Mineralogical and geochemical studies of Ypresian marly clays and silica rocks of phosphatic series, Gafsa-Metlaoui basin, southwestern Tunisia: implication for depositional environment

Thick Ypresian marly clays and silica rich rocks constitute the so-called interbedded facies of phosphatic series, in the Gafsa-Metlaoui basin, southwestern Tunisia. Mineralogical and geochemical investigations are carried out on selected samples from Kef Eddour section. The data obtained by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Energy Dispersive X-ray microanalysis (EDX) and chemical analysis and IR of clay fractions indicate that the bulk rock samples are composed of cristobalite-tridymite and dolomite. Observation by SEM showed the occurrence of dolomite as a well-crystallized and a weathered shape. Whereas, cristobalite-tridymite observed in the cherty bed appears as a spheres, filling the pore spaces. However, samples which contain moderately a significant amount of feldspars, clinoptilolite and pyrite occur in the upper part of Kef Eddour section. Clay minerals consist mainly of smectite, palygorskite, and sepiolite. In this clay assemblages, the N-alkane (m/z = 57) distributions, revealed that organic matter has phytoplankton and bacterial origin. The Pristane/Phytane ratio indicated that the organic matter is deposited in a reduced environment. In this latter, sulfate-reducing bacteria generate hydrogen sulfur, which is incorporated on the organic biomolecules that occurred at the lower part of Kef Eddour section. However, the high Fe activity on the depositional environment favored the occurrence of pyrite in the upper part.