Mise en évidence d’un Sénonien gypseux sous la série phosphatée du bassin des Ouled Abdoun: Un nouveau point de départ pour l’origine des zones dérangées dans les mines à ciel ouvert de Khouribga, Maroc

Abstract

In the Ouled Abdoun sedimentary basin (Morocco), the phosphatic series is composed of regular interbedded phosphatic and marly limestone layers. Exploitation of the phosphate in some deposits in this basin collides frequently with problems bound to the existence, in the phosphatic series, of disturbed areas (sterile bodies) qualified as derangements by the mining engineers of the Office Cherifian Phosphate Group (OCP). Their presence in the phosphatic layers causes two kinds of problems: (1) since the whole phosphatic sequence is overlain by a Quaternary cover, we do not know their volume proportion in the phosphatic layers, and therefore the reserves estimations can be wrong (2) they are generally hard, so they complicate the phosphate extraction. Indeed, in an area containing sterile bodies, boring grid may always be tightened and boreholes filled with dynamite. The required borehole tools and use of explosives are time consuming and therefore increase drastically the cost of phosphate extraction. Their localisation would permit the mining engineers to get around them during the exploitation.&#13; &#13; Several geophysical works have carried out in the achieved in Khouribga area to localize and delimit these sterile bodies. Electric resistitivy was established as a suitable geophysical parameter to map them, but the slowness and difficulty of data collection hinders the application of these geophysical methods to the whose phosphatic deposits area (about 25000 ha). Their application in all the Ouled Abdoun basin requires the comprehension of the origin of the sterile bodies, in order to specify their formation process and to predict consequently their spacial distribution in each phosphatic deposit.&#13; &#13; Our study concerns to sedimentological and diagenetic analysis of disturbed aereas and their immediate vicinity. It made it possible to identify for the first time the existence of an evaporitic series intensely karstified at the top of the Senonian, under the phosphatic series. The senonian karsts are undoubtedly at the origin of the derangements, which are defined as the collapsing phenomena at the base of underground cavities.&#13; &#13; These bodies have a polyphase structuring and a genesis that is the consequence of on several processes (fracturing, infiltration, dissolution, collapse). Their installation began from the end of Senonian, and continues up to the Quaternary.<br><br>[fr] Dans le bassin sédimentaire des Ouled Abdoun (Maroc), la série phosphatée est formée d’une intercalation régulière de niveaux phosphatés et marno-calcaires sur environ 50m de puissance. Localement, cette régularité est perturbée par la présence fréquente de structures communément appelés «<i>dérangements</i>». Il s’agit de masses non stratifiées, stériles, qui perturbent et alourdissent les travaux d’exploitation des couches phosphatées. Les études géophysiques expérimentales, réalisées dans les zones dérangées de la série phosphatée, ont démontré la possibilité de les cartographier sous couvertures à l’échelle décamétr...

Geochemical investigation of the taphonomy, stratigraphy, and palaeoecology of the mammals from the Ouled Abdoun Basin (Paleocene-Eocene of Morocco)

“…The Ouled Abdoun Basin is one of the ancient epicontinental marine basins along the Atlantic coast of Africa, where large amounts of phosphorite accumulated during the late Cretaceous -early Eocene period. The basement rocks are covered by Cenomanian-Turonian carbonates followed by yellow marls with possible age within the Coniacian-Campanian interval (El Assel et al, 2013), which were eventually succeeded by the phosphorite series.…”

Geophysical Prospecting for Groundwater Resources in Phosphate Deposits (Morocco)

“…Beneath the 70 m depth, the top of moderately resistant layers is locally observed. They correspond to the compact gray marl with gypsum of the Senonian, which can be seen in the OCP's recognition wells at this depth [43]. Above the conductive horizon, moderately resistant and resistant soils consisting of phosphatic layers with very low water content with intercalated limestone and flint layers occur.…”

“…Other resistant bodies also cut the phosphatic series. They are attributed to the disturbed areas of the phosphatic series, known as "derangements" by the OCP mining engineers [43][44][45][46]. These structures are often accompanied by a localized silicification event.…”

Quantification of the Disturbances of Phosphate Series Using the Box-Counting Method on Geoelectrical Images (Sidi Chennane, Morocco)

“…These bodies are characterized by an irregular geometrical shape and dispersed without any order or privileged direction under a Quaternary cover. The geological investigations carried out in Sidi Chennane deposit have revealed that the origin of these structures is linked to the dissolution of the Senonian gypsiferous layers situated at the base of the phosphate series [3]. Their detection and delimitation required in many cases several exploration works, such as drilling wells that remain not particularly effective and increase drastically the cost of phosphate extraction.…”