Etude geochimique de la fraction carbonatee d'un facies de bordure de depot gypseux (exemple du gypse ludien du Bassin de Paris) (geochemical study of a carbonate-rich facies in the northern border-area of the Ludian gypsum in the Paris Basin)

Abstract

The bore-hole of Mont-Pagnotte (Oise, France), located in the northern border-area of the Ludian gypsum in the Paris Basin, shows a facies richer in carbonate than those of the centre of the basin. It is possible to relate this facies to the typical formations of the centre of the basin. Samples from the Middle Bartonian (Calcaire de Saint-Ouen) to the Stampian (argile verte) are studied. In this part of the core, the proportion of argillaceous minerals is very low, but there are many variations which are related to the evolution of the area of sedimentation. Alizarine-coloured thin-section study and X-ray analysis show that the microfacies is constant: it is always a calcimicrite. This constancy is very important in geochemical study, because we can eliminate a possible action of mineralogic and diagenetic factors on the distribution coefficient of the different elements studied. Results of the atomic absorption analysis of the trace elements (on the carbonate part of the samples) are positive for 8 elements (Sr, Na, K, Mg, Zn, Mn, Cr, Ni). After a critical discussion (based on much experimental data) of the comportment of those elements during carbonate sedimentation, the Sr, Na, K, Mg, Zn contents appear to represent an indicator of the salinity of the water in the area of sedimentation; the Mn concentration as an indicator of oxydo-reduction conditions in the sediment and the Ni and Cr contents as an indicator of variation in the quality of continental supplies (in ionic or detritic form). It is possible to relate this variation to weathering conditions on the continent. The study shows an evolution of the salinity in the Middle Bartonian: the basal part has the characteristics of a marine deposit, the top has those of a continental (fresh-water) deposit. The Mg contents are constant in the carbonate part, but this is related to a competition between an argillaceous phase and a carbonate phase towards magnesium. This evolution of the sedimentation area is confirmed by the distribution of Characeae in the Middle Bartonian. The Ludian gypsums of the Paris Basin are really a continental deposit, the lateral, calcareous facies equivalent to the gypsum deposits always present the geochemical characteristics of a continental deposit: carbonates in gypsum formations always have the lowest concentrations of Sr, Na, K, Mg and Zn. Comparison between the evolution of argillaceous and carbonate sediments shows that argillaceous sedimentation is not affected by variations in salinity, but it is very sensitive to variations of the quality and quantity of continental supplies in the sedimentation area. To conclude, an evolutionary paleogeography of this region of the Paris Basin from the middle Bartonian to the Stampian (salinity of water, weathering on the continent…) is proposed.