Economic phosphorite from the Ediacaran Doushantuo Formation, South China, and the Neoproterozoic-Cambrian Phosphogenic Event

Abstract

Phosphorite of the Ediacaran Doushantuo Formation accumulated in the central Guizhou province during the Neoproterozoic-Cambrian Phosphogenic Event, producing the first true phosphorite giant in Earth history. Deposition is interpreted to have occurred on a wave-dominated epeiric platform along the northern margin of the Qianzhong Uplift. Lithofacies stacking patterns and two paleokarst surfaces record phosphogenesis through at least two sea-level cycles. As in younger Phanerozoic epeiric sea phosphorites, the delivery of upwelled phosphorus is interpreted to have stimulated phosphogenesis in a range of shallow-water environments.Ten phosphatic and associated lithofacies are recognized in the Doushantuo Formation. Wavy and horizontal laminated pristine facies were the locus of phosphogenesis and reflect the authigenic precipitation of francolite in accumulating fine sediment. Granular, cross-stratified phosphorite is interpreted to have been produced through storm-wave winnowing of pristine facies and the subsequent transport, redeposition, and concentration of phosphatic peloids into economic beds. Beds with the highest concentration of phosphorus formed in shallow, wave-agitated environments around paleo-topographic highs. Paragenetic analysis suggests that secondary meteoric processes further enriched the phosphorus content (>35 wt%) of these granular beds during sea-level lowstands.Such aerially extensive, upwelling-related phosphorite contrasts Paleoproterozoic and Neoproterozoic phosphatic deposits. Throughout much the Proterozoic francolite precipitation was restricted to shallow-water, photosynthetic oxygen oases where redox sensitive phosphogenic processes were focused in the sediment. The Doushantuo Formation is interpreted to record the oxygenation of the water column and concomitant expansion of phosphogenic environments during the Neoproterozoic Oxygenation Event. This progressive ventilation of the deep ocean forever changed the biogeochemical cycling of phosphorus in the marine realm and set the stage for the deposition of phosphorite giants that punctuate the Phanerozoic stratigraphic record.