A lee-side cavity fill diamicton under the Durance valley glacier: Insights from hoodoos of the Remollon Arch, Southern French Alps

Deep and narrow gullies dissecting reliefs of the Southern French Alps are filled with 10's to 100's meters thick accumulation of unsorted, boulder-rich muddy material (i.e., diamicton) left behind by glaciers that repeatedly covered the region during Pleistocene glaciations. The modern erosion and removal of this diamicton forms renowned peculiar hoodoo landscapes. Internal stratigraphic architectures in the diamicton show clinothems with topsets unconformably overlying steep, up to 20°, foresets dipping in the inferred down-ice direction. Interpretation of depositional processes reveals subglacial till in topsets, alternations of glaciogenic cohesive debris flows and more turbulent density flows in foresets and occasional glacio-lacustrine muds with ice-rafted debris at the toe of the foresets. We interpret this diamicton as a lee-side cavity fill. In alpine ice caps, ice flow can be accommodated by the deformation of a plastic subglacial till that can be advected in subglacial topographic depressions behind an obstacle made by resistant lithology, i.e., in a pressure shadow zone regarding the ice-flow direction, referred to as lee-side cavity. Lee-side cavity fills are rarely documented in the literature, with existing references limited to subglacial settings beneath ice sheets. To date, no such features have been reported in alpine glacier environments. Recognizing this process expands the range of known subglacial mechanisms active beneath alpine ice caps, offering new insights into how these processes shape modern landscapes.