THE ROLE OF MICROBIAL ACTIVITY IN THE GENERATION OF LOWER CRETACEOUS MIXED FE-OXIDE-PHOSPHATE OOIDS FROM THE PROVENCAL DOMAIN, FRENCH MARITIME ALPS

Early Hauterivian mixed Fe-oxide-phosphate ooids from the Lower Cretaceous condensed succession of the Alpine Provencal Domain are ellipsoidal, 200-2000 mu m in diameter, and the main component of a sedimentary body, 0-100 cm thick, that can be traced laterally for tens of kilometers. Cathodoluminescence, EDS microprobe, backscattered electron imaging, epifluorescence, XRD, and micro Raman spectroscopy reveal that ooid cortices consist of variable proportions of Fe-oxide-rich and Ca-phosphate-rich layers. SEM observations show that phosphate-rich layers are composed of a dense matrix of micrometer-size, rod-shaped, commonly curved phosphate grains, whose size and shape strongly suggest a microbial origin. Conversely, Fe-oxide-rich ooid cortical layers consist of aggregates of micrometer-size, plate-like crystals of hematite that point to an inorganic precipitation. We suggest that the ooidal condensed interval records a long sedimentary history in an open shelf environment subject to a background accumulation of organic-rich, fine-grained sediments. It was periodically interrupted by high-energy, storm-related events that resulted in winnowing, reworking, and oxygenation that favored precipitation of Fe-oxides around nuclei of biogenic origin. Such precipitation was not uniform on the outer surface of ooids and influenced their ellipsoidal shape. After the return to background conditions, Fe-ooids were buried and driven from an oxic to a post-oxic subbottom geochemical environment where rod-shaped Fe-reducing bacteria colonized Fe-ooid outer surfaces. Decomposition of organic matter and release of P bound on Fe-oxides increased pore-water concentration of phosphate, which precipitated at the outer surfaces of the ooids, thus preserving the microbial community. The bimineralic composition of these ooids discloses a complex and prolonged history of cyclic alternations of oxic and post-oxic geochemical conditions, which are in turn related to alternating sediment accumulation and winnowing phases with exposure at the seafloor.