Navegando por Assunto "Pirabas Formation"

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Flora e fauna do neógeno das áreas de manguezais de lagoas costeiras da plataforma equatorial do Brasil: processo de piritização
(Universidade Federal do Pará, 2021-04-02) MATA, Giovanni Alvaro Teixeira da; AGUILERA SOCORRO, Orangel Antonio; http://lattes.cnpq.br/5854051483674293; https://orcid.org/0000-0003-4418-8351
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.
Acesso aberto (Open Access)
Gênese e distribuição estratigráfica das concreções carbonáticas da ilha de Baunilha Grande, região do Quatipuru - Pará
(Universidade Federal do Pará, 2025-06-27) GARCIA, Danilo Sena; SOARES, Joelson Lima; http://lattes.cnpq.br/1345968080357131
In the Baunilha Grande Ecofacies, defined by Petri (1957) and located on Baunilha Grande Island, in the Quatipuru region, Pará, carbonate concretions known as carcinolites are found. However, near these carcinolites, there are other concretions whose origin and stratigraphic position had not yet been described. These concretions are part of the Pirabas Formation, from the Lower Miocene, composed of fossiliferous limestones and sedimentary deposits that record a shallow coastal marine environment influenced by tides. This study aims to characterize the formation of carbonate concretions on Baunilha Grande Island and their relationship with the diagenetic and stratigraphic processes of the Pirabas Formation. The methodology included: (I) Petrographic, mineralogical, and geochemical analysis of the concretions, using techniques such as optical microscopy, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS); (II) Stratigraphic correlation of the concretions with similar deposits in Salinópolis and Maracanã, relating them to sedimentation events, bioturbation, and diagenesis. The results indicate that the concretions exhibit different shapes and compositions, classified into four main types: spherical, tubular, rough, and bulbous. Mineralogical analysis revealed the predominant presence of quartz and calcite, along with kaolinite and kutnohorite in some samples. Pyrite is present in all concretions, particularly associated with organic fossils and the diagenesis of diatoms. The presence of these concretions suggests early cementation processes in sandstone lenses and bioturbation in shales, influenced by sea-level variations and storm events. The high pyrite content in the concretions indicates a reducing depositional environment favorable for organic matter preservation and sulfide formation. The relationship between concretions and sedimentary structures suggests that these features are restricted to the top of the Pirabas Formation, near its contact with the Barreiras Formation. Thus, the occurrence of these concretions can serve as a stratigraphic marker for the top of the Pirabas Formation.