Multi-proxy analysis of boreholes in remolded Quaternary paraglacial deposits (Avignonet landslide, French Western Alps)

Abstract The Avignonet landslide in the Trieves area is made of Quaternary paraglacial sediments among which imbricated till, glacio-lacutrine and fluviatil layers. Numerous boreholes (drillings and corings) were conducted during the mid-1980s and the mid-2000s to assess the geological and geotechnical setting of the inhabited, southern part of this slow-moving earthslide. Heterogeneous paramaters available from these different boreholes make interpretation difficult. Gamma-ray logging along with magnetic susceptibility and pocket vane tests on core samples allowed to distinguish between the different lithological and geotechnical units. Mechanical strength obtained from drilling parameter recording acquired during drilling was correlated to corings and their combination, along with outcrop observations, was used to delineate the lithological units and shear surfaces. The joint analysis of the dataset with outcrop observations and chronological data reveals a high geological complexity with the intrication of various sedimentary units. Two ancient landslides, several tens of metres in width, were furthermore identified within the large Avignonet landslide, almost 2 km in width. All the results point toward a high lithological and hydrogeological complexity that impacts the dynamics of the landslide. This work shows the interest of a multi-proxy approach in studies involving Quaternary sedimentary units. The methodology developed here could be applied to other sites with different palaeoenvironmental contexts initially exhibiting sedimentary compaction, such as lacustrine or coastal environments.