Dunas quaternárias do médio rio São Francisco: proveniência sedimentar com base em minerais pesados
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Data
2022-01-06
Autores
Sforcin, Ana Maria
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Universidade Estadual Paulista (Unesp)
Resumo
Minerais pesados têm sido utilizados para estudo de proveniência sedimentar em sistemas costeiros no Brasil. No entanto, em sistemas continentais existem poucas contribuições a respeito. Na região noroeste do Estado da Bahia (médio rio São Francisco) ocorre o maior campo de dunas interiores do Quaternário no Brasil. Abrangendo cerca de 8.000 km² na margem esquerda do rio, e cerca de 200 km² na direita, sua grande extensão é resultado de fatores como alto suprimento de areia, alta energia dos ventos de direção E-W e SE-NW, e a presença da Serra do Estreito na margem oeste do sistema eólico, que age como uma barreira física, impedindo o transporte eólico e favorecendo a acumulação dos sedimentos. A partir de análises granulométricas, da assembleia de minerais pesados, e de variedades de turmalina e zircão, buscou-se identificar as fontes sedimentares dos campos eólicos, entender os mecanismos de deposição e verificar se a proveniência sedimentar variou ao longo do tempo. A maior parte dos depósitos analisados é composto por areia média moderadamente selecionada. Quanto à mineralogia, predominam espécies ultraestáveis e estáveis (turmalina, estaurolita e zircão). A assembleia de minerais pesados nos campos das duas margens é similar, entretanto na margem esquerda o mineral mais abundante é estaurolita, enquanto na direita é turmalina. Análise de variedadesvarietal de turmalina e zircão demonstra que predominam grãos anédricos moderadamente a bem arredondados. Porém, na margem direita a proporção de grãos anédricos subangulosos e angulosos aumenta. Com a análise estatística da assembleia mineralógica, concluiu-se que, nas duas margens, os terraços fluviais parecem ter atuado como fonte principal dos sedimentos. Na margem esquerda o rio São Francisco agiu como fonte subordinada. Já na direita, o embasamento neoproterozoico (Grupos Paraguaçu e Chapada Diamantina) foi a fonte subordinada, assim como os terrenos granítico-gnáissico-migmatíticos do embasamento do Cráton São Francisco, veios hidrotermais de turmalina e quartzo e rochas ígneas vulcânicas ácidas e intrusivas básicas relacionadas ao Supergrupo Espinhaço, o que explicaria a presença marcante de turmalina e zircão. O fato de a proporção de grãos anédricos subangulosos e angulosos de turmalina e zircão crescer nesta margem pode ser explicado como resultado entre a curta distância entre a área fonte subordinada e a deposição dos sedimentos. Os índices RZi e TZi não possuem variações consideráveis, então entende-se que não houve mudanças na área fonte nem na seleção hidráulica ao longo do tempo.
Heavy minerals have been used to study sedimentary provenance in Brazil’s coastal systems. However, on continental systems there are few contributions regarding the subject. In the northwest region of Bahia State (medium São Francisco River) occurs the biggest interior dune field from Brazilian quaternary. Encompassing about 8.000 km2 on the left margin of the river, and about 200 km2 on the right, it bigger extension results from factors as high supply of sand, high energy from the winds of direction E-W and SE-NW, and the presence of Serra do Estreito on the west margin of the eolian system, that acts like a physical barrier, preventing the eolical system and favoring the accumulation of sediments. From granulometrics analysis, of the heavy mineral assemblies, and tourmaline and zircon varieties, this dissertation aims to identify the sedimentaries sources of the eolian field, understand the deposition mechanisms and verify if the sedimentary provenance varied through time. Most part of the analyzed deposits are composed by moderately selected medium sand. Regarding the mineralogy, predominate ultrastable and stable species (tourmaline, staurolite and zircon). The heavy minerals assembly on both margins is similar, however, on the left margin the most abundant mineral is staurolite, while in the right margin is tourmaline. Varietal analysis from tourmaline and zircon demonstrates the predominance of moderate to well rounded, anhedral grains. However, the proportion increases for subangular and angular anhedral grains on the right margin. Observations of the mineralogical assembly statistical analysis, allows to conclude that, on both margins, the fluvial terrains seem to act as the primary source of sediments. On the left margin, the São Francisco River acted as secondary source of sediments. On the right margin, the Neoproterozoic basement (Paraguaçu Group and Chapada Diamantina) was the secondary source, as the granitic-gneissic-migmatitic terrains from the São Francisco Craton basement, tourmaline and quartz hydrothermal veins and extrusive felsic and basic intrusive rocks related to the Espinhaço Supergroup, which explains the outstanding presence of tourmaline and zircon. The increased proportion of tourmaline anhedral angulous and subangulous grains and anhedral angulous and subangulous grains of zircon on this margin can be explained as a result of the short distance from the subordinate source area and the sediments deposition. The RZi and TZi indexes do not vary considerably; therefore, there were not changes in the source area nor the hydraulic selection over time.
Heavy minerals have been used to study sedimentary provenance in Brazil’s coastal systems. However, on continental systems there are few contributions regarding the subject. In the northwest region of Bahia State (medium São Francisco River) occurs the biggest interior dune field from Brazilian quaternary. Encompassing about 8.000 km2 on the left margin of the river, and about 200 km2 on the right, it bigger extension results from factors as high supply of sand, high energy from the winds of direction E-W and SE-NW, and the presence of Serra do Estreito on the west margin of the eolian system, that acts like a physical barrier, preventing the eolical system and favoring the accumulation of sediments. From granulometrics analysis, of the heavy mineral assemblies, and tourmaline and zircon varieties, this dissertation aims to identify the sedimentaries sources of the eolian field, understand the deposition mechanisms and verify if the sedimentary provenance varied through time. Most part of the analyzed deposits are composed by moderately selected medium sand. Regarding the mineralogy, predominate ultrastable and stable species (tourmaline, staurolite and zircon). The heavy minerals assembly on both margins is similar, however, on the left margin the most abundant mineral is staurolite, while in the right margin is tourmaline. Varietal analysis from tourmaline and zircon demonstrates the predominance of moderate to well rounded, anhedral grains. However, the proportion increases for subangular and angular anhedral grains on the right margin. Observations of the mineralogical assembly statistical analysis, allows to conclude that, on both margins, the fluvial terrains seem to act as the primary source of sediments. On the left margin, the São Francisco River acted as secondary source of sediments. On the right margin, the Neoproterozoic basement (Paraguaçu Group and Chapada Diamantina) was the secondary source, as the granitic-gneissic-migmatitic terrains from the São Francisco Craton basement, tourmaline and quartz hydrothermal veins and extrusive felsic and basic intrusive rocks related to the Espinhaço Supergroup, which explains the outstanding presence of tourmaline and zircon. The increased proportion of tourmaline anhedral angulous and subangulous grains and anhedral angulous and subangulous grains of zircon on this margin can be explained as a result of the short distance from the subordinate source area and the sediments deposition. The RZi and TZi indexes do not vary considerably; therefore, there were not changes in the source area nor the hydraulic selection over time.
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Palavras-chave
Granulometria, Sistema eólico, Fontes sedimentares, Sistema de rotas sedimentares, Granulometry, Eolian system, Sedimentary sources, Sediment routing system