Flora e fauna do neógeno das áreas de manguezais de lagoas costeiras da plataforma equatorial do Brasil: processo de piritização

Abstract

The carbonate-siliciclastic rocks from the early to middle Miocene Pirabas Formation in the equatorial margin of Brazil show a brackish ecofacies of mangrove and coastal lagoons paleoenvironments under tidal influence. The studied section has dark mudstone at the top, characterized by a microbial methanogenesis zone where pyritized trunks, leaves, micro- and macrofossils, and trace fossils, have been investigated. The petrographic characterization and crystallographic analyses distinguish mostly framboids crystal for trunk fragments to the octahedric and cubic crystals from invertebrate shells. The geochemistry analyzes revealed that Fe and S are concentrated both in the fossiliferous content of invertebrate constituents and in the matrix that hosts the trunk, while the other elements are mainly linked to invertebrates. The preferential distribution of these elements is in accordance with the presence of FeS2 compounds replacing fossils, reflecting the anoxic and reducing conditions of the environment. The pyrite-rich lithostratigraphic section was deposited in a shallow water environment, where pyrite mineralization was developed during the early diagenetic stage under anoxic conditions, plenty of organic matter, warm and mixohaline water. The integration of faciological, stratigraphic and chemical data from the carbonate deposits of the Pirabas Formation, in addition to reconstructing the stratigraphic behavior of these units in the period studied, would also assist in understanding the paleoenvironmental and paleogeographic changes of the Bragantina Platform and its possible relationship with global events.