Changes in depositional paleoenvironment of black shales in the Permian Irati Formation (Paraná Basin, Brazil): Geochemical evidence and aromatic biomarkers

• Published in: Marine and petroleum geology Vol. 126; p. 104917
• Main Authors: Nascimento, Caroline Adolphsson do, Souza, Eliane Soares de, Martins, Laercio Lopes, Severiano Ribeiro, Hélio Jorge Portugal, Santos, Victor Hugo, Rodrigues, René
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.04.2021
• Subjects: Aryl isoprenoids; Benzohopanes; Irati Formation; Lanostane; Multivariate statistical analysis; Sempervirane; Simonellite; Source rock; Lanostane; Multivariate statistical analysis; Sempervirane; Aryl isoprenoids; Benzohopanes; Simonellite; Irati Formation; Source rock
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2021.104917

Physicochemical changes that occurred during deposition of Irati black shales were investigated via outcrop samples collected in the northeastern and central-eastern Paraná Basin. Various traditional geochemical indicators were used, in addition to newly detected biomarkers: C32 lanostane, simonellite, sempervirane, and C32–C35 benzohopanes. Bulk geochemistry parameters, mineralogy, and conventional saturated biomarkers, such as tetracyclic polyprenoids (TPPs), reflect restricted and hypersaline marine deposition with freshwater input and increased terrigenous contribution toward the top of the studied outcrops. This led to stratification of the water column, establishing euxinic conditions in the photic zone and reducing conditions at the bottom of the water column, which enhanced preservation of organic matter. Principal component and hierarchical cluster analyses were performed to group the samples according to their similar geochemical characteristics: prevailing marine environment (basal outcrop samples) and higher freshwater inflow with greater terrigenous contribution (top outcrop samples). These geochemical observations were corroborated by aromatic biomarkers such as triaromatic steroids, trimethylnaphthalenes, simonellite, and sempervirane. The aryl isoprenoid ratio (AIR) indicates persistence of photic zone euxinia (PZE) during Irati black shale deposition. The presence of C32 lanostane and C32–C35 benzohopanes is directly related to the hypersaline depositional environment of the analyzed samples. Finally, the C32-33/C32-35 benzohopane ratio is proposed as a thermal maturity proxy because variations in the ratio can be linked to the presence of a diabase sill and temperature catalyst minerals near one of the outcrops. •Tetracyclic polyprenoids (TPPs) indicate freshwater incursion into the Irati Sea.•C32 lanostane indicates high salinity and a sulfur-rich paleoenvironment.•Simonellite and sempervirane support terrigenous input into the Irati Sea.•C32-33/C32-35 benzohopane ratio proposed as a proxy for thermal maturity.•Aryl isoprenoid ratio (AIR) indicates persistence of photic zone euxinia.