The Moroccan High Atlas phosphate-rich sediments: Unraveling the accumulation and differentiation processes

The phosphate series of the Moroccan High Atlas accumulated during the Upper Cretaceous-Paleogene phosphogenic period, on a biologically-productive shallow-water platform. These phosphate-rich sediments were deposited during a relative sea-level cycle generating major stratigraphic surfaces. Five different types of phosphatic lithofacies are recognized based on their petrographic and sedimentologic features. The pristine phosphate lithofacies corresponds to phosphatic marls formed by francolite authigenesis within the outer platform domain below the storm wave-base (primary phosphogenesis zone). This primary phosphatic lithofacies can be differentiated as a result of the interaction between hydrodynamic autocyclic processes and allocyclic forcing. Accordingly, the granular phosphate lithofacies was formed by in-situ storm and bottom currents that winnowed the pristine phosphate. This winnowed lithofacies, composed of densely-packed peloids, shows relatively high P₂O₅ concentrations. The turbiditic phosphate resulted from the basinward transport and deposition of other phosphate types by gravity flows leading to normally-graded phosphatic sediment. The phosphatic lags were formed by wave reworking and transporting of pristine and associated granular facies within the inner platform. The karst-filling phosphate corresponds to former phosphate types transported by early transgressive currents and trapped within karstic pockets as phospharudites. The distribution of the different phosphate types across the margin and within the depositional sequences is controlled by the effects of relative sea-level changes. The pristine phosphate and associated winnowed facies are found above the major maximum flooding surface, which is dated by nannoflora to the Selandian-Thanetian transition. The phosphatic turbidites occurred during periods of high sea-level. The karst-related phosphatic facies accumulated during early transgressive stages. The phosphatic lags accumulated mainly during the regressive phases of third and fourth-order sequences. The accumulation processes of the different phosphatic lithofacies control the ore grade of the phosphatic sediments: repeated storm-induced winnowing of pristine phosphate contributes to the enrichment of the phosphate ore grade.