Origin of paleovalleys on the Rio Grande do Sul Shield (Brazil): Implications for the extent of late Paleozoic glaciation in west-central Gondwana

• Published in: Palaeogeography, palaeoclimatology, palaeoecology Vol. 531; p. 108738
• Main Authors: Fedorchuk, Nicholas D., Isbell, John L., Griffis, Neil P., Montañez, Isabel P., Vesely, Fernando F., Iannuzzi, Roberto, Mundil, Roland, Yin, Qing-Zhu, Pauls, Kathryn N., Rosa, Eduardo L.M.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.2019
• Subjects: Detrital zircon geochronology; Itararé Group; Late Paleozoic ice age; Paraná Basin; Pull-apart basin; Rio Bonito Formation; Paraná Basin; Pull-apart basin; Itararé Group; Late Paleozoic ice age; Detrital zircon geochronology; Rio Bonito Formation
• ISSN: 0031-0182; 1872-616X
• DOI: 10.1016/j.palaeo.2018.04.013

The location, longevity, and geographic extent of late Paleozoic ice centers in west-central Gondwana remain ambiguous. Paleovalleys on the Rio Grande do Sul Shield of southernmost Brazil have previously been interpreted as fjords carved by outlet glaciers that originated in Africa and emptied into the Paraná Basin (Brazil). In this study, the sedimentology, stratigraphy, and provenance of sediments infilling two such paleovalleys (the Mariana Pimentel and Leão paleovalleys) were examined in order to test the hypothesis that an ice center over present day Namibia drained across southernmost Brazil during the Carboniferous and Permian. Contrary to previous findings, the facies assemblage from within the paleovalleys is inconsistent with a fjord setting and no clear evidence for glaciation was observed. The facies show a transition from a non-glacial lacustrine/estuarine environment, to a fluvial-dominated setting, and finally to a restricted marine/estuarine environment. Detrital zircon results present a single population of Neoproterozoic ages (c. 800–550 Ma) from the paleovalley fill that matches the ages of underlying igneous and metamorphic basement (Dom Feliciano Belt) and is incongruent with African sources that contain abundant older (Mesoproterozoic, Paleoproterozoic, and Archean) zircons. Furthermore, results suggest that the formation of the paleovalleys and the deposition of their fill were controlled by the reactivation of Neoproterozoic basement structures during the Carboniferous and Permian. The lack of evidence for glaciation in these paleovalleys highlights the need for detailed studies of supposed late Paleozoic glacial deposits. These results are supportive of the hypothesis that well-established glacial sediments on the Rio Grande do Sul Shield (southern margin of the Paraná Basin) may be the product of a separate lobe extending north across Uruguay, rather than a single, massive ice sheet draining west from Africa. •Paleovalleys on the southernmost margin of Paraná Basin are not paleofjords.•The Mariana Pimentel and Leão paleovalleys likely have a tectonic origin.•Sediments from paleovalleys do not have an African provenance.•Separate ice centers are likely for eastern and southern margins of Paraná Basin.