Middle Loire Valley settlement: First chronology using ESR on quartz grains

Abstract

Over the past two decades, stepped-terrace systems within the Loire River basin's tributaries have undergone comprehensive multidisciplinary studies, encompassing Quaternary geology, prehistory and geochronology. Surprisingly, the geochronological exploration of the Loire River itself, the longest river in France, and its fossil stepped terraces system has been relatively limited, especially in the middle section near Orléans. However, for about fifty years, this region has been pivotal in providing evidence of the historical confluence of two ancient rivers, shaping the current Loire Valley. Researchers have suggested the existence of a Plio-Pleistocene Loire paleo-river, flowing northward from the Massif Central to the Seine valley and eventually reaching the English Channel. Subsequently, this Loire River would have underwent a reorientation of its flow westward, from the Blois area to the Atlantic Ocean, after being disconnected from its previous course.

The absence of alluvial deposits in the area between the current Loire valley and the Seine valley, attributed to substantial erosion, prevents direct exploration of evidence related to the south-north paleo-river. To address this challenge, the hypothesis suggests that the preserved alluvial terraces in the intermediate sector were established post-capture, offering an opportunity to determine when this significant geological event occurred. Through the application of Electron Spin Resonance (ESR) quartz dating, a chronological model for the Middle Loire has been then constructed. This model identifies five distinct phases in the evolution of the Middle Loire system, spanning from 800 ka to the present day. These phases indicate a gradual capture process, initiating between 900 and 800 ka, resulting in alterations to fluvial dynamics and ultimately leading to the establishment of the current course approximately 250 ka.