Sedimentary Context and Diagenetic History of Phosphatic Microbialites, Ediacaran Una-Bambuí Carbonate Platform, Eastern Brazil

Abstract:

The Una and Bambuí Groups of Northeastern and Central Brazil are remnants of a vast intracratonic carbonate platform formed on the São Francisco Craton during the Ediacaran Period. Their basal stratigraphic units contain early phosphatic cements and phosphatic intraclasts in association with distinctive sedimentary structures: elongate digitate stromatolites in the more northern Irecê and Salitre paleobasins (Salitre Formation), and microbial laminites and aragonite crystal fan pseudomorphs in the more southern São Francisco paleobasin (Sete Lagoas Formation). Previous studies have drawn comparisons to other Precambrian phosphorites, as well as modern phosphogenetic environments. Proposed mechanisms for phosphate accumulation, retention, and mineralization invoke microbial sulfide oxidation, oxygenic photosynthesis, chemical inhibitors to carbonate mineralization, and the role of phosphate-adsorbing detrital and authigenic iron minerals. However, there is a lack of data available to test these mechanisms and explain the limited spatial and textural extent of phosphate in these rocks. Here, we present new sedimentological and stratigraphic data that characterize the depositional setting of the Salitre and Sete Lagoas Formations as a shallow, wave-dominated, high-energy carbonate platform. We also present compositional and mineralogical data, at meso- and micro-scale spatial resolutions, that elucidate the paragenesis of phosphatic and non-phosphatic facies, showing limited to no spatial association between primary and early diagenetic phosphate minerals and oxidized iron minerals, nor between phosphate minerals and chemical inhibitors to carbonate micrite formation. Rather, we find that the extent of early phosphate mineralization is predicted by the spatial extent of stromatolitic textures, particularly those with digitate morphology. Early carbonate mineralization occurred in stromatolitic facies as well, but not in digitate stromatolites. This suggests that the early formation of phosphate minerals in the Salitre and Sete Lagoas Formations was dependent not just on shallow redox gradients or physical environment, but on microbial-sediment interactions in the context of digitate stromatolite formation.