The silicification of phosphate series of Ouled Abdoun (Maastrichtian Lutetian-Morocco): Sedimentology, Mineralogy, Geochemistry and Genetic Context

The Middle East and North Africa regions are known by phosphates of the Cenozoic age with a workable commercial quality. And Morocco is by far the main producer with more than three quarters of the world's sedimentary phosphate reserve. Moroccan phosphates from upper Cretaceous - lower Eocene, attracted the interest of many scientists for years. Several investigations have been focused on the first extremely diagenetic phenomena and since then, the sedimentological studies still offer lots of fascinating results. Indeed, they are the referential deposits for the geological studies and particularly the diagenetic Post and Syn sedimentary phenomena. The Silicification is then the object of this study that aim to inventory all the forms and the facies siliceous and silicified, in addition to determine the origin and the diagenetic processes that control the neoformation of silica. And that has been performed on the deposits of Ghar Tajer, Sidi El Maâti, Sidi Chennane and EL Halassa, all a part of the Ouled Abdoun Basin; and go from the Maastrichtian to the Lutetian. At first, a sedimentological and petrographic approach was applied on the different terms of the phosphate series, in order to characterize the silicification and deduce the probable links between this diagenetic phenomenon and the facies and forms containing the silica. As a result, the silicification turns to be diversified and abundant, usually in the form of flint, opal and chalcedony, as lenticular layers of asymmetric lenses with zonation, nodules or rognons called nougat facies and silicified rocks.The second step consisted on a mineralogical and geochemical approach on the same silica and silicified facies of phosphate series of Ouled Abdoun basin. The mineralogical results showed their quality of crystalline minerals, clastic quartz and microcrystalline or fibrous quartz, and marked major peaks 3.34 Å and 4.26 Å. On an other hand, the opal types of silica was presented by the intense and large peaks 4.10 Å, 4.33 Å and 2.50 Å, that correspond to opal CT (Cristobalite-Tridymite opal) and opal T (Tridymite). According to the mineralogical phases’ reports, three main phases: the silicate phase, the carbonate phase and the clay phase. Regarding geochemistry, major and trace elements were measured, then geochemical associations of SiO2 and all elements seemed having significant relationships with the increased degree of diagenetic silicification. The Diagenetic phenomena and silicification have been conditioned by the factors pH, the depositional environment of phosphates and also the rise of upwelling currents.