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 strong sub- atmospheric alteration processes throughout the basin. A wide range of facies, depositional and weathering, were recognized in an interval of 5 Myr and include floodplain fine-grained alluvium, palustrine limestones, playa dolostones, mottled palaeosols, vadose and phreatic calcretes, and phreatic dolocretes. Palygorskite was invariably found in laminar, massive and honeycomb vadose calcretes and phreatic dolocretes, as well as in playa dolostones, and only exceptionally in floodplain siltstones, nodular calcretes and palustrine limestones. Regardless of the facies, palygorskite is associated with smectites. An authigenic 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 environments is interpreted as a sign of a stable seasonal semi-arid climate and low detrital input during a period of low lake level (-0.5 Myr). The occurrence of authigenic palygorskite in siltstones around the lake was attributed to the rising of the water table, retention of the already saturated water in the pore spaces of the sediment, and its subsequent evaporation in a confined environment. A hydrogeological model is proposed for the distribution of palygorskite. The elongated shape of the Provence basin enhanced the influence of lateral inflow of freshwater during the low lake period which explains the distribution of palygorskite in different environments of the Provence basin 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.