Middle Loire Valley settlement: first chronology using ESR on quartz grains.

Stepped-terraces systems of several tributaries of the Loire River basin were during the two last decades particular objects of multidisciplinary study, involving Quaternary geology, prehistory and geochronology. The Loire River itself, the longest stream of France, and its fossil stepped terraces system have remained little studied until now from the geochronological point of view, particularly in its middle section, near Orléans. This section is however considered for some fifty years as a key area evidencing the existence in the past of the connection of two older rivers leading to the formation of the current Loire valley. Several researchers have hence proposed the hypothesis of the existence of a Plio-Pleistocene Loire paleo-river flowing northward from the Massif Central to the Seine valley and the English Channel. This Loire River was then disconnected from this one and its flowing was orientated to the west from the Blois area until the Atlantic Ocean. The lack of alluvial deposits in the area between the present-day Loire valley and the Seine one, in relation to significant erosion, means that it is not possible to work directly on ancient evidence of the south-north paleo-river. According to the hypothesis that the alluvial terraces preserved in the intermediate sector were put in place after the connection of the two former rivers (capture), the dating of these sedimentary formations would make it possible to determine when this capture took place. Thus, a series of ESR quartz dates have made it possible to establish a chronological model for this Middle Loire. Five different phases in the evolution of the Middle Loire system can now be described, between 800ka and the present day. These phases seem to indicate a gradual capture, beginning between 900 and 800 ka, with changes in fluvial dynamics, leading to the current course of the river around 250 ka ago. [ABSTRACT FROM AUTHOR]