Navegando por Assunto "Sedimentology"

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Mineralogical and geochemical influences on sediment color of Amazon wetlands analyzed by visible spectrophotometry
(Instituto Nacional de Pesquisas da Amazônia, 2013-09) GUIMARÃES, José Tasso Felix; COHEN, Marcelo Cancela Lisboa; FRANÇA, Marlon Carlos; SILVA, Any Kelly Terra da; RODRIGUES, Suyanne Flavia Santos
Based on sedimentological and geochemical data, this work relates spectrophotometric measurements with sediment composition and its application in palaeoecological studies of Amazon wetlands. The CIELAB values are directly related to mineralogical and chemical composition, mostly involving quartz, iron oxyhydroxides and sulfides (e.g. pyrite), and total organic carbon. Total organic carbon contents between 0.4-1%, 1-2%, 3-5% and 15-40% were related to L* (lightness) data of 27, 26-15, 7-10 and 7 or less, respectively. The CIELAB values of a deposit in Marabá, Pará, were proportional to variations in quartz and total organic carbon contents, but changes in zones of similar color, mainly in the +a* (red) and +b* (yellow) values of deposits in Calçoene, Amapá and Soure, Pará, indicate a close relationship between total organic carbon content and iron oxyhydroxides and sulfides. Furthermore, the Q7/4 diagram (ratio between the % re?ectance value at 700 nm to that at 400 nm, coupled with L*) indicated iron-rich sediments in the bioturbated mud facies of the Amapá deposit, bioturbated mud and bioturbated sand facies of Soure deposit, and cross-laminated sand and massive sand facies of the Marabá core. Also, organic-rich sediments were found in the bioturbated mud facies of the Amapá deposit, lenticular heterolithic and bioturbated mud facies of the Soure deposit, and laminated mud and peat facies of the Marabá deposit. At the Marabá site, the data suggest an autochthonous influence with peat formation. The coastal wetland sites at Marajó and Amapá represent the development of a typical tidal flat setting with sulfide and iron oxyhydroxides formation during alternated ?ooding and drying.
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Proveniência sedimentar dos depósitos cretáceos da Formação Alter do Chão, Bacia do Amazonas, região de Ponta do Curuá, Prainha-PA
(Universidade Federal do Pará, 2020-08-03) SANTOS, Raiza Renne Leitão dos; SOARES, Joelson Lima; http://lattes.cnpq.br/1345968080357131
Deposits of the Alter do Chão Formation are found along the right bank of the Amazon River and are well registered in the locality of Ponta do Curuá, municipality of Prainha, the western region of the State of Pará. The outcrops are characterized by thick layers of fine to coarse sandstones interbedded with siltstones and claystones, and subordinate subordinate conglomerates. The facies analysis carried out in these outcrops allowed the individualization of 8 sedimentary facies, grouped in three facies associations (AF), configuring a high sinuosity and mixed load fluvial meandering depositional system. AF1 comprises facies with a thicker granulometry of the studied succession and was interpreted as the filling of the fluvial channel, characterized by massive to stratified conglomerates, massive sandstones, sandstones with trough and tabular cross-bedding, sandstones with convoluted laminations and climbing crosslaminations. AF2 was interpreted as abandoned channel filling deposits and corresponds to thick layers of massive to laminated mudstones that are sporadically associated with thin layers and very thin sandstone lenses forming wavy-linsen bedding. Well-preserved leaves and trunks are present in this association. AF3 corresponds to the flood deposits, composed of massive to laminate mudstones, massive sandstones, and sandstones with trough, tabular and sigmoidal cross-stratifications. The study of heavy minerals in the sandstones of this formation showed an assembly with a predominance of ultra-stable minerals such as zircon, tourmaline, rutile and anatase, and less frequent minerals such as kyanite, staurolite, sillimanite, andalusite, and garnet. Metasedimentary sources are suggested due to the presence of aluminum-rich metamorphic minerals. Whereas, rutile and tourmaline are present in both igneous and metamorphic rocks, being more common in the latter. Well-rounded minerals suggest an origin from sedimentary deposits, in the same way that subhedral and euhedral minerals suggest first cycle sediments. The high stability of this assembly is attested by the high compositional maturity exhibited by these sandstones, with values of the ZTR index ranging from 69% to 99%. These high values indicate that chemically unstable minerals have been eliminated over geological time, probably due to the action of interstitial fluids that circulate in the porous during weathering and diagenetic processes. However, the analyzed mineral assembly is more compatible with conditions of exposure to intense chemical weathering. The effects of intrastate dissolution in the area is directly proportional to the increase in the ZTR index, being attested by the presence of corrosion textures on the surface of the minerals that were analyzed from SEM images. Thus, the mineralogical assembly would reflect the effects of the intense chemical weathering, under a humid tropical climate, imposed on the deposits of the Alter do Chão Formation that would be associated with the lateritization event responsible for the genesis of bauxite deposits during the Paleogene. Recently, the luminescence (LC) characteristics of quartz grains have been used as provenance indicators, mainly in rocks affected by weathering or rich in quartz. In this work, luminescence signals were acquired by polychromatic cathodoluminescence to establish the relationship between luminescent signals and the genesis of this mineral. The studied grains present red, blue, violet and brown colors, with different intensities: the reddish-brown tones, are generally attributed to metamorphic rocks; dark blue luminescence is a characteristic observed in quartz of plutonic origin; grains with intense LC such as blue, red and violet of high bright, are volcanic quartz. Geochronological analyzes carried out on detrital zircons using the U-Pb method allowed to determine that the main sources for the Alter do Chão Formation are Paleoproterozoic ages, subdivided into two groups: (1) 1771 to 1906 Ma; and (2) 1957 to 2037 Ma, in addition to a small Archean contribution (Neoand Meso-Archean) aged between 2529 and 2977 Ma. The data from the first group, 1771 and 1906 Ma, were correlated with the age of provenance of metasedimentary rocks from the Araguaia Belt, which occurs on the eastern edge of the Amazon Craton. Ages around 1957 and 2037 Ma are consistent with rocks from the Maroni-Itacaiúnas Province, located on the eastern edge of the basin. It is possible to mention rocks associated with late to post-collisional orogenic magmatism such as the Igarapé Careta Intrusive Suites (2065 ± 33 Ma), Parintins (2030 ± 3 Ma) and paleoproterozoic indistinguishable granitoids occurring in the Amapá Block and Carecuru Domain, in addition to rocks related to post-orogenic magmatism such as the Granodiorito Sant'Ana (1986 ± 5 Ma) of the Bacajá Domain. Archean contributions can be associated with rocks from the Central Amazon Province or even to Archean nuclei distributed in the region.
Acesso aberto (Open Access)
Sedimentação recente e palinologia do Talude Continental Superior Amazônico-Maranhão
(Universidade Federal do Pará, 2025-04-24) AZEVEDO, Gabriela Miranda de; EL-ROBRINI, Maâmar; http://lattes.cnpq.br/5707365981163429; https://orcid.org/0000-0001-7850-1217
The continental slope off the Amazonas, coast of Pará is a geologically complex region, influenced by tectonic phenomena and sedimentary processes associated with the proximity of the Amazon River mouth. The Amazon River discharges approximately 5.7 × 10¹² m³ of water per year and 1.2 × 10³ m³·s⁻¹ of solid load. The continental slope displays a diverse sedimentary cover distribution, which varies regionally depending on the origin of the sediment sources. Few studies (in sedimentology, palynology, and paleontology) have been conducted on the continental slope of the northern region. The main objective of this study is to analyze the sedimentological characteristics (grain size distribution, calcium carbonate content/CaCO₃, organic matter/OM, and organic carbonate/OC) and investigate the occurrence of fertile sediments in the sediment cover of the upper continental slope off Pará, based on two core samples (T66 and T144). The methodology involved: (1) Bibliographic research on scientific databases; (2) Acquisition of sub-surface core samples using a piston corer; (3) Macroscopic description of cores (color, stratigraphic arrangement, and sediment analysis) and core subsampling; (4) Grain size analysis of the sampled layers, including separation of the silt/clay fraction; (5) Quantification of OM and CaCO₃ contents; (6) Selection of fertile samples for palynological analysis. The results indicate that the sediments are predominantly muddy, with a dominance of coarse silt fraction. They are mostly well-sorted, with approximately symmetrical skewness and platykurtic kurtosis. OM content in the cores ranged from 10.64% to 24.42%, with an average of 16.39% (T66) and 10.64% to 24.42% with an average of 16.39% (T144). Preliminary palynological investigation confirms the presence of pollen grains in the samples. The sediments from the continental slope off Pará consist of a mixture of allochthonous and autochthonous material, with the occurrence of palynomorphs.