Navegando por Assunto "Ediacarano"

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Acesso aberto (Open Access)
A capa carbonática marinoana do Sul do Cráton Amazônico: multiproxies aplicados na reconstituição paleoceanográfica e geobiológica do início do Ediacarano.
(Universidade Federal do Pará, 2024-08-30) SANTOS, Renan Fernandes dos; SANSJOFRE, Pierre; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
The post-Marinoan glaciation (~650-635 Ma) was the most severe event from the late Cryogenian period of the planet. The snowball Earth conditions induced a climate anomaly, triggering major changes in the paleoenvironmental and ocean chemistry recorded in the carbonate cap sequences. The dramatic effects on the global sea level were caused by glacial-isostatic adjustment (GIA) and ice gravity on the coastal zones associated with the ocean thermal expansion under greenhouse conditions. The fast input of meltwaters contributed to a stable density ocean stratification formed by hypersaline deep waters and meltwater surface layers. The ocean destratification occurred in a timescale ranging from tens of thousands to thousands of years. The Puga cap carbonate (~635 Ma), the basal deposits of the Araras-Alto Paraguai basin from the southern Amazon Craton, is revisited in the classical sections from the Tangará da Serra and Mirassol d'Oeste, Mato Grosso State. This succession is one of the best records for evaluating the supersaturation events under GIA and transgression conditions that controlled the accommodation space in the southern Amazon Craton. Sedimentological and stratigraphic data were integrated with new paleoceanographic, and paleo-redox data combined with diagenetic, crystallographic, geochemical (primarily rare earth elements and trace metals), and isotopic (whole rock 87Sr/86Sr, εNd(t), δ 13C, δ 18 O, Sm/Nd) results providing further insights to understand the post-Marinoan conditions. The Puga cap carbonate sequence spans approximately 90 meters, with the first 10 meters composed of glaciomarine deposits, diamictites, and dropstones from the Puga Formation. The basal contact with diamictites is plastically deformed, indicating rapid carbonate precipitation. The cap dolostone consists of stratiform doloboundstones with gypsum pseudomorphs and domal doloboundstones with tubestone, which were deposited in a shallow platform with intense microbial activity. Peloidal dolomudstone/dolopackstone with laminations parallel to bedding planes and peloidal dolograinstones/dolomudstone with quasi-planar laminations and low-angle truncation were formed in a wave-influenced shallow platform. The cap limestone conformably overlies the cap dolostone deposits, marked by dolomitic marlstone with calcite crystal fans (aragonite pseudomorphs) interbedded with mega-rippled limestone. The facies association of the cap limestone indicates moderately deep-water conditions dominated by waves and storms transitioning to a deep platform supersaturated with CaCO3. The main diagenetic process is dolomitization during syn-depositional and shallow burial stages. Rare earth element + yttrium patterns have been analyzed in these deposits. Low Y/Ho ratios (<36) in the cap dolostone suggest a mixture of meltwater and seawater, while the base records superchondritic Y/Ho values up to 70 and high Eu/Eu* values up to 3, indicating upwelling of hypersaline seawater with hydrothermal fluid interaction, suggesting dolomite precipitation during ocean destratification. The radiogenic isotopic compositions of Nd, combined with other proxies such as δ 13C and 87Sr/86Sr, indicate the influence of continental and marine contributions. The Nd isotopic system, less susceptible to diagenetic exchanges, revealed distinct signatures of water masses and enhanced weathering of the Amazon Craton during deglaciation. This process is indicated by geochemical trends (e.g., Y/Ho) and 87Sr/86Sr, εNd(t), δ 13C values. 87Sr/86Sr data in the cap dolostone range from 0.7264 to 0.7084, higher than pre- and postglacial seawater values. More radiogenic 87Sr/86Sr values associated with less radiogenic εNd(t) values, similar to those found in diamictites, reinforce coastal weathering contribution to meltwaters. This multiproxies approach is a reconciliation with the previous rapid cap carbonate precipitation model following the short-term timescale for ocean destratification. Redox-sensitive trace metal data, U, Mo, V, Ni, Cu, P, and δ13C isotopes, indicated paleo-redox states and paleoproductivity during the post-glacial transgression. The cap dolostone precipitated under oxygenated conditions with extensive microbial community contributions, transitioning to predominantly dysoxic conditions with wave action in the last deposition phase. The abrupt sea-level rise altered the biogeochemical cycle, indicating a direct relationship between oxygen production and rapid microbial community colonization. The rapid sea-level rise and continental weathering reduced seawater Mg/Ca ratios with substantial Ca2+ input, also demonstrated by εNd(t) values, causing the change of dolomitic platform to CaCO3 -supersaturated seas in the Amazon Craton margin. Additionally, low siliciclastic content in the cap dolostone is consistent with the siliciclastic starvation model, and the abrupt increase caused the decline of microbial communities coincident with the predominance of dysoxic conditions and longterm transgression. During specific Cryogenian-Ediacaran deglaciation scenarios, anomalous sedimentary and geochemical processes generated one of the most complex paleoenvironmental disturbances in the biogeochemical cycle. They strongly influenced the rapid primary productivity, directly impacting microbial life. The analysis of Precambrian scenarios in the Amazon Craton unraveling the extreme climates sheds critical light on extremophile life proliferation and has strong implications for understanding other planetary surfaces.
Acesso aberto (Open Access)
Geoquímica isotópica Sr-Nd-Pb na capa carbonática marinoana de Tangará da Serra - MT.
(Universidade Federal do Pará, 2023-11-25) SOUZA, Nayra Michelly das Chagas; MILHOMEM NETO, João Marinho; http://lattes.cnpq.br/6010783520758482
In the Neoproterozoic post-glacial scenario, cap carbonates are systematically found over diamictites having been deposited immediately after these glacial sediments. Cap carbonates are wordlwide of great interest as they provide information about paleoenvironmental and paleoclimatic conditions for carbonate precipitation, being able to arrive at the composition of sea water at the time of its deposition, in periods of the ocean great disturbance. On the southern edge of the Amazonian craton, the Mirassol d'Oeste and the Guia formations constitute the Marinoan cap carbonate of the Araras Group. At the Calcário Tangará Quarry (Tangará da Serra – MT), these formations are composed of dolomite deposits from moderately deep platform (Mirassol d'Oeste Formation) covered by limestone and bituminous shales from a CaCO3-saturated deep platform (Guia Formation), which overlay the Puga Formation diamictites. Eleven samples located in the middle and upper part of the profile (from 45m to 65m) were used, with the methodologies of Pb-Pb geochronology by Neptune ICP-MS mass spectrometry, Sr isotopic geochemistry by sequential leaching in a MAT-262 TIMS mass spectrometer, and Sm-Nd isotopic geochemistry by total dissolution of the carbonate phase in a Neptune ICP-MS mass spectrometer. The Pb-Pb geochronological data for the Guia Formation confirmed the Marinoan age for cap carbonate deposition, with 622 ± 30 Ma, which is in agreement with previous works. Regarding the Sr isotopic compositions at the base of such a Formation, these provided an 87Sr/86Sr ratio of 0.7071–0.7073, which are among the lowest values found in the world for the cap marinoan. For the upper portion of the Guia Formation, higher values were found (0.70702 ± 00014 to 0.70769 ± 000012; 2σ), which are in agreement with the base of this Formation and with other surrounding Marinoan cap carbonate around the world. Globally, this abrupt increase in Sr isotopic composition is also observed in the various Marinoan cap carbonate, ranging from 0.7070 to 0.7120. The Nd-TDM model ages ranged between 1.85 and 2.54 Ga, with ԐNd(635Ma) values ranging from -11.1 to -4.7, but without an increasing or decreasing trend. The auxiliary methodologies consisted in the identification of the mineralogical composition, through petrography in the optical microscope for the carbonate phase, X-Ray diffraction for the siliciclastic phase, and the quantification of the siliciclastic material, which confirmed the presence of terrigenous material (quartz, feldspar and mica) in the Guia Formation carbonates in different proportions in the samples (from 5.9% to 19.8%). Some diagenetic aspects were also observed in petrography, such as dolomitization and replacement of aragonite fans by calcite. Variations in the isotopic composition of Sr and Nd may reflect a rapid change in the isotopic composition of seawater, possibly related to continental influx, with the incorporation of such elements during diagenesis. The Nd-TDM model ages of the Guia Formation carbonates are comparable to those of the continental crust found in rocks from the Paleoproterozoic provinces of the Amazonian Craton, which forms the basement of the Araras Group. The negative values of ԐNd(635Ma) probably indicate the presence of Nd from suspended material transported from continental to seawater from which the carbonates precipitated. As for Sr, the random variations of Nd-TDM ages and ԐNd values suggest important and rapid changes in the continental contribution to the post-Marinoan oceans.
Acesso aberto (Open Access)
Paleoambiente e evolução dos estromatólitos gigantes da capa carbonática marinoana do Cráton Amazônico, Tangará da Serra-MT
(Universidade Federal do Pará, 2020-03-10) SANTOS, Renan Fernandes dos; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
Giant domical stromatolites, associated with tubstone structures, are recorded here for the first time the sequence of the Puga cap carbonate the southeast of the Amazonian Craton region of Tangará da Serra, Brazil. The well-preserved giant mounds reach 12 m in diameter and up to 10 m in height, forming a 200 m laterally continuous biostrom, with individualized domes, which display an expressive synoptic, metric to decametric relief. This work aimed to de- scribe and interpret the first occurrence of giant stromatolites and attest to their biogenicity, considering their macro, meso and microscopic aspects. It also determined the stratigraphic biogenic facies relationship, formed by organomineralization processes, whether bioinduced or bioinfluenced together with the other inorganic facies, originated by precipitation and/or sedimentary (mechanical) processes of the Puga carbonate sequence and finally, proposed an evolutionary model for giant stromatolites with the other facies, providing new inferences for the hydrodynamics of the epicontinental sea, of the SE of the Amazonian craton, with pale- oenvironmental and bioevolutionary implications for the beginning of Ediacaran. The begin- ning of Ediacaran marked by epicontinental, shallow and stratified seas, formed by dense and hypersaline waters, contrasting with the melting waters, light and rich in nutrients. The origin of the giant stromatolites is the product of a sum of anomalous paleoenvironmental and biotic factors, which occurred after the Marinoana glaciation (635Ma). The diamicton substrate was colonized by extremophiles, halophilic microbial communities in hypersaline waters with little or no direct influence of hydrodynamic processes, forming stratiform stromatolites with gypsum pseudomorph cement. With the advancement of greenhouse conditions, there was a continuous generation of accommodation space, caused by rising sea levels, influenced by syn-deglacial transgression and by the glacio-isostatic adjustment (GIA) that caused the uplift of the coastal zone and the mixing of the waters. The mixture of the waters that occurred at the end of the glaciation was essential for the development of microbial mats, since the cool water was, rich in nutrients. The microbial communities developed initially in balance with the constant migration of the photic zone, caused by the gradual rise in sea level, with the end of the GIA's influence. The accommodation space is controlled only by post-glacial transgres- sion, resulting in a more significant impact on hydrodynamic processes, registered in the ex- pressive synoptic relief. The sum of these conditions would have propitiated the development of microbial communities that would become giant stromatolitic mounds, with metric to dec- ametric synoptic relief. The constant increase in energy would result in the reworking of mi- crobial mats, generating macropeloids, agglomerates of micropeloids. The record demon- 1 strates an alternation of macropeloids and micropeloids blades, which suggests that in times of stabilization, there was an attempt to colonize microbial communities, that were reworked continuously by the wave. The decline of giant stromatolites, in the southern of the Amazoni- an craton, would be related to the height of greenhouse conditions, with an sudden increase in the influence of post-glacial transgression, which would condition a massive siliciclastic en- try, promoting the burial of microbial communities. Thus, the stromatolitic strata were suc- ceeded by a limestone factory, induced mainly by inorganic processes, in a sea saturated with CaCO3. Giant stromatolites are considered here as an essential record for understanding post- glacial conditions, from the beginning of Ediacaran.
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)
A seção-tipo da Formação Serra do Quilombo, Grupo Araras, Neoproterozoico da Faixa Paraguai Norte, Mato Grosso
(2013-06) MILHOMEM NETO, João Marinho; NOGUEIRA, Afonso César Rodrigues; MACAMBIRA, Moacir José Buenano
After the last Cryogenian glaciation (ca. 635 Ma), large carbonate platforms were developed in several cratonic regions of the Earth and, despite the intense dolomitization of these deposits, paleoceanographic and paleoenvironmental pieces of information are preserved. One of the most important examples of this period in Brazil are the dolostones of Serra do Quilombo Formation, which belong to the upper portion of the Araras Group, exposed in the northern Paraguay Belt, south of the Amazonian Craton. The stratigraphic revaluation of this formation in its type-section and in a reference section in the Nobres region, based on facies and stratigraphic analyses, allowed to enlarge the paleoenvironmental interpretations and to propose a depositional model. The study succession of 140 m thick includes Serra do Quilombo Formation in sharp basal contact with the limestones of Guia Formation and the gradual contact, to the top, with the sandy dolostones of Nobres Formation, at the top. Serra do Quilombo Formation comprises a shallowing upward succession, which includes two facies associations: 1) deep to moderately shallow carbonate platform, composed of laminated dolostone rich in organic matter and a massive to laminated dolostone; and 2) storm influenced shoreface, consisting of hummocky/swaley to planar stratified sandy dolostone, sandy/oolitic dolostone with wave cross laminations and matrix-supported dolomitic breccias. Serra do Quilombo Formation represents the progradational record of a high-stand system tract, in a homoclinal carbonate ramp, installed on the southern part of the Amazonian Craton during the Ediacaran.