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

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. Knupfer 1967; Serpagli 1967, both using form taxonomy). Sweet & Bergstrom (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 & Bergstrom 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. Bergstrom 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' ('Pelites a 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 Lindstrom & Pelhate (1971) and Paris et al. (1981). A latest Ordovician Hirnantian brachiopod fauna from Camaret (Crozon Peninsula) has also been reported by Melou (1987).

The limited natural outcrop, located 2 km SW of Saint-Hilaire-la-Gerard in the Sees 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 Universita di Modena e Reggio Emilia-IPUM' at the Dipartimento di Scienze Chimiche e Geologiche, Universita degli Studi di Modena e Reggio Emilia, Modena, Italy.

CONODONT FAUNA

All samples produced conodonts. Brachiopods, bryozoans, ostracodes, gastropods and silicified echinoderm fragments were found as well. An unexpectedly abundant collection of conodonts of several thousand elements representing 12 multielement taxa was recovered. Preservation of specimens is highly variable, ranging from well-preserved specimens to more or less encrusted elements. The conodont specimens exhibit a CAI (Colour Alteration Index; cf. Epstein et al. 1977) of 4-5, indicating a heating of 300-400[degrees]C. No significant change in the composition of the conodont fauna through the study material was observed. The following taxa were identified:

Amorphognathus duftonus Rhodes, 1955

Amorphognathus ordovicicus Branson & Mehl, 1933

?Baltoniodus sp.

Dapsilodus mutatus (Branson & Mehl, 1933)

Eocarniodus gracilis (Rhodes, 1955)

Hamarodus brevirameus (Walliser, 1964)

Icriodella cf. superba Rhodes, 1953

Istorinus erectus Knupfer, 1967

Panderodus gracilis (Branson & Mehl, 1933)

Sagittodontina robusta Knupfer, 1967

Scabbardella altipes (Henningsmoen, 1948)

?Walliserodus sp.

Amorphognathus and Scabbardella are dominating the fauna but Hamarodus and Sagittodontina are also common, with the former being abundant in some intervals. Remarkably, the Vaux fauna contains Hamarodus brevirameus, which appears to be absent in the Rosan fauna from Brittany (Lindstrom & Pelhate 1971; Paris et al. 1981). Simple-cone taxa (Dapsilodus, Panderodus) are less frequent. The presence of A. ordovicicus, A. duftonus and Sagittodontina robusta indicates that the fauna represents the A. ordovicicus Zone and is possibly of middle Katian age (Stage Slices 3-4 of Bergstrom et al. 2009). Similar co-occurrences of A. ordovicicus and A. duftonus are also documented in continental Europe from the Carnic Alps (e.g. Serpagli 1967; Bagnoli et al. 1988; Ferretti & Schonlaub 2001), Sardinia (Ferretti & Serpagli 1991, 1999), Poland (Dzik 1999), Spain (Ferretti 1992; Del Moral & Sarmiento 2008) and possibly Bohemia (Ferretti 1998). The Vaux conodont fauna strikingly differs in diversity and composition from coeval middle-upper Katian faunas from the United Kingdom, such as the Sholeshook Limestone from South Wales (Ferretti et al. 2013, 2014a) and the Keisley Limestone of northern England (Rhodes 1955; Bergstrom & Ferretti, in press) and the Portrane Limestone of Ireland (Ferretti et al. 2014b).

The composition of the conodont fauna indicates that it represents the Sagittodontina robusta-Scabbardella altipes biofacies of Sweet & Bergstrom (1984), which is characteristic of this limestone interval in the Upper Ordovician of continental Europe.

doi: 10.3176/earth.2014.21

Acknowledgements. The authors would like to thank M. Robardet for providing detailed information for locating the outcrop, and C. Corradini, S. Gouwie and P. Serventi for their help in the 2006 conodont sampling. We acknowledge useful comments on the manuscript by Guillermo Albanesi and Peep Mannik. M. Tonelli (Centro Interdipartimentale Grandi Strumenti CIGS, Modena, Italy) and M. Barbieri are recognized for the skilled assistance during SEM investigations and figure preparation, respectively. This paper is a contribution to IGCP Project 591 'The Early to Middle Palaeozoic Revolution--Bridging the Gap between the Great Ordovician Biodiversification Event and the Devonian Terrestrial Revolution'.

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Annalisa Ferretti (a), Andrea Messori (a) and Stig M. Bergstrom (b)

(a) Dipartimento di Scienze Chimiche e Geologiche, Universita degli Studi di Modena e Reggio Emilia, 1.go S. Eufemia 19, I-41121 Modena, Italy; ferretti@unimore.it, andrea.alexander.messori@gmail.com

(b) School of Earth Sciences, Division of Earth History, The Ohio State University, 125 S. Oval Mall, Columbus, Ohio 43210, USA; Bergstrom.1@osu.edu

Received 30 June 2014, accepted 11 September 2014
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Author:Ferretti, Annalisa; Messori, Andrea; Bergstrom, Stig M.
Publication:Estonian Journal of Earth Sciences
Article Type:Report
Geographic Code:4EUFR
Date:Dec 1, 2014
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