COEVAL PERMIAN BENTONITES IN BRAZIL AND SOUTH AFRICA

Early Permian bentonite beds in Brazil and South Africa record episodes of silicic explosive volcanism. Despite their distance from each other today, present day Brazil and South Africa were proximal to each other during the Permian along an active subduction zone, suggesting that volcanism in the area would likely be common. The focus of this study is to determine if ash beds can be correlated between the presumably coeval Whitehill Formation of the Ecca Group in South Africa and the Irati Formation in Brazil using chemical finger printing, to indicate a similar source. Black shale is the dominant facies of both formations, with the Irati having more organic matter. Samples analyzed were distal to the volcano of origin, verified by the lack of phenocrysts in all samples. Initial research was done with X-Ray Diffraction (XRD) in order to determine clay composition of samples. Mixed layer clays were found in both South Africa and Brazil, but the compositions differed. South Africa contained a mixed illite/smectite, whereas samples from Brazil contained mostly smectite, with some kaolinite. The difference in clays is due to differing post-depositional histories. Brazilian samples containing kaolinite are more weathered than samples taken from drier South Africa. In order to determine concentrations of major and trace elements, X-Ray Fluorescence (XRF) was used to analyze select samples. Data from XRF using Nb/Y and Zr/TiO2 indicated a complete overlap between the Irati and all of the Ecca Group (Collingham, Whitehill and Prince Albert) Formations, signifying a similar dacitic and rhyolitic source. There is a separation of the end member tephras into dacitic and rhyolitic groups, and both groups are present in all of the stratigraphic units.