DEGEO - Departamento de Geologia

URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8

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    Peritidal microbialites in the upper Araras Group : Morphotypes, potential preservation and the relation with the Ediacaran-Cambrian unconformity in the Araras-Alto Paraguai Basin, southern Amazon Craton.
    (2022) Romero, Guilherme Raffaeli; Santos, Renan Fernandes dos; Nogueira, Afonso César Rodrigues; Rudnitzki, Isaac Daniel; Fairchild, Thomas Rich
    The Proterozoic biosphere was dominated by shallow-marine and intertidal bacterial biota, as evident from a robust record of microbialites in carbonate rocks. The lower Ediacaran successions in the southeastern Amazon Craton, Brazil represented by carbonates rocks of the Araras Group, record excellent occurrences of microbialites implying significant evidence for shallow marine colonization post-Snowball Earth Events (∼635 Ma). Microbialites occur at the lower and upper units of this group -Mirassol D'Oeste and Nobres, respectively with the lower associated to the Marinoan glaciation event. The upper unit, Nobres Formation, is here described with outcrop-based facies analysis discontinuously exposed in the Araras-Alto Paraguai basin. This allowed the paleoenvironmental reconstructions of the carbonate peritidal settings, organized in tidal flats and sabkha meter-scale cycles. Fifteen levels of microbialites have been described, with 4 morphotype associations. These microbialites colonized upper tidal flat zones forming stromatolite deposits of bulbous domes, stratiform, pseudo-columnar, and “cerebroid” forms. The recurrent cyclicity indicates a residence time of hydrodynamic and climatic variations for a long time producing minimum morphological changes without any decline evidence. In addition, no metazoan competition was observed in these strata. The siliciclastic inflow observed in the top of Nobres Formation is interpreted as seasonal variations that imprint turbidity in the shallow waters causing a diversification of the morphology of microbialites. The microbialite record in the Nobres Formation do not show any evolutionary trend or apparent decline, that has been attributed to the evolution of substrate-modifying metazoans, but suggest a continuous record truncated by the Ediacaran-Cambrian unconformity found at the upper portion of the Araras Group.
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    Ichnologic evidence of a Cambrian age in the southern Amazon Craton : implications for the onset of the Western Gondwana history.
    (2017) Santos, Hudson Pereira; Mángano, Maria Gabriela; Soares, Joelson Lima; Nogueira, Afonso César Rodrigues; Bandeira, José; Rudnitzki, Isaac Daniel
    Colonization of the infaunal ecospace by burrowing bilaterians is one of the most important behavioral innovations during the Ediacaran-Cambrian transition. The establishment of vertical burrows by suspension feeders in high-energy nearshore settings during Cambrian Age 2 is reflected by the appearance of the Skolithos Ichnofacies. For the first time, unquestionable vertical burrows typical of the Skolithos Ichnofacies, such as Skolithos linearis, Diplocraterion parallelum and Arenicolites isp., are recorded from nearshore siliciclastic deposits of the Raizama Formation, southeastern Amazon Craton, Brazil. Integration of ichnologic and sedimentologic datasets suggests that these trace fossils record colonization of high-energy and well-oxygenated nearshore sandy environments. Chronostratigraphically, the presence of these vertical burrows indicates an age not older than early Cambrian for the Raizama Formation, which traditionally has been regarded as Ediacaran. Therefore, the Raizama ichnofauna illustrates the advent of modern Phanerozoic ecology marked by the Agronomic Revolution. The discovery of the Skolithos Ichnofacies in these shallow-marine strata suggests possible connections between some central Western Gondwana basins.