The Neoproterozoic successions of the São Francisco Craton: Phosphogenesis, chemostratigraphy and possible correlations
|Location||International Geological Congress,oslo 2008|
|Author||Misi, Aroldo۱; Sanches, Andreia Lima۲; Kaufman, Alan Jay۳; Azmy, Karem۴|
|Holding Date||15 September 2008|
The Neoproterozoic sedimentary sequences of the São Francisco Craton are formed by thick carbonate and siliciclastic strata accumulated in epicontinental seaways, and on passive cratonal margins. At least two transgressive-regressive sea level cycles have been recognized during the evolution of the carbonate mega-sequences. These sequences, forming the Vazante, Bambuí, and Una Groups, lay above basal glacio-marine diamictites of probable Sturtian age, and a late glacio-marine diamictite, possibly of the same age, was observed in upper units of the Vazante Group. The broad similarities of lithofacies, as well as the presence of phosphate and sulfide (Zn,Pb) deposits restricted to narrow stratigraphic intervals, suggest that the Vazante, Bambuí and Una Group sequences may be correlative. The Sr isotope data from well preserved carbonates and carbonate fluorapatite from each of these successions, ranging from 0.70763 to 0.70794, support the general correlation and are indicative of seawater composition around 650 Ma, although the lower Sr/Sr values of 0.70614 at the Vazante carbonates may suggest that this unit is older than the Bambuí and Una sediments.
Carbon isotope data of carbonate fluorapatite reveal sharp negative excursions between -3.47 and -12.25‰ PDB in the cratonic area, and between -1.3 and -9.61‰ VPDB in the carbonate fluorapatite hosted by the passive margin Vazante carbonates, suggesting that primary P concentration was accumulated in strong anoxic environments in both geotectonic settings. Carbonate hosting the phosphate deposits in these units are moderately enriched in C, ranging from positive δC values of +2.0 and + 9.6‰ VPDB in the different sections. Most primary phosphate accumulations occur above glaciogenic sequences. We suggest that this may be related to high organic productivity after glacial events. Neoproterozoic phosphorites are present in several continental areas around the world and represent a widespread episode of phosphogenesis. The phosphate deposits of the Neoproterozoic basins of the São Francisco Craton are characterized by an intimate association with stromatolitic structures, suggesting the influence of microorganisms in controlling the phosphogenesis.
Some of these deposits rest unequivocally above glaciogenic sequences, some few others do not, but the presence of negative δC signals immediately below these deposits may suggest that discrete glacial events could have occurred before the phosphate accumulation. Regardless of the nature of the carbonate fluorapatite formation and the uncertainties concerning the precise ages of these successions, the depositional setting represented by the Vazante and the Una Groups is conducive to the formation of phosphorite. Bacterial degradation of organic matter in the cyanobacterial mats probably was responsible for local phosphate enrichment of the pore waters.