Carbonate cements in contemporaneous beachrocks, Jaguaribe beach, Itamaracá island, northeastern Brazil: petrographic, geochemical and isotopic aspects
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Data
2005-06-01
Autores
Guerra, Núbia C.
Kiang, Chang H. [UNESP]
Sial, Alcides N.
Título da Revista
ISSN da Revista
Título de Volume
Editor
Academia Brasileira de Ciências
Resumo
Os recifes de arenito holocênicos da praia de Jaguaribe, Pernambuco, Nordeste do Brasil, consistem de camadas de areias de praia dispostas de forma horizontal, cimentadas por carbonatos, com aproximadamente 40 cm de espessura. O cimento ocorre sob três tipos: a) calcítico, circundando os grãos siliciclásticos, b) micrítico, com uma franja acicular; e c) preenchendo os poros na forma de calcita criptocristalina. A cimentação, precoce, aconteceu ao nível freático abaixo da crista praial, onde condições geoquímicas, hidrodinâmicas e, talvez, microbiológicas favoreceram uma rápida precipitação de aragonita e/ou calcita magnesiana. delta13C varia de -1.8 a +1.5 para carbonato dissolvido em água intersticiale de +0.2 a +2.1 para componentes bioclásticos.delta18O varia de -2.8 a +0.5 na água do mar, água docee intersticial. delta13C e feições diagenéticas sugerem que a cimentação ocorreu em água meteórica-vadosa e/ou marinha-freática, por perda de CO2 durante a evaporação da água intersticial. Localmente, cimento calcítico pouco magnesiano superimposto sugere influência de água doce subseqüente. A composição do cimento das camadas de recifes de arenito empilhadas verticalmente na praia de Jaguaribe mostra que a camada do topo do recife de arenito é mais velha do a situada no presente nível freático. Isto implica em uma cimentação de cima para baixo, retratando provavelmente rebaixamento do nível do mar.
Holocene beachrocks of the Jaguaribe beach, State of Pernambuco, northeastern Brazil, consist of horizontal, cemented layers approximately 40 cm thick. The cement shows three textural varieties: (a) calciferous, surrounding siliciclastic grains, (b) micritic, with an acicular fringe; and (c) cryptocrystalline calcite in pores. Early cementation took place at the water table below beach ridges, where geochemical, hydrodynamic and, perhaps, also microbiological conditions favored rapid precipitation of aragonite and/or high-Mg calcite. delta13C values range from -1.8 to +1.5 for dissolved carbonate in interstitial water and from +0.2 to +2.1 for bioclastic components. delta18O values range from -2.8 to +0.5 for seawater, freshwater and interstitial water. delta13C values and diagenetic features suggest that cementation occurred in meteoric-vadose and/or marine-phreatic water by loss of CO2 during evaporation of the interstitial water. Locally, superimposed low-Mg calcite cements point to subsequent freshwater influence. Total-rock cement composition of vertically stacked beachrock beds at the Jaguaribe beach shows that the highest beachrock bed is older than the one (of same petrographic composition) situated at the current groundwater level. This implies a downward progression of cementation, which probably followed the sea-level fall after a local high stand.
Holocene beachrocks of the Jaguaribe beach, State of Pernambuco, northeastern Brazil, consist of horizontal, cemented layers approximately 40 cm thick. The cement shows three textural varieties: (a) calciferous, surrounding siliciclastic grains, (b) micritic, with an acicular fringe; and (c) cryptocrystalline calcite in pores. Early cementation took place at the water table below beach ridges, where geochemical, hydrodynamic and, perhaps, also microbiological conditions favored rapid precipitation of aragonite and/or high-Mg calcite. delta13C values range from -1.8 to +1.5 for dissolved carbonate in interstitial water and from +0.2 to +2.1 for bioclastic components. delta18O values range from -2.8 to +0.5 for seawater, freshwater and interstitial water. delta13C values and diagenetic features suggest that cementation occurred in meteoric-vadose and/or marine-phreatic water by loss of CO2 during evaporation of the interstitial water. Locally, superimposed low-Mg calcite cements point to subsequent freshwater influence. Total-rock cement composition of vertically stacked beachrock beds at the Jaguaribe beach shows that the highest beachrock bed is older than the one (of same petrographic composition) situated at the current groundwater level. This implies a downward progression of cementation, which probably followed the sea-level fall after a local high stand.
Descrição
Palavras-chave
recife de arenito, diagênese recente, isótopos de O e C, Holoceno, beachrocks, early diagenesis, O and C isotopes, Holocene
Como citar
Anais da Academia Brasileira de Ciências. Academia Brasileira de Ciências, v. 77, n. 2, p. 343-352, 2005.