Lithofacies analysis, sedimentary dynamics and genesis of Maastrichtian-Eocene phosphorites of BouCraa deposit (Southern Morocco)

The BouCraa phosphate deposits of the Oued Eddahab Basin (southern Morocco) is a part of the Upper Cretaceous-Paleogene phosphate province of North-West Africa. Their origin and genesis are still controversial. X-ray diffraction and infrared studies show that the BouCraa phosphate deposits are composed of a low-crystalline carbonate fluorapatite. The phosphatic particles are generally well-rounded, pellet-like, coated grains or phosphatic mudclasts with an average of 59.25 wt% in these deposits. These phosphate grains have no internal structure, they are generally of homogenous textures, but sometimes they can contain detrital or biodetrital particles, such as quartz grains and bone fragments. Phosphatic bioclasts (bones, teeth) makes up an average of 13.72 wt%. Their cavities and pores are filled with mud matrix of identical facies than that of phosphatic mudclasts. All phosphatic particles contain amorphous organic matter of primary origin, associated with framboidal pyrite. In terms of granulometry, the economic phosphate layers show well-sorted sediments and homogeneous textures. The enhanced concentration of phosphate-rich particles is a result of winnowing caused by the dynamic agitation of wave-process and amalgamation/cannibalism. Scanning electron microscope observations of phosphate grains show some pores and cavities with bud-like structures and others reveal porous nanostructures of spherical and ovoid shapes. These structures are phosphatized bacteria in the form of rods or microspheres of 750 nm width and 1 μm length, on average. This is a characteristic of phosphatic grains formed under bacterial mediated upwelling conditions, further suggesting that the phosphatic particles had been reworked from pre-existing authigenic phosphorites. As far as sequence stratigraphy is concerned, the identified phosphatic facies suggest transgressive deposits of large extensions, accumulated in a shallow oxygenated shelf environment. Therefore, all the phosphate levels of the BouCraa deposit are related to recurrent sedimentation of authigenic offshore phosphorites, initially formed in an anoxic environment during a Highstand Systeme Tract, then, at a Lowstand System Track, they are reworked and later, during a transgression period, phosphatic grains were transported to near-shore in an oxic environment where these particles are concentrated under storm wave actions allowing the formation of economic phosphorite layers