PHOSPHORUS AND PHOSPHORITES - SEDIMENTOLOGY AND ENVIRONMENTS OF FORMATION

Phosphatic sediments form not only beneath upwelling systems, but also on oceanic islands, atop seamounts, guyouts and plateaus, and on some shelves, slopes and in epeiric environments that may not have experie nced upwelling at all. We evaluate the formation of phosphorites and t he sources of phosphorus in each of these settings. We define primary phosphogenesis, which produces primary phosphorite, as the process of carbonate fluorapatite precipitation within sediments, at the sediment /water interface and/or during diagenesis. Primary phosphogenesis is d istinct from the hydraulic and biological reworking processes by which phosphatic grains are concentrated into granular phosphorite. Non-tra nsported phosphorites may occur in any or all of the principal phospho rite settings. Granular phosphorites, which we believe are derived fro m primary phosphorites, are accumulations which developed on ancient s helves and in epeiric seas and appear to dominate phosphorite occurren ces in the Phanerozoic record. These are variously cemented phosphorit e siltstones, sandstones and conglomerates composed chiefly of phospha tic particles, many of which show some evidence of transport and depos ition under the influence of bottom currents. Current activity plays a n important role for many phosphorite deposits, not only for concentra ting dispersed authigenic precipitates, but also for prolonging phosph ate precipitation at critical shallow burial levels. In some cases, re deposition as phosphorite turbidites and tempestites has also been imp ortant. We summarize 13 important topics which require future research .