Reconstructing fluvial incision rates based upon palaeo‐water tables in Chalk karst networks along the Seine valley (Normandy, France)

Carole Nehme (1) , Andrew Farrant (2) , Daniel Ballesteros (1) , Dominique Todisco (1) , Joël Rodet (3) , Diana Sahy (2) , J. Michael Grappone (4) , Jean‐claude Staigre (5) , Damase Mouralis (1)

1 IDEES - Identité et Différenciation de l’Espace, de l’Environnement et des Sociétés
2 BGS - British Geological Survey
3 M2C - Morphodynamique Continentale et Côtière
4 University of Liverpool
5 CNEK - Centre normand d'étude du karst et des cavités du sous-sol

Quantifying rates of river incision and continental uplift over Quaternary timescales offer the potential for modelling landscape change due to tectonic and climatic forcing. In many areas, river terraces form datable archives that help constrain the timing and rate of valley incision. However, old river terraces, with high-level deposits, are prone to weathering and often lack datable material. Where valleys are incised through karst areas, caves and sediments can be used to reconstruct the landscape evolution because they can record the elevation of palaeo-water tables and contain preserved datable material. In Normandy (N. France), the Seine River is entrenched into an extensive karstic chalk plateau. Previous estimates of valley incision were hampered by the lack of preserved datable fluvial terraces. A stack of abandoned phreatic cave passages preserved in the sides of the Seine valley can be used to reconstruct the landscape evolution of the region. Combining geomorphological observations, palaeomagnetic and U/Th dating of speleothem and sediments in eight caves along the Lower Seine valley, we have constructed a new age model for cave development and valley incision. Six identified cave levels up to ∼100 m a.s.l. were formed during the last ~1 Ma, coeval with the incision of the Seine River. Passage morphologies indicate that the caves formed in a shallow phreatic/epiphreatic setting, modified by sediment influxes. The valley's maximum age is constrained by the occurrence of late Pliocene marine sand. Palaeomagnetic dating of cave infills indicates that the highest-level caves were being infilled prior to 1.1 Ma. The evidence from the studied caves, complemented by fluvial terrace sequences, indicates that rapid river incision occurred during marine isotope stage (MIS) 28 to 20 (0.8–1 Ma), with maximal rates of ~0.30 m ka−1, dropping to ~0.08 m ka−1 between MIS 20–11 (0.8–0.4 Ma) and 0.05 m ka−1 from MIS 5 to the present time. © 2020 John Wiley & Sons, Ltd.

Domaines: Géographie; Etudes de l'environnement

Carole Nehme, Andrew Farrant, Daniel Ballesteros, Dominique Todisco, Joël Rodet, et al.. Reconstructing fluvial incision rates based upon palaeo‐water tables in Chalk karst networks along the Seine valley (Normandy, France). Earth Surface Processes and Landforms, 2020, 45 (8), pp.1860-1876. ⟨10.1002/esp.4851⟩. ⟨halshs-02572133⟩