Análise paleoambiental dos estromatólitos de Santa Rosa de Viterbo: morfologia e geoquímica de ocorrências singulares de microbialitos no Permiano da Bacia do Paraná
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Data
2019-12-09
Autores
Antunes, Gabriel Correa
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Universidade Estadual Paulista (Unesp)
Resumo
Microbialitos são depósitos biossedimentares carbonáticos formados pela ação de microbiotas bentônicas e ocorrem sob variadas formas dependendo de seu ambiente de formação, batimetria, condições hidrodinâmicas ou mesmo da comunidade bacteriana presente. Os microbialitos do Permiano da Bacia do Paraná, especialmente aqueles descritos na Formação Irati, são ainda pouco estudados do ponto de vista de sua morfologia, geoquímica e paleoecologia. Desta forma, o presente trabalho buscou preencher esta lacuna no conhecimento geológico e caracterizar o paleoambiente em que os microbialitos de Santa Rosa de Viterbo, Membro Assistência (Formação Irati) se desenvolveram. Na área estudada foram definidas 9 fácies sedimentares distintas: i) arenito muito fino bioturbado; ii) brecha carbonática; iii) grainstone peloidal; iv) boundstone com laminação plano-paralela; v) boundstone pseudo-colunar; vi) boundstone com laminação convexa; vii) boundstone com laminação convexa à parabólica/turbinada; viii) marga; e ix) siltitos laminados. As variações morfológicas sugerem deposição em um ambiente marcado por mudanças importantes na energia hidráulica, taxa de sedimentação e/ou espessura da lâmina d’água. A partir da análise morfológica dos estromatólitos também foi possível identificar uma forte orientação preferencial dos mesmos na direção NE/SW, sugerindo uma paleolinha de costa com direção aproximada NW/SE. A análise de isótopos de C e O e geoquímica de elementos maiores e traço possibilitou identificar alguns padrões interessantes nos microbialitos, entre os quais: i) o trend de variação isotópica do C (valores crescentes da razão 13C/12C) corrobora parcialmente com o modelo deposicional aceito para a Fm. Irati; ii) o trend de variação isotópica do O indica diminuição da temperatura e da salinidade da água em direção ao topo; iii) as razões Ca/Mg baixas ao longo de toda a sucessão sugerem que ocorreu, ao menos, dolomitização parcial nos carbonatos durante a diagênese; iv) as razões Mn/Sr e Fe/Sr se enquadram em limites definidos na literatura indicando que não há alteração pós-deposicional dos valores originais. Esta afirmativa é corroborada pelos dados de razões Eu/Eu* que não sugerem alteração significativa por fluidos hidrotermais; v) presença de pirita juntamente com uma anomalia positiva de As e aumento de Fe para o topo da sucessão indica espessamento da lâmina d’água e ambiente progressivamente mais redutor. As razões Ce/Ce* também indicam fundo pouco oxidado ou anóxico; e vi) razões Rb/Sr relativamente elevadas no topo da sucessão correspondem ao período de maior aporte de terrígenos na bacia.
Microbialites are carbonate biosedimentary deposits formed by the action of benthic microbiots and occur in various forms depending on their formation environment, bathymetry, hydrodynamic conditions or even the present bacterial community. The Permian microbialites of the Paraná Basin, especially those described in the Irati Formation, are still poorly studied from the point of view of its morphology, geochemistry and paleoecology. Thus, the present work sought to fill this gap in geological knowledge and to characterize the paleoenvironment in which Santa Rosa de Viterbo microbialites, Assistência Member (Irati Formation), developed. In the studied area, 9 distinct sedimentary facies were described: i) very fine bioturbed sandstone; ii) carbonate breccia; iii) pelloidal grainstone; iv) boundstone with plane-parallel lamination; v) pseudo-columnar boundstone; vi) boundstone with convex lamination; vii) boundstone with convex to parabolic lamination; viii) marl; and ix) laminated siltstones. Morphological variations suggest deposition in an environment marked by significant changes in hydraulic energy, sedimentation rate and/or water depth. From the morphological analysis of the stromatolites it was also possible to identify a strong preferential orientation in the NE/SW direction, suggesting an approximate NW/SE paleo-coastline. The analysis of isotopes of C and O and geochemistry of larger and trace elements made it possible to identify some interesting patterns in microbialites, including: i) the trend of isotopic variation of C (increasing 13C/12C ratio) partially corroborates the accepted depositional model for Fm. Irati; ii) the isotopic variation trend of O indicates a decrease in water temperature and salinity towards the top; iii) Low Ca/Mg ratios throughout the succession suggest that at least partial carbonate dolomitization occurred during diagenesis; iv) Mn/Sr and Fe/Sr ratios are inside the limits defined in the literature, indicating that there is no post-depositional alteration from the original values. This statement is corroborated by data from Eu/Eu* ratios that suggests absence of significant alteration by hydrothermal fluids; v) the presence of pyrite,along with a positive As anomaly and increase of Fe towards the top of the succession indicates a great increase in water depth and a progressively more reducing environment. Ce/Ce* ratios also indicate low oxidized or anoxic bottom; and vi) relatively high Rb/Sr ratios at the top of the succession correspond to the period of greatest input of terrigens in the basin.
Microbialites are carbonate biosedimentary deposits formed by the action of benthic microbiots and occur in various forms depending on their formation environment, bathymetry, hydrodynamic conditions or even the present bacterial community. The Permian microbialites of the Paraná Basin, especially those described in the Irati Formation, are still poorly studied from the point of view of its morphology, geochemistry and paleoecology. Thus, the present work sought to fill this gap in geological knowledge and to characterize the paleoenvironment in which Santa Rosa de Viterbo microbialites, Assistência Member (Irati Formation), developed. In the studied area, 9 distinct sedimentary facies were described: i) very fine bioturbed sandstone; ii) carbonate breccia; iii) pelloidal grainstone; iv) boundstone with plane-parallel lamination; v) pseudo-columnar boundstone; vi) boundstone with convex lamination; vii) boundstone with convex to parabolic lamination; viii) marl; and ix) laminated siltstones. Morphological variations suggest deposition in an environment marked by significant changes in hydraulic energy, sedimentation rate and/or water depth. From the morphological analysis of the stromatolites it was also possible to identify a strong preferential orientation in the NE/SW direction, suggesting an approximate NW/SE paleo-coastline. The analysis of isotopes of C and O and geochemistry of larger and trace elements made it possible to identify some interesting patterns in microbialites, including: i) the trend of isotopic variation of C (increasing 13C/12C ratio) partially corroborates the accepted depositional model for Fm. Irati; ii) the isotopic variation trend of O indicates a decrease in water temperature and salinity towards the top; iii) Low Ca/Mg ratios throughout the succession suggest that at least partial carbonate dolomitization occurred during diagenesis; iv) Mn/Sr and Fe/Sr ratios are inside the limits defined in the literature, indicating that there is no post-depositional alteration from the original values. This statement is corroborated by data from Eu/Eu* ratios that suggests absence of significant alteration by hydrothermal fluids; v) the presence of pyrite,along with a positive As anomaly and increase of Fe towards the top of the succession indicates a great increase in water depth and a progressively more reducing environment. Ce/Ce* ratios also indicate low oxidized or anoxic bottom; and vi) relatively high Rb/Sr ratios at the top of the succession correspond to the period of greatest input of terrigens in the basin.
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Microbialitos, Geoquímica, Paleoambiente, Microbialites, Geochemistry, Paleoenvironment