# The Centre-Val de Loire region (France) in relation to the Middle Miocene climatic optimum: a more accurate stratigraphic dating

Adrien de Perthuis, Cyril Gagnaison, Sébastien Potel

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Adrien de Perthuis, Cyril Gagnaison, Sébastien Potel. The Centre-Val de Loire region (France) in relation to the Middle Miocene climatic optimum: a more accurate stratigraphic dating. Jean-Yves Reynaud; Eric Armynot du Châtelet. Short Papers from the 19th French Congress of Sedimentology, Lille, 84, Association des Sédimentologistes Français, pp.27-30, 2025, 2-907205-83-8. 10.70665/NJSM4169. hal-05005325

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Submitted on 1 Apr 2025

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# THE CENTRE-VAL DE LOIRE REGION (FRANCE) IN RELATION TO THE MIDDLE MIOCENE CLIMATIC OPTIMUM: A MORE ACCURATE STRATIGRAPHIC DATING

Adrien DE PERTHUIS, Cyril GAGNAISON and Sébastien POTEL

Institut Polytechnique UniLaSalle UPJV, B2R (GeNumEr), U2R 7511, F-60026 Beauvais cedex France

Corresponding author: adrien.deperthuis@unilasalle.fr

**Abstract:** This study is focusing on eight paleontological sites of the Miocene formations (Aquitanian to Messinian) of the Centre-Val de Loire Region (France). By a multi-approach (sedimentology, geochemistry, palaeontology), the aim is, firstly, to give a more precise age to those Miocene formations; and, secondly, a better understanding of the paleoenvironment evolution in the Loire sedimentary basin during the Miocene, related to the Middle Miocene Climatic Optimum.

**Keywords:** Middle Miocene Climatic Optimum, Paleoenvironment, Palaeontology, Absolute Dating.

## 1 INTRODUCTION

Between -16.9 and -13.8 Ma a hyperthermal event called the Middle Miocene Climatic Optimum (MMCO) took place (Hansen et al. 2013). During the MMCO, the climate in Western Europe was subtropical to semi-arid (Costeur & Legendre 2008), with temperatures 4 to 5 °C higher than today (Hansen et al., 2013). The Centre-Val de Loire region, located south-west of the Paris Basin, was characterised by various depositional environments (lacustrine, fluvial and shallow-marine; Fig. 1) (Mégnien et al. 1980), recorded in 5 Formations encompassing the Miocene and thus providing geological data for the entire MMCO: 1- The Beauce lacustrine limestone formation (Oligocene-Aquitanian), which covers a large part of Beauce and Sologne (Ginsburg, 2000). 2- The Orléanais and Blésois sand and marl formation (Burdigalien-Langhien), which extends between Orléans and Angers, corresponds to a series of terrigenous sediments of fluvio-lacustrine origin. These sediments are channelized by folded plicative East-West tectonic structures inherited from the Alpine orogeny. 3- The Faluns Formation (Langhien), which is a shallow marine lateral equivalent of the Orléans and Blésois sands and marls. It is exposed from the Loire-Atlantique to Blois. Locally, the marine sedimentation continues until the Tortonian (Gagnaison 2020). 4 & 5- The fluvio-lacustrine formation of the Sologne sands and clays (Middle Miocene-Pliocene), which outcrops in the east of the Loire sedimentary basin and grades westwards to the Redonian marine sands formation. The aim of this study is to refine the dating of these different sedimentary formations to characterize the palaeoenvironmental changes associated to the MMCO.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Miocene formations of the Loire valley (Gagnaison 2020, modified). Black stars: sites studied. Beige: Beauce lacustrine limestone (Oligocene-Aquitanian). Red: Fluvial sands and clays (Burdigalian-Pliocene). Yellow: faluns (Langhian-Tortonian).</figcaption></figure>
# 2 MATERIALS AND METHODS

## 2.1 Sites studied

Eight outcrops with geological or palaeontological data that are still accessible have been selected (Fig. 1): Selles-sur-Cher in the Beauce lacustrine limestone formation (Aquitanian); Mauvières, La Guimardière, Pont-Boutard, Tavers and Contres in the Orléanais and Blésois sands and marls formation (Burdigalien-Langhien); Doué-la-Fontaine and Saint-Hilaire-de-Clisson in the Faluns formation (Tortonian). A systematic review of the geological and palaeontological data for these sites is ongoing.

## 2.2 Dating

Previous to this study, the age of the site of Mauvières was determined using absolute dating (U/Pb, U/Th; Monchal *et al.* 2025) on calcite nodules from palaeosols. This method will be applied to other sites if possible. Three sites have been dated using strontium isotope stratigraphy on samples of shark tooth enameloid. The Tavers site has been dated by sequence stratigraphy, based on the maximum flooding of the Langhian (Miller *et al.* 2005).

## 2.3 ATR-FTIR spectroscopy

Several enamel and bone samples from the different study sites have been analysed to assess the secondary recrystallization rates of the samples (Aufort *et al.* 2019).

# 3 RESULTS AND DISCUSSION

## 3.1 Dating

Former absolute ages obtained on the Mauvières site, established the base of the formation of the Orléans and Blésois sands at -19.3 Ma (Monchal *et al.* 2025). In addition to absolute dating, sequence stratigraphy has been used to date the Tavers site at -15.5 Ma (Perthuis *et al.* *accepted*), making it the site closest in time to the Miocene Climatic Optimum (-15.6 Ma; Kochhann *et al.* 2017). At this site, molluscs (Neritidae, Pachychilidae and Dreissenidae; de Perthuis *et al.*, *accepted*) showing a marine influence were found at the top of the conglomerate, which correlates with the maximum marine transgression of the Langhien at -15.5 Ma (Miller *et al.* 2005). Strontium isotope stratigraphy yielded an age of -15.3 Ma for the Contres site and of -9.7 Ma and -7.4 Ma respectively for the Doué-la-Fontaine and Saint-Hilaire-de-Clisson sites. All these dates constitute a new dataset for the geological sites of the Loire depression (Fig. 2), the ages of which was previously known only from the biostratigraphy of continental mammals (Ginsburg 2000).

<figure><img src="image_3.png" /><figcaption>Figure 2. Lithostratigraphic column of the Miocene formations in the Loire valley.</figcaption></figure>
## 3.2 ATR-FTIR spectroscopy

Dental enamel samples do not show secondary recrystallization, unlike bone samples, which, depending on the considered site, show recrystallization between 40 and 50%. Future results from isotopic analyses of dental enamel will therefore not be influenced by secondary recrystallization.

## 4 CONCLUSION

Application of combined dating methods, in particular U/Pb dating and strontium isotope stratigraphy, enabled five sites to be precisely dated between the Lower Miocene and Upper Miocene in the Loire valley. At the same time, a review of the sedimentological and palaeontological data from these sites has been conducted using new field and laboratory data (Perthuis de et al., accepted). The clay mineral composition and a taxonomic review of the fossil remains is in progress. As a first result, ATR-FTIR spectroscopy analyses show that dental enamel of the various sites is not affected by recrystallization. Geochemical analyses (δ¹³C and δ¹⁸O) have been launched on the dental enamel of several groups of mammals (Rhinocerotidae, Suidae and Proboscidea). All these new data will enable a more precise reconstruction of the palaeoenvironments of the various sites studied and to trace their evolution on the scale of the sedimentary basin in direct link with the MMCO. This study could then serve as a methodological example for future studies in other European sedimentary basins affected by rapid temperature changes.

## 5 ACKNOWLEDGMENTS

The authors would like to thank Étienne Balan and Julie Aufort (Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, UMR 7590, Sorbonne Université) for their invaluable help for ATR-FTIR spectroscopy analysis; Sidonie Revillon (SEDISOR) for carrying out the strontium isotope stratigraphy analysis; and the Chaire GéoAgro Énergies (UniLaSalle-Beauvais) for the funding. This paper was reviewed by Victorien Paumard and Sébastien Rohais.

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