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

Abstract

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.

Stratigraphical influence on chalk cave development in Upper Normandy, France: implications for chalk hydrogeology

“…Cave development is linked to the Quaternary incision of the Seine River, which defines the regional base level. During the last one million years, repeated phases of cave formation, cold-stage valley incision and subsequent terrace aggradation, followed by renewed conduit development has created a stacked sequence of ancient cave levels preserved between 5 and 100 m altitude above current base level (Nehme et al, 2020). Cave development is also influenced by the influx of sediment into the conduit system from the overlying Cenozoic cover (Chédeville et al, 2015).…”

“…Repeated influxes of sediment into phreatic conduits causes dissolution to be focused upwards on the conduit ceiling as the passage floors are mantled with sediment. Over time, this leads to the upwards enlargement of the conduit, a process known as paragenesis (Farrant & Smart, 2011;Nehme et al, 2020), also named per ascensum speleogenesis (Mocochain et al, 2009) or antigravitative erosion (Pasini, 2009). Paragenetic cave development is common in the Normandy chalk caves.…”

“…In general, scallops preserved on the conduit walls indicate that ancient groundwater flowed to the northeast. The passage was largely filled with clay, silt and sand, capped by speleothem and breakdown deposits (Nehme et al, 2020). Part of this sediment fill has been removed, leaving remnants of speleothem false floors in some parts of the system between the Trou du Chien and Jacqueline passages (Fig.…”

“…Over 2 m of sediment is preserved in a section at the southern entrance, with several other 0.5 to 1 m sections elsewhere in the cave. The influx of sediment has led to paragenetic overprinting and continued upwards enlargement of the passage (Nehme et al, 2020).…”

“…Caves formed in the phreatic and epiphreatic zones have a distinctive passage morphology, and past flow directions can be ascertained through the examination of asymmetric dissolutional flow markings known as scallops. The wider geomorphological and hydrogeological setting can be derived from the geometry, spatial and vertical distribution of conduit networks (Nehme et al, 2020).…”