Tafonomia de alta resolução da Camada Amargosa, Formação Marizal, Cretáceo Inferior, Sub-bacia do Tucano Central, Bahia: implicações para a dinâmica paleoambiental e paleoecológica
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Data
2022-01-07
Autores
Souza, Talita Gabriela Lucas de
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Universidade Estadual Paulista (Unesp)
Resumo
Nesta dissertação, reportamos a biota da Camada Amargosa, como pertencente a um novo depósito do tipo Konservat-Lagerstätte, do Cretáceo Inferior, da Formação Marizal, Sub-Bacia do Tucano Central, NE do Brasil. A análise de uma seção colunar de detalhe, com aproximadamente 4 metros de espessura, próximo à Vila Amargosa, Euclides da Cunha, Bahia, revelou uma sucessão de argilitos bem laminados, lamitos, siltitos e arenitos muito finos. Os fósseis estão restritos ao intervalo inferior da seção, com aproximadamente 1 metro de espessura, onde sete níveis (N0-N6), com atributos sedimentológicos e tafonômicos distintos foram reconhecidos. Estes contêm ostracodes, “conchostráceos”, camarões palaemonídeos, peixes e restos de plantas. Os fósseis ocorrem em alta concentração em pelo menos quatro níveis (N2, N3, N5, N6), formando assembleias politípicas dominadas por um grupo particular de fósseis. Exceto no caso dos ostracodes, que formam pavimentos densos com menos de um milímetro de espessura, os fósseis estão dispersos ou fracamente empacotados e sem orientação preferencial na matriz sedimentar. Da base para o topo, o primeiro nível ricamente fossilífero (N2) é dominado por carapaças de “conchostráceos" não fragmentadas, várias ainda articuladas, sugerindo condições de baixa energia e nenhum transporte hidráulico. O segundo nível (N3) é formado por espécimes de camarões lateralmente comprimidos, desarticulados, total ou parcialmente articulados e preservados como filmes carbonosos. Camarões total ou parcialmente articulados sugerem reduzido tempo de exposição na interface água-sedimento em fundos pobremente oxigenados. Peixes fósseis excepcionalmente preservados são comuns em dois níveis (N5-N6), na parte superior do intervalo fossilífero estudado. Os indivíduos estão dispersos na matriz e lateralmente comprimidos, indicando a ausência de quantidades consideráveis de gases produzidos durante a decomposição e nenhuma reorientação das carcaças devido à flutuabilidade. Os peixes estão, em geral, totalmente articulados, alguns exibindo coluna vertebral ligeiramente curvada, nadadeiras e bocas abertas, provavelmente refletindo contrações musculares involuntárias (i.e., tetania), devido à anoxia e/ou variações acentuadas na salinidade e pH das águas. As assembleias são formadas principalmente por elementos autóctones a parautóctones e a mistura temporal deve ter sido variável, porém, não muito extensa. Um atributo-chave dos níveis ricos em fósseis (e.g., N3, N5, N6) é a preservação, como filmes carbonosos, de invertebrados com exoesqueleto fracamente biomineralizado e/ou órgãos com tecidos moles (i.e., olhos de peixes), os quais são tipicamente propensos à rápida destruição pela necrólise, ataque de necrófagos e/ou bioturbação. A natureza politípica das assembleias fósseis, intercaladas com intervalos afossilíferos, sugere eventos de mortalidade em massa, causados por mudanças abruptas na oxigenação (anoxia), salinidade, e pH das águas. Localmente, a natureza finamente laminada das rochas indica ausência de vida bentônica de infauna ou, pelo menos, que essa foi muito limitada nos fundos relativamente “profundos” e pobres em oxigênio do lago. Condições de anoxia e salinidade elevada, associadas ao clima semi-árido durante o Cretáceo Inferior podem ter desempenhado fatores essenciais para a preservação excepcional de alguns fósseis (camarões e peixes). Finalmente, os dados aqui apresentados fornecem um melhor entendimento da distribuição vertical dos grupos e dos processos tafonômicos associados à complexa gênese do intervalo fossilífero da Camada Amargosa na sua localidade tipo.
Herein, we report the Amargosa Bed and associated biota, as a new Lower Cretaceous Konservat-Lagerstätte, from the virtually unfossiliferous, fluvial deposits of the Marizal Formation, Central Tucano Sub-Basin, NE Brazil. A detailed ~4-m-thick columnar section measured in outcrops near the Amargosa Village, Euclides da Cunha, Bahia, revealed a succession of well-laminated claystones, mudstones, siltstones, and very fine-grained sandstones. Fossils are not found across the entire bed, instead, they are restricted to its lower ~1-m-thick interval, where seven bedding planes (L0-L6) with distinct sedimentological and taphonomical attributes were identified. These contain ostracods, spinicaudatan carapaces, palaemonid shrimp, fish, and comminuted plant remains. Fossils occur in high concentration in at least four bedding planes (L2, L3, L5, L6), forming polytypical assemblages that are, however, dominated by one group of fossils. Except in the case of the ostracod molds that form sub-millimeter-thick, dense pavements, the fossils are typically dispersed to loosely packed in and randomly oriented the matrix. From the base to the top, the first fossil-rich bedding plane (L2) are mainly dominated by non-fragmented, spinicaudatan carapaces, various articulated, suggesting low energy bottom conditions and no hydraulic transport. The second fossil-rich level (L3) is formed by disarticulated, as well as fully and partially articulated shrimp corpses, which are preserved as laterally compressed, brownish films. Fully articulated and partially articulated shrimp may suggest short term exposition at the sediment/water interface and preservation in oxygen-deficient bottoms. Fossil fish are common in two well-defined bedding planes (L5-L6) at the upper part of the fossil-rich sedimentary interval. These are laterally dispersed individuals, indicating the absence of considerable amounts of gases produced during decay, and no fish carcasses re-orientation due to buoyancy. Fish are mostly fully articulated, some showing slightly bent spinal column, flabellate fins, and open mouth, probably reflecting involuntary muscular contractions (i.e., tetany) due to water anoxia, salinity, and/or alkalinity sharp variations. A key attribute of some fossil-rich strata (L3, L5, L6) is the preservation of light-biomineralized invertebrates and/or complete soft-tissue organs (i.e., fish eye), as carbon films, which are typically prone to destruction due to rapid decay or bioturbation. The assemblages are formed mainly by autochthonous to parautochthonous elements, and the temporal mixing was variable, but limited, in general. The polytypical nature of those fossil-rich assemblages, interbedded with unfossiliferous intervals suggests mass mortality events, probably caused by abrupt changes in water oxygenation (anoxia), salinity, and pH. The finely laminated nature of the claystone, siltstone, and mudstone indicates that benthic infaunal life was absent or very scarce in a locally, relatively deep, oxygen-poor lake bottom. Anoxia and high salinity linked with local semi-arid conditions during the Lower Cretaceous may have played key roles in the exceptional preservation of some fossils (shrimps, fish). Finally, our data provide a more comprehensive understanding of the temporal distribution of taxa and taphonomic processes associated with the complex genesis of the fossil-bearing interval of the Amargosa Bed in its type locality.
Herein, we report the Amargosa Bed and associated biota, as a new Lower Cretaceous Konservat-Lagerstätte, from the virtually unfossiliferous, fluvial deposits of the Marizal Formation, Central Tucano Sub-Basin, NE Brazil. A detailed ~4-m-thick columnar section measured in outcrops near the Amargosa Village, Euclides da Cunha, Bahia, revealed a succession of well-laminated claystones, mudstones, siltstones, and very fine-grained sandstones. Fossils are not found across the entire bed, instead, they are restricted to its lower ~1-m-thick interval, where seven bedding planes (L0-L6) with distinct sedimentological and taphonomical attributes were identified. These contain ostracods, spinicaudatan carapaces, palaemonid shrimp, fish, and comminuted plant remains. Fossils occur in high concentration in at least four bedding planes (L2, L3, L5, L6), forming polytypical assemblages that are, however, dominated by one group of fossils. Except in the case of the ostracod molds that form sub-millimeter-thick, dense pavements, the fossils are typically dispersed to loosely packed in and randomly oriented the matrix. From the base to the top, the first fossil-rich bedding plane (L2) are mainly dominated by non-fragmented, spinicaudatan carapaces, various articulated, suggesting low energy bottom conditions and no hydraulic transport. The second fossil-rich level (L3) is formed by disarticulated, as well as fully and partially articulated shrimp corpses, which are preserved as laterally compressed, brownish films. Fully articulated and partially articulated shrimp may suggest short term exposition at the sediment/water interface and preservation in oxygen-deficient bottoms. Fossil fish are common in two well-defined bedding planes (L5-L6) at the upper part of the fossil-rich sedimentary interval. These are laterally dispersed individuals, indicating the absence of considerable amounts of gases produced during decay, and no fish carcasses re-orientation due to buoyancy. Fish are mostly fully articulated, some showing slightly bent spinal column, flabellate fins, and open mouth, probably reflecting involuntary muscular contractions (i.e., tetany) due to water anoxia, salinity, and/or alkalinity sharp variations. A key attribute of some fossil-rich strata (L3, L5, L6) is the preservation of light-biomineralized invertebrates and/or complete soft-tissue organs (i.e., fish eye), as carbon films, which are typically prone to destruction due to rapid decay or bioturbation. The assemblages are formed mainly by autochthonous to parautochthonous elements, and the temporal mixing was variable, but limited, in general. The polytypical nature of those fossil-rich assemblages, interbedded with unfossiliferous intervals suggests mass mortality events, probably caused by abrupt changes in water oxygenation (anoxia), salinity, and pH. The finely laminated nature of the claystone, siltstone, and mudstone indicates that benthic infaunal life was absent or very scarce in a locally, relatively deep, oxygen-poor lake bottom. Anoxia and high salinity linked with local semi-arid conditions during the Lower Cretaceous may have played key roles in the exceptional preservation of some fossils (shrimps, fish). Finally, our data provide a more comprehensive understanding of the temporal distribution of taxa and taphonomic processes associated with the complex genesis of the fossil-bearing interval of the Amargosa Bed in its type locality.
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Palavras-chave
Fossilização excepcional, Paleoambiente, Aptiano, Formação Marizal, Depósito lacustre, Exceptional fossilization, Paleoenvironment, Aptian, Marizal Formation, Lacustrine Deposit