Ichnological signatures of the Rio Bonito sedimentary succession in the central block of the Iruí coal mine, Rio Grande do Sul (Brazil)

Abstract

The subsurface deposits of the Rio Bonito sedimentary succession in the central block of the Iruí coal mine (Cachoeira do Sul, Rio Grande do Sul State, S Brazil) is composed of sedimentary facies associations that represent deposition in dominantly marginal-marine and shallow-marine settings, the later in minor scale. Ichnofabrics of Chondrites, Chondrites-Helmintopsis-Planolites, Cylindrichnus-Thalassinoides, Helminthopsis, Macaronichnus, Ophiomorpha, Palaeophycus, Planolites, Thalassinoides, and Thalassinoides-Palaeophycus occur in the marginal marine settings, whereas the Thalassinoides-Palaeophycus-Helminthopsis ichnofabric is present in the shallow marine deposits. The marginal-marine ichnofabrics are characterized by the low bioturbation index, the reduced size of burrows, and the low ichnodiversity, whereas the marine ones show a moderate to high degree of bioturbation and low ichnodiversity. The ichnofabric pattern, its stratigraphic distribution, and its sedimentological relationships allow recognizing four ichnological signatures, three suggesting stress caused by changes in the salinity gradient, and one substrate-controlled. Ichnofabrics of Helmithopsis, Palapeophycus, and Planolites in lithofacies SiltArg suggest the dominance of oligohaline to freshwater conditions. Ichnofabrics of Thalassinoides-Palaeophycus and Cylindrichnus-Thalassinoides refl ect the dominance of mesohaline conditions, each one representing, respectively, more quiet and stable environments (impoverished Cruziana Ichnofacies suite), and moderate to high energy settings (mixed Skolithos-Cruziana Ichnofacies suite). The Thalassinoides-Palaeophycus-Helminthopsis ichnofabric represents an arquetypical Cruziana Ichnofacies suite, indicating shallow marine settings. The reduced size of the burrows, however, suggests dominance of polyhaline rather than stenohaline conditions and its local distribution allow infer deposition in embayments, which mimic the environmental conditions found in the lower shorefce/ offshore transition zones. The Thalassinoides ichnofabric is substrate-controlled, representing a Glossifungites Ichnofacies suite. It demarks two autogenic stratigraphic surfaces, and one allogenic stratigraphic surface interpreted as a sequence boundary. The integrated ichnological and sedimentological analysis in high resolution scale, allowed refining the existent paleoenvironmental and stratigraphic interpretations of the Rio Bonito sedimentary succession in the study area.

Key words: ichnology, sedimentology, ichnofabrics, ichnological signatures, coastal deposits.