Brachiopodes

RÉÉVALUATION DE LA FAUNE À BRACHIOPODES DU FAMENNIEN SUPÉRIEUR (STRUNIEN) DE LA RÉGION DE BOURBON-LANCY (MORVAN, MASSIF CENTRAL FRANCAIS)

par Marie LEGRAND-BLAIN* et Georges GAND **

* "Tauzia" 216 Cours Général de Gaulle – F 33170 GRADIGNAN France. Publié en février 2005
E-mail : legrandblain@wanadoo.fr
** UMR 5561 Biogéosciences, Centre des Sciences de la Terre, Université de Bourgogne,
6 Bd Gabriel –F 21000 DIJON, France. E-mail$: georges.gand@wanadoo.fr

article paru en français le bull N° 189

Mots-clés : BRACHIOPODES, FAMENNIEN TERMINAL (STRUNIEN), MORVAN (MASSIF CENTRAL, FRANCE).
Keywords : BRACHIOPODA, UPPERMOST FAMENNIAN (STRUNIAN), MORVAN (MASSIF CENTRAL, FRANCE).

Résumé : Un gisement proche de Bourbon-Lancy, découvert par Michel-Lévy (1908), a été revu ; il est daté par les céphalopodes, ostracodes et les trilobites, du Strunien (Dévonien terminal), peu avant l'événement "Hangenberg". Il contient une faunule de brachiopodes ici décrits : Retichonetes (?), Araratella (?) sp., Athyridoid gen. nov. (?), "Rigauxia" gen. nov. (?), Cyrtiopsis (?) sp. Les coquilles, décalcifiées, de petite taille, sont peu abondantes dans des siltstones noirâtres, en environnement anoxique de volcanisme d'arrière-arc. Cette faune de brachiopodes est très différente des associations contemporaines du Strunien néritique.

Abstract : A locality described near Bourbon-Lancy by Michel-Lévy (1908), has been recently revised. It is dated by cephalopods, ostracods and trilobites as Strunian (uppermost Devonian) before the "Hangenberg" event. It yields a brachiopod faunule : Retichonetes (?), Araratella (?) sp., Athyridoid gen. nov. (?), "Rigauxia" gen. nov. (?), Cyrtiopsis (?) sp. The decalcified small shells occur rarely in dark silstones interpreted as an anoxic environment in a volcanic back-arc system. That brachiopod fauna looks strongly different from the contem-poraneous Strunian associations described in neritic environments.

ABRIDGED ENGLISH VERSION

GEOLOGICAL SETTING ; THE LOCALITY "PONT DU VEZON" (Fig. 1)

The Morvan, NE edge of the French Massif Central, lies in the internal Variscan belt. In SW Morvan, siltstones and sandstones, dated Famennian – Tournaisian, are frequently interrupted by volcanic episodes (DELFOUR et al., 1984 – 1990).
MICHEL-LEVY (1908), in an authoritative monograph, described the Famennian "gisement fossilifère du Moulin du Roi". It is located 2,5 km SE of Bourbon-Lancy, along the D 192 road, 30-50 m after the bridge crossing the stream Vezon. The name "Pont du Vezon" is more convenient, since the locality Moulin du Roi lies 500 m to the NE.

Figure 1
The Pont du Vezon locality and section

1-1French Massif Central, Morvan and Boubon-Lancy (BL),
1-2 Location of the "Pont du Vezon" section in regional geology( Delfour & al 1990; geological map Bourbon-Lancy 1/50000 structural scheme slightly modified) with a=granites , b=Famenien volcano-sedimentairy, c=LowerDinantien volcano-sedimentairy, d=mesozoïc and cenozoïc sediments, G=Gilly, SA=Saint-Aubin;
1-3: Location of the"Pont du Vezon" section on the topographic map IGN n° 2727 Est.Molinet 1/25000
1-The"Pont du Vezon" section lithology,field marks and fossis with l(*)=Cephalopods, (=)=Trilobites, (+)=Ostracods,
(O)=Plant fragments , (.) =Brachiopods

The measured Pont du Vezon section (Fig. 1-3, 1-4 ; marks A-H) is continuous along the road, with a dip of 25° S-E. Dark siltstones are interlayered with volcanic beds : cinerite, flows, tuffs. Some 30 m below, the Givallois Andesite (ROBELIN, 1981, DELFOUR et al., 1989), exposed in an ancient quarry W of the road, is a volcanic flow conformably interbedded within the siltstones.
Fossils (MICHEL-LEVY, published 1908 ; G.G. and Société d’Histoire Naturelle d’Autun members, 1975 - 1988 ; M. L-B., 1998-1999) occur scarcely on 16 m thickness of the measured section - indeterminate terrestrial plant fragments ; Ostracods (Determination F. LETHIERS) : Richterina (R.) striatula, R. (R.) costata, Kuzminaella n.sp. A, Bythocyproidea ? sp. ; Trilobites, usually disarticulated (Det. R. FEIST) Dianops sp., Phacops wedekindi, Drevermannia schmidti, Pseudowaribole ; Cephalopods (Determination D. KORN) : Finiclymenia sp., Cymaclymenia involvens and Orthoceratid fragments ; indeterminate Bivalves, Gastropods ; Hyolithids ; Crinoid columns and ossicles ; Brachiopods (this paper) : Retichonetes (?) sp., Araratella (?) sp., Athyridoid gen. nov., "Rigauxia" gen. nov., Cyrtiopsis (?). Palynomorphs are absent (COQUEL, 2001, litt.), so as probably conodonts, not investigated in unfavourable sediments.

BIOSTRATIGRAPHY (TABLE)

Clymenids, trilobites and ostracods characterize the upper Wocklumeria cephalopod zone = lower-middle praesulcata conodont zone, below the "Hangenberg event" (LEGRAND-BLAIN et al., 2000) = uppermost Famennian, according to the Devonian/Carboniferous boundary GSSP (FEIST et al., 2000) and recent biostratigraphic papers (CHLUPAC et al., 2000 ; GROOS-UFFENORDE et al., 2000).

SEDIMENTARY PALAEOENVIRONMENTAL DATA.

In a siltstone locality below the Givallois andesite, current-ripple microcycles were deposited under weak currents, in intertidal to subtidal dysoxic bottoms (ROBELIN, 1981). At Pont-du-Vezon, the siltstones are devoid of ripple-marks. The presence of pelagic faunas might indicate a temporary deepening of the sea bottom. Plant fragments give evidence of a neighbouring emerged land. The ostracod assemblage, Entomozoid ecozone, characterizes dysaerobic environments. In the whole area, geochemical data give evidence of permanent volcanic contribution (DELFOUR et al., 1984) : the volcanic flows came from a center located at Chizeuil, 7 km East of Pont-du-Vezon

THE BRACHIOPOD FAUNA

In the Pont-du-Vezon section, brachiopods are rare, decalcified : external-internal moulds, usually isolated valves. All genera either have a strictly Devonian range, especially Araratella described from uppermost Devonian in Eurasia (BRICE et al., 2000 ; SARTENAER & PLODOWSKI, 2003), or may be new. If compared to Strunian brachiopods from standard platform environments (CONIL et al., 1986), the present assemblage differs by : its low diversity ; the probable absence of Productoids ; the minute size of Spiriferoids, "Rigauxia" gen. nov. (?) attaining maturity at smaller size than usual.

FIGURE 2 : Brachiopods of Pont-du-Vezon section,. Bars = 2 mm on all figures, except Fig. 2.13 : bar = 1 mm. 2.1 : Retichonetes (?) sp. V.v. interior, ML 1081 d 1 ; 2.2 - 2.7 : Athyridoid gen. nov. (?) ; 2.2 : D.v. internal mould crushed against V.v. ; faint radial markings on anterior left border, AML MR b 1 ; 2.3 : Counter-part of the same specimen, V.v. beak and dental plates internal mould, AML MR B 1 bis ; .2.4. : D.v. internal mould, G.1 A1 ; 2.5 : counter-part of the same specimen, V.v, G.1 A 2 ; 2.6 : D.v. mould, median septum, cardinal plate (?), visceral foramen, R 206 ; 2.7 : D.v. internal mould crushed against V.v, R 203 ; 2.8 : Araratella (?) sp. D.v. internal mould crushed against V.v., R 201 ; 2.9 - 2.13 : Cyrtiopsis sp. ; 2.9, 2.10 : V.v. internal mould, ventral view and interarea, R 205 ; 2.11, 2.13 : V.v. external mould, macro and micro-ornament, ML 1101 a 1 ; 2. 12 : internal mould of the same specimen, ML 1101 a 1 bis ; 2.14 - 2.18 Rigauxia gen. nov. (?) ; 2. 14 : V .v. internal mould, anterior growth lamellae, ML 1081 e ; 2.15 - 2.17 : V.v. internal mould ; 2.15 profile ; 2.16 : interarea, lamellose pseudo-deldidial plate inserted on the delthyrial edge ; 2.17 : posterior view, R 204 ; 2.18: V.v. external mould of the same specimen, R 207.

Why is the Pont du Vezon assemblage so peculiar ? According to brachiopod literature, soft dysoxic substrates (MARTINEZ-CHACON &t al., 2003) and volcanic environments (LOCKLEY, 1990, ZEZINA et al., 2004) are notably difficult environments. The associated cephalopods and ostracods look more normal in size and associations, probably because of their vagile mode of life. The Pont du Vezon brachiopods differ totally from the lower-middle Tournaisian faunas, described from other Morvan localities (LEGRAND-BLAIN, 1997). They are presently described in open nomenclature, because of their rarity and poor preservation. They are deposited in different places : Paris VI University for AML = Michel-Levy collection ; and the Autun Natural History Museum for MLB = Legrand-Blain, G = Genot, R = Robbe collections.