A HYDROGEOLOGICAL MODEL FOR PALYGORSKITE FORMATION IN THE DANIAN CONTINENTAL FACIES OF THE PROVENCE BASIN (FRANCE)

The Danian of the Provence basin (SE France) is characterized by a str ong subatmospheric alteration processes throughout the basin. A wide r ange of facies, depositional and weathering, were recognized in an int erval of 5 Myr and include floodplain fine-grained alluvium, palustrin e limestones, playa dolostones, mottled palaeosols, vadose and phreati c calcretes, and phreatic dolocretes. Palygorskite was invariably foun d in laminar, massive and honeycomb vadose calcretes and phreatic dolo cretes, as well as in playa dolostones, and only exceptionally in floo dplain siltstones, nodular calcretes and palustrine limestones. Regard less of the facies, palygorskite is associated with smectites. An auth igenic origin, based on the morphology of the fibres and the vertical distribution of clay minerals, is proposed for palygorskites in all of the facies. The simultaneous occurrence of palygorskite in these envi ronments is interpreted as a sign of a stable seasonal semi-arid clima te and low detrital input during a period of low lake level (similar t o 0.5 Myr). The occurrence of authigenic palygorskite in siltstones ar ound the lake was attributed to the rising of the water table, retenti on of the already saturated water in the pore spaces of the sediment, and its subsequent evaporation in a confined environment. A hydrogeolo gical model is proposed for the distribution of palygorskite. The elon gated shape of the Provence basin enhanced the influence of lateral in flow of freshwater during the low lake period which explains the distr ibution of palygorskite in different environments of the Provence basi n as well as spatial distribution of phreatic calcretes and dolocretes . During the high lake level, palygorskite formed in the few remaining ephemeral ponds and on the floodplain along the lake margin.