Chemostratigraphy of the Tamengo Formation (Corumbá Group,Brazil): A contribution to the calibration of the Ediacaran carbon-isotope curve |
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| Authors: | Paulo C Boggiani Claudio Gaucher Alcides N Sial Marly Babinski Cynthia M Simon Claudio Riccomini Valderez P Ferreira Thomas R Fairchild |
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| Institution: | 1. Instituto de Geociências, Universidade de São Paulo, Rua do lago 562, São Paulo, SP, Brazil;2. Departamento de Geología, Facultad de Ciencias, Iguá 4225,11400 Montevideo, Uruguay;3. Institute of Geography and Geology, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark;4. NEG-LABISE, Department of Geology, Federal University of Pernambuco, Recife, PE, Brazil |
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| Abstract: | The Corumbá Group, cropping out in the southern Paraguay Belt in Brazil, is one of the most complete Ediacaran sedimentary archives of palaeogeographic, climatic, biogeochemical and biotic evolution in southwestern Gondwana. The unit hosts a rich fossil record, including acritarchs, vendotaenids (Vendotaenia, Eoholynia), soft-bodied metazoans (Corumbella) and skeletal fossils (Cloudina, Titanotheca). The Tamengo Formation, made up mainly of limestones and marls, provides a rich bio- and chemostratigraphic record. Several outcrops, formerly assigned to the Cuiabá Group, are here included in the Tamengo Formation on the basis of lithological and chemostratigraphical criteria. High-resolution carbon isotopic analyses are reported for the Tamengo Formation, showing (from base to top): (1) a positive δ13C excursion to +4‰ PDB above post-glacial negative values, (2) a negative excursion to −3.5‰ associated with a marked regression and subsequent transgression, (3) a positive excursion to +5.5‰, and (4) a plateau characterized by δ13C around +3‰. A U-Pb SHRIMP zircon age of an ash bed interbedded in the upper part of the δ13C positive plateau yielded 543 ± 3 Ma, which is considered as the depositional age ( Babinski et al., 2008a). The positive plateau in the upper Tamengo Formation and the preceding positive excursion are ubiquitous features in several successions worldwide, including the Nama Group (Namibia), the Dengying Formation (South China) and the Nafun and Ara groups (Oman). This plateau is constrained between 542 and 551 Ma, thus consistent with the age of the upper Tamengo Formation. The negative excursion of the lower Tamengo Formation may be correlated to the Shuram–Wonoka negative anomaly, although δ13C values do not fall beyond −3.5‰ in the Brazilian sections. Sedimentary breccias occur just beneath this negative excursion in the lower Tamengo Formation. One possible interpretation of the origin of these breccias is a glacioeustatic sea-level fall, but a tectonic interpretation cannot be completely ruled out. |
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| Keywords: | Neoproterozoic Cloudina Chemostratigraphy Paraguay Belt Brazil |
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