Desilicification of flint in flint clay formations in the western Paris Basin

In NW Europe, chalk areas are generally covered with clay-with-flints (CWF). These formations are made of flints with a clayey or silty-clayey matrix. In the literature devoted to CWF, the flint dissolution during the weathering of chalk was poorly studied. In this study, we provide an estimation of this phenomenon in the western Paris Basin, by comparisons between the flints of CWF and those of chalks based on mineralogy, size, cortex and surface cavities of flints. Our results show that the mineralogy and thickness of flints have not been affected by the weathering. Only the volumes of cortex and surface cavities have increased. These two parametres allow us to quantify the flint dissolution. Flint dissolution is low: the amount of dissolved silica in flints varies between 0.9 and 7.3%. These low values show that weathering of chalk who has led to the creation of CWF has not reached the intensity of a lateritic type weathering, as is the case in the ferralitic soil.