of a deglaciated depocenter: The early Permian Rio do Sul and Rio Bonito formations, southern Brazil

This paper describes fades relationships and stratigraphic architecture of early Permian deglacial deepwater to fluvio-deltaic deposits exposed in the eastern border of the Parana Basin, southern Brazil. The studied succession is located in the Rio do Sul depocenter and comprises an upward transition between the topmost interval of the Itarare Group (Rio do Sul Formation) and the lowermost unit of the Rio Bonito Formation (Triunfo Member). Geological mapping and facies analysis allowed the recognition of the following fades associations, vertically stacked: thin and thick-bedded turbidites, hyperconcentrated and concentrated density-flow deposits (hyperpycnites), mass-transport deposits, fine-grained heterolithic beds of prodelta, delta-front, shoreface and delta plain deposits, as well as fluvial beds resting on incised valleys. The stratigraphic analysis through the interpretation of the facies associations, stacking pattern and paleocurrent dispersal trends were enough to define the clinoform stratigraphic architecture, composing a progradational-aggradational pattern. The paleoflows for all facies associations show a main trend towards the south-southwest. In this context, hyperpycnal beds can represent a glacial source from the northeast, because of striated and faceted clasts immersed in the conglomeratic facies, whereas turbidites have been understood as the products of their dilution. Likewise, mass-transport deposits are related to the collapse of the shelf-margin delta, represented by allochthonous blocks composed of deltaic beds within the mass-flow diamictites. Thus, these mass-transport deposits do not evolve downslope to turbidity currents. This configuration was likely for building the relief of the slope system, probably a shelf-edge delta susceptible to mass failure. Paleo-ice flows towards SW recorded in the Rio do Sul Formation, genetic related to fluvio-deltaic deposits of Rio Bonito Formation, highlight the scenario of final stages of the Late Paleozoic Ice Age in the SW of Gondwana and support the previous researches which point to the presence of diachronous ice-spreading centers from paleohighs.