1 - Introduction

1Over the two last decades, stepped-terraces systems of several tributaries of the Loire River basin have been the subjects of multidisciplinary studies, involving Quaternary geology, prehistory and geochronology. These studies have contributed to reconstructing the history of hydrologic networks covering the last 1.7 Ma in the Creuse Valley, 1.4 Ma in the Cher Valley and 1.1 Ma in the Loir Valley (Despriée et al., 2011). However, the Loire River, France's longest stream, and its fossil stepped terraces system have received limited attention from a geochronological perspective, especially in its middle section, near the city of Orléans. For nearly fifty years, this area has been considered as a key area evidencing the existence in the past of the connection between two rivers leading to the formation of the current Loire valley.

2Since the 1970s, several researchers have proposed the hypothesis of the existence of a Plio-Pleistocene paleo-Loire River flowing northwards, from the Massif Central following a pathway similar to the current Loing and Seine valleys, to reach the English Channel. This hypothesis is supported by the presence of some alluvial sedimentary formations attributed to this paleo-Loire River deposited along this south-north axis. These deposits have been dated by different authors to periods ranging from the Mio-Pliocene to the Lower Pleistocene (Cavelier et al., 1993; Pomerol, 1993; Tourenq & Pomerol, 1995).

3While the existence of such a paleo-river seems to be proven during the Pliocene times (corresponding then to the deposition of the “Sables de Lozère” formation), questions remain about the existence of a Pleistocene paleo-Loire River flowing northwards. This question is mainly due to the scarcity of chronological data available for the Loire's alluvial remnants, both for the fossil alluvial formations following the current river course or on potential older fluvial evidence located along this paleo-river’s course.

4The objective of this thesis was to characterize the altimetry, mineralogical composition and ages of the alluvial deposits remnants of the Loire valley, including ancient alluvial evidence attributed to the “south-north” paleo-Loire River. This research aimed to understand the Pleistocene setting and evolution of this significant French river. The main study area encompassed the region between the cities of Briare, Orleans and Blois in the Centre Val-de-Loire region (fig. 1A). Downstream of Briare, the Loire River begins its westward bend towards the Atlantic. This thesis proposes a first chronology of the establishment of the fluvial system for the Middle Loire area as well as an original reconstitution of the evolution of the Loire system since the Lower Pleistocene in this region.

2 - Methods

5In this work, the extent of the different sedimentary formations was determined through spatial processing using GIS. Their altitude was estimated in relation to the maximum incision under the current Loire alluvial floodplain. Subsequently, these formations were characterized thanks to field and laboratory works and dated. For this purpose, 48 sediment samples were collected from various alluvial remnants of the Paleo-Loire or the Loire rivers and dated by the Electron Spin Resonance (ESR) method. ESR analyses of quartz grains were performed using the multi-centre approach, involving the measurement of both aluminium (Al) and titanium-lithium (Ti-Li) centres (Toyoda et al., 2000; Tissoux et al., 2007; Duval and Guilarte, 2015). Ti-H centers were not considered in regard to the ancient ages expected. In the discussion, this corpus was augmented by 19 samples analysed in previous studies (Liard et al, 2017; Serin-Tuikalepa, 2017; Tissoux, pers. comm.). Heavy minerals from the samples were also extracted and identified using X-ray diffraction (XRD) to specify the geological sources of the material.

3 - Results

6The Loire valley between Briare and Blois features the presence of several stepped alluvial terraces, mainly located on the left bank of the present river's course. Using an innovative semi-predictive mapping approach (Lacquement et al., 2016), we identified eight topographic flat areas, ranging from +10 m rel. to +65 m rel., interpreted as alluvial terraces: T1a, T1b, T2, T3a, T3b, T4a, T4b, and T5. T0a and T0b, part of the current alluvial plain, were not considered in this study.

7In addition, two other ancient formations, potentially unrelated to the (Paleo-)Loire course, were also taken into account:

8- the “Cailloutis Culminants” (culminating gravels, CC) ranging from +59 to + 69 m rel. and located on the current interfluve of the Loire and Seine systems

9- “Fu-pre-Loire” (Fu-pL), an alluvial deposit identified in the current Loing valley but attributed to the Paleo-Loire River in the literature (fig 1A).

10Dating results are presented on figures 1B & 1C. The ages of the culminating gravels indicate a fluvial deposition around ca 0.8 - 1 Ma ago. This formation could belong to either the Loire or the Seine systems, but is observed at a similar elevation than the undated T5 Loire formation (figs. 1A & 1C). The confirmed incision and deposition by the Loire River, flowing westwards, began around ca 700 ka (T4). This age is quite recent compared to the onset of the incision in the tributaries valleys of the Loire River, which occurred several hundred thousand years earlier. Incision then accelerated between 600 ka and 400 ka, resulting in multiple of sub-contemporary sediment depositions at different altitudes. After 400 ka, incision slowed and there was a tendency for the alluvial deposits to become imbricated. These results, compiled along the synthetic transect of the valley (fig. 1B), confirm the dual climatic-tectonic constraint of the development of the terraces.

11The Fu-pL formation appeared to be located at low relative altitude not in agreement with deposition by the Paleo-Loire River (fig. 1C), especially regarding ESR ages ranging between 550 and 300 ka. This formation was certainly deposited by the Loing River, tributary of the Seine River, and is unrelated to the Loire’s history.

12Constructing the longitudinal profile reveals two groups of terraces (fig. 1C). The oldest formations, i.e. the highest (T5, T4a and T4b, and T3b), are characterised by a metamorphic signature in relation to Massif Central sources and exhibit a slight increase in relative altitude to the current base level from upstream to downstream. In contrast, the lowest - and therefore most recent - terraces (T1a, T1b, T2 and T3a), still originating from the Massif Central have volcanic origin and show relative altitudes decreasing from upstream to downstream. This significant difference between the two groups suggests that the history of the highest terraces identified in the study area differs from that of more recent formations and the present-day Loire River. A capture or an abandon event may have occurred between the two deposition groups.

13Unfortunately, the Paleo-Loire deposits couldn’t be directly dated in this work. However, the absence, of deposits older than 800 ka in the Middle Loire valley, while several Loire tributaries started their incision several hundred kiloyears before this date, may suggest that the Loire River was flowing northwards at that time.

14In conclusion, five distinct phases in the evolution of the Middle Loire system can now be described between 800 ka ago and today (fig. 2). These phases could indicate a westward shift of the river course between 900 and 800 ka ago (possibly due to a capture?). Between 800 ka and around  250 ka ago, when the river cut its current alluvial plain, the river migrated northward and underwent changes in dynamics and in source of material.

A/ Map of the alluvial deposits attributed to the middle Loire River between the cities of Briare and Blois (Central France); B/ Synthetic cross-section of the middle Loire river system; C/ Longitudinal Profile of the Loire valley and projection of the fossil terraces

15SERIN‑TUIKALEPA I., 2022 - Évolution du cours de la Loire moyenne au Pléistocène, relation avec la géodynamique et les premiers peuplements du Bassin parisien. Thèse de Doctorat, Muséum national d’Histoire naturelle, Paris, 280 p.