Microbial mediation in phosphatogenesis: new data from the Cretaceous phosphatic chalks of northern France

Abstract

Abstract Detailed petrographic study of the Upper Cretaceous phosphatic chalks of northern France, particularly by scanning electron microscopy, shows that phosphate occurs predominantly as phosphatized bacterial remains. Shiny phosphatic crusts on hardgrounds and anisotropic phosphate coatings on grains are interpretated as phosphate mineralized microbial (‘microstromatolitic’) structures. Phosphatic remains of ovoid bacteria and of microbial colonies with botryoidal surfaces have been observed in chambers of foraminiferans and bryozoans, and within other bioclasts, and occur as pore linings in peloids, coprolites and lenses of phosphatic chalk incorporated into hardgrounds. The microbial community which formed the crusts and grain coatings developed by surface accretion, whereas the other community grew solely in the intragranular porosity of grains and in the extragranular porosity of semilithified sediments. Analysis of these two types of microbial growth, which have also been documented from other phosphorite deposits, permits a better but as yet incomplete understanding of the mechanisms of phosphatization.