Geomorfologia e geologia do quaternário da planície flúvio-eólica do Médio rio São francisco, NE do brasil
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Data
2020-05-06
Autores
Santos, Landerlei Almeida
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Universidade Estadual Paulista (Unesp)
Resumo
Compreender a evolução paleoclimática do Quaternário na região Nordeste do Brasil (NEB) é parte fundamental para entender como tem funcionado a circulação atmosférica na faixa tropical da América do Sul. Nesse contexto, o campo de dunas eólicas estabilizadas do Médio Rio São Francisco é um “geoproxy” de grande potencial principalmente por fornecer informações sobre o comportamento dos paleoventos durante os estágios de formação e desenvolvimento das dunas eólicas presentes nesta região. A magnitude e tipo das dunas arenosas presentes neste campo eólico são características particulares desta área em relação a outros registros eólicos da América do Sul tropical. Assim, o principal objetivo deste trabalho é propor um modelo de organização espacial dos padrões eólicos predominantes, inferir mecanismos de desencadeamento e expansão do campo de dunas, e contribuir com o entendimento paleoclimático do NEB a partir de resultados de datações por luminescência optica dos principais padrões de dunas eólicas integrados com outros dados proxies da região. Baseado na combinação de métodos de sensoriamento remoto, morfoestratigrafia e datação por luminescência óptica, definimos as unidades morfosedimentares do campo eólico e, secundariamente, da planície aluvial do Rio São Francisco. Devido a pouca informação de subsuperfície, analisamos dados prévios da planície flúvio-eólica com objetivo de entender a estruturação do arcabouço no qual ocorreu a expansão do campo eólico. Usamos mapa prévio derivado de dados de anomalia magnética, descrições de sondagens e imagens de radar para obter evidências da estruturação dessa planície. Revelamos que a planície regional do Médio São Francisco é caracterizada por um padrão alternado de planície de ampla sedimentação e planície restrita (knickzonas). Na mesoescala, mapeamos oito unidades morfosedimentares da planície aluvial: (U1) planície de acreção de bancos, (U2) planície flúvio-eólica, (U3) planície de espiras de meandro, (U4) planície impedida, (U5) planície-ilha laterítica e três níveis de terraços (T1, T2, T3). No campo eólico, identificamos, caracterizamos e mapeamos os principais padrões eólicos: dunas source-bordering, megadunas, dunas parabólicas alongadas (simples e composta), cadeias de dunas parabólicas, rampa arenosa, lençol arenoso com blowouts e lençol arenoso marginal. Sequências dunares particulares estão dispostas em três corredores eólicos entre a margem da planície fluvial (área fonte) e a Serra do Estreito (barreira topográfica). As áreas proximais e distais do campo eólico apresentam fortes indicativos da influência morfotectônica regional no padrão eólico. Análises morfosedimentares e geocronológicas sugerem que a formação das megadunas e dunas parabólicas alongadas ocorreram no Pleniglacial Médio. Correlacionamos dados proxys obtidos no campo eólico, com dados prévios de espeleotemas e travertines na mesma região, e demonstramos consistentemente que as megadunas se desenvolveram entre o MIS3 e MIS2. Esse longo período de aridez foi interrompido pela fase pluvial do HS5 e pela pequena fase do HS4. Concluímos que durante o MIS3 ocorreu, na região, o pico mais importante do clima árido e maior intensidade dos ventos.
The paleoclimatic knowledge of the Quaternary period of Northeastern Brazil (NEB) is key to understanding how atmospheric circulation has worked in the tropical zone of South America. In this context, the inactive dune field in the Middle São Francisco River is a “geoproxy” of great potential mainly because it provides information on the behavior of paleowinds during the formation and development stages of the aeolian dunes present in this region. The magnitude and type of sandy dunes present in this dune field are particular characteristics of this area in relation to other wind records in tropical South America. Thus, the main objective of this work is to propose a model of spatial organization of the prevailing aeolian patterns, to infer mechanisms of triggering and expansion of the dune field, and to contribute to the paleoclimatic understanding of NEB from results of dating by optical luminescence of the main patterns aeolian dunes integrated with other proxies data from the region. Based on the combination of remote sensing, morphostratigraphy and optical luminescence dating, were define the morphosedimentary units of the dunefield, and secondarily of the alluvial plain of the São Francisco River. Due to scarcity of subsurface information, we analyzed previous data from the fluvial-aeolian plain in order to understand the structure of the framework in which the expansion of the dunefield occurred. Were used a previous map derived from magnetic anomaly data, sedimentary descriptions and radar images to obtain evidence of the structure of this plain. Thus, it was revealed that the regional plain of the middle São Francisco is characterized by an alternating pattern of plain of wide sedimentation - restricted plain (knickzonas). On the meso-scale, were mapped eight morphosedimentary units of the alluvial plain: (U1) bank accretion plain, (U2) fluvial-aeolian plain, (U3) meandering scrolls plain, (U4) impeded plain, (U5) plain-island lateritic and three levels of terraces (T1, T2, T3). In the dunefield, were identify, characterize and map the main dunes patterns: source-bordering dunes, Megadunes, elongated parabolic dunes (simple and compound), parabolic dune chains, sandy ramp, sandy sheet with blouwouts dunes and marginal sandy sheet. Particular dune sequences are arranged in three aeolian corridors between the margin of the alluvial plain (source area) and Estreito Ridge (topographic barrier). The proximal and distal areas of the dunefield have strong indications of the regional morphotectonic influence on the dune pattern. Morphosedimentary and geochronological analyzes suggest that the formation of elongated Megadunes and parabolic dunes occurred in the Middle Pleniglacial. Were correlate proxies data obtained in the dune field, with previous data from speleothems and travertines in the same region, and consistently demonstrated that the Megadunes developed between MIS 3 and MIS2. This long period of dryness was interrupted by the HS5 rain phase and the small HS4 phase. Thus, concluded that that during MIS3, the most important peak of the arid climate and the highest intensity of winds occurred in the region.
The paleoclimatic knowledge of the Quaternary period of Northeastern Brazil (NEB) is key to understanding how atmospheric circulation has worked in the tropical zone of South America. In this context, the inactive dune field in the Middle São Francisco River is a “geoproxy” of great potential mainly because it provides information on the behavior of paleowinds during the formation and development stages of the aeolian dunes present in this region. The magnitude and type of sandy dunes present in this dune field are particular characteristics of this area in relation to other wind records in tropical South America. Thus, the main objective of this work is to propose a model of spatial organization of the prevailing aeolian patterns, to infer mechanisms of triggering and expansion of the dune field, and to contribute to the paleoclimatic understanding of NEB from results of dating by optical luminescence of the main patterns aeolian dunes integrated with other proxies data from the region. Based on the combination of remote sensing, morphostratigraphy and optical luminescence dating, were define the morphosedimentary units of the dunefield, and secondarily of the alluvial plain of the São Francisco River. Due to scarcity of subsurface information, we analyzed previous data from the fluvial-aeolian plain in order to understand the structure of the framework in which the expansion of the dunefield occurred. Were used a previous map derived from magnetic anomaly data, sedimentary descriptions and radar images to obtain evidence of the structure of this plain. Thus, it was revealed that the regional plain of the middle São Francisco is characterized by an alternating pattern of plain of wide sedimentation - restricted plain (knickzonas). On the meso-scale, were mapped eight morphosedimentary units of the alluvial plain: (U1) bank accretion plain, (U2) fluvial-aeolian plain, (U3) meandering scrolls plain, (U4) impeded plain, (U5) plain-island lateritic and three levels of terraces (T1, T2, T3). In the dunefield, were identify, characterize and map the main dunes patterns: source-bordering dunes, Megadunes, elongated parabolic dunes (simple and compound), parabolic dune chains, sandy ramp, sandy sheet with blouwouts dunes and marginal sandy sheet. Particular dune sequences are arranged in three aeolian corridors between the margin of the alluvial plain (source area) and Estreito Ridge (topographic barrier). The proximal and distal areas of the dunefield have strong indications of the regional morphotectonic influence on the dune pattern. Morphosedimentary and geochronological analyzes suggest that the formation of elongated Megadunes and parabolic dunes occurred in the Middle Pleniglacial. Were correlate proxies data obtained in the dune field, with previous data from speleothems and travertines in the same region, and consistently demonstrated that the Megadunes developed between MIS 3 and MIS2. This long period of dryness was interrupted by the HS5 rain phase and the small HS4 phase. Thus, concluded that that during MIS3, the most important peak of the arid climate and the highest intensity of winds occurred in the region.
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Palavras-chave
Médio Rio São Francisco, Interação eólica-fluvial, Campo de dunas eólicas, Quaternário, Paleoclima, Middle São Francisco River, Aeolian-fluvial interaction, Dune field, Quaternary, Paleoclimate