Pleistocene cryostructures and landslide at Petit-Bost (southwestern France, 45°N)

Remnants of a Pleistocene landslide have been observed during field investigations of Quaternary alluvial formations of the Isle River at Petit-Bost, southwestern France. The main features of the landslide-induced deformation give a good indication of its size and depth. The surface of rupture is located in a 0.5 m thick, very plastic mottled clay at the top of an alluvial gravel. The clay is overlain by 1.7 m thick loamy colluvial deposits with a polygonal net of bleached wedges that testify to past permafrost conditions. The mechanical parameters used in stability calculations for the case of an infinite planar sliding surface, that fits well the geomorphological data, are given by triaxial tests. Stability calculation indicates that the slope is very stable at present and that rupture can only have occurred in fully unconsolidated and undrained conditions. This likely happened during permafrost degradation and subsequent thawing of ice-rich soil layers. Oedometric tests indicate post-sliding over-consolidation of the clay. With regards to the debate on permafrost distribution in France during the Pleistocene, this study gives support to a wide extent of permafrost, at least to the latitude 45°N southward, during the coldest periods. It also points out that frost-induced slope instability may have played a major, but still underestimated, role in many French regions north to this latitude.