Résumé
Modifications of fluvial systems at the Pleniglacial-Lateglacial transition and during the Holocene: The example of the Somme River basin (northern France). Recent research on Late Glacial and Holocene sequences of the Somme basin are based on large sections of alluvial plains completed by investigation of numerous archeological profiles. Field and bioclimatic data confrontation result in a global vision of the Somme basin valleys evolution between the end of the Weichselian Upper Pleniglacial and the middle of the Holocene. Within this evolution it is possible to identify important changes in fluvial morphology and sedimentation linked to climatic modifications. The first major modification of fluvial morphology is dated from the the Upper Pleniglacial/Lateglacial transition (13 000 BP): valley response to the climatic improvement is marked by an incision and the passage from a braided channel pattern to a transitional system (multi-channel). The first organic deposits infilling these channels are dated from the Bolling interstadial. During the Allerod we observe the deposition of organic silts in a large single channel system. The end of the Lateglacial, is characterized by a very important infilling of the entire valley by fine calcareous silts attributed to the Younger Dryas. These overbank deposits are linked to the overflowing of a single meandering channel. A second major incision phase, in a single channel, is related at the beginning of the Holocene at about 10 000 BP. Then, between the Preboreal and the middle of the Atlantic (10 300 to about 5 500 BP), the entire valley is progressively infilled by a peat bog with two lateral channels where organic silts were deposited. Finally, a last phase of erosion and incision, whose climatic origin is not obvious, appears in the Atlantic between about 6000 and 5500 BP. This last fluvial pattern modification is followed by the deposition of laminated organic silts in a large single channel and by an important calcareous tufa development.