Estruturas discoides na Formação Cerro Negro (Grupo La Providencia), Argentina e suas implicações para a paleontologia de macrorganismos pré-cambrianos
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Data
2018-04-12
Autores
Inglez, Lucas
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Universidade Estadual Paulista (Unesp)
Resumo
A Formação Cerro Negro, Grupo La Providencia (Região das cidades de Olavarría e Barker, Argentina), compreende uma sucessão siliciclástica marinha com idade ediacarana. A unidade é composta por uma intercalação de pelitos e arenitos ricos em estruturas microbialmente induzidas (MISS), depositada sob influência de tempestades em ambiente de inter- a inframaré. Nesse contexto, camadas de evento de arenitos finos a muito finos, ricos em estruturas de deformação por sobrecarga apresentam em seu topo centenas de estruturas discoides preservadas como relevo completo (epirelevo positivo + endorelevo + epirelevo negativo). Os discos densamente distribuídos no acamamento ocorrem como protuberâncias centimétricas (5 – 140 mm) de epirelevo liso em oposição a sua superfície inferior radialmente enrugada e marcada por uma depressão central. Tais feições, juntamente com a distribuição de frequências de tamanho e presença de deformação mútua de indivíduos em contato lateral levaram a interpretação dos discos como tafomorfos do plexus Aspidella. Entretanto, os contextos sedimentares e tafonômicos das estruturas discoides ornamentadas (EDO) argentinas, bem como sua morfologia interna complexa, impedem sua associação direta com o registro de macrobiotas em outras unidades ediacaranas. Assim, de modo a elucidar a gênese e preservação das ED em termos de sua natureza biótica ou abiótica, foi proposta a aplicação de uma série de técnicas analíticas, tais como: 1) contagem e medições sistemáticas das estruturas em campo, observando a distribuição de classes de tamanho em distintas populações de discos, 2) secção e escaneamento de slabs de rocha, de modo a acessar suas feições internas, 3) análise secções delgadas ao microscópio óptico e eletrônico de varredura (MEV), 4) análise por microtomografia computadorizada (Micro-CT), compondo modelos tridimensionais das estruturas. Assim, o uso combinado de diferentes técnicas permitiu discutir as EDO diante de duas hipóteses alternativas, HA-I e II, as quais compreendem: I- Uma origem biológica, de acordo com a qual, perturbações em subsuperfície seriam produzidas por uma comunidade infaunal produzindo tubos verticais simples (icnofósseis tipo-Skolithos), que podem ou não culminar em discos horizontalmente radiados (icnofósseis tipo-Asterosoma). II – Uma natureza abiótica, implicando nos discos como produtos da ascensão de sedimento fluidificado interagindo com o substrato microbialmente estabilizado. Tal hipótese explica a correlação entre os discos e as feições de liquidificação observadas, o que lhe confere maior poder de síntese. Além do mais, a HA-II independe do rompimento de paradigmas biológicos e paleontológicos, sendo compatível com a interpretação presente na literatura para ocorrências semelhantes. Dessa forma, conclui-se que as EDO sejam estruturas problemáticas, com gênese provavelmente associada a processos de fluidificação e injeção de sedimentos em interação com substratos bioestabilizados, caracterizando assim um pseudofóssil.
The Cerro Negro Formation, La Providencia Group (Olavarría and Barker municipalities Argentina) comprises a marine siliciclastic succession of ediacaran ages. The unit is composed of intercalated mudstones and sandstones rich in microbially induced structures (MISS) and deposited under the influence of storms in inter- to subtidal settings. In this context, event beds of fine to very fine sandstones are rich in overload deformation features (liquidization processes). They also commonly exhibit on its upper surface hundreds of discoidal strucutures preserved in full relief. The densely distributed discs occur as centimetric protuberances (5 to 140 mm) with a smooth upper surface and a radially symmetric wrinkled underside, which is marked by a central depression. These characters, along with their size frequency distribution and the occurrence of mutually deformed individuals, led to the interpretation of the discs as probable taphomorphs of the plexus Aspidella. However, the sedimentary and taphonomic context of the Argentinian Ornamented Discs (OD), as well as their complex internal morphology, prevent direct association with macrofossils from other Ediacaran units. Thus, in order to elucidate the genesis and preservation of the discs in terms of their biotic or abiotic nature, I proposed the application of a series of analytical techniques, including: 1) Systematic counting and measuring the discs in the field, understanding the size distribution of different disc populations; 2) sectioning and scanning rock slabs in order to observe internal features; 3) analyses of thin sections under optical and electron scanning microscopes (MEV); 4) scanning with Micro-computorized Tomography (Micro-CT) to observe the tridimensional distribution of internal features. The combined use of these techniques allowed the discussion of the OD under the light of two main alternative hypothesis, AH-I and AH-II, that comprise: AH-I) A biogenic genesis, in which subsurface perturbations were originated by an infaunal community producing simple vertical tubes (Skolithos-like traces), that may or may not culminate in horizontal radiated discs (Asterosoma-like traces). AH-II) An abiotic origin, in which the discs arethe product of the ascension of fluidized sediments interacting with a microbially bounded substrate. This hypothesis explains the observed correlation between discs and the liquidization features, thereby providing it greater synthesizing power. The AH-II is also independent of the breaking of biological and paleontological paradigms and it is compatible with the interpretation of similar occurrences in the literature. As such, I consider the OD as a problematic structure probably associated to migrating fluidized sediments, trapped under a biostabilized substrate. In this interpretation, therefore, they are considered pseudofossils.
The Cerro Negro Formation, La Providencia Group (Olavarría and Barker municipalities Argentina) comprises a marine siliciclastic succession of ediacaran ages. The unit is composed of intercalated mudstones and sandstones rich in microbially induced structures (MISS) and deposited under the influence of storms in inter- to subtidal settings. In this context, event beds of fine to very fine sandstones are rich in overload deformation features (liquidization processes). They also commonly exhibit on its upper surface hundreds of discoidal strucutures preserved in full relief. The densely distributed discs occur as centimetric protuberances (5 to 140 mm) with a smooth upper surface and a radially symmetric wrinkled underside, which is marked by a central depression. These characters, along with their size frequency distribution and the occurrence of mutually deformed individuals, led to the interpretation of the discs as probable taphomorphs of the plexus Aspidella. However, the sedimentary and taphonomic context of the Argentinian Ornamented Discs (OD), as well as their complex internal morphology, prevent direct association with macrofossils from other Ediacaran units. Thus, in order to elucidate the genesis and preservation of the discs in terms of their biotic or abiotic nature, I proposed the application of a series of analytical techniques, including: 1) Systematic counting and measuring the discs in the field, understanding the size distribution of different disc populations; 2) sectioning and scanning rock slabs in order to observe internal features; 3) analyses of thin sections under optical and electron scanning microscopes (MEV); 4) scanning with Micro-computorized Tomography (Micro-CT) to observe the tridimensional distribution of internal features. The combined use of these techniques allowed the discussion of the OD under the light of two main alternative hypothesis, AH-I and AH-II, that comprise: AH-I) A biogenic genesis, in which subsurface perturbations were originated by an infaunal community producing simple vertical tubes (Skolithos-like traces), that may or may not culminate in horizontal radiated discs (Asterosoma-like traces). AH-II) An abiotic origin, in which the discs arethe product of the ascension of fluidized sediments interacting with a microbially bounded substrate. This hypothesis explains the observed correlation between discs and the liquidization features, thereby providing it greater synthesizing power. The AH-II is also independent of the breaking of biological and paleontological paradigms and it is compatible with the interpretation of similar occurrences in the literature. As such, I consider the OD as a problematic structure probably associated to migrating fluidized sediments, trapped under a biostabilized substrate. In this interpretation, therefore, they are considered pseudofossils.
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Palavras-chave
Discos ediacaranos, MISS, Pseudofósseis, Escape de fluido, Ediacaran discs, Pseudofossils, Fluid escape