Sequências deposicionais do Grupo Itararé (Carbonífero e Eopermiano), Bacia do Paraná, na área de Dr. Pedrinho e cercanias, Santa Catarina, Brasil: turbiditos, pelitos e depósitos caóticos

Abstract

Outcrops of sedimentary strata from Mafra and Rio do Sul formations (Itararé Group, Paraná Basin) constitutes the record of the Gondwana permocarboniferous glaciation in Santa Catarina state, southern Brazil. These deposits were mainly deposited in glacially influenced prodeltaic and in relatively deep and distal settings related to marine glaciers. They are constituted by sandy and conglomeratic turbidites, slowly settled pelites, deposited below storm wave base and chaotic strata, originated by debris flows, slumps and slides. These rocks had been studied in regional scale, including sections next to Alfredo Wagner, Vidal Ramos, Aurora, Rio do Sul, Trombudo, Presidente Nereu and Lontras cities, as well in the scale of detail, in Dr. Pedrinho region. Five depositional sequences, with regional extent, were correlated along 300km, from Santa Catarina to the north of the Paraná state. Each sequence initiates with the abrupt entrance of sandstones and conglomerates over muddy beds, marking a downward shift of depositional facies to the basin interior, in consequence of the relative sea level fall. Sequences 1 to 4, main targets of this study, are equivalent the Itararé Group, reaching a thickness of 450 m. The glacial influence in these sequences is notable, registering the phase when the Gondwana was situated next to the polar region during Carboniferous and early Permian. These glacio-influenced units present a change in the climatic pattern, from sequences 1 up to 4. Sequences 1 and 2 records colder conditions, passing to a mild, temperate climate, marked by glacier retraction and concurrent establishment of fluvial systems and vegetation, already during sequences 3 (final) and 4, as the Gondwana moves away from the polar region. The lowstand systems tract of sequences 1 to 4 is characterized by thick turbidites, generated by subaqueous outwash flows, produced by retreating marine glaciers and by hiperpicnal flows, produced by catastrophic fluvial floods. Tillites are very rare deposits, formed in moments of significant advance of glaciers in the basin, causing glacio-eustatic sea level falls. Prodeltaic pelites and fine-grained deposits related to tidewater glaciers are interbedded with thin-bedded turbidites and chaotic strata, constituting the most common deposits of the transgressive systems tract of sequences 1 to 4. Rapid progradations, developed during the highstand systems tract, propitiated the deposition of unstable pelitic packages, frequently remobilized as slumps and more evolved debris flows, resulting in thick chaotic beds. The change in paleocurrent directions indicates an important modification of sediment supply in the basin during the “Itararé time”. Paleocurrents of sequences 1 to 3 indicates a sediment transport to the north, while paleocurrents from the upper part of sequence 3 and 4 indicates an increasing sediment supply to south and southwest, preceding the bigger tectonic inversion registered by Fm. Rio Bonito. Sequence 5 is composed of postglacial Rio Bonito formation sediments, deposited in deltaic, fluvial, and shallow marine (waves and tides) contexts.