Phosphorites et évaporites : deux formations de milieux sédimentaires voisins étudiés dans la coupe du Paléogène de Foum Selja (Metlaoui, Tunisie)

Abstract

The sedimentary phosphatic series of the Gafsa area is strongly tilted to the West of Metlaoui where they are cut by the Selja river. A detailed study of this excellent natural cross section has been carried out, and numerous samples obtained. The observed litho-logical superposition may be explained by sequences established on basis of the variations of a «chemical index», defined as the ratio between the abundance of material of chemical and biochemical origin, and the abundance of material of detrital origin. On this way the different facies may be classified by their dominant minerals in the following order : clay, calcite, dolomite, phosphate or gypsum. Clays are mainly represented by detrital montmorillonites and kaolinite, whereas attapulgite and sepiolite strongly indicate more chemical environments. The apatite is characteristically fluor -carbonate-apatite with 4 % F and 5 to 6 % CO₂. The entire El Haria -Metlaoui -Souar series corresponds to three sequences, which do not coincide with the stratigraphical divisions. The lower and upper sequences are gypsiferous, bracketing an intermediate phosphatic sequence. Gypsum and apatite can be considered as equivalent in terms of their position in the sequence ; though invariably separate minerals, their appear at the same stadium of the sequential evolution, the former chemical, the latter biochemical (with its usual suite of characteristic trace elements, particularly the group V, Ni, Cr, Zn). The behaviour of strontium is also interesting, since the study of the Sr -P2O5 diagram suggests that apatite forms by transformation of a calcium carbonate, in particular aragonite. The composition of the deposits and their succession seem to be the result of three factors working simultaneously : a vertical movement which supplies detrital material ; the opening and the closing of the connection between the basin and the open ocean, favouring either chemical restriction, or biological development ; a geochemical factor, not completely understood, which is induced by a greater or lesser abundance of magnesium.