ROLE OF DRY GRAIN FLOWS IN THE FORMATION OF A CERTAIN TYPE OF STRATIFIED SCREE

Experiments were carried out in a gravel pit near Belesten (Pyrenees, France) with artificially triggered dry grain flows. In a quarry near Remollon (French Alps) a man-made scree cone was studied which is building up below a conveyor belt exclusively by dry grain flows. Stratigraphy and sedimentological properties were described, Both Series of observations allow the development of a new model for stratification development by dry grain flows, In this model, layers of fine material (indicated as the 'sole' of the flows) consist of material that infiltrates rapidly towards the base of a flow of openwork clasts. The flow is in a dilated condition because of multiple interparticle collisions. Once formed, the 'sole' acts as a slip surface for subsequent grain flows which thus can reach ever lower positions on the slope. Layers of coarse, openwork clasts showing a well developed coarsening upward arrangement together with a marked upslope imbrication correspond to the terminal lobes of the flows. The lobes are gradually fossilizing their slip planes by a process of accumulation in the upslope direction. This model of stratification development by dry grain flows explains the main characteristics of the relict stratified slope deposits at Belesten, which seem representative for many such accumulations in the Pyrenees and in the Alps. A condition for the frequent occurrence of dry grain flows is a continuous recharge of the debris stock at the head of the scree slope. Bedrock very sensitive to frost weathering, giving relatively small clasts but without producing much very fine material, is most favourable. Dry grain flows are an ubiquitous process, observed in warm and dry regions as well as in the temperate and periglacial zones. As such, relict stratified slope deposits with predominance of dry grain flows thus do not have any palaeoclimatic significance. It is however clear that during the cold periods of the Pleistocene and within the context of the mid-latitudes, open landscapes with a strong debris production by frost weathering were a favourable environment for the occurrence of dry grain flows.