ABSTRACT

The predominantly siliciclastic Upper Proterozoic sediments of the Taoudeni basin have been buried to depths of 2.5 to 3 km. In a normal geothermal gradient 25°C/km), maximum burial temperatures of the section studied would be no higher than 100°C. However, detailed petrographic and fluid inclusion data of sandstones and shales from three areas in Mauritania and Mall along the western side of the basin indicate two major diagenetic events: an earlier low temperature (< 70°C) quartz cementation of the sandstones and a later high temperature (135-170°C diagenesis that affected both shales and sandstones. These latter temperatures are too high to have occurred simply in response to burial. The sediments are cut by numerous Jurassic diabase intrusions, which crop out es ecially commonly in the south and the northeast. The emplacement of these diabases was related to the opening of the South Atlantic ocean. Contact metamorphism and hydrothermal alteration are visible near intrusive contacts. In western Mall (southern part of basin) where the largest exposed sill in the basin occurs, at 50 to 60 km from any outcropping massive diabase, the late diagenetic paragenesis consists of micas, illite, chlorite, kaolinite, siderite and ankerite. High-temperature diagenesis is also observed in Adrar of Mauritania (northwestern part of basin) where outcropping diabases are extremely rare. There, calcite cement is associated with illite and chlorite. Fluid inclusions indicate formation temperatures of 135°C for the ankerite and 170°C for the calcite.

Thus, in spite of a thin sedimentary pile, the Taoudeni basin experienced a high-temperature diagenesis. We propose that this resulted from circulation of hot fluids, generated during the diabase emplacement, over extensive areas within the basin.