Taciba Formation: The last glacial episode in Paraná State

Abstract

The Late Paleozoic Ice Age (LPIA) and the transition to fully postglacial conditions are considered unique climatic events within the geological history. The main record of these events is encompassed by the gondwanic sedimentary basins, that suggest a dynamic glaciation punctuated by repeated glacial and interglacial events. The Paraná Basin holds the thickest and geographically more significative record of the LPIA, referred to as the Itararé Group. This work aims to analyze the record of the demise of the Late Paleozoic glaciers in northern Paraná Basin and the nature of the contact with the postglacial Rio Bonito Formation. The Ibaiti region in northeast Paraná State has been chosen as study area because it contains one of the thickest successions of the upper Itararé Group (Taciba Formation) and because of the need of high-resolution studies capable to refine its stratigraphy and depositional history. For this, we conducted sedimentological analysis (facies and facies association description and measurement of stratigraphic logs) and geological mapping. Furthermore, we performed stratigraphic correlations using gamma-ray logs from deep wells. The Taciba Formation is ca. 150 m-thick and was subdivided into three mappable genetic depositional units: 1) proglacial deposits, stacked in a fining-upward motif, including conglomerates, sandstones, diamictites and mudstones; 2) deltaic deposits, with a coarsening- upward pattern, and including rhythmites, heterolithics and sandstones; and 3) resedimented glaciomarine deposits encompassing diamictites stacked in a fining-upward pattern. Glacial affinity within units 1 and 3 and the decrease/ absence of glacial evidence within unit 2 suggest two distinct glacial advances interspersed with an interglacial phase. The depositional stacking patterns defined in outcrops extents for hundreds of kilometers into de basin, according to the correlations. The glacial-postglacial transition is sharp, defined by an erosive surface related to paleosol and incised valleys development.