A DIVISÃO TRIPARTITE DO SILURIANO DA BACIA DO PARANÁ

Abstract

A previously poorly, understood , and partly unknown, thick, and widespread Silurian Sequence is recognized in the brazilian portion of the Parana Basin. Based on electrical log response and lithological differences, this sequence can now be subdivided into three major lithostratigraphic units, from base to top: Rio Ivai (a new formal unit, here described), Vila Maria, and Furnas Formations. They represent a complete transgression-regression cycle strongly influenced by glaciaticn. The Rio Ivai Formation is a predominantly sandy unit, with intercalated diamictites, of continental character at the base becoming marine-influenced toward its upper part. The Vila Maria Formation is represented by intercalations of sandstones and fossiliferous shales, some diamictites, and indicates the peak of the marine transgression. The Furnas Formation is a sandy pile of stacked anastomosing paleo-channels, probably formed as outwash in a periglacial fringe (regressive phase). The only unit surely dated as Early Silurian is the Vila Maria Farmation. The Silurian age of the other formations is indicated by stratigraphic relationship . The Rio Ivai Formation rests unconformably, in some places, over Cambro-Ordovician sedimentary and volcanic rocks; while the Vila Maria grades vertically into the Furnas Formation, that is covered unconformably by the Emsian to Frasnian Ponta Grossa Formation. Known maximum thicknesses of the Silurian Sequence in Brazil are in the order of 500 m, and it thickens towards the southwest into Paraguay, where the correlatable Itacurubi and Caacupe Groups can attain thicknesses in the order of 1,000 m.