BIEFS A SILEX - PERIGLACIAL DEPOSITS OF CLAY AND FLINTS IN THE PARIS BASIN

Clay with flints in the western parr of the Paris Basin occurs in two different morphological contexts: the plateaus and the valley sides. T he geometric study of the deposits and the lithological characterizati on of the clay with flints occurring in the valley sides, named ''bief s a silex'' in France, have enabled a typology of these materials. The typology depends on the slope of the valley sides. The analytical com parison (grain-size distribution, flint content and percentage of brok en surfaces of flints) between the clay with flints of the plateaus an d those of the valley sides (''biefs a silex'') demonstrates that form ations constitute two distinct facies. The clay with flints occurring on the valley sides is equivalent to ''head'' and ''massive grezes'' T his facies results from periglacial reworking of the plateau clay with flints. the reworking process involved solifluction during the down-c utting of the valleys, in Quaternary. The geometry of the ''biefs a si lex'' comprises either a single or double bevel; the latter case requi res a minimum of two main stages of the reworking, which occurred betw een - 800000 and - 20000 years ago.