Thermal and depositional history of Early-Permian Rio Bonito Formation of southern Paraná Basin – Brazil

Abstract

The Paraná Basin is a large volcano-sedimentary basin with a complex depositional history from Ordovician to the Cretaceous period. During Permian significant amounts of organic-rich beds accumulated within the Rio Bonito and Irati formations representing deposition in coastal (deltaic and barrier lagoon) and shallow marine environments, respectively. In this work, the thermal history of these beds is examined from 16 vitrinite reflectance analyses of coal beds (Rio Bonito Formation), integrated with previous data on coal rank, and 21 apatite fission-track (AFT) analyses, collected from 17 well profiles in the south of the Paraná Basin. Coal samples showed three different maturity levels. Two with reflectances of 0.4–0.6 and 0.6–0.8%Rr formed by the natural burial history of the basin (the latter is restricted to the Torres Syncline area). In contrast, the third group reached much higher values (1.0–5.0%Rr) related to intrusive igneous rocks. The AFT data show thermal consistency with the reflectance values. The sandstone samples related to the two lower reflectance patterns were partially reset, preserving AFT ages older than the stratigraphic age of the bed. Otherwise, the samples severely affected by the magmatism have Cretaceous and Early Cenozoic ages, but some were much younger than the last magmatic event, implying deep burial before cooling onset. The mean track length distribution of the partially reset samples is short due to the inheritance of tracks preserved from basement cooling history. However, the population of fully reset samples gives crucial information on the maximum temperature reached and the age when cooling started. At these conditions, around 1.0 to 3.0 km of the Paraná Basin sequences would have been removed from the surface of the studied areas by erosion since Cretaceous. Regional variations in the amounts of removed sections were controlled by the structural framework, mostly regional NW-SE and NE-SW oriented structures.