https://repositorio.ufba.br/handle/ri/6394
Tipo: | Artigo de Periódico |
Título: | Isotope stratigraphy of the Lapa Formation, São Francisco Basin, Brazil: Implications for Late Neoproterozoic glacial events in South America |
Título(s) alternativo(s): | Precambrian Research |
Autor(es): | Azmy, Karem Kaufman, Alan J. Misi, Aroldo Oliveira, Tolentino Flávio de |
Autor(es): | Azmy, Karem Kaufman, Alan J. Misi, Aroldo Oliveira, Tolentino Flávio de |
Abstract: | The Lapa Formation is a thick carbonate sequence (∼900 m) that constitutes the upper part of the Vazante Group on the São Francisco craton, Brazil. It conformably overlies a previously unrecognized glacial diamictite unit of poorly constrained age. The sequence, above the glacial unconformity, consists predominantly of organic-rich shale, subtidal rhythmic dolomicrites and microbialaminites, and intertidal stromatolites. Four boreholes, spanning different depositional settings, were sampled at high-resolution and investigated for their petrographic and chemical criteria to evaluate their degree of preservation. The δ13C and δ18O values of well preserved Lapa carbonate microsamples range from −8.2‰ to 3.3‰ (VPDB) and from −13.6‰ to −0.9‰ (VPDB), respectively. Each of the δ13C profiles of the investigated cores reveals two strong negative excursions of up to 8‰, an event in post-glacial dolomicrites immediately above the glaciogenic unit and a 10 m interval of organic-rich shale, and a second near the top of the sequence associated with a shale interval. Based on the observation of dropstones and sedimentary iron formation in the underlying diamictite, as well as the distinguishable carbon isotope trends, the Lapa Formation is considered as a cap carbonate lithofacies. The age of the Lapa Formation is presently unknown but the least radiogenic 87Sr/86Sr value (∼0.7068), associated with a negative carbon isotope excursion, matches that from the Rasthof Formation in Namiba on the Congo craton, which is radiometrically constrained to be younger than ca. 750 Ma. |
Palavras-chave: | Neoproterozoic carbonates Stable isotope stratigraphy Lapa Formation Brazil |
Editora / Evento / Instituição: | Precambrian Research |
URI: | http://www.repositorio.ufba.br/ri/handle/ri/6394 |
Data do documento: | 2006 |
Aparece nas coleções: | Artigo Publicado em Periódico (IGEO) |
Arquivo | Descrição | Tamanho | Formato | |
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GG.pdf | 1,11 MB | Adobe PDF | Visualizar/Abrir |
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