Geochemical paleoredox indicators in organic-rich shales of the Irati Formation, Permian of the Paraná Basin, southern Brazil

Abstract

Paleoredox indicators were used to assess the depositional environment of the Permian Irati Formation, and discuss the controls on the accumulation of organic-rich sediments. Geochemical data were measured for 43 drill-core samples in southern Paraná Basin. We infer that redox boundary was located at the sediment-water interface during the accumulation of these sediments. Trace-element data demonstrate that Irati shales were similar to average shales, except during deposition of two organic-rich beds in which U and Mo are enriched, suggesting fully anoxic conditions. The variable degree of oxygenation of bottom waters is also attested by DOP T , Fe T /Al, and sedimentological indicators of episodic ventilation. DOP T and Fe and S correlation points to an iron-limited environment, where the main sulfur sink is organic matter rather than pyrite. Cu and Cu/Mo ratio suggests bottom-water anoxia driven by increased organic flux. The Irati Formation was probably deposited in a salinity-stratified body of water, with positive water balance. The main control on the accumulation of the organic-rich sediments was the high organic carbon flux resultant from high primary productivity in surface waters. Anoxia arose as a consequence of increased productivity, not as the primary driver in the formation of these organic-rich rocks.KEYWORDS: black shales; paleoredox proxies; Irati Formation; Permian. RESUMO: Indicadores da paleo-condição redox foram utilizados para interpretar o ambiente deposicional da Formação Irati (Permiano) e discutir os controles na acumulação de sedimentos ricos em matéria orgânica. Os dados geoquímicos foram obtidos em 43 amostras de quatro testemunhos de sondagem de poços da Bacia do Paraná no Sul do Brasil. A interpretação desses dados permitiu inferir que a interface redox normalmente coincidia com a interface água-sedimentos durante a acumulação desses sedimentos. Dados de elementos-traços demonstram que os folhelhos da Formação

Was Mesosaurus a Fully Aquatic Reptile?

“…Intriguingly, the largest mesosaurids known so far (Piñeiro, 2002(Piñeiro, , 2006 come from crystalline levels (gypsum), and are associated with desiccation marks and a thin bentonitic layer. The alternative hypothesis suggesting a more fully aquatic mesosaurid taxon (Modesto, 1996) or simply that the very mature individuals lived in deeper waters is not supported by the existent evidence: the largest individuals are solely preserved in the marginal (shore line) area of the basin (e.g., those from the Mangrullo Formation); the suggested presence of parental care in Mesosaurus supports the hypothesis that mature and juvenile individuals lived very close to each other at least seasonally, and the main food item of mesosaurids (see Silva et al, 2017) should have been most abundant in coastal areas, because of the unfavorable environmental conditions for bentonic biota such as the high salinity and anoxia of the deepest basin bottoms (Goldberg and Humayun, 2016).…”

An integrated approach to understanding the depositional environment and organic matter enrichment factor in Carboniferous source rocks, Junggar Basin, NW China

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“…When the organic matter breaks down by aerobic bacteria, oxygen depletion results, then the residual organic matter is preserved in relatively good conditions for promoting organic matter accumulation (suboxic to anoxia). The δ 13 C org of the samples of Irati Formation, Lower Permian increased from −27‰ to between −25‰ and − 22‰, indicating the occurrence of algal blooms (Goldberg & Humayun, ), which represent the elevation of primary productivity.…”

Textural and geochemical characteristics of late Pleistocene to Holocene fine-grained deep-sea sediment cores (GM6 and GM7), recovered from southwestern Gulf of Mexico

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