The floodplain of Soucy-les-Grandes-Pieces (Yonne, France): Pluridisciplinary reconstruction of the dynamics of a Middle Pleistocene river.

The floodplain of Soucy-les-Grandes-Pieces (Yonne, France) : Pluridisciplin ary reconstruction of the dynamics of a Middle Pleistocene river. Six Lower Palaeolithic sites within Middle Pleistocene alluvial deposits were recove red from a gravel pit quarried at Soucy in the Yonne valley (France). The a lluvium age is assigned to marine isotopic stages 10 and 9, according to am inochronology and ESR dating. Malacofaunas from the upper sandy and loamy s and formations indicate continental temperate conditions. Reconstruction of the river dynamics is based on the morphology of the deposits, C-M diagram , palaeosol features observed on thin sections and molluscan successions. S everal channels can be distinguished. The western bank of older channel A i s edged by a point bar. Several secondary incisions occur on the eastern si de of channel A. These shallow channels are infilled by sands and gravels i ndicative of high-energetic but intermittent discharges. The incision of th e more recent channel B results from the eastward migration of the river, a t a time of open-wood environment, as registered by molluscan assemblages. Ongoing processes of aggradation of the major river bed allows the construc tion of a levee. Later on, an alluvial greyish soil develops, marking a sta bilization phase, while the forested environment tends to disappear, accord ing to molluscan spectra. Afterwards, alluvial deposition starts again with floodplain deposits. The development of xerophilous molluscs species indic ates open-ground environments which may point out to the climatic degradati on characterizing the end of an interglacial.