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

Abstract

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 phosphate 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 phosphate 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 phosphate 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 microconglomeratic phosphate. 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 3 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 phosphate lithofacies control the ore grade of the phosphatic sediments: repeated storm-induced winnowing of pristine phosphate contributes to enrichment of the phosphate ore grade.

Phosphate Rocks: A Review of Sedimentary and Igneous Occurrences in Morocco

“…The most important sedimentary phosphates in Morocco are distributed mainly over four basins: Ouled Abdoun, Ganntour, Meskala, and Oued Ed-dahab (Figure 4). However, phosphates are also reported in other sites of less economic importance: Middle Atlas, Beni Mellal, High Atlas, the Marrakesh High Atlas northern basins, Souss, and the Ouarzazate basins (Figure 4) [18]. These deposits form part of the Mediterranean (Tethyan) phosphogenic province [80].…”

“…These marine sediments developed between the underlying Senonian continental series and the overlying Neogene erosive continental deposits. Phosphatic facies include granular phosphate, phosphatic marls, and phosphatic sandstones and microconglomerates [18]. The Souss Basin, located at the southern edge of the High Atlas, embodies the thickest known Maastrichtian sequence in Morocco, and contains various phosphatic sediments, including phosphatic sandstones and microconglomerates.…”

“…These conditions favor a partial degradation of organic matter to release phosphorus into the interstitial microsystems, preventing its recycling into seawater [86,90]. On the other hand, the exceptional accumulation of phosphate in Morocco reflects a mechanical concentration of phosphatic elements via hydrodynamic processes as a part of a multistage post-phosphogenesis evolution of primary phosphate that involves reworking, transport, and secondary accumulation on the marine platform [18,[86][87][88]. On a global scale, the concentration of phosphate in phosphate plateaus in Morocco seems to be controlled mainly by paleogeographic and eustatic forcing [103].…”

“…Moroccan sedimentary phosphate rocks belong to the upper Cretaceous to lower Eocene stratigraphic interval. They outcrop in several sedimentary basins of different surface area and content [18]. This discontinuous geographical character reflects important paleointeractions with the local and the regional geodynamic context, which continues to be the subject of several controversial hypotheses [18].…”

“…They outcrop in several sedimentary basins of different surface area and content [18]. This discontinuous geographical character reflects important paleointeractions with the local and the regional geodynamic context, which continues to be the subject of several controversial hypotheses [18]. These Moroccan phosphate rocks represent a unique case study to understand all of the paleo-processes involved in phosphogenesis.…”