A rare ammonite assemblage in the Romaniceras ornatissimum Zone (Middle Turonian) from the Uchaux Massif, Vaucluse, France

Abstract

The Uchaux Massif (Vaucluse, SE France) is one of the rare places in Europe where ammonites collecting can be led from a high middle Turonian through Late Turonian. That incited Alcide d'Orbigny in 1852 to quote Uchaux as "the French type of the Turonian stage" to complement the Saumur, Tourtenay, Montrichard and Tours localities mentionned in 1842 when the stage was created. In 2014, the monograph by Robaszynski et al. synthetized and actualized the informations available on the litho- and biostratigraphy of the Turonian from the Uchaux Massif. More recently, a new outcrop at Mondragon, in a place named Prés Morel, exposes the whole carbonate siltstones with Spongia of the Peyras Member (UL3). Were collected : Lewesiceras peramplum (Mantell, 1822), Romaniceras (Romaniceras) marigniacum Amédro & Châtelier, 2020, R. (Yubariceras) ornatissimum (Stoliczka, 1864), R. (Y.) kanei Jones, 1938, Collignoniceras woollgari regulare (Haas, 1946), C. carolinum (Orbigny, 1841), C. papale (Orbigny, 1841), C. turoniense (Orbigny, 1841) and a new species of Collignoniceras : C. rhodanicum Amédro, Baudouin & Delanoy sp. nov. This assemblage, unprecedented in the studied area, is characteristic of the Romaniceras ornatissimum Zone which is the third of the four ammonite zones of the middle Turonian (Tm 3). At the top of this third zone, the surprise was to find the Romaniceras marigniacum bio-horizon defined in 2020 in the southern part of the Paris Basin at the top of the Tuffeau de Bourré and at the base of the Tuffeau Jaune de Touraine. Moreover, we now think that the Peyras Member has to be fully assigned to the Romaniceras ornatissimum Zone. Lastly, the assemblage includes a rich population of Collignoniceras carolinum (24 specimens), the most important known to-day over the world. All these ammonites allow to know the spectrum of their morphological variations and show a dimorphism through the existence of micro- and macroconchs.