The Drac paleo-valleys: a long-term archive of fluvial dynamics in the periglacial zone of the western French Alps

The evolution of the Alpine mountain belt during the Quaternary is strongly controlled by periodic glaciations and deglaciations. The significant erosion during these glacial/interglacial cycles has left very few sedimentary archives to study the surface dynamics within the mountain belt over the last hundreds of thousands of years. Valleys within the periglacial zone are the best candidates to target long-term geological archives in the Alps because they potentially preserve ancient fluvial deposits that have been preserved from glacial abrasion. The Drac River in the French western Alps preserves the alluvial fills of three generations of paleo-valleys, which were filled in response to glacial damming of the river and subsequently re-incised during glacial retreat. Detailed 3D mapping of the paleo-valleys was carried out to constrain their geometry and reconstruct the evolution of the Drac fluvial profile over time. The age of the fills of the three paleo-valleys was constrained by measuring the luminescence signal of feldspars, targeting sandy intervals within the coarse fluvial deposits. Dating these fills allows to quantitatively constrain the alluviation and incision dynamics of the Drac paleo-valleys. The onset of alluviation of the most recent paleo-valley occurred before the Last Glacial Maximum, between 40 ka and 90 ka BP (MIS 3 -5). The fill of the intermediate paleo-valley is dated to the previous cold period at 134±20 ka BP (MIS 6). Finally, the oldest paleo-valley was filled more than 200 ka ago. The filling periods correspond to the global climatic cooling stages and are much longer than the incision phases, which took place during global warm intervals. The pattern of sedimentary filling implies it is controlled by an increase in sediment flux in the context of glacial advance, while the incision phases are due to rapid base-level lowering linked to the retreat of glaciers damming the Drac basin. Complementary luminescence dating is currently carried out on the terraces, at the tops of the fillings, and will lead to a better understanding of the control of glaciations on the dynamics of alluvial deposits in the periglacial zone.