Navegando por Assunto "Grupo Araras"

Agora exibindo 1 - 4 de 4

Acesso aberto (Open Access)
Carbon and oxygen isotope geochemistry of Ediacaran outer platform carbonates, Paraguay Belt, central Brazil
(2007-09) RICCOMINI, Claudio; NOGUEIRA, Afonso César Rodrigues; SIAL, Alcides Nóbrega
After the late Cryogenian glaciation the central region of Brazil was the site of extensive deposition of platformal carbonates of the Araras Group. This group includes a basal cap carbonate sequence succeeded by transgressive, deep platform deposits of bituminous lime mudstone and shale. Facies and stratigraphic data combined with carbon and oxygen isotopic analyses of the most complete section of the transgressive deposits, exposed in the Guia syncline, were used to evaluate the depositional paleoenvironment and to test the correlation of these deposits along the belt and with other units worldwide. The studied succession consists of 150 m thick tabular beds of black to grey lime mudstone and shale with predominantly negative d13CPDB values around –2.5 to -1 . The d13CPDB profile of Guia syncline shows a clear correlation with the upper portion of Guia Formation in the Cáceres region, about 200 km to the southwest. The d13CPDB profile of the Araras Group is comparable with d13CPDB profiles of Ediacaran units of the southern Paraguay Belt, western Canada, and the Congo and Kalahari cratons. Moreover, facies distribution, stratigraphy and the carbon isotopic profile of the Araras Group match the middle Tsumeb Subgroup in Namibia, which reinforces the Ediacaran age assigned to the Araras Group.
Acesso aberto (Open Access)
Eventos diagenéticos e hidrotermais do sul do Cráton Amazônico: desvendando a origem das brechas carbonáticas e os processos de dolomitização da Formação Serra do Quilombo do Ediacarano, sudeste do Estado de Mato Grosso
(Universidade Federal do Pará, 2024-04-01) SILVA, Leandro Freitas Sepeda da; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998; https://orcid.org/0000-0002-5225-9255
The Ediacaran period was characterized by the establishment of extensive platforms during CaCO3 supersaturation events, and by the exceptional creation of accommodation space associated with the aftermath of the Marinoan glaciation (~635 Ma). The carbonate deposits in the Araras Group recorded sedimentation from the beginning of the Ediacaran period, stage 1, in the Amazon Craton. The Serra do Quilombo Formation is a distinct unit characterized by the occurrence of strongly fractured dolomites and faults, along with the presence of dolomitecemented breccias (BC) overlying thick packages of limestone associated with diagenetic and hydrothermal modifications. This evidence introduces complexity to these scenarios. The origin of this deposit is still uncertain, as work has mainly focused on paleoenvironmental issues. This study aims to unravel the origin of cemented breccias and diagenetic/burial processes within the unit, focusing on dolomitization processes. The dolomite samples were analyzed using petrography, scanning electron microscopy, microprobe, Raman microscopy, cathodoluminescence, and isotopic analyses (δ13C, δ18O, 87Sr/86Sr) to unravel their burial history. BCs occur as subvertical to subhorizontal bodies with complex geometries, generally cutting through bedding at a high angle, indicative of hydrofracturing processes related to vertical flows of hydrothermal fluids (hydraulic breccia), in addition to exhibiting the typical cockade texture of expansion breccias in dilatational faults. The paragenetic assemblage of these rocks includes dolomite, quartz, calcite, alkali-feldspar, apatite, pyrite, chlorite, bitumen, and iron oxide, with the replacement features (RD) and dolomite cementation (DC) being the main targets of analysis. The almost micritic substitutive matrix (RD1/RD2) is the primary constituent of the Serra Quilombo Formation, with its low correlation between the values of δ13C and δ18O (R²=0.009), well-preserved fabric, and similarity to the isotopic values (C and Sr) documented for Ediacaran carbonates, suggesting syndepositional dolomitization of this constituent under conditions of shallow burial, possibly involving seawater. The first generation of dolomite cement (DC1) and the last phase of dolomite cementation (saddle dolomite - DC3) occur by filling pores, BCs, and fractures. The cockade texture of the breccias highlights a low precipitation rate or a pause in precipitation between DC1 and DC3. Concomitantly, DC1 has isotopic signals of δ18O = -4.34 ± 1.32‰ (n=18) and 87Sr/86Sr = 0.708831 (n=2), while DC3 has values of δ18O = -9.57 ± 2.51‰ (n=15) and 87Sr/86Sr = 0.711464 (n=3). The significant isotopic difference between the two cementation phases indicates distinct dolomitizing fluids. This relationship shows an increase in 87Sr in the fluid as the temperature increases. Furthermore, the enrichment of 87Sr in the fluid can be explained by its interaction with crystalline basement rocks. Thus, faults with deep roots spatially close to tectonically active zones are likely the main conduits for the ascent of this radiogenic fluid. The occurrence of BCs, essentially in the Serra do Quilombo Formation, is attributed to the limestone-dolomite contact and was interpreted as facilitating the development of fracture corridors, which assist in the percolation of hydrothermal fluids. Finally, it is understood that the conduits with carbonate breccia are subsequent to the silicification event of the evaporites of the Nobres Formation, which acted as a sealing rock for hydrothermal fluids. The presence of tectonic stylolites cutting the cemented breccias and the subverticalized zebra-like stratified structures indicates that the BCs were already formed during the installation of post-Ordovician transtensional structures, preceding the installation of the Paleozoic Basins on the South American Platform.
Acesso aberto (Open Access)
Paleoambiente e quimioestratigrafia da Formação Serra do Quilombo, Neoproterozóico da Faixa Paraguai Norte, regiões de Cáceres e Nobres (MT)
(Universidade Federal do Pará, 2012-05-03) MILHOMEM NETO, João Marinho; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998; MACAMBIRA, Moacir José Buenano; http://lattes.cnpq.br/8489178778254136
After the last criogenian glaciation (ca. 635 Ma), large carbonate platforms were developed in several cratonic regions of the Earth and, despite of the intense dolomitization of these deposits, many of paleoceanography and sedimentary processes can still be rescued, contributing to the understanding of the paleoenvironment and the isotopic geochemistry of the Neoproterozoic seawater. One of the most important testimonies of this period in Brazil is the Serra do Quilombo Formation, upper portion of the Araras Group, exposed in the northern Paraguay Belt, southern of the Amazonian Craton. This unit has been interpreted in its type section in the Caceres region, Mato Grosso, as deposits of shallow to moderately deep platform influenced by storms and earthquakes. The stratrigraphic and facies revaluation in combination with C, O and Sr isotopes data in the Caceres (type-section) and Nobres regions, allowed enlarging the paleoceanography and paleoenvironmental interpretations, aiming at building a chemostratigraphic framework for the unit. The 140 m thick of the study succession includes the Serra do Quilombo Formation in sharp basal contact with the Guia Formation and the gradual contact, to the top, with the Nobres Formation. In this succession were described five depositional facies that correspond the organic matter rich laminated dolostone (Dl), massive to laminated dolostone (Dml), hummocky/swaley to planar stratified sandy dolostone (DAh), sandy/oolitic dolostone with wave cross laminations (DAl) and matrix-supported dolomitic breccia (BDm). The sedimentary facies were grouped into two facies associations (FA) that indicate environments of deep to moderately shallow carbonate platform (FA1: Dl and Dml) and storm influenced shoreface (FA2: DAh, DAl and BDm). The Serra do Quilombo Formation comprises a shallowing upward succession, which represents the progradational record of a highstand system tract, in a homoclinal carbonate ramp, installed on the southern part of the Amazonian Craton during the Ediacaran. C and O isotopic analyses were performed in 141 samples (among laminated limestones and dolostones, sandy/oolitic dolostones and clasts, cement and matrix of dolomitic breccias), of which 6 were also selected for the Sr analysis. Major (Ca, Mg and Fe) and trace (Rb, Sr and Mn) elements contents, performed on 20 samples, assisted in the selection of those for Sr isotopic analysis and evaluation of the primary nature of the isotopic signal of study carbonates. The δ13C values, interpreted as representing the original sea water, showed a trend ranging from negative, around -2 ‰ at the base of the succession (Guia Formation) to above 0‰ at the top of the succession. The type-section of the Serra do Quilombo Formation presents a homogeneous C-isotope curve, with δ13C values around 0 ‰, which allows its perfect correlation with other sequences, for example, its section reference in Nobres, far about 200 km. The 87Sr/86Sr follow the general C-isotopes trend, increasing towards the top of the succession, ranging from 0.7077 to 0.7083, directly related to the environmental shallowing and the consequent influx of siliciclastic grains recorded in the top portion of the unit (AF2). The δ13C and δ18O data obtained for the dolomitic matrix-supported breccias (BDm) indicate, in general, a similarity between the isotopic signal of both matrix and clasts, suggesting a sindeposicional and primary process. On the other hand, for the dolomitic cemented breccia, the values obtained for the sparry dolomite cements indicate lower values of δ13C and, notably, of δ18O in relation to the clasts, suggesting that its origin is secondary, probably resulting from the interaction with meteoric water or hydrothermal fluids during the breccias formation. The establishment of a moderately deep to shallow carbonate platform environment with C isotopic patterns, tending to positive values, is clearly distinct of the postglacial deposits of the base of the Araras Group, with strongly negative values of C, and may represent the restoration of normal sedimentation conditions. Likewise, the growing trend of Sr isotopic ratio indicates the increasing in the influx continental in a scenario completely devoid of glacial influence (Snowball/slushball Earth hypothesis). Sr values indicate ediacaran age, between 560 and 580 Ma for the Serra do Quilombo Formation deposition.
Acesso aberto (Open Access)
Reavaliação paleoambiental e estratigráfica da Formação Nobres do Grupo Araras, Neoproterozóico da Faixa Paraguai, região de Cáceres (MT)
(2012-12) RUDNITZKI, Isaac Daniel; NOGUEIRA, Afonso César Rodrigues
Outcrop-based facies and stratigraphic studies in the region of Cáceres, State of Mato Grosso, Brazil, subdivided the Nobres Formation into: lower member, composed of dolostone, intraclastic dolopackstones, sandy dolostones, stratiform stromatolites and evaporitic molds, interpreted as tidal flat/sabkha deposits; and upper member, constituted of dolostone, dolomitic sandstone, stratiform to domal and wrinkled stromatolites, evaporite molds, sandstones and mudstones, interpreted as mixed tidal flat deposits. The stacking of these deposits up to 200 m thick is composed by meter-thick shallowing/brining upward cycles related to a hot arid climate. The peritidal cycles also suggest continuous and recurrent generation of accommodation linked to tectonic subsidence. The siliciclastic inflow at the end of the deposition of the Nobres Formation that hindered the carbonate sedimentation and is attributed to the uplift of source areas linked to the initial phase of closure of the Clymene Ocean, during the Pampean-Araguaia Orogeny, at the limit Neoproterozoic-Cambrian.