Alpine debris flows and their sedimentary properties. A case study from the French Alps

Abstract

AbstractDebris‐flow tracks are present at many sites within the French Alps. The vertical zonation (mainly between 1700 m and 2800 m altitude) of these medium‐scale debris flows means that they are active within a mild periglacial environment, where the flows are triggered by high‐intensity summer rainstorms.The present paper describes the grain‐size distribution, particle shape and fabric of some recent to fresh (1 day old) flow deposits. It was found that characteristic differences exist between the different parts of the levees and the terminal lobes. An interpretation of these phenomena is given with regard to processes involved in their origin.It is concluded that the coarse openwork external parts of the debris‐flow levees are formed during the passage of the flow surge(s), and that the matrix‐rich, coarsening upward, channel‐side parts of the levees, as well as the terminal lobes, mainly consist of material transported in the waning phases of the surges.The existence of these sedimentologically different zones within thedeposits of a debris flow that are related to flow morphology has consequences for the recognition of debris‐flow sediments within fossilized deposits. A correct interpretation is only possible if sections allow a sufficiently detailed analysis.