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ARQUITETURA ESTRATIGRÁFICA, TRATOS DEPOSICIONAIS E PALEOGEOGRAFIA DA BACIA DO PARANÁ (BRASIL) NO NEO-ORDOVICIANO / EO-SILURIANO

2017; Sociedade Brasileira de Geologia; Volume: 28; Issue: 1 Linguagem: Inglês

2177-4382

Mário Luís Assine, José Alexandre J. Perinotto, Carlos José Souza de Alvarenga, Setembrino Petri,

Geography and Environmental Studies

The Late Ordovician / Early Silurian Rio Ivai Group is the lowermost stratigraphic cycle at the Brazilian side of the Parana basin and is made up of three formations: Alto Garcas (base), Iapo and Vila Maria. A regional stratigraphic analysis was carried out and new stratigraphic sections were described along the north and northwestern margin of the basin, where the Ordovician / Silurian rocks crops out discontinuously with variable thicknesses. The Rio Ivai Group displays an asymmetrical geometry and its thickness increases westward. The transgressive nature, from coastal to marine in the Alto Garcas Formation, was interrupted by a short but widespread glacial event in the Ordovician / Silurian boundary, which caused the deposition of diamictites (Iapo Formation. The stratigraphic architecture reveals an onlap from west to east, toward the original borders of the basin. The Iapo Formation overlaps the eastern limits of the Alto Garcas Formation, resting right on the crystalline basement. After the end of glacial times, a widespread Early Llandoverian marine transgression led to shelf shale deposition (Vila Maria Fomation). Westward paleocurrents in the Alto Garcas Formation suggest that the Asuncion Arch had not yet attained a geomorphic expression in Ordovician / Silurian times, when Parana and Chaco basins were geographically continuous. This range of events resulted in a similar stratigraphic succession in several basins bordering the Gondwana continent. As a result, coastal to shelf sandstones are covered by diamictites of glaciogenic origin, which were succeeded by marine transgressive shales and finally by shallow sea sandstones.