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

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

La grotte du Funiculaire (Le Mesnil sous Jumièges, Seine-Maritime) Spéléogenèse et étude d'un remplissage ferro-manganique

(Le Mesnil sous Jumièges , Seine-Maritime, France). The Funicular cave is a major cave of the Lower Seine. Completely exca¬ vated 40 years ago by its owner , it presents a spatial organization model of drains for the paleokarsts of the great valley. Among the infillings, we discovered a brown-black deposit with a surprising concentration of 90 % iron and of 7 % manganese , without organic matter or nickel mineral.La grotte du Funiculaire est une cavité majeure de la Basse Seine. Entièrement désobstruée depuis 40 ans par son propriétaire, elle offre une organisation spatiale de ses conduits qui est un modèle pour les "paléokarsts" de la grande vallée. Parmi les remplissages, a été remarqué un dépôt brun-noir composé de 90 % de fer et de 7 % de manganèse, sans matière organique ni nickel.Coquerel Gérard, Lefebvre Dominique, Rodet Joël, Staigre Jean- Claude. La grotte du Funiculaire (Le Mesnil sous Jumièges, Seine-Maritime) Spéléogenèse et étude d'un remplissage ferro-manganique . In: Karstologia : revue de karstologie et de spéléologie physique, n°22, 2e semestre 1993. pp. 35-42