L'évolution tardi et postglaciaire des lits fluviaux au nord-est de Paris (France). Relations avec les données paléoenvironnementales et l'impact anthropique sur les versants/The evolution of river beds in the North-East region of Paris (France) during the Lateglacial and theHolocene. Relation to the palaeoenvironments and to the human impact on the slopes

Abstract

Abstract The dynamics of the river beds in the lower sections of the Marne and the Oise rivers are analyzed in relation to the evolution of the slopes during the Lateglacial and the Holocene. The results of the study of the morphosedimentary responses are confirmed by the palaeoenvironmental variations recorded by malacological, palynological and archaeological data. The Lateglacial evolution appears to be tightly linked to the climatic changes. Two main periods of sedimentation can be attributed to the Pleniglacial I Lateglacial transitional phase and to the Younger Dryas; they are separated by an erosion and soil formation phase which may have occurred during the B0lling and АИегфа phases. At the beginning of the Holocene, an important phase of vertical cutting can be allocated to the first half of the Preboreal period ; thereafter river beds tend to be more stable. During the Boreal and the Atlantic phases, the stabilization of the slopes and of the river beds by the vegetation cover plays an important role in the morphogenic evolution. The detrital input remains very low while the river beds are slowly filled in by highly organic material, often dominated by peat formation processes. During the second half of the Atlantic, the various indicators do not show any environmental degradation, but the pollen data show, during the Middle Neolithic times, the expansion of agricultural activities with punctual opening of the landscape and rude- ration. The first episodes of silt sedimentation occur during the second half of the Subboreal, in relation to the erosion of the loess covering the slopes. The river discharges increase during the Bronze Age, thus frequently creating channels which will be filled in during the La Tène period. During the Roman times, the accumulation of sediment in the mean water channels continues in the main valleys; afterwards, the sedimentary dynamics seem to slow down with the decreasing agricultural pressure during the Late Middle Ages. An important erosion crisis occurs during the modern times, marked by an important deposition of silt in the valleys of minor tributaries.