The tidally-influenced river deposits of the pleistocene Seine system: the example of the Tourville-la-Riviere terrace (NW France)
Résumé
Since the 20th century, the Pleistocene Seine terraces system from Les Andelys to Le Havre was mainly studied using geomorphological tools (Lautridou et al., 1999). The lower Seine Valley was formed by the Plio-Pleistocene fluvial incision and shows presently large enclosed meanders. Few studies focused on fluvial to tidal environment transition during Pleistocene. An understanding of both continental and marine processes at the lower Seine Valley scale is needed to comprehend the stepped terraces system. On the one hand, the Seine River has incised a 125 m-deep and 2-4 km-wide valley. The substratum is composed of Cretaceous chalks with flints overlain by an irregular Cenozoic sedimentary cover (clay-with-flints and sands) and Plio-Pleistocene fluvio-marine facies (Lozere sand Formation and Saint Eustache sand Formation). The braided Pleistocene Seine River flow was able to erode, transport and rework bed material over large distances. On the other hand, alternating glacial and interglacial conditions during the Pleistocene period resulted in large shoreline fluctuations between the coastal watersheds of the Bay of Seine (Seine, Touques, Dives and Orne) and the Channel. During the interglacial periods (i.e., Marine Isotopic Stages 5e, 7 or 11), several marine incursions occurred along different courses of the Seine fluvial system. Consequently, the Pleistocene Seine Valley started to trap silty and sandy sediments supplied by the river.