Navegando por Assunto "Paleoclimatologia"

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Acesso aberto (Open Access)
Paleoambiente e paleoclima da Formação Pedra de Fogo da Bacia do Parnaíba e sua correlação com os eventos globais de silicificação.
(Universidade Federal do Pará, 2019-09-02) ANDRADE, Luiz Saturnino de; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
The Pedra de Fogo Formation of the Parnaíba Basin, Northern Brasil is a sedimentary unit that has one of the most important sedimentary records of the early Permian (Cisuralian), characterized mainly by intense silicification. Although previous works have contributed to the understanding of its paleoenvironment, important gaps regarding the sedimentary and paleoclimatic conditions that favored the great concentration and preservation of silica, due to the global changes occurred at the beginning and along of the Permian, still remain inconclusive. Problems as to which sources of silica contributed most to the expressive chert content were never satisfactorily explained. There is no reference to the organic and/or inorganic origins, nor is little known about the conditions and processes that led to the preservation of silica deposits and concretions, as well as the genesis of the known occurrence of carbonates. In order to fill these gaps and/or contribute to a better understanding of the depositional processes in the Pedra de Fogo Formation. This study made facies and stratigraphic analysis, and petrography, complemented by cathodoluminescence image, XRD and SEM-EDS analysis of the permian deposits exposed in the east and southeast portions of the Parnaíba Basin. The main sedimentary facies were grouped into facies associations representative of braided fluvial-aeolian systems from the top Piauí Formation (Carboniferous). This deposits are overlain by lacustrine-sabkha system, wave-dominated nearshore-lacustrine and ephemeral streamsdeposits of the Permian Pedra de Fogo Formation. In general, this unit was deposited in an arid, endorheic lacustrine system, often affected by storm regimes and fed by unconfined flows. Although the arid climate was predominant, this system keeps seasonally, relatively high levels of moisture sufficient to maintain and proliferate its thriving taphoflora, formed mainly by Ferns and Gymnosperms tree. This flora colonized the lakeshore, both in relatively wet and dry periods, as a way to compensate for the low humidity in macroenvironmental. Cyclic variations between the records of organosedimentary structures (microbial mats and stratiform stromatolites) and gymnosperm stem in life position were interpreted as the recurrent migration of the lacustrine shoreline, in response to the expansion and contraction phases of these lakes, triggered by climatic seasonality in the southwestern portion of Pangea. Probably, the flora of the Pedra de Fogo Formation was an important catalyst for the expressive silicification that characterizes this predominantly syndepositional / eodiagenetic unit. This silicification is formed largely by microquartz, under conditions of supersaturation in silica, high enough to preserve the delicate cyanobacteria filaments, as well as pinnules ferns and gymnosperm stems in life position. The occlusion of fractures and dissolution voids (secondary pores), by mosaic megaquartz mosaic, chalcedony spherulites and two generations of chalcedonic overlays, in addition to large crystals (mm) in coarse mosaic calcspar are indicative of polycyclic silicification and circulation of carbonate fluids to mesogenetic zones. The presence of gridwork microtexture indicates that the genesis of silicification is similar to the Magadi chert-type (Rift Valley of Kenya), but from distinct sources, given the absence of volcanic sources associated with Pedra de Fogo deposits.
Acesso aberto (Open Access)
O Pensilvaniano da Bacia do Parnaíba, norte do Brasil: implicações paleoambientais, paleogeográficas e evolutivas para o Gondwana Ocidental
(Universidade Federal do Pará, 2019-02-21) MEDEIROS, Renato Sol Paiva de; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998
The end of the Paleozoic Age was marked by landmasses tectonic movements that were forming the supercontinent Gondwana, which stretched mainly in the southern hemisphere, surrounded by the Panthalassa oceans to the west and Tetis to the east. During the Carboniferous period, there were several cycles of climatic variations in the earth's history, evidenced in glaciogenic icehouse and greenhouse carbonate deposits, both in the paleocontinent Gondwana and in Laurasia. These global cycles of eustatic variation were recorded in the epicontinental seas, as they represent the thawing phase and eustatic level increase in a greenhouse period, which together with a favorable paleogeography form extensive marine transgressions over the continental blocks, with cyclic stacking patterns, called cyclothems (eg., terrestrial; terrestrialmarine mixed; marine and restricted marine / evaporitic pattern). Records of these events in northern Brazil are found in the intracratonic basins, particularly in the Balsas Group of Parnaíba Basin, where the exhibits allow us to assess the Pennsylvanian sedimentary history. The sedimentary succession studied belongs to the Piauí Formation Upper Member, described among the cities of José de Freitas, União, Miguel Alves and Lagoa Alegre, and exhibits richly fossiliferous carbonate deposits overlapped by thick pelitic packages and progradant clinoforms. Seventeen sedimentary facies were grouped into four facies associations (FA), representative of a shallow carbonate platform, adjacent to a coastal dune field, later replaced by lacustrine-delta deposits. The FA 1- coastal dune field/interdune comprises well selected, intensely bioturbed, fine to medium sandstones with plane-parallel stratification, tabular crossbedding and climbing translatent lamination. The shallow-sea deposits FA 2 consists of a succession of fossiliferous carbonate rocks, laterally continuous for hundreds of meters, interspersed with bituminous shale. These carbonates were dolomitized and have negative δ13Ccarb values covariate with positive values of δ18Ocarb, suggesting that the supersaturated fluid volume was sufficient to change not only δ18O but also δ13C. The FA 3- suspension lobes/mouth bar and FA 4- lacustre prodelta consist respectively of sigmoidal cross-stratification and planeparallel stratification sandstones and intercalated fine sandstones and pelites. The thick politic layers of prodelta in contact with FA 2 feature quartz grains with morphology texture of windorigin sediments, with textures such as bulbous and smooth edges, upturned plates, irregular depressions, and percussion marks. Subarea exposure surfaces in carbonate marked by shrinkage cracks and dissolution features indicate the end of carbonate sedimentation (eg., Marine Sequence - Highstand System Tract) with the Pennsylvanian Sea Retreat and Confinement in an extensive lacustrine system. (eg., Continental Sequence – High Accommodation System Tract) in the central portion of Gondwana. The FA 4 mineral clay assembly confirms the aridest climatic pattern to the top of the studied succession, presenting mainly smectites and illite. This marine retraction was concomitant with the Appalachian orogeny (300 Ma) that caused the uplift in western Gondwana and definitively disconnected the Itaituba-Piaui epeiric sea from the Panthalassa ocean to the west. Restricted seas or lakes were progressively beset by prograding hipopycnal flows with the establishment of the most extreme arid conditions triggered during Pennsylvanian.
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.