Révision des données sédimentologiques et biostratigraphiques des gisements à vertébrés des sables de l'Orléanais, à Beaugency, Tavers et Le Bardon (Miocène Moyen; Loiret, France)

Revision of sedimentological and biostratigraphic data from vertebrate deposits in the “Orléanais sands”, at Beaugency, Tavers and Le Bardon (Middle Miocene; Loiret, France). We present a review of sedimentological, biostratigraphic, and structural data from the vertebrate sites of the Sables de l'Orléanais (Orléans sands Formation) in Beaugency, Tavers, and Le Bardon outcrops (Middle Miocene; Loiret, France). Known since the 19th century, the Middle Miocene sands of the Beaugency, Tavers, and Le Bardon region have yielded remains of large mammals such as Rhinocerotidae and Proboscidea. These deposits have only been sporadically studied and temporally correlated with the upper Orleanian (biozone MN5, c. late Burdigalian-Langhian). However, a comprehensive study of these deposits, integrating geological and paleontological data, has yet to be conducted. Between 2022 and 2023, a data recovery campaign was launched in the field (structural context and taphonomy) and in the collections (paleontology). The geological series include the “Formation du calcaire de Beauce” (Oligocene–Aquitanian; Gigout 1974), the “Formation des sables et marnes de l'Orléanais” (FSMO) from the Middle Miocene (Orleanian, MN5, c. late Burdigalian-Langhian) as well as any surface formations (alluvium, colluvium and plateau silts) from the Quaternary. The understanding of the geometric arrangement of the Cenozoic formations highlights a tectonic division into blocks subsequent to the sedimentation. Two assemblages of fossil vertebrates are presented in this study: 1) the rare presence of Oligocene mammals Ronzotherium romani Kretzoi, 1940 and Palaeogale minuta (Gervais, 1848) (reworked in the FSMO); and 2) the assemblage of Orleanian (in situ in the FSMO) is composed of 66 taxa of vertebrates and 16 taxa of invertebrates typical of the MN5 biozone (Orleanian, c. late Burdigalian-Langhian). Some taxa are represented by well-preserved specimens, as evidenced by the skull of Chelydropsis aff. sansaniensis (Bergounioux, 1935) and the carapace of Mauremys aff. pygolopha (Peters, 1869), respectively, the skulls of the beaver Steneofiber depereti Mayet, 1908, and of the horse Anchitherium aurelianense (Cuvier, 1825) (coll. BG). The occurrence of tropical euryhaline molluscs allows a better understanding of the stratigraphic position of the FSMO during the maximum of the Langhian transgression, corresponding to the Mid-Miocene Climatic Optimum. The faunas of Beaugency, Tavers, and Le Bardon comprise the most diverse large mammal faunas in Europe that seamlessly fit into the European landscape where a gradient of faunal similarity is observed from East to West, as shown by the Dice similarity index. Although rodents are sparsely represented in the localities of the Loire Valley, large mammals, particularly evidenced by the northernmost presence of the rhinocerotids Hispanotherium matritense (de Prado, 1864) and Iberotherium rexmanueli (Antunes & Ginsburg, 1983), exhibit a strong affinity with the Iberian region. Due to its regional geological conditions, exceptional fossiliferous diversity, quality of its fossils, and precise stratigraphic calibration, this coherent set of palaeontological sites deserves to be included in the French national list of geological heritage sites to be preserved.

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