Composition and significance of the Katian ( Upper Ordovician ) conodont fauna of the Vaux Limestone ( Calcaire des Vaux ) in Normandy , France

Study of new conodont collections from the Vaux Limestone exposed at its classical locality at Saint-Hilaire-la-Gérard in the Sées syncline, the conodont fauna of which was previously described in a preliminary way by Weyant et al. (1977, Un épisode calcaire ashgillien dans lest du Massif armoricain; incidence sur lâge des dépôts glacio-marins fini-ordoviciens. Comptes Rendus de l'Académie des Sciences, Paris, 284, Série D, 11471149), has provided significant new information about Late Ordovician conodonts from Normandy. A more precise age of this formation has been established based on an unexpectedly abundant conodont fauna of low diversity. Representatives of Amorphognathus, Hamarodus, Sagittodontina, Scabbardella and Eocarniodus are present and the fauna is referable to the middle Katianlower Hirnantian A. ordovicicus Zone. Our study, the first illustrating Ordovician conodonts from Normandy, shows that the fauna is closely similar to middle Katian faunas from other parts of continental Europe and represents the Sagittodontina robustaScabbardella altipes biofacies of the Mediterranean


INTRODUCTION
The Late Ordovician limestone unit exposed locally in continental Europe has long been known as being the only significant calcareous deposit in Ordovician successions otherwise dominated by siliciclastic sediments.Its thickness varies from a few centimetres (e.g. in Germany : Ferretti & Barnes 1997) to some dozens of metres or more (e.g. in Spain: Del Moral & Sarmiento 2008).Shelly fossils such as trilobites and brachiopods, echinoderms and bryozoans are generally abundant.Conodonts have been reported as well from several localities (e.g.Knüpfer 1967;Serpagli 1967, both using form taxonomy).Sweet & Bergström (1984) made the first attempt of a global assessment of the conodont fauna from continental Europe identifying a specific Mediterranean Province, distinct from the approximately coeval British and Baltoscandic provinces, and documented there two different biofacies: the dominant Sagittodontina robusta-Scabbardella altipes biofacies (Spain, northwestern France and Thuringia) and the Hamarodus brevirameus-Dapsilodus mutatus-Scabbardella altipes (HDS) biofacies (Carnic Alps).Sardinia was later assigned to the same HDS biofacies (Ferretti & Serpagli 1999), suggesting that this area and the Carnic Alps in Katian time occupied a lower latitude as the same biofacies had been reported in the British and Baltoscandic provinces (Sweet & Bergström 1984).More recently, several papers have been published establishing the age of these European faunas in terms of the new global classification of the Ordovician System (e.g.Bergström et al. 2009).For a review of these studies, see Ferretti et al. (2014a).
The purpose of this paper is to briefly describe, discuss and establish the age of a new conodont fauna that has recently been recovered from the Vaux Limestone ('Calcaire des Vaux') in southern Normandy, northern France.

GEOLOGICAL SETTING
Although Ordovician and Silurian rocks are well represented in France, they are predominantly developed in clastic facies with only rare limestone occurrences.Different palaeogeographic regions have been documented.Particularly significant is the Armorican Massif in northwestern France (Fig. 1A), as the Palaeozoic series exposed there played a crucial role in the sedimentary and faunal evolution of the Gondwana margin (Vidal et al. 2011).
As regards the Upper Ordovician exposed in southern Normandy, based on conodonts and other fossils, the Vaux Limestone as well as limestone olistoliths in the overlying 'Tillite de Feugueroles' ('Pélites à fragments') were dated as Ashgillian (Weyant et al. 1977), an age later confirmed by chitinozoans (Babin et al. 1988).Conodonts were reported in form taxonomy, but no description or illustration of the elements was provided.The 'Calcaire de Rosan' (Rosan Formation) in Brittany, which traditionally has been regarded as coeval with the Vaux Limestone (Fig. 2), has also yielded a conodont fauna described by Lindström & Pelhate (1971) and Paris et al. (1981).A latest Ordovician Hirnantian brachiopod fauna from Camaret (Crozon Peninsula) has also been reported by Mélou (1987).The limited natural outcrop, located 2 km SW of Saint-Hilaire-la-Gérard in the Sées syncline (Fig. 1B) and described earlier by Weyant et al. (1977), was restudied and sampled for conodonts during fieldwork in 2006 and 2007.A calcareous succession is exposed there in a discarded quarry that is now part of the Normandie-Maine Regional Natural Park.It represents a single 1-m-thick bed that has been faulted and repeated several times in the diverse spots of the little outcrop, with erosion surfaces present both below and above the Vaux Limestone, indicating gaps in the succession (Avoine et al. 2011).The unit consists of hard massive micritic limestone without any distinct macrofossils exposed.The dominant microfacies in the conodont-productive intervals consists of a bryozoan-echinoderm packstone with subordinate trilobites, gastropods and brachiopods.A distinctive iron ooid horizon was also located in a single exposure.This ferruginous level represents a widespread and recurrent time-specific facies known from the Upper Ordovician as well as from other specific intervals in the Palaeozoic (Brett et al. 2012).
A total of 13 levels were sampled and more than 90 kg of limestone was processed in formic acid using standard methods of conodont extraction.All samples were productive and yielded abundant conodonts.Lithological samples, residues and conodont elements are housed in the 'Inventario Paleontologia Università di Modena e Reggio Emilia-IPUM' at the Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia, Modena, Italy.

Fig. 1 .
Fig. 1.A, geological sketch of the Armorican Massif (redrawn after Vidal et al. 2011).Stippling indicates Palaeozoic units, horizontal lines represent Mesozoic units, white refers to Proterozoic and Cadomian units and to igneous Variscan units.The black star indicates the investigated outcrop in the Sées syncline.B, C, sketch-map showing the location of the studied section about 2 km SW of Saint-Hilaire-la-Gérard (B) and generalized map (C) of the Vaux Limestone locality in Normandy (northern France).