Le Néogène du bassin Guercif-Taourirt (Maroc oriental) : Paléoenvironnement, caractérisation et potentiel de valorisation des Argiles

The Neogene Guercif-Taourirt Basins, located in the South Rifain Corridor (Morocco), includes large marl-clay deposits. These deposits were studied using a micro-paleontological (planktonic and benthic foraminifera) and mineralogical (clays) approach in order to reconstruct the evolution of palaeoenvironmental conditions over the late Miocene. In addition, the economic interest of this mixed sedimentation was evaluated for ceramic applications.
The Neogene sedimentation of the Guercif-Taourirt basins, covers the late Miocene deposits to the Pliocene. Five lithological units observed in the studied sections of the Guercif basin. The Neogene sedimentation begins by conglomerate-marl continental deposits (Draa Sidi Saada unit) followed by Tortonian marine deposits (calcareniticsmarls of the Ras El Ksar unit, blue marl subunit and sandstone-marl subunit). The whole Tortonian marine deposits represent a transgressive sequence, followed by regressive deposits made by gypsiferous marls. These are capped by the transitional regressive sandstone-marl deposits of the Messinian Kef Ed Deba unit and then by the continental Pliocene deposits of unit Bou Irhardaiene. In the sections of the Taourirt basin, three lithological units were identified, based on the Zobzit-Safsafat composite section. The sedimentary record begins by green marl unit (equivalent to the blue marls subunit of the Tortonian), followed by the sandstone-marl unit (equivalent to the gypsiferous marls subunit of the Messinian). The unit represented by the alternation of sandstone, sand and laminated clays represents a partial equivalent of the Kef Ed Deba unit of the Messinian. These are surmounted by conglomerates of the Plio-Quaternary Bou Irhardaiene unit.
The chronostratigraphical data allow to reconstruct the evolution of the South Rifain Corridor over the late Miocene. The marine deposits were deposited between the Middle/Late Tortonian and Early Messinian in the Guercif basin and Late Tortonian to Early Messinian in the Taourirt basin. The main change in the planktonic foraminifera assemblages defines the transition and lithostratigraphical continuity between Late Tortonian and Messinian in the Guercif-Taourirt basins.
The benthic foraminifera associations evidence an evolution of the sedimentary environments of the upper Miocene deposits of the Guercif basin, with a gradual deepening of the basin. For the basal deposits of Middle and Late Tortonian, the micropaleontological content revealed species of infralittoral towards circalittoral environment. The environment gradually changes to epibathiyal conditions observed for upper Tortonian deposits where the transgression reaches its maximum. At the Tortonian/Messinian boundary (7,24 Ma), the sedimentation environment returns back to circalittoral in relation to the rapid shrinking of the South Rifain Corridor. Synchronously clay-rich Tortonian marls were deposited in the Taourirt basin. During the Messinian, a decrease of bathymetry is observed, with a transition from a circalittoral to a infralittoral environment. The emergence of the basin occurs at the end of the Messinian (ca. 6.1 Ma) with the closure of the South Rifain Corridor. This event initiates the Messinian Salinity Crisis.
The analysis of clay associations indicates a mineralogical diversity conditioned by contemporary paleogeographic factors, with no diagenetic influence. The clay fraction is characterized by the presence of illite, chlorite, palygorskite, smectite, kaolinite and interstratified illite-smectite. At the scale of the basin, the appearance of palygorskite during upper Tortonian and its presence over the Messinian indicate evaporitic conditions.
According to their mineralogical and geotechnical properties, the Tortonian and Messinian sediments of the Guercif basin and the Tortonian sediments of the Taourirt basin belong to the common clays type. These clays can be applied in ceramic industry, especially as building materials. However the valorization of these sediments as fired bricks requires pretreatments (i.e., crushing and carbonate elimination). In the Taourirt basin, the Messinian sediments and gray bentonite can both be used as clay barriers in landfill sites, taking into account the requirements and criteria recommended in this field.