Navegando por Assunto "Estratigrafia"

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Desenvolvimento da vegetação e morfologia da foz do Amazonas-PA e rio Doce-ES durante o Quaternário tardio
(Universidade Federal do Pará, 2013-11-05) FRANÇA, Marlon Carlos; PESSENDA, Luiz Carlos Ruiz; http://lattes.cnpq.br/0425441943533975; COHEN, Marcelo Cancela Lisboa; http://lattes.cnpq.br/8809787145146228
This work compares the vegetation and morphological changes occurred along the littoral of the Marajó Island, Amazonian littoral, and the coastal plain of the Rio Doce, southeastern Brazil, during the Holocene and late Pleistocene/Holocene, respectively, focused specifically on the response of mangroves to sea-level fluctuations and climate change, which have been identified in several studies along the Brazilian coast. This integrated approach combined radiocarbon dating, description of sedimentary features, pollen data, and organic geochemical indicators (δ13C, δ1₵N and C/N). On coastal plain of the Doce River between ~47,500 and ~29,400 cal yr BP, a deltaic system was developed in response mainly to sea-level fall. The post-glacial sea-level rise caused a marine incursion with invasion of embayed coast and broad valleys, and it favored the evolution of a lagoonal/estuary system with wide tidal mud flats occupied by mangroves between at least ~7400 and ~5100 cal yr BP. Considering the Marajó Island during the early and middle Holocene (~7500 and ~3200 cal yr BP) mangrove area increased over tidal mud flats with accumulation of estuarine/marine organic matter. It was a consequence of the marine incursion caused by post-glacial sea-level rise, further driven by tectonic subsidence. Dry conditions in the Amazon region during this time led to a rise is tidal water salinity and contributed to mangrove expansion. Therefore the effect of relative sea-level (RSL) rise was determinant to the mangrove establishment in the southeastern and northern region. During the late Holocene (~3050 – 1880 cal yr BP) the mangroves in both regions were retracted to a small area, with some areas replaced by freshwater vegetation. This was caused by the increase in river discharge associated to a wet period recorded in the Amazon region, and considering the coastal plain of the Doce River (southeastern Brazil), the mangroves shrank in response to an increase in fluvial sediment input associated to a sea-level fall.
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Estratigrafia da faixa de dobramentos Paraguai-Araguaia no centro-norte do Brasil
(1997-12) HASUI, Yociteru; ABREU, Francisco de Assis Matos de; SILVA, José Maurício Rangel da
The Paraguai-Araguaia Fold Belt, constituted during the Brasiliano Cycle, extends to the north of Bananal Island, with submeridian trend. Between the 9º 30'S and 3º30'S parallels, the fold belt is represented by the Baixo Araguaia Group, an ophiolitc belt, some granitic bodies and by the Rio das Barreiras Formation. The Baixo Araguaia Group comprises three formations named, from the bottom to the top, Estrondo, Couto Magalhães and Pequizeiro, the latter one being of magmatic-sedimentary characted related to the ophiolitic belt. The ophiolitic belt is associated to the Tocantins-Araguaia geosuture, which seems to be manifested at the surface only in the northern region by a thrust fault, where glaucophanitic greenschist facies rocks have been recognized. The fold belt evolution shows a western trending polarity for the geosynclinal phenomena, except for the basic-ultrabasic magmatism.
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Estratigrafia e eventos da transição Neoarqueano-Paleoproterozoico da Bacia de Carajás, sudeste do Cráton Amazônico
(Universidade Federal do Pará, 2020-09-18) ARAÚJO, Raphael Neto; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
The Neoarchean-Paleoproterozoic transition is marked by a series of paleoenvironmental, paleoclimatic and tectonic changes that resulted in dramatic events, which imposed to the Earth novel conditions, some of them with irreversible characteristics. On the paleoenvironmental aspect, it is highlighted the rise of oxygen in the hydrosphere-atmosphere system, onset the Great Oxidation Event (GOE) at around ca. 2.45 Ga. The rise of this gas caused consequently the decrease of the greenhouse gases such as CH4, which promoted the emergence of glacial episodes at around ca. 2.45–2.22 Ga, generically termed the Huronian Glacial Event (HGE). Although several studies support the hypothesis that these glacial episodes represent the first global glaciation of the Earth's history (Paleoproterozoic snowball Earth), stratigraphic and geochronological contradictions impose doubt as to its global extension. Strangely, although this set of events is widely recognized in several cratonic areas around the globe, these events are still poorly understood and/or not yet reported in the Amazonian Craton. In this study, the stratigraphic, sedimentological and geochronological investigation of the volcano-sedimentary succession (ca. 5-km-thick) of the Carajás Basin, situated in the southeastern Amazonian Craton, northern Brazil, allowed the recognition and sequencing of some of these events in this basin. Two new units are being formally proposed to this basin: the Serra Sul and Azul formations. Glacial diamictite intervals of the Siderian–Rhyacian (ca. 2.58–2.06 Ga) occur stacked within the Serra Sul Formation, and are the first reported occurrence of glacial deposits of that age in South America. In paleogeographic terms, the occurrence of Paleoproterozoic glacial deposits in this part of the globe, expands the reach of these glaciations to the Amazonian Craton for the first time, although the Serra Sul diamictite may be correlated with any of the know Paleoproterozoic glaciations, or none of them. Well-preserved textures, such as glacial foliation and dropstone features, indicate that the deposition of the Serra Sul Formation occurred in a coastal subglacial setting, in which glaciogenic sediments were resedimented in submarine fan system, and through ice rafting process in distal waters of the marine environment. The Serra Sul glacial system was developed immediately above of pre-glacial strata represented by the Neoarchean banded iron formation and volcanic rock units, which not was the main substrate, but also was the main source of sediments to this glacial system. Additionally, the stratigraphic results indicate that the immediately above of the Serra Sul diamictite, rhythmite deposits of the Azul Formation, locally enriched in manganese, were deposited in a shallow marine environment (offshore and offshore transition/shoreface zones), as a result of the sea level rise during the deglaciation phase. The manganese-bearing strata were possibly deposited in association with black shale deposits—which allowed the formation of rhodochrosite during diagenesis—in deep zones of the marine basin. Petrographic and mineralogical evidences, supported by field observation, indicate that manganese oxides were secondarily remobilized through faults to zones with low strain and high permo-porosity within red bed strata of the Azul Formation, similarly to that observed in hydrocarbon migration. In stratigraphic terms, the Azul Formation represents the same interval previously arranged in the lower member of the Águas Claras Formation. This formation was redefined to designate exclusively sandstone, conglomerate and jasper conglomerate strata, deposited in a braided fluvial system, which occur in unconformably immediately above of the Azul Formation. Moreover, it is suggested that the Azul and Águas Claras formations are the stratigraphic record associated with a transgressive-regressive sequence (T-R). The dating (U-Pb) of detrital zircon grains separated from the Azul and Águas Claras formations indicate that Meso- to Neoarchean rocks, possibly of the Rio Maria and Carajás domains, were the main source of sediments. The 207Pb/206Pb Age distribution of the 76 concordant analysis of the Azul Formation indicate a youngest population at ca. 2.27 Ga, interpreted as the maximum deposition age of this unit. The occurrence of Rhyacian to Siderian zircon grains in this unit strongly suggest that the Bacajá Domain may have been a subordinated source of sediments, and in paleogeographic terms, suggest a possible connection between this domain and the Carajás Domain at that time period. The integration of the results obtained from this study, supported by previous data on the regional geology, allowed the proposition of a tectono-sedimentary evolutive model to the Paleoproterozoic succession of the Carajás Basin. It is envisaged that this basin evolved during the greater part of the Paleoproterozoic in a foreland style, as result of the collision of the Bacajá and the Carajás domains during the Transamazonian orogenetic cycle at ca. 2.2–2.0 Ga. The convergent movement of these blocks caused the gradual uplift of the Carajás protocontinent; the closure of the Azul Sea, and installation of a wide fluvial-alluvial system, in which the Águas Claras and Gorotire formations were deposited. This scenario of profound changes is directly related to the Columbia supercontinent assembly at the end of the Paleoproterozoic, that promoted the continentalization and amalgamation of the ancient landmasses that later formed the proto-Amazonian Craton at the end of Paleoproterozoic.
Acesso aberto (Open Access)
Estratigrafia e paleoambiente da Formação Pastos Bons, Jurássico-Cretáceo da Bacia do Parnaíba.
(Universidade Federal do Pará, 2019-03-06) CARDOSO, Alexandre Ribeiro; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
The Jurassic-Cretaceous transition was marked by the fragmentation of the West Gondwana supercontinent and consequent opening of the Atlantic Ocean. The pre-rupture stages were characterized by epeirogenic uplifts associated with voluminous magmatic accumulation in the infracrust. Additionally, expressive volcanic flows occurred in the central portion of the West Gondwana, composing the Central Atlantic Magmatic Province (CAMP). A post-CAMP thermal subsidence stage allowed the installation of massive lakes coincidently with the depocenters of the Parnaíba Basin, which is recorded in the Jurassic-Cretaceous Pastos Bons Formation (PBF). The PBF is a predominantly constituted of thick reddish mudstones interbedded to tabular sandstones. The basal portion is composed of fossiliferous black shales, the Muzinho Shale. Due to discontinuous exposures and fault displacements, the stratigraphy of the Mesozoic of the Parnaíba Basin keeps poorly understood and the is a necessity for more detailed faciological and sedimentological studies. In this sense, this research performed a sedimentological lecture of these deposits in order to elucidate the paleoenvitonment and Paleogeography of the PBF in the context of the West Gondwana, through outcrop-based facies analysis and cyclostratigraphy. The provenance of this succession was investigated through compositional sandstones diagram, quartz hot cathodoluminescence and heavy minerals analyses. The Muzinho Shale beds were evaluated through petrography, XRD and SEM/EDS. The PBF is composed of four facies associations, interpreted as central lake (FA1), sheet-like delta front (FA2), lakeshore (AF3) and ephemeral fluvial channels (FA4). FA1 is composed of drying/shallowing upward cycles, defined by millimeter-scale black shales interlayered with limestones, that grade to reddish shales and laminated/stratified sandstones. Shales are composed by quartz, illite, smectite and calcite. The fossiliferous levels include young and adult macroforms in the same horizon, sandwiched by crinkly laminations with organic rich Fe-smectites. FA1 indicate sedimentation in the center of the lakes, in eutrophic and anoxic conditions. Mass mortality events were probably induced by contamination of the ater column due to H2S release by cyanobacteria. The transition to mudstones and sandstones reflects the evolution of underfilled to overfilled lakes, as the sediment and water supply were increased. FA2 is composed of tabular sandstones in thickening upward cycles, which record unconfined flows and progressive lake filling, with consequent reworking of the top of the beds by wave action. FA3 is constituted of shallowing upward cycles, marked by wave marks, adhesion structures or mud cracks. FA4 is defined by fining upward cycles developed by ephemeral fluvial channels, with conglomerates and sandstones grading to mudstones. This succession defines open and stratified lakes, dominated by settling and unconfined flows, in hyperpycnal regime. The stratigraphic framework of the PBF is composed of four depositional cycles, constituted of centimeter to millimeter-scale cycles, bounded by flooding surfaces and unconformities. These cycles define a retrogradational-progradational-retrogradational stacking pattern, with increasing accommodation space upward conditioned by post-CAMP thermal subsidence pulses and variations in sediment supply. The Mesozoic succession suggests migration of the West Gondwana toward Equatorial regions during Jurassic-Cretaceous, with aridity attenuation relatively to the Permian-Triassic. The sandstones of the PBF indicate provenance from recycled orogens and craton interior, whereas cathodoluminescence data indicate predominantly volcanic sources. In order to test possible correlations with adjacent units, we verified the heavy minerals assemblage of the PBF is very similar to the Corda Formation, and both differ from the fluvial deposits of the Grajaú Formation. The ZTR, GZi and RZi indexes are higher for sandstones of the PBF and Corda Formation, and lower for the Grajaú Formation. The fluvial deposits distinguish mainly by sillimanite and high hornblende content (>50%). These data indicate polycyclic minerals and mixed sources for sandstones of the Parnaíba Basin. The Mearim Group exhibits volcanic contribution supplied by CAMP basalts and low to medium grade metapelitic sources. This last was possibly supplied by Neoproterozoic rocks of the Médio Coreaú Domain, Borborema Province. Differently, the Grajaú Formation was supplied by type-I Brazilian granites. This geological evolution indicates change in provenance areas or exhumation of common source areas during the Mesozoic of the Parnaíba Basin.
Acesso aberto (Open Access)
Evolução da paisagem da porção centro-oriental da Amazônia do Cretáceo ao Paleógeno.
(Universidade Federal do Pará, 2024-06-23) MOURA, Matheus Ramos de; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
This study presents a new perspective for the evolution of the relief in Central and Eastern Amazonia based in geomorphologic and stratigraphic analysis of two areas: the first one at the surroundings of Paragominas and the second includes territories from Juruti, Belterra and Santarém counties, both at the state of Pará. The results of the relief analysis demonstrate a diversity of morphological modellings that indicate a polycyclic evolution involving periods of development of flat plateaus, associated with the development of lateritic profiles, alternated with intervals that tend to erosional activity from unstable and collapsing slopes, being the planation reliefs representative of the Paleocene-Eocene and Early-Middle Miocene, while the dissection modellings mark ages from the Neoeocene-Neooligocene and Neomiocene-Pliocene. The ages of the planation modellings are confirmed by age estimations of Cenozoic paleosurfaces that occur along the Amazon region (with which they are correlatable) and by global eustatic and climatic tendencies. The sedimentological and stratigraphic analysis identified two typical profiles for these study areas – PWH (profiles with well-defined horizons) and PMC (profiles with massive conglomerates), distinguished by their in situ/reworking characteristics. Regarding to the PMC profiles, there were observed two facies associations: FA1: an association of conglomerates of diverse supportings and arenites assumed to be formed by proximal fan deposits generated through debris flows; and FA2: packs of clay displaying mostly a massive framework, interpreted as distal fan deposits generated through sheet flow and mudflow. Both associations have subaerial exposure evidence and erosive surfaces in their bases, that allow to conclude a deposition at embedded valleys through rapid pulses of detritus, and hence were interpreted as colluvial and alluvial fan deposits, that formed along the most erosive periods of the regions, having been initiated in the vicinities of the main drainages of the analyzed areas (Capim and Amazonas rivers). The results of this study demonstrate a more intense sedimentary and tectonic activity than that generally accepted for the Amazon region during the Cenozoic Era, showing that this region’s landscape was more affected than it was believed to be by another tectonic remarkable events in the South America continent, such as: the Andes Uplift, the deposition of the Amazonas and Marajó basins and Bragantina Platform neogene formations, the Tortonian Regression and the install and evolution of the Amazonas River.
Acesso aberto (Open Access)
Evolução geológica no cenozóico da região norte entre Açailândia (MA) e Ligação (PA)
(Universidade Federal do Pará, 2000-06-14) CALAF, José Maria Calaf; KOTSCHOUBEY, Basile; http://lattes.cnpq.br/0096549701457340
The studied area is located along the BR-010 Highway (Belém-Brasília) between the cities of Açailândia (MA) and Ligação do Pará (PA), in the southern part of the Bauxiti-bearing Province of Paragominas and has an extension of about 115 km. In it, two surfaces were distinguished, each one supported by a specific alteritic-sedimentary sequence. The highest surface corresponds to the tops of vast residual plateaus from 360 to 220 m in altitude and presents a slight slope to the north. The other surface, embedded in the first, culminates between 210 m, to the south, and 150 m, to the north, and shows a slight swell. The old lateritic-bauxitic cover that supports the higher surface is constituted by a saprolitic horizon originating from the alteration of clayey or archosean sandstones and claystones from the “Itapecurú Deposits” of the Upper Cretaceous. On the saprolite rests a powerful armor, essentially ferruginous in the extreme south, which, towards the north, is progressively enriched in gibbsite. The first signs of bauxitic levels, still diffuse, are found about 40 km north of Açailândia. This trend is confirmed to the north with the presence of increasingly clear and consistent bauxitic horizons, above and at the base of the ferruginous horizon. Superimposed on this cover and in extremely sharp contact, there is a thick clay capping, the Belterra Clay. The lateritic-bauxitic cover developed in three phases. During the first phase, the ferruginous armor was formed through the ferruginization of the Itapecurú sediments, during the Lower Tertiary. Afterwards, the armor was affected by partial degradation and physical reworking of its products. Remains of older alterites may have also been involved in this process that resulted in deposition of nodular to pseudopisolithic gravel and sandy clay. The third phase, essentially chemical, consisted of the generation of bauxite in areas that presented more favorable climatic and geomorphological conditions. This event probably occurred at the end of the Eocene. The exclusively ferruginous lateritic armor that supports the lower surface has as substrate a reddish clayey sandstone containing small lateritic fragments and quartz pebbles. The cuirass itself has a massive, columnar or nodular structure, and the degraded stone-layer can be found in several places. Its cap is formed by a yellowish clayey-sandy material, rich in quartz grains and disseminated letheritic fragments. This laterite probably formed in the late Pliocene – early Pleistocene. Its substrate can be correlated with the Barreiras Group, while its capping would correspond to the unit called Post-Barreiras further north, in the Ipixuna-Aurora region. The stone-layers quite frequent in the area are located in the same stratigraphic position as the more recent lateritic armor and resulted from in situ degradation of this armor, without significant physical rework. However, locally, colluvial deposits of the glacis type or pediments composed of fragments of ancient armor were identified. The following stages characterized the evolution of the two alteritic-sedimentary sequences: 1- formation of a ferruginous armor from the Itapecurú Deposits; 2- partial degradation of the armor; 3- Bauxitization in the Paleogene; 4- deposition of sediments that later change to Argila de Belterra; 5- major erosive phase and individualization of vast plateaus; 6- filling of the valleys and lowered areas separating the plateaus by sandy-clay sediments correlated with the Barreiras Group and degradation products of the old armor; 7- ferruginization of the sediments from step 6; 8- more or less accentuated degradation of the ferruginous crust from stage 7, giving rise to stone-layers; 9- deposition of clayey-sandy sediments correlated with the Post-Barreiras on the armoring of stage 7 and the stone-layers; 10- dissection and establishment of the current drainage network. Climatic variations and periodic tectonic (epirogenetic) reactivations throughout the Cenozoic were the main factors responsible for this evolution.
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.
Acesso aberto (Open Access)
Geologia e geocronologia da região a sul de Paraíso do Tocantins-TO
(Universidade Federal do Pará, 1996-04-22) SOUZA, Silvia Helena Pereira de; MOURA, Candido Augusto Veloso; http://lattes.cnpq.br/1035254156384979
The most recent geochronological studies carried out in the basement of the northern segment of the Araguaia Belt indicated the occurrence of rocks from both the Archean and the Lower Proterozoic. The oldest rocks, predominantly represented by trondhjemitic orthogneisses, with ages around 2.85 Ga, were grouped in the Colméia Complex. On the other hand, granitic orthogneisses, with a minimum age of 1.85 Ga, called Cantão Gneisse, constitute the youngest rocks, hitherto known, from the basement of the Araguaia Belt. In its southern segment, the basement of the Araguaia Belt is represented by tonalitic gneisses, associated with calciosilicate gneisses, which were correlated to the Colméia Complex, and which serve as host for the Serrote Granite, located south of the city of Paraíso do Tocantins. Calciosilicate gneisses, quartzites and tonalitic gneisses were also described, gathered in a distinct unit called Complexo Rio dos Mangues. Although the units mentioned above were considered Archean in age, there is practically no geochronological information about them. This work, therefore, aims to determine the ages of the basement rocks in the southern portion of the Araguaia Belt, and to investigate the correlation or not of these rocks with those outcroppings within the dome structures of the northern portion of this belt. This information is important for the organization of the lithostratigraphic framework of the basement of the southern portion of the Araguaia Belt, which in turn is fundamental for understanding the geological evolution of this belt. In the geochronological investigation, the method of evaporation of Pb in zircon single crystals (Pb-Pb in zircon) was used, which provides the crystallization age of zircon obtained from its gradual heating, in a thermal ionization mass spectrometer. A plateau age of the zircon grain is then calculated, on a 207Pb/206Pb versus temperature plot. Granodiorite gneisses with associated leucosomes and calciosilicate gneisses originally correlated to the Colméia Complex (samples SH12, SH15 and SH40), tonalytic gneisses and syenitic gneisses included in the Rio dos Mangues Complex (samples SH36 and SH33) and a sample of Serrote Granite. In the set of rocks that had been correlated to the Colméia Complex, the ages obtained were concentrated around 2.0 Ga, and the zircons of the leucosomes of the calcosilicate gneisses provided a relatively younger age of 1.8 Ga. In the group of rocks mapped as Complexo Rio dos Mangues, the age obtained for the zircons of the syenitic gneisses was relatively younger (1.0 Ga). These gneisses, due to their proximity to the alkaline gneisses of Serra da Estrela and because they are compositionally similar to them, should be part of the Monte Santo Suite. In the analysis of the zircons of the tonalytic gneisses, a plateau age was not defined and, on the contrary, it showed a pattern of continuous growth of the ratio , 207Pb/206Pb, and at higher temperatures, the ages systematically presented values slightly higher than 2.0 Ga . In this case, the oldest ages were assumed as minimum crystallization ages for these zircons. For the Serrote Granite, the age obtained through this method was 1851 ± 41 Ma (2σ). The ages of the gneissic rocks in the basement of the southern segment of the Araguaia Belt, located between 1.8 and 2.1 Ga, indicate the absence of gneisses of Archean ages in this portion of the belt. Consequently, it is advised against correlating them with those of the Colméia Complex. It is additionally suggested to abandon the aforementioned term and extend the name of Complexo Rio dos Mangues to the calciosilicate and orthogneisses that have expression in the vicinity of Paraíso de Tocantins. The crystallization ages of Serrote Granite (1851 ± 41 Ma, 2σ) and Cantão Gneiss (1846 ± 64 Ma) are very similar. Due to this similarity, an additional geochronological study was carried out using the Pb-Pb method in whole rock and feldspars, aiming to investigate the correlation between these granitic rocks. For the Serrote Granite, an age of 1872 ± 140 Ma (1σ) was obtained and for the Cantão Gneiss, an age of 1744 ± 27 Ma (1σ). These ages, when compared with those obtained by the Pb-Pb method in zircon, show that the Pb-Pb system in whole rock was partially opened, during a thermo-tectonic event subsequent to the crystallization of these rocks, rejuvenating their ages. A comparative study was also carried out between these units and some anorogenic granites of Proterozoic age from the Amazon region and the Median Massif of Goiás, where it was verified that the Serrote Granite is very similar to the same ones, also adjusting to the curves of evolution of Pb, proposed in the plumbotectonics model. The Cantão Gneiss does not fit the plumbotectonic model, due to the high 206Pb/204Pb ratios found in the feldspars of the samples from this unit. These high Pb ratios in these rocks reveal that they are enriched in U, and this enrichment may have come from the magma that generated them or have been acquired in a metamorphic event subsequent to its crystallization.
Acesso aberto (Open Access)
Geomorfologia, mudanças na fonte de matéria orgânica e vegetação em planícies de maré próximas a foz do rio Amazonas durante o Holoceno
(Universidade Federal do Pará, 2011-11-11) GUIMARÃES, José Tasso Felix; COHEN, Marcelo Cancela Lisboa; http://lattes.cnpq.br/8809787145146228; 8809787145146228
Geomorphological data, sedimentary facies, pollen, spores, carbon and nitrogen isotopes records, C/N ratio and radiocarbon dates allowed the identification and discussion about the relationship between the main morphological process, climatic signals and its influence on vegetation patterns of tidal flats near the mouth of the Amazon River during the last 5500 cal yr B.P. Therefore, data from the margin of Amazon River (Macapa site) indicate marine influence related to mangrove presence over a tidal mud flat between 5560 - 5470 cal yr BP and 5290 - 5150 cal yr BP. Afterward, the mangrove area shrank following the return of more humid conditions and increase of Amazon River discharge. A common reworking process of the tidal flat through the lateral migration of a meandering creek occurred in the study site, with later development of transitional vegetation under freshwater influence. Following the natural vegetation succession under stable climate and hydrological conditions, the expansion of varzea forests occurred since 600 - 560 cal yr BP until the present. Furthermore, regarding the tidal flats located west of the mouth of Amazon River (Amapa site), these stable conditions also allowed the mangrove maintenance over mudflats with deposition of marine organic matter during at least the last 2350 - 2300 cal yr BP. Processing and interpretation methods in optical and synthetic aperture radar (SAR) images, data of water salinity, maximum height of flood, historical rain series and sedimentary facies were used in the analysis of the morphological and geobotanical units, and its short-term changes to understand the main processes acting on the tidal flats downdrift of the Amazon River mouth (Calcoene site) during the last 30 years. Thus, the study area was subdivided in two main compartments: coastal plain and plateau. The coastal plateau presents a flat to gently undulated surface shaped by erosive process. Avulsion of alluvial channel and birdfoot feature possibly related to six deltaic lobes were also identified in this compartment. The vegetation is represented by varzea (flooded freshwater forests) and savannah. The coastal plain has an average length of 10 km, and it presents the tidal-fluvial channel, paleochannels, lakes, varzea, inundated field (herbaceous field), mangrove, chenier ridges, elongated tidal mud bars, tidal mud and mixed flat (non-vegetated). The sedimentary facies indicate tide and wave-dominated environments. The presence of lakes and lake belts, coexistence of herbaceous field and mangrove in the coastal plain may be related to silting and abandonment of tidal channels. The temporal analysis of these features indicates the savannah contraction, expansion of varzea and mangrove areas and formation of extensive mud tidal flats during drier period under the influence of El Nino. The wetter period under the influence of La Nina probably allowed the increase of varzea and lakes over the savannah areas, and the expansion of mangrove. Therefore, the decrease of rainfall index during El Nino may have reduced the Calcoene River inflow and favored an increase of tidal propagation, transport and deposition of mud along the tidal-fluvial channel and its secondary channels with subsequent development of mangrove and stabilization of muddy substrate near the coastline in La Nina. Considering a longer time scale during the mid and late Holocene, the analysis of morphology, sedimentary facies, palynological, carbon and nitrogen isotopes, C/N and radiocarbon data from tidal flats in the Calcoene site, indicates that vegetation development during the mid and late Holocene occurred influenced by the interaction relative sea-level and tidal channel dynamic. The data indicate a mud-rich tidal flat that presents alternations between supra and intertidal environments. The proximal portion of the tidal flat is related to the transitional sector between the costal plateau and plain and represents the final filling stage of the concave-up feature formed by the abandoned channel that contributed to water accumulation under very low energy flows, fern and other terrestrial vegetation establishment surrounding the formed lake since 5280 - 5160 cal yr BP. During the last 2840 - 2750 cal yr BP, the source of mud ceased and autochthonous organic material became prevalent as well as the increase in contribution of terrestrial organic matter (C3 plants), mainly represented by varzea vegetation. Herbaceous field already colonized the tidal flat at least during the last 3170 - 2970 cal yr BP. However, part of distal portion of the tidal flat related to the herbaceous field was covered by chenier ridges between 3170-2970 and 220-140 cal yr BP. The mangrove establishment, characterized by estuarine organic matter, Rhizophora and Avicennia pollen, occurred after 1350-1290 cal yr B.P and 220-140 cal yr B.P. on G3 and G2 site, respectively. This stacking pattern of sediments indicating a retrogradation, with distal facies over proximal facies, and gradual transition of herbaceous to mangrove sediment (e.g. G3 site) suggesting that the creation of accommodation space may be produced during an increase of wave action, tidal inundation frequency and evolution of secondary channels in the study site as a result of progressive relative sea-level rise. The integration of all these data suggests that the morphological process, vegetation patterns and sources of organic matter from tidal flats of Calcoene, Amapa and Macapa sites was influenced and conditioned by the interaction between relative sea-level, climatic and hydrological changes, and tidal channel dynamic during the Holocene.
Acesso aberto (Open Access)
Organização lito-estrutural do duplex Salobo-Mirim Serra dos Carajás
(Universidade Federal do Pará, 1990-06-06) SIQUEIRA, José Batista; COSTA, João Batista Sena; http://lattes.cnpq.br/0141806217745286
The Salobo Group embodies a sequence of volcanosedimentary rocks, which overlain a sialic basement, and is linked to the evolution of the Cinzento Transcurrent System, situated immediately to the north of the transcurrent system which provided the estabelishment of the Grão-Pará Group. The macrostructure is formed by two major shear zones which join at the west and east end parts of the studied area, defining a WNW-ESE eliptcal structure. This structure, which is interpreted as an extensional duplex, comprises, internally, minor shear zones that converge to the main zones. NW-SE, NNE-SSW and NE-SW strike-slipe zones modify the major geometric pattern. The duplex lithologies underwent metamorphism of the amphibolite facies during regional transpression, followed by expressive hidrotermalism and greenschist facies transformation trought the progress of the deformation. Significant mineralizations of bornite, chalcocite and in some cases chalcopirite, with molibdenite, gold and silver, are mainly hosted by lenses of iron formations, besides magnetite. These sulphides are distrubuted as inclusions in the grains of silicate gangue minerals and oxides or between them, as well as along the ductile and ductile-ruptile features, such as milonitic S-C foliations and R, R', p and y=D fractures, showing the manifold origin of such mineralizing fluids. Two kinematics models can be visualized to explain the main litho-structural units: sinistral stike-slip system involving transtension, followed by transpression and then transtension; and crotonic reworking related to transtension regime.
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Petrografia, alterações hidrotermais e eventos mineralizantes do Bloco Norte do depósito aurífero Volta Grande, Domínio Bacajá (PA), Cráton Amazônico
(Universidade Federal do Pará, 2021-09-22) SOUZA, Hugo Paiva Tavares de; VASQUEZ, Marcelo Lacerda; http://lattes.cnpq.br/4703483544858128; https://orcid.org/0000-0003-2729-9404; FERNANDES, Carlos Marcello Dias; http://lattes.cnpq.br/0614680098407362; https://orcid.org/0000-0001-5799-2694
The southeastern region of the Amazonian Craton has been the target of several mineral survey programs over the past few years, which have recently led to the identification of the world-class Volta Grande gold deposit, with reserves of ~3.8 Moz at 1.02 g/t, which provides an expectation of 17 years of operation. The deposit is in the municipality of Senador José Porfírio in Pará and is housed in Rhyacian granitoids (2.15 Ga) that occur associated with the volcano-sedimentary Siderian sequence (2.45 Ga) of the Três Palmeiras Group. These units are in the Bacajá Domain, which is formed by belts of high-grade para- and orthoderived rocks and greenstone belt of Archean to Siderian protoliths, reworked during the orogenesis of the Transamazonian Cycle (2.26–2.06 Ga). Granitoids and charnockites sectioned this set in Rhyacian. Part of the mineralization at the Volta Grande is housed in granitoids metamorphosed under medium to high-grade conditions. Local kinematic indicators suggest dip-slip movement in which the greenstone moves up relative to the intrusive rocks. Petrographic descriptions carried out in this work revealed: 1) gray to greenish mylonitic granodiorite, with intense deformation of the main minerals that make up them, such as quartz, biotite, and feldspars. The texture in this lithotype is mainly porphyroclastic. Main metamorphic foliation (S1) is defined by biotite and amphibole, as well as reveals concordant quartz veins and venules. The highest gold contents are distributed in upper amphibolite facies zones. In these, the ore occurs mainly as isolated grains in cm-sized quartz veins and venules associated with pervasive carbonatic alteration that was synchronous to dynamic metamorphism, as well as in a fracture-controlled style. Part of the gold is also associated with a low sulfides content disseminated in the veins and host rock; 2) The metamafic rocks comprise foliated fine- to medium-grained amphibolite and andesite with a dark grayish-green color and nematoblastic texture. Chlorite, calcite, sericite, and opaque minerals are the main secondary phases. These relationships are compatible with lode-type gold systems, usually developed in the transition between greenschist to amphibolite metamorphic facies. Lava flows and dykes of isotropic rhyodacite, rhyolite, and plutonic rocks such as quartz monzonite, granodiorite, monzodiorite, and minor microgranite cut the mineralizing event previously described. Plutonic rocks are medium- to coarse-grained, have a gray color with reddish and greenish portions throughout the profiles, inequigranular texture with quartz, feldspar, biotite, and amphibole. Apatite, zircon, calcite, epidote, and opaque minerals are primary accessories. In turn, volcanics have light gray, black or dark red colors, porphyritic to aphyric texture, and microlithic or felsophyric groundmass. They reveal phenocrysts of plagioclase, amphibole, potassic feldspar, and quartz. This volcano-plutonic system contains potassic, propylitic, intermediate argillic, and/or carbonate hydrothermal alterations in selective, pervasive, or fracture-controlled styles. In hydrothermalized zones, gold occurs as isolated grains disseminated or associated with sulfides, as well as in cm-sized quartz veins in a stockwork arrangement. These characteristics are like those of shallow intermediate- to lowsulfidation epithermal systems already identified in the Amazonian Craton. The Volta Grande data suggest a second overprinted mineralizing event, common in high-tonnage productive gold deposits in China, Finland, and other areas of the planet and represents a new exploration guide for the Bacajá Domain. Several mineralizing events are critical to the economic feasibility and longevity of world-class gold deposits. Thus, new geochemical, geochronological, microthermometric, and stable isotope data will be obtained to better define the genetic modeling of the Volta Grande gold deposit.
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.
Acesso aberto (Open Access)
Stratigraphy, tectonics, paleoclimatology and paleogeography of northern basins of Brazil
(University of California, 1984-02-15) CAPUTO, Mário Vicente; CROWELL, John C.
Paleozoic basins in northern Brazil contain thick sequences of sedimentary rocks, including diamictites. Because several different geological environments may generate diamictites a study of tec-tonism, stratigraphy, paleoclimatology and paleogeography was made in order to deduce the processes involved in their origin. A large part of northern Brazil is underlain by metavolcanic and metasedimentary sequences steeply folded and metamorphosed during many tectonic events from about 3600 to 1000 m.y. ago. Northeast Brazil was also affected by the Brazilian tectonic cycle from about 700 to 450 m.y. ago. The pre-basin weak zones and resulting trends are responsible for the shape and geometry of 3 huge intracratonic basins developed during Paleozoic time: the Soliaes, Amazonas and Parna(ba basins. The three basins had a similar geologic development during Paleozoic times; from Ordovician to Early Carboniferous time only clastic rocks were deposited and from Late Carboniferous to Permian time carbonate and evaporites were laid down. Tectonism that affected basins is related to uplift and collapse that preceded the break up of Pangea and subduction activity along the Soliges basin, in the western side of the continent. Climate •has influenced the characteristics of each formation. Paleolatitudes based on paleoclimatic indicators such as tillites, eolian sands, coal, bauxite, red beds, evaporites, limestone, fauna and flora, changed from polar and circumpolar to equatorial during Phanerozoic times. Glaciation was recorded in Ordovician-Silurian, Late Devonian and Early Carboniferous times. A Late Devonian glaciation left a clear imprint as shown by sedimentary facies. Diamictites with striated, faceted and polished pebbles; rhythmites with dropstones; erratic boulders; striated pave-ments and deformed sandstones document glacial conditions. Study of the migration of glacial centers based on the available literature and new data from Brazil shows that they closely follow published paleomagnetic wander data and that there is a close rela-tionship between all Paleozoic glaciations and the Brazilian gla-ciations. Ice centers moved from northern Africa to southwestern South America from Late Ordovician to Early Silurian time. From Mid-Silurian to early Late Devonian time no record of glaciation is known. In Late Devonian time intermittent glaciation initiated again in central South America and, from Late Devonian to Late Permian time ice centers migrated toward Antarctica across South America and South Africa. The Devonian and Ordovician-Silurian glaciations together with the Permo-Carboniferous glaciations may all have primarily resulted from the shifting position of the Gondwana continent with respect to the South Pole.
Acesso aberto (Open Access)
Strontium isotope stratigraphy of the Pelotas Basin
(2014-03) ZERFASS, Geise de Santana dos Anjos; CHEMALE JUNIOR, Farid; MOURA, Candido Augusto Veloso; COSTA, Karen Badaraco; KAWASHITA, Koji
Strontium isotope data were obtained from foraminifera shells of the Pelotas Basin Tertiary deposits to facilitate the refinement of the chronostratigraphic framework of this section. This represents the first approach to the acquisition of numerical ages for these strata. Strontium isotope stratigraphy allowed the identification of eight depositional hiatuses in the Eocene-Pliocene section, here classified as disconformities and a condensed section. The reconnaissance of depositional gaps based on confident age assignments represents an important advance considering the remarkably low chronostratigraphic resolution in the Cenozoic section of the Pelotas Basin. The recognition of hiatuses that match hiatuses is based on biostratigraphic data, as well as on global events. Furthermore, a substantial increase in the sedimentation rate of the upper Miocene section was identified. Paleotemperature and productivity trends were identified based on oxygen and carbon isotope data from the Oligocene-Miocene section, which are coherent with worldwide events, indicating the environmental conditions during sedimentation.
Acesso aberto (Open Access)
Transição siluro-devoniana na borda Sul da bacia do Amazonas, entre Uruará- Rurópolis, Oeste do Estado do Pará, Norte do Brasil.
(Universidade Federal do Pará, 2019-08-04) SILVA, Eduardo Francisco da; SILVA JUNIOR, José Bandeira Cavalcante da; http://lattes.cnpq.br/8615194741719443
The Silurian-Devonian transition is characterized by drastic geographical variations in the paleocontinents geographical setting. On the Amazonas Basin South margin, whose register is observed in the contact between Pitinga and Maecuru Formations. These deposits were studied among Rurópolis and Placas regions, in the west of the State of Pará, aiming recognition and association of their sedimentary facies for paleoenvironmental reconstitution and its relation with collisional events of the supercontinent Gondwana. The deposits provenance is also desired. In this study, eight sedimentary facies were identified and grouped into two facies association: a) Shallow platform with tidal and wave influences (AF-1) corresponding to Pitinga Formation and; b) Proximal braided lowland (AF-2) corresponding to Maecuru Formation. The contact between Pitinga and Maecuru Formations is interpreten as regional erosive discordance observed in more than 300 km along all the Amazonas Basin south margin, which is tied up to accretionary stages from the supercontinent Gondwana during the Precordillera and/or glacio-eustatic effects caused by the end of Silurian glaciation. Sandstones from Pitinga Formation were classified as fine- to medium-grained, moderately well-sorted sub-arkoses and quartz-arenites. While the sandstones of Maecuru Formation are medium- to very coarse-grained, moderate- to poorly-sorted arkoses and quartz-arenites. The petrographic analysis of these deposits suggests quartz enrichment due to diagenetic processes that were also effective on the elimination of less stable minerals as indicated by the presence, in great amount, of secondare pores. The quartz-grains imaging by cathodoluminescence (CL) indicated the predominance of quartz-grains from igneous and metamorphic rocks. This technique results together with paleocurrent measurements provided substantial data for deducing that the Maecuru Formation are lithotypes from the Bacajá and Iriri-Xingu Domain. The rock bodies alignment from the source areas corroborate these interpretations. Even with reliable and convincing CL data, the lack of paleocurrent measurements made it unfeasible to accurately interpret the Pitinga Formation provenance.