Sedimentological and stratigraphic constraints on depositional environment for Ediacaran carbonate rocks of the São Francisco Craton: Implications for phosphogenesis and paleoecology

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 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, suggesting mechanisms of phosphogenesis dependent on the accumulation of porewater phosphate via microbial activity in low flow velocity environments, such as relatively quiescent tidal flats, where an absence of wave-driven advection and porewater refreshment could have allowed oversaturation with respect to carbonate fluorapatite. Here, we present new sedimentological data that characterize the depositional setting of the Salitre and Sete Lagoas formations as a shallow, wave-swept carbonate platform notable for its extensive high-energy lithofacies. In this setting, phosphatic and non-phosphatic stromatolite buildups formed within the same depositional facies, often in close (meter- and decimeter-scale) spatial association with one another. These data suggest that early phosphate mineralization of the Salitre and Sete Lagoas formations was likely not a function of low porewater advection in paleogeographically sheltered regions, but rather highly local processes of phosphate enrichment on a high-energy, wave-swept platform environment.