DEGEO - Departamento de Geologia
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8
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Resultados da Pesquisa
Item 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 RichThe 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.Item Insights into vase-shaped microfossil diversity and Neoproterozoic biostratigraphy in light of recent Brazilian discoveries.(2019) Soares, Luana Pereira Costa de Morais; Lahr, Daniel José Galafasse; Rudnitzki, Isaac Daniel; Freitas, Bernardo Tavares; Romero, Guilherme Raffaeli; Porter, Susannah M.; Knoll, Andrew H.; Fairchild, Thomas RichVase-shaped microfossils (VSMs) occur in dolostone clasts within conglomerates, breccias, and diamictites of the Neoproterozoic Urucum Formation, Jacadigo Group, southwest Brazil. Although their taphonomic history is distinct from those of other VSM assemblages, morphometric comparison of Urucum fossils with five others described previously from North America and Europe show that two of the Urucum species—the long-necked Limeta lageniformis Morais, Fairchild, and Lahr in Morais et al., 2017 and the funnel-necked Palaeoamphora urucumense Morais et al., 2017—occur in the Kwagunt and Callison Lake assemblages, as does Pakupaku kabin Riedman, Porter, and Calver, 2017 recently described from the Togari Group, Tasmania. Obelix rootsii (Cohen, Irvine, and Strauss, 2017) new combination, previously known only from the Callison Lake Formation, is documented here from the Kwagunt Formation. In addition, Trigonocyrillium horodyskii (Bloeser, 1985) and Bonniea dacruchares Porter, Meisterfeld, and Knoll, 2003, first described from the Kwagunt assemblage, have now been found in the Urucum Formation. In light of this survey, 16 of the 18 validly described VSM species are now known to occur in the Kwagunt Formation and 13 in the Callison Lake Formation, with 12 of them shared by both formations. The fact that the Urucum VSM assemblage exhibits six of seven species in common with the Kwagunt Formation—L. lageniformis, P. urucumense, Cycliocyrillium simplex Porter, Meisterfeld, and Knoll, 2003, C. torquata Porter, Meisterfeld, and Knoll, 2003, B. dacruchares Porter, Meisterfeld, and Knoll, 2003, and T. horodyskii (Bloeser, 1985)—and all but the last of these in common with the Callison Lake Formation supports correlation of these three assemblages and indicates that the source of the fossiliferous clasts within the Urucum Formation may well have been a now-vanished late Tonian carbonate platform.Item Carbonaceous and siliceous Neoproterozoic vase-shaped microfossils (Urucum Formation, Brazil) and the question of early protistan biomineralization.(2017) Soares, Luana Pereira Costa de Morais; Fairchild, Thomas Rich; Lahr, Daniel José Galafasse; Rudnitzki, Isaac Daniel; Schopf, James William; Garcia, Amanda K.; Kudryavtsev, Anatoliy B.; Romero, Guilherme RaffaeliVase-shaped microfossils (VSMs) occur in dolomitic extraclasts of indeterminate provenance within the basal diamictite of the Neoproterozoic Urucum Formation (Jacadigo Group) of west-central Brazil, having an age constrained between 889±44 Ma (K-Ar; basement rocks) and 587±7 Ma (40Ar/39Ar age of early metamorphic cryptomelane in overlying manganese ore). Early isopachous carbonate cement entombed these VSMs, preserving rare direct evidence of original wall composition that is carbonaceous (now kerogenous) in practically all specimens. Some tests are siliceous or composed of a quartz-kerogen mixture; secondary replacement explains some features of these tests, but original biomineralization seems more likely for others. This interpretation, coupled with test morphology, suggests affinity to arcellinid testate amoebae. Five VSM taxa are recognized in the deposit: Cycliocyrillium simplex Porter, Meisterfeld, and Knoll, 2003, and C. torquata Porter, Meisterfeld, and Knoll, 2003, originally described in the Chuar Group (USA), and three new monospecific genera—Palaeoamphora urucumense n. gen. n. sp., Limeta lageniformis n. gen. n. sp., and Taruma rata n. gen. n. sp. Most of the taxonomically important characteristics of these VSMs occur also in extant testate amoebae, but the combinations of some characters, such as organic-walled tests having exceptionally long necks that exhibit terminal apertures (L. lageniformis n. gen. n. sp.), are evidently novel additions to the known diversity of Neoproterozoic VSMs. Evidence of glacially influenced deposition in the conformably overlying Santa Cruz Formation may indicate that the Urucum Formation slightly preceded or was penecontemporaneous with a major Neoproterozoic glaciation, although the VSM-hosting extraclasts must be older, possibly rivaling the age of the testate amoebae of the Chichkan Formation (766±7 Ma) that are currently regarded as the oldest record of protists in the geological record.Item Geobiological and diagenetic insights from Malvinokaffric devonian biota (Chapada Group, Paraná Basin, Brazil) : paleobiological and paleoenvironmental implications.(2017) Kerber, Bruno Becker; Osés, Gabriel Ladeira; Curado, Jessica Fleury; Rizzutto, Márcia de Almeida; Rudnitzki, Isaac Daniel; Romero, Guilherme Raffaeli; Alves, Silvio Yuji Onary; Benini, Victoria Giopato; Galante, Douglas; Rodrigues, Fabio; Buck, Pedro Victor; Rangel, Elidiane Cipriano; Ghilardi, Renato Pirani; Pacheco, Mírian Liza Alves ForancelliThis study tests the presence of differential preservation in the Devonian Malvinokaffric fauna from the Chapada Group (Parana´ Basin, Brazil). Results of EDXRF, EDS, Raman Spectroscopy, and petrographic analyses show differential preservation of shells that were originally calcite as hematite and goethite fossils, while organisms with original calcium phosphate shells tend to be preserved inside phosphatic concretions. Both preservation types are commonly associated with pseudoframboids, while calcium sulfate minerals are commonly associated with hematized fossils. From this evidence, a diagenetic model for these fossils is proposed. The model includes an early diagenetic phase (characterized by anaerobic sulfate reduction and precipitation of pyrite and carbonate-fluorapatite) and a second, near-surface chemical weathering phase (characterized by the oxidation of pyrite and precipitation of iron oxyhydroxides and calcium sulfates). Acidic conditions in both phases may account for the dissolution of less stable minerals compared to calcium phosphate. It is considered that this model may assist in understanding other similarly preserved biotas, as well as enhancing understanding of the taphonomic overprint that may occur within this important and endemic Devonian biota.