The sedimentology and geochemistry of phosphatic and associated strata in Jordan: Implications for phosphogenesis and the formation of economic phosphorite.

摘要

Sedimentary, authigenic, and biological processes are preserved within the Upper Cretaceous (Campanian) Alhisa Phosphorite Formation (AP) in central and northern Jordan. The AP formed near the eastern extremity of the South Tethyan Phosphorite Province (STPP), a carbonate-dominated Upper Cretaceous to Eocene “phosphorite giant” that extends from Colombia, North Africa to the Middle East. Multidisciplinary research of the AP and associated cherts, chalks, and oyster buildups indicate that phosphatic strata formed on a highly productive, storm-dominated, east-west trending epeiric platform along the south Tethyan margin. The onset of phosphogenesis and the accumulation of economic phosphorite coincided with a rise in relative sea level that onlapped peritidal carbonates of the Ajlun Group. Authigenic precipitation of phosphate occurred in a broad array of sedimentary environments—herein termed a “phosphorite nursery”—that spanned the entire platform. Sedimentologic data indicate that pristine phosphates were concentrated into phosphatic grainstones through storm wave winnowing, and storm-generated, shelf-parallel geostrophic currents. Economic phosphorites formed through the amalgamation of storm-induced event beds. Stratigraphic packaging of phosphatic strata indicates that temporal variations in storm frequency were a prerequisite for the formation of economic phosphorite. Syndepositional phosphogenesis, reworking, and amalgamation to form phosphorites contrasts sharply with the concepts of “Baturin Cycling”. A transgressive systems tract coupled with high surface productivity created detritally starved settings for the establishment of a “phosphorite nursery” and amalgamation of storm-generated event beds formed economic phosphorite within a single systems tract.; Coated phosphate grains were investigated to elucidate the processes governing phosphogenesis. Stable isotopic data (δ13C _{carbonate fluorapatite}) indicate that coated grains precipitated in association with the suboxic to anoxic microbial respiration of organic matter. The micro stratigraphies of some grains suggest that phosphogenesis is commonly accompanied by changes in pore water redox chemistry. These changes reflect fluctuations in the biological oxygen demand within suboxic pore water environments resulting from variations in the surface productivity and/or ecological dynamics in the overlying water column. Coated phosphate grains record low and/or net negative sediment accumulation rates and are the granular equivalent to condensed beds.; The trace element chemistry (Mg and Sr) of skeletal calcite from the Cretaceous oyster, Oscillopha figari was analyzed in sclerochronological profile in order to determine the temperature and salinity regime that prevailed over the Jordanian shelf. Although there is significant uncertainty in interpreting the data, the results provide clear objectives for future research, and support sedimentologic evidence that suggests oysters developed on a productive epeiric platform that experienced periods of intense upwelling.