Caracterização paleoambiental dos depósitos pensilvanianos da formação Itaituba através da análise de microfácies e invertebrados marinhos, borda norte da Bacia do Amazonas

Abstract

Thus, the present work aimed to carry out the taxonomic classification of marine paleoinvertebrates, as well as the paleoenvironmental reconstruction of the Itaituba Formation deposits, outcropping in the region of Monte Alegre, northern edge of the Amazonas Basin, contributing with new data for the paleofaunal and paleoenvironmental refinement of this region. The fossiliferous content of carbonates and shales is predominantly represented by brachiopods of the families Athyrididae, Rhipidomellidae, Spiriferidae, Productidae, Productellidae and Strophomenidae, associated with bryozoans of the families Fistuliporidae, Rhomboporidae and Fenestellidae, and the crinoids represented by the Family Anthinocrinidae and the genera Baryschyr and Ciclocaudex, benthic foraminifers miliolid and fusulinid, and undefined trilobites. The occurrence of the Baryschyr crinoid restricts the age of these deposits to the Middle Pennsylvanian (Moscovian Age). Four Standard Microfacies (SMF) and two Faciologic Zones (FZ) have been individualized: a) FZ 7, representing a lagoon environment with open circulation, characterized by siliciclastic wackestones (SMF 09) and siliciclastic wackestones/packstones (SMF 10) deposits, consisting of brachiopods, crinoids, echinoids, miliolids and fusulinids foraminifers, trilobites, and bryozoans fossils. b) FZ 8, representing a sheltered lagoon environment, characterized by deposits of peloidal siliciclastic wackestones (SMF 16) and mudstones (SMF 23), with a fossil assemblage limited to brachiopods, echinoids and miliolid foraminifera. Therefore, the paleoenvironment of these deposits started in a lagoon environment with open circulation that enabled the formation of wackestones and siliciclastic packstones (SMF 09 and 10) and the development of a diversified marine invertebrate fauna that attest to the open, eutrophic marine environment, with waters warm, clear, well oxygenated, normal salinity and moderate energy. Subsequently, the environment evolved into a sheltered lagoon with restricted water circulation that allowed the formation of siliciclastic wackestones and mudstones (SMF 16 and 23) and the development of a restricted fauna composed of organisms adapted to stressful environmental conditions due to large fluctuations in the rates of salinity, temperature, oxygen and energy. This limited biota is consistent with an environment of more saline and nutrient-depleted waters, while the presence of siliciclastic grains indicates a proximal environment influenced by continental sedimentation. The biostratinomic features, characterized by a low degree of fragmentation, disarticulation, and partial abrasion, as well as the encrustation of cystoporate bryozoans (Fistuliporidae) in brachiopod carapaces and columnar segments of crinoids, suggest exposure of the skeletons for a short-time at the water-sediment interface and rapid burial in low-energy environmental conditions. In general, this work details the lagoon deposits and reinforces the Moscovian Age attributed to the Itaituba Formation, as well as paleoenvironmental interpretations, which suggest that a carbonate platform was established in the Amazonas Basin during the Pennsylvanian Period.