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| Irati Formation | |
|---|---|
| Name | Irati Formation |
| Period | Permian |
| Type | Geological formation |
| Region | Paraná Basin |
| Country | Brazil, Argentina, Paraguay, Uruguay |
| Namedfor | Irati |
Irati Formation is a Permian-aged stratigraphic unit of the Paraná Basin noted for organic-rich shales, fossiliferous limestones, and economically significant bituminous horizons. The unit crops out across parts of South America and has been the focus of multidisciplinary studies in paleontology, sedimentology, geochemistry, and petroleum geology. Research on the unit involves collaborations among institutions studying the Paraná Basin, Gondwana paleogeography, and Permian ecosystems.
The Irati Formation lies within the succession of the Paraná Basin and is typically positioned above the Sao Bento Formation-equivalent units and below the Rio Bonito Formation-equivalent beds in regional chronostratigraphic schemes; regional correlation involves the Passa Dois Member, Assistencia Member, and adjacent units recognized in the stratigraphic columns of São Paulo State, Paraná (state), Santa Catarina, and Rio Grande do Sul. Stratigraphic studies integrate lithostratigraphy, sequence stratigraphy, and chemostratigraphy used by research teams from the Universidade de São Paulo, Universidade Federal do Rio Grande do Sul, and the Brazilian Geological Survey. Correlative units in neighboring basins include sequences described by geologists working with the Servicio Geológico Minero Argentino and the Instituto Geológico y Minero de España in comparative Permian frameworks.
Biostratigraphic, radiometric, and palynological data constrain the Irati Formation to the Middle to Late Permian, often assigned to the Kungurian to Guadalupian intervals in global chronostratigraphic charts used by the International Commission on Stratigraphy. Studies citing conodont biostratigraphy, fusulinid assemblages, and radiometric ages published by teams from the Utrecht University and the University of Buenos Aires provide tighter age brackets. Correlations are routinely made with Permian stages in Eurasian and North American sequences studied by institutions such as the Natural History Museum, London and the Smithsonian Institution.
The formation comprises organic-rich black shales, laminated siltstones, carbonate nodules, and occasional sandstones interpreted as lacustrine to restricted marine deposits. Sedimentological analyses performed by researchers at the Federal University of Rio de Janeiro and the Universidade Estadual Paulista invoke low-oxygen bottom waters, euxinic conditions, and stratified water columns analogous to settings studied in the Kupferschiefer and Posidonia Shale by European teams. Geochemical fingerprinting with stable isotopes and total organic carbon measurements has been reported in collaborations involving the Max Planck Institute for Chemistry and the University of São Paulo.
Fossil assemblages include diverse marine and freshwater taxa such as brachiopods, bivalves, gastropods, conodonts, fish remains, coprolites, plant fragments, and palynomorphs. Vertebrate findings described by paleontologists at the Museu Nacional (Brazil) and the Museo Argentino de Ciencias Naturales include temnospondyl amphibians and early synapsid remains that inform Gondwanan faunal interchanges described in literature by authors affiliated with the Field Museum and the Natural History Museum of Los Angeles County. Microfossil and palynological studies, often compared to assemblages cataloged at the British Geological Survey and the Australian Museum, support paleoenvironmental reconstructions. Trace fossil studies referencing ichnotaxa cataloged at the American Museum of Natural History provide evidence of benthic activity under dysoxic conditions.
The Irati Formation is notable for its hydrocarbon potential, hosting shale gas and shale oil source rocks evaluated by exploration groups from Petrobras, international energy companies, and independent consultants from the International Energy Agency-affiliated studies. Organic-rich horizons have been analyzed for total organic carbon and kerogen type by researchers associated with the International Petroleum Technology Conference and the Society of Economic Geologists. Additionally, the formation contains minerals such as phosphate-bearing nodules and carbonate concretions studied by teams from the Brazilian Mining Association and academic departments at the Federal University of Paraná.
Outcrops and subsurface extents of the formation occur across the Paraná Basin in southern and southeastern Brazil, extending into northeastern Argentina, southeastern Paraguay, and northern Uruguay. Key localities include exposures near the cities of Irati (city), São Bento do Sul, sites in the Iguazu region, and boreholes drilled in the Campos Basin-adjacent margin for comparative stratigraphy. Field campaigns have involved collaborations between the Universidade Federal do Paraná, the Universidade Estadual de Maringá, and regional museums that curate specimens and cores.
The unit was first described in regional geological surveys conducted during the late 19th and early 20th centuries by geologists associated with the Imperial Brazilian Geological Commission and later refined in systematic studies by researchers at the Serviço Geológico do Brasil (CPRM), the Universidade de São Paulo, and international collaborators from the University of Oxford and the University of Buenos Aires. Nomenclatural debates and stratigraphic revisions have been addressed in symposia organized by the Brazilian Geological Society and at international meetings such as those of the International Geological Congress.
Category:Geologic formations of South America Category:Permian