Long-term fluvial archives in NW France: response of the Seine and Somme rivers to tectonic movements, climatic variations and sea-level changes

The Seine and the Somme are the two main rivers flowing from northwestern F rance into the Channel. During the Pleistocene cold stages both rivers were tributaries of the River Manche which was exporting sediments into the cen tral deeps of the Channel. The River Seine has a very well developed terrac e system recording incision that began at around 1 Mn. The same age is prop osed for the beginning of the main incision in the Somme Valley on the basi s of morphostratigraphy, pedostratigraphy, palaeontology, palaeomagnetism a nd ESR datings. The uplift rate deduced from analysis of the Seine and Somm e terrace systems is of 55 to 60 m/Ma since the end of the Lower Pleistocen e. The response of the two rivers to climatic variations, uplift and sea-le vel changes is complex and variable in the different parts of the river cou rses. For example, the evolution of the lower Seine system is influenced by uplift and climate changes but dominated by sea-level changes. In the midd le Seine the system is beyond the impact of sea-level variations and shows a very detailed response to climatic variations during the Middle and Upper Pleistocene in a context of uplift. The Somme Valley response appears to b e more homogeneous, especially in the middle valley, where the terrace syst em shows a regular pattern in which incision occurs at the beginning of eac h glacial period against a general background of uplift. Nevertheless, the lower Somme Valley and the Palaeo-Somme in the Channel area indicate some s trong differences compared with the middle valley: influence of sea-level v ariations and probably differences in rates of tectonic uplift between the Channel and the present continent. The differences in the responses of the two river valleys during the Pleistocene an related to differences in the s ize of the fluvial basins, to the local tectonic characteristics, to the ge ometry of the platform connected to the lower parts of the valleys and to t he hydrodynamic characteristics of each river. Finally, it is shown from th ese examples that the multidisciplinary study of Pleistocene rivers is a ve ry efficient tool for the investigation of neotectonic activity. (C) 2000 E lsevier Science B.V. All rights reserved.