Mixed carbonate-siliciclastic microfacies from Permian deposits of Western Gondwana: Evidence of gradual marine to continental transition or episodes of marine transgression?

Abstract

At the end of the Paleozoic, several intracratonic basins developed in Gondwana, including the Paraná Basin in Western Gondwana. Permian sedimentary units include the Passa Dois Group, which encompasses four formations: the Irati, Serra Alta, Teresina, and Rio do Rasto, and represents a transition from marine to continental depositional environments. Its strata correlate extensively, from Argentina to South Africa, and the Irati and Teresina formations in particular present mixed carbonate-siliciclastic successions. This interval has been speculated to have formed under several different paleoenvironmental scenarios; however, Permian Gondwana basin microfacies, with no open sea influence, have rarely been studied. In addition, in these basins there is a scarcity of micropaleontological studies and geochronological calibration, besides an absence of marine index fossils. In view of this, a comprehensive microfacies study for the unit is developed, with a large and representative surface and subsurface sample analysis of the Passa Dois Group, divided into three large geographic domains: north, central, and south of the Paraná Basin, Brazil. Thin sections distributed among the four units were analyzed with an emphasis on carbonate layers where the microfossils have a better chance to be recognized. The data were interpreted within a revised biochronostratigraphic framework and integrated with bivalves, macroflora, palynology, conchostracans, and vertebrates biozones. Microfacies analysis resulted in the recognition of 13 microfacies: four siliciclastic, eight carbonate, and one siliceous (chert). We observed a predominance of fine siliciclastic microfacies, rich in organic matter, in the Southern Domain; rudaceous carbonate microfacies in the Central Domain, with diverse allochemicals and the greater diversity of bioclasts; and an increase on siliciclastic sediments influx in the Northern Domain of the basin, where only the Irati and Teresina formations occur. A shortage of microfossils was observed in the South Domain, where the carbonate microfacies are less varied than those to the north. In the Central Domain, coarse-grained carbonates are present. Bivalves, ostracods, and sponge spicules are the main bioclasts on rudstones and grainstones from the Irati and Teresina formations, and have been interpreted as allochthonous to parautochthonous. The association of detrital and authigenic glauconite to bioaccumulations of sponge spicules is interpreted as episodes of marine transgression.