Analysis of the stromatolites from the Itaiacoca Group (Proterozoic) from south Itapeva, State of São Paulo, Brazil / Análise dos Estromatólitos do Grupo Itaiacoca (Proterozóico), ao Sul de Itapeva, SP

AUTOR(ES)

William Sallun Filho

DATA DE PUBLICAÇÃO

1999

RESUMO

Stromatolites were studied at nine localities south of Itapeva, São Paulo, Brazil, generally in light-gray metadolostones and secondarily in dark-gray metalimestones of the Itaiacoca Group, a Mesoproterozoic volcanosedimentary unit of the Ribeira Belt. Five columnar forms were distinguished, the most common consisting of unbranched, coniform columns, with centimetric to decimetric diameters and heights, attributed to Conophyton. The other four forms exhibit convex, but not coniform lamination and differ in size, silhouette and style/frequency of branching. Differences in stromatolite preservation are related to the differing tectonic behavious of the purer and more competent metadolostones and the more argillaceous metalimestones which behaved more plastically. In the best exposures in this area the stromatolites are grouped into Conophyton bioherms, without any evidence of subaerial exposure or reworking by waves, which suggests that they formed in a calm and relatively deep setting (perhaps up to several tens of meters in depth), probably below the base of fairweather water. Conophyton from Itapeva is similar to other coniform stromatolites in the Itaiacoca Group near Abapã (Paraná), about 100 km SW of Itapeva, but differs from other forms, including Conophyton cylindricum and C. metulum, from Proterozoic successions associated with the western margin of the São Francisco Craton. The Conophyton from the Itaiacoca Group is most similar to forms in the ex-Sovietic Union that are usually found in the Mesoproterozoic or lowest Neoproterozoic, which is consistent with available radiometric age dates that place this unit near the end of the Mesoproterozoic.

ASSUNTO(S)

paleontologia grupo itaiacoca itaiacoca group pré-cambriano estromatólito proterozóico stromatolite proterozoic precambrian paleontology

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