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 buil ding up below a conveyor belt exclusively by dry grain flows. Stratigr aphy and sedimentological properties were described, Both Series of ob servations allow the development of a new model for stratification dev elopment by dry grain flows, In this model, layers of fine material (i ndicated as the 'sole' of the flows) consist of material that infiltra tes 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 fl ows which thus can reach ever lower positions on the slope. Layers of coarse, openwork clasts showing a well developed coarsening upward arr angement 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. Th is model of stratification development by dry grain flows explains the main characteristics of the relict stratified slope deposits at Beles ten, which seem representative for many such accumulations in the Pyre nees and in the Alps. A condition for the frequent occurrence of dry g rain flows is a continuous recharge of the debris stock at the head of the scree slope. Bedrock very sensitive to frost weathering, giving r elatively small clasts but without producing much very fine material, is most favourable. Dry grain flows are an ubiquitous process, observe d 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 i s however clear that during the cold periods of the Pleistocene and wi thin the context of the mid-latitudes, open landscapes with a strong d ebris production by frost weathering were a favourable environment for the occurrence of dry grain flows.