Ciclicidade de alta frequência e quimioestratigrafia da Formação Tagatiya Guazu, Grupo Itapucumi, Paraguai: evidências da passagem Ediacarano-Cambriano
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Data
2022-06-15
Autores
Antunes, Gabriel Correa
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Universidade Estadual Paulista (Unesp)
Resumo
O intervalo final do Período Ediacarano e sua transição para o Cambriano reúnem algumas das maiores mudanças já ocorridas na história da Terra, como a formação do supercontinente Gondwana, distintos padrões e anomalias geoquímicas, bem como sua relação com o surgimento dos animais e a “Explosão Cambriana”. Com o objetivo de compreender tais fenômenos, diversas sucessões ao redor do mundo vêm sendo intensamente estudadas. Nesse contexto se insere a Formação Tagatiya Guazu (Grupo Itapucumi, Paraguai). No entanto, ao contrário de sucessões muito bem estudadas na China, Namíbia e Rússia, a sucessão superior da Formação Tagatiya Guazu ainda carece de estudos de detalhe quanto seus aspectos estratigráficos e geoquímicos. Frente a esta perspectiva, duas seções colunares de detalhe foram estudadas. As fácies sedimentares descritas constituem típica associação de rochas carbonáticas de perimaré depositadas em ambiente lagunar, organizadas em ciclos de raseamento ascendente, formados por grainstones em sua base, trombólitos, estromatólitos planares, margas e mudstones no topo, verticalmente configurando sucessões maiores de progradação e retrogradação. A presença de espécimes in situ de Cloudina sp. e Corumbella sp. bem preservados atestam a idade ediacarana terminal. Os valores entre +1 e +5 ‰ obtidos para δ13Ccarb se enquadram no platô positivo globalmente observado em sucessões contendo Cloudina (EPIP, Late Ediacaran Positive Carbon Isotope Plateau), reforçando a idade ediacarana terminal. Variações cíclicas de alta frequência nos valores de δ13Ccarb resultam de um gradiente isotópico gerado pela estratificação redox do corpo aquoso ou pela diminuição das taxas de produtividade juntamente à oxidação de matéria orgânica, mostrando forte relação com as fácies e batimetria. A presença de icnofósseis com hábitos complexos permite afirmar que a Fm. Tagatiya Guazu registra a passagem para o Cambriano e o Éon Fanerozoico, resultando em quatro hipóteses a serem testadas: (a) a anomalia isotópica negativa da base do Cambriano (BACE, Basal Cambrian Negative Carbon Isotope Excursion) não possui caráter global; (b) a anomalia BACE ocorre acima do limite E-C; (c) um aumento na restrição durante a deposição da porção de topo da unidade impediu o registro do sinal oceânico global, ou (d) processos orgânicos e/ou inorgânicos locais obliteraram o sinal global. De qualquer maneira, o incomum grau de preservação da assembleia fóssil presente e o detalhamento e originalidade do sinal isotópico preservado fazem com que a Fm. Tagatiya Guazu se configure como seção-chave para o entendimento do Ediacarano terminal na porção oeste de Gondwana.
The final interval of the Ediacaran Period and its transition to the Cambrian bring together some of the biggest changes ever to take place in Earth's history, such as the formation of the supercontinent Gondwana, different geochemical patterns and anomalies, as well as their relationship with the emergence of animals and the so-called “Cambrian Explosion”. In order to understand such phenomena, several successions around the world have been intensively studied. In this context, the upper portion of the Tagatiya Guazu Formation (Itapucumi Group, Paraguay), a unit deposited in the terminal Ediacaran, is inserted. However, unlike the very well-studied successions in China, Namibia and Russia, the upper succession of the Tagatiya Guazu Fm. still lacks detailed studies regarding its stratigraphic and geochemical aspects. With this perspective, two columnar sections were studied in detail. The described sedimentary facies constitute a typical association of peritidal carbonates deposited in a lagoonal environment. The studied successions are organized in shallowing-upward cycles, formed essentially by grainstone at the base, thrombolite, planar stromatolite, marl and mudstone at the top, vertically configuring larger successions of progradation and retrogradation. The presence of well preserved in situ specimens of Cloudina sp. and Corumbella sp. attests the terminal Ediacaran age. The obtained δ13Ccarb values varies between +1 and +5 ‰, matching the globally observed positive plateau in successions containing Cloudina (EPIP, Late Ediacaran Positive Isotope Plateau), reinforcing the terminal Ediacaran age. High-frequency cyclical variations in δ13Ccarb values result from an isotopic gradient generated by the redox stratification of the aqueous body or by decreasing primary productivity rates together with organic matter oxidation, showing a strong relationship with facies and bathymetry. The presence of trace fossils with complex habits allows us to affirm that the Tagatiya Guazu Fm. records the passage to the Cambrian and the Phanerozoic Eon, resulting in four hypotheses to be tested: (a) the basal Cambrian negative isotopic anomaly (BACE, Basal Cambrian Negative Isotope Excursion) does not have a global character; (b) the BACE anomaly occurs above the E-C limit; (c) an increase in restriction during deposition of the upper portion of the unit prevented the recording of the global oceanic signal, or (d) local organic and/or inorganic processes obliterated the global signal. Anyway, the unusual degree of preservation of the fossil assemblage and the detail and originality of the preserved isotopic signal makes the Tagatiya Guazu Fm. a key section for understanding the terminal Ediacaran in western Gondwana.
The final interval of the Ediacaran Period and its transition to the Cambrian bring together some of the biggest changes ever to take place in Earth's history, such as the formation of the supercontinent Gondwana, different geochemical patterns and anomalies, as well as their relationship with the emergence of animals and the so-called “Cambrian Explosion”. In order to understand such phenomena, several successions around the world have been intensively studied. In this context, the upper portion of the Tagatiya Guazu Formation (Itapucumi Group, Paraguay), a unit deposited in the terminal Ediacaran, is inserted. However, unlike the very well-studied successions in China, Namibia and Russia, the upper succession of the Tagatiya Guazu Fm. still lacks detailed studies regarding its stratigraphic and geochemical aspects. With this perspective, two columnar sections were studied in detail. The described sedimentary facies constitute a typical association of peritidal carbonates deposited in a lagoonal environment. The studied successions are organized in shallowing-upward cycles, formed essentially by grainstone at the base, thrombolite, planar stromatolite, marl and mudstone at the top, vertically configuring larger successions of progradation and retrogradation. The presence of well preserved in situ specimens of Cloudina sp. and Corumbella sp. attests the terminal Ediacaran age. The obtained δ13Ccarb values varies between +1 and +5 ‰, matching the globally observed positive plateau in successions containing Cloudina (EPIP, Late Ediacaran Positive Isotope Plateau), reinforcing the terminal Ediacaran age. High-frequency cyclical variations in δ13Ccarb values result from an isotopic gradient generated by the redox stratification of the aqueous body or by decreasing primary productivity rates together with organic matter oxidation, showing a strong relationship with facies and bathymetry. The presence of trace fossils with complex habits allows us to affirm that the Tagatiya Guazu Fm. records the passage to the Cambrian and the Phanerozoic Eon, resulting in four hypotheses to be tested: (a) the basal Cambrian negative isotopic anomaly (BACE, Basal Cambrian Negative Isotope Excursion) does not have a global character; (b) the BACE anomaly occurs above the E-C limit; (c) an increase in restriction during deposition of the upper portion of the unit prevented the recording of the global oceanic signal, or (d) local organic and/or inorganic processes obliterated the global signal. Anyway, the unusual degree of preservation of the fossil assemblage and the detail and originality of the preserved isotopic signal makes the Tagatiya Guazu Fm. a key section for understanding the terminal Ediacaran in western Gondwana.
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Geoquímica, Geologia isotópica, Geologia estratigráfica, Carbono, Geochemistry, Isotope geology, Stratigraphic geology, Carbon