Navegando por Assunto "Neoproterozoico"

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Acesso aberto (Open Access)
Estratigrafia e análise paleoambiental de rochas Neoproterozóicas do Cinturão Araguaia, região de Redenção–PA, Brasil
(Universidade Federal do Pará, 2020-12-26) MEDINA HIGUERA, Adriana Nataly; SOARES, Joelson Lima; http://lattes.cnpq.br/1345968080357131
The Neoproterozoic age was marked by intense climatic changes that were accompanied by the reorganization of the continental masses at a global level. These climatic events are represented by the Sturtian and Marinoan Snowball Earth glaciations registered in the rocks of the Cryogenic age. The rearrangement of the continental masses was marked by the rupture of the Rodinia (870 - 750 Ma) and subsequent intracratonic collisions (600 Ma) that generated the amalgamation of Gondwana, which in turn originated several orogens such as the Araguaia Belt. This Neoproterozoic geotectonic unit located in the northern portion of the Tocantins Province, is composed of the sedimentary rocks of the Baixo Araguaia Supergroup, which subdivided into the Estrondo and Tocantins Groups. In the region of Redenção, the company WMC Resources Ltda drilled several cores, called SMD, to unveil the genesis of the São Martim prospect. Initially, the carbonate and siliciclastic rocks without evidence of metamorphism, in these cores (SMD 08 and SMD 15), were positioned in the Couto Magalhães Formation. However, the original definition of this unit involves rocks with low-grade metamorphism. This fact led to a lithostratigraphic review of the Couto Magalhães Formation that occurs on a subsurface in the Redenção region, in addition to the paleoenvironmental reconstitution proposed in the original work. Thus, this unit was redefined, in the study region, exclusively based on its lithological characteristics and named São Martim Formation (carbonate rocks) and Rio Arraias Formation (siliciclastic rocks). Due to the few testimonies that exist, it is not possible to define its lateral extension in subsurface. To achieve the proposed objectives, faciological, sedimentological and geochemical analyzes were performed, as well as interpretations of sedimentary microfacies and glacial microstructures. The most basal portion of the cores is characterized by the occurrence of banded iron formations (BIF) that are interpreted as the basin floor, these formations have age and compositional similarity correlated with the BIF’s of the Carajás Formation. In the overlying sequence, nineteen sedimentary facies were defined, which were grouped into four facies associations corresponding. In the São Martim Formation, carbonate rocks are interpreted as the record of a carbonate platform environment in an epicontinental sea (AF1) that is in discordant contact with the BIF’s. The Rio Arraias Formation is characterized by deposits of mud-sand-rich slope plain turbidites (AF2), glacial (AF3) and sand-rich slope turbidites (AF4). The ages of these rocks were inferred from the bibliographic survey. Meso-Archean ages (2,952.3 ± 7.3 Ma and 2,994.0 ± 5.8 Ma) of the BIF´s were determined based on U-Pb (SHRIMP) data of detritical zircons. There are no ages available for carbonates from the São Martim Formation. The ages obtained for the siliciclastic rocks of the Rio Arraias Formation are based on the Pb / Pb methodology in clastic pyrite (716 to 670 Ma), diagenetic pyrites (668–616 Ma) and vein pyrite (438 Ma - 394 Ma). As well as Sm / Nd ages of sedimentary provenance between 1660 Ma, 1710 Ma and 1720 Ma in diamictite clasts of the Rio Arraias Formation. Additionally, rhyolitic pyroclastic rocks (ages U-Pb) dated 634 ± 21 and 624 ± 11 Ma superimposed directly on the glacial deposits of the Rio Arraias Formation. According to these geochronological ages, a Cryogenian age for the carbonate-siliciclastic sedimentary sequence is inferred in this work. The first sedimentation event after the formation of BIF was the deposition of carbonate sediments that make up AF1, the contact between these two associations is abrupt and characterized by the presence of a thin layer of black shale. The AF1 is constituted in the basal portion by pseudonodular to pseudolaminar mudstone, which present structures generated by chemical compaction such as dissolution seams and stylolites, in addition to fine levels of volcanic tuff, in the upper portion are the carbonate and siltstone breccia facies with wavy lamination. The second deposition event was possibly marked by the subsidence of the basin generated by the post-collisional closure of the Amazon with São Francisco / Congo cratons, where the AF2 was deposited, which is composed of massive sandstones and with plane-parallel lamination, truncated by wave and deformed, rhythms with plane-parallel lamination and truncated by wave, massive mudstones and with plane-parallel lamination. The third sedimentation event occurred at the maximum glacial advance and subsequent lowering of sea level, with the deposition of materials corresponding to AF3. This association is made up of foliated and massive diamictites associated with the deposition of sediments from glaciers that advance in water bodies. The transition between AF2 and AF3, presents layers with deformations possibly produced by glaciotectonic. The fourth sedimentation event is marked by a rapid thaw accompanied by an increase in sea level which increased the potential for preservation of subglacial deposits and possibly controlled the deposition of AF4 materials. This association is composed of massive sandstones with planar-parallel, wavy, wave-truncated and deformed lamination, to a lesser extent rhythms with planar and wave-truncated lamination and thin layers of massive mudstones, in addition to two levels of siliciclastic breccias. In summary, a new Neoproterozoic lithostratigraphic definition for the Araguaia Belt is proposed that suggests for the first time that the platform, in certain periods, was subjected to glacial processes possibly within the context of Snowball Earth, precisely to the Marinoan glaciation.
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)
A transição neoproterozoico-eopaleozoico no Graben Pimenta Bueno, NW da bacia dos Parecis, estado de Rondônia
(Universidade Federal do Pará, 2016-05-30) AFONSO, Jhon Willy Lopes; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
The Pimenta Bueno Graben (PBG) represents a sub-basin localized at northwestern portion of the Parecis Basin, established over the southwestern of Amazon Craton. The PBG is filled by a succession of siliciclastic rocks, and subordinately carbonate rocks with approximately 1000 m of thick on subsurface. The Paleozoic siliciclastic succession exposed in PBG overlay crystalline basement rocks and carbonatic-siliciclastic deposits which were previously considered as Paleozoic age. Outcrop-based facies and stratigraphic analysis around the cities of Cacoal, Pimenta Bueno and Espigão d'Oeste, Rodônia State, allowed the redescribing and redefinition of PBG basal units, previously included in the Cacoal and Pimenta Bueno formations. The Cacoal Formation unconformably overlies Precambrian crystalline basement rocks. This was redefined in two units: one homonymous, which was characterized by diamictite and sandstones, and another named Espigão d'Oeste Formation, interpreted as a Neoproterozoic cap carbonate, consisting of pinkish dolomite, dolomite-siltstone rhythmite and a thick siltstone sucession. Both units were inserted in the base of the Ediacaran, and related, respectively, to the glacial and post-glacial global Marinoan event (635 Ma) linked to the hypothesis of Snowball-Slushball Earth. These units belong to basement coverage of PBG, unrelated with the Paleozoic succession of the Parecis Basin. The pink dolomites were interpreted as Neoproterozoic cap carbonates unconformably overlain by Ordovician-Silurian Pimenta Bueno Formation. Twenty sedimentary facies grouped in eight facies associations (FA) have been interpreted as glacial-marine, carbonate and siliciclastic platform and coastal deposits. The glacio-marine deposits (AF1) of Cacoal Formation consist of diamictite containing faceted and striated clasts (blocks and pebbles of granite, gneiss, volcanic rock, phyllite, sandstone, pelite and chert), interbedded with fine- to medium grained sandstones, which consists of climbing ripple cross-laminated and rare dropstones. The cap carbonate includes two facies associations, AF2 and AF3. The AF2 consist pinkish finely laminated doloboundstones and dolomudstones/dolopackstones with abundant micro and macropelóides. This association was deposited in wave-influenced shallow marine platform grading towards the top to lagoonal dolostone/siltstone rhythmites (AF3). The δ13C values among - 3.66 to – 3.03 ‰ founded in AF2 and AF3 are typical of Marinoan cap carbonate. Eosilurian Pimenta Bueno Formation consists by massive diamictite and pelites with dropstones occurring interbedded with sigmoidal cross-bedded sandstone, punctually containing dump structure. This succession is organized in coarsening and thickening upward cycles interpreted as advance and retreat deposition of coastal glaciers and melt-out deltas (AF4). The post-glacial sea level rise allowed the installation of wave- and storm-influenced siliciclastic platform consisting by hummocky cross-stratified sandstone and laminated mudstones (AF5), as well as, shoreface deposits (AF6) indicated by swaley cross stratified fine- to medium-grained sandstone. Cross-bedded sandstone with mud drapes in the foresets and sandstone-pelite rhythmite suggest deposition in subtidal zone (AF7). Tidal processes in this facies indicate probable oceanic connection with Silurian sea almost devoid of glacial influence. Laminated mudstones and massive sandstone with cross lamination, locally bioturbated by Skolithos, organized in coarsening upward cycles indicate progradation of suspended lobes in lakes or restricted sea (AF8). The recurrence of glacial events in the studied succession indicates extreme climatic conditions of Snowball-Slushball Earth expanding for the more southwestern portion of Amazonian Craton during the Neoproterozoic. The glaciers returned to this part of Amazonia only in the end of Ordovician with important records in the Brazilian intracratonic basins.