Navegando por Assunto "Diagenesis"

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Acesso aberto (Open Access)
Efeitos hidrotermais em rochas carbonáticas-siliciclásticas da Formação Itaituba, Pensilvaniano da Bacia do Amazonas, região de Uruará (PA)
(Universidade Federal do Pará, 2020-10-26) SOUZA, Isabele Barros; SOARES, Joelson Lima; http://lattes.cnpq.br/1345968080357131
Carbonate and siliciclastic rocks from the Neocarboniferous period are widely recorded in the western and central-western portions in the Uruará region, State of Pará, on the southern edge of the Amazon Basin. These deposits are represented by the Itaituba Formation, from which a 35 m drill was described and five carbonate microfacies (calcimudstone, dolomudstone, bioclastic wackestone, bioclastic packstone and bioclastic grainstone rich in terrigenous) and eigth siliciclastic facies (massive claystone, black shale, siltstone with truncated crosslamination, siltstone with low-angle cross-lamination, siltstone with parallel laminate, sandstone with truncated cross-laminate, sandstone with parallel laminate and massive sandstone) were describe. From petrographic analysis, cathodoluminescence, X-ray diffraction, scanning electron microscopy and dispersive energy spectroscopy, it was possible to identify the main hydrothermal and diagenetic processes that affected these rocks. In the carbonate rocks, micritization, dissolution, calcitization, mechanical and chemical compaction and cementation were observed. In sandstones, quartz overgrowth, carbonate cementation and chemical compaction occur. During the Triassic-Jurassic period, great volcanism event occurred in the central portion of the West Gondwana supercontinent, known as the Central Atlantic Magmatic Province (CAMP), with the placement of dikes and basalt sills represented in the Amazon Basin by the Penatecaua Magmatism. The percolation of hydrothermal fluids in the late phase of this magmatism, promoted several changes in the mineralogy and textures of these rocks, among them hydraulic fracturing, formation of vugular porosity, precipitation of mineral assemblages typical of hydrothermalism such as - saddle dolomite, calcite, apatite, pyrite, chalcopyrite, galena, sphalerite, iron and titanium oxides, chlorites, talc, garnet, fluorcarbonate enriched in REE, saponite and corrensite - in the venules, and silicification of the matrix and carbonate grains. Organic geochemistry analysis in the samples of carbonate and shale, exhibit extremely low total organic carbon values, with low maturity, generating type IV kerogen, probably originated from oxidized organic matter. Thus, the present work intends to expand the study of rocks that occur in subsurface in the Uruará region, enabling the understanding of depositional and diagenetic processes active in their formation and its alteration due to the hydrothermal effect.
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.