Unravelling the evolution of the Frébouge polygenetic cone in Val Ferret (Mont Blanc Massif)

Abstract

Proglacial settings in the Alps are typically polygenetic, often characterized by a complex and discontinuous interplay between glacial, fluvial and gravitational processes. These processes yield high volumes of sediments, which usually exceed their transportation capacity. The excessive proglacial sediment load leads to accumulation on slopes, and thus, to subsequent failures such as rock avalanches. The northern slopes of the Ferret and Veny valleys in the Mont Blanc Massif are home to several polygenetic cones and are a stunning field laboratory for the exploration of the interplay between the glacial, fluvial and gravitational processes. This study investigates a well-preserved polygenetic cone, the Frébouge cone, to disentangle the geomorphic processes that contributed to its formation and to reconstruct its evolution. To achieve these goals, detailed field and remote mapping, ¹⁰Be surface exposure dating, and runout modelling with DAN3D^® were used. The geomorphological map revealed complex interactions of glacial, fluvial, debris flow, as well as rock and snow avalanche processes. The established chronology indicates two major episodes of debris flows, the first one at c. 2 ka, and the second at c. 1 ka. In addition, a rock mass with a maximum volume of up to 12±3 Mm³ collapsed in the upper reaches of the cone at 1.3±0.1 ka and overran the cone, travelling more than 100 m up onto the opposite valley slope. Afterwards, the Frébouge Glacier overrode the cone several times leaving moraines and till, reaching its maximum extent c. 300 years ago. This study underscores the untwisting of the complex interaction of surface processes in the Alpine valleys, which are prone to hit the urban areas and infrastructures.