Etude preliminaire des argiles eocenes, mio-pliocenes et actuelles de la region de Metlaoui-M'Dilla et du cap Bon (Tunisie)

Eocene and Mio-Pliocene formations of the Gafsa region are differentiated by their clay minerals. The clays of the El-Haria formation are composed of equal amounts of montmorillonite and kaolinite. The overlying Eocene marine clays of Djebel M'Dilla are composed of montmorillonite, illite and traces of kaolinite, and include a bed of clay containing montmorillonite and sepiolite-attapulgite. The continental Mio-Pliocene deposits around Djebel M'Dilla contain a suite of minerals including kaolinite, attapulgite, illite, montmorillonite and chlorite. These minerals apparently do not have authigenic characteristics. Diffractograms obtained from these deposits indicate pronounced crystallinity of the minerals. Mineralogic variation in the clay lenses shows that, from the base to the top, illite and, to a lesser degree, kaolinite, gain importance at the expense of the montmorillonite. At Cape Bon, the Oligocene is rich in kaolinite with subordinate amounts of illite and montmorillonite. Vindobonian, Pliocene and Recent deposits contain the same phyllites but with kaolinite and montmorillonite in equal proportions and a lesser percentage of illite. The uniformity exhibited by the Miocene clays--suggesting monotonous accumulation from a single source of supply--reaffirms the fact that the composition of clays depends more on the nature of the source of supply than on the diagenetic modification produced in the course of sedimentation. The montmorillonitic mineral of these deposits is an unstable phyllite intermediate between a classic montmorillonite in the Eocene and an ammersoite in the post-Eocene.