Carnets de Géologie [Notebooks on Geology]: Article 2012/06 (CG2012_A06)

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Contents

[1. Introduction] [2. Description of the thuoux section proposed as the GSSP of Oxfordian stage]
[3. The Callovian-Oxfordian boundary and the fossil record]
[4. Satisfaction of geological requirements for Thuoux GSSP proposal]
[5. Correlations and comparisons with the alternative candidate GSSPs]
[Bibliographic references]


Proposal for the Thuoux section as a candidate for
the GSSP of the base of the Oxfordian stage

Dominique Fortwengler*

Le clos des Vignes, F-26160 La Begude de Mazenc (France)

Didier Marchand*

8a, avenue Ste Claire, F-06100 Nice (France)

Alain Bonnot*

12, rue des Vergers, F-21121 Hauteville-les-Dijon (France)

Rémi Jardat

29, rue Sainte Colombe, F-94800 Villejuif (France)

Daniel Raynaud

7, rue Albert Ier, F-94120 Fontenay-sous-Bois (France)

* Chercheurs libres au Centre des Sciences de la Terre, Université de Bourgogne, 6 boulevard Gabriel, 21000 Dijon (France)

Manuscript online since 14 July 2012
[Editor: Bruno Granier; copy editor: Christian C. Emig; language editor: Stephen Carey]

Click here to download the PDF version!

Abstract

The Thuoux section, located in South-Eastern Basin of France (coordinates: 44°30'55"E; 5°42'25"N), is a section that satisfies numerous demanding criteria as reference section (GSSP) for the base of the Oxfordian stage. Sedimentation was continuous in that the abundant ammonitic fauna yields no detectable hiatuses. The stratigraphic boundary is located between the Lamberti Zone and the Mariae Zone or more precisely between the Paucicostatum horizon (Marchand, 1979) and the Thuouxensis horizon (Fortwengler & Marchand, 1994a). In this section, there is a perfect mixing between Boreal ammonites (Cardioceratinae) whose species are used as stratigraphic markers and Submediterranean/Subboreal ammonites (Hecticoceratinae, Peltoceratinae and Perisphinctinae) that provide further possibilities for wide correlation of this boundary. Finally, parallel ammonite zonations have been established with great  precision (biohorizons and sometimes "sub-biohorizons") in various areas of France, with different palaeoenvironments. The Thuoux section permits correlations with central and eastern Europe as well as North America, based on Cardioceratinae successions, and with South America, using Peltoceratinae. Thickness of the Callovian-Oxfordian transitional beds in the Thuoux section has allowed detailed sedimentological studies and astronomical calibration of the Lower Oxfordian (Boulila, 2008; Boulila et al., 2008). The Thuoux section is located at the centre of a set of more than thirty sections where the base of the Oxfordian stage is visible (Fortwengler, 1989; Fortwengler & Marchand, 1994a, b, c, d).

Key Words

Ammonites; upper Callovian; lower Oxfordian; GSSP Oxfordian.

Citation

Fortwengler D., Marchand D., Bonnot A., Jardat R. & Raynaud D. (2012).- Proposal for the Thuoux section as a candidate for the GSSP of the base of the Oxfordian stage.- Carnets de Géologie [Notebooks on Geology], Brest, Article 2012/06 (CG2012_A06), p. 117-136.

Résumé

Proposition de candidature de la coupe de Thuoux (France) pour le GSSP de la base de l'étage Oxfordien.- La coupe de Thuoux, localisée en France dans le Bassin du Sud-Est,  (coordonnées : 44°30'55"E; 5°42'25"N) satisfait à la majorité des critères  demandés pour devenir le GSSP de la base de l'étage Oxfordien. La sédimentation est continue à l'échelle de l'horizon ammonitique et en particulier des horizons à Paucicostatum (Marchand, 1979) et à Thuouxensis (Fortwengler & Marchand, 1994a). Dans cette coupe, il y a un mélange constant entre les ammonites boréales (Cardioceratinae) et les ammonites subboréales et sub-méditérranéennes (Hecticoceratinae, Peltoceratinae et Perisphinctinae) ce qui autorise de larges corrélations géographiques. De plus, la présence simultanée de ces 4 sous-familles permet un découpage temporel très précis et permet aussi de proposer des corrélations temporelles entre l'Europe Occidentale, le nord de l'Amérique (grâce aux Cardioceratinae) et l'Amérique du Sud (grâce aux Peltoceratinae). La forte épaisseur des sédiments à Thuoux a aussi permis des études sédimentologiques et une calibration astronomique de l'Oxfordien inférieur (Boulila, 2008 ; Boulila et al., 2008). Enfin, plus d'une trentaine de coupes ont été analysées en détail autour de Thuoux, ce qui augmente la fiabilité des résultats obtenus sur la coupe où la base de l'Oxfordien est visible (Fortwengler, 1989 ; Fortwengler & Marchand, 1994a, b, c, d).

Mots-Clefs

Ammonites ; Callovien supérieur ; Oxfordien inférieur ; GSSP Oxfordien.


1. Introduction

The Thuoux and Savournon sections were proposed as candidates for Global Boundary Stratotype Section and Point (GSSP) of the Oxfordian Stage more than 15 years ago (Fortwengler & Marchand, 1994a, b, c, d). Both these sections were assessed as excellent insomuch as in 1996 (in Meléndez) a vote by Oxfordian Working Group yielded the following results:

Subsequently, we have focused our efforts on the Thuoux section which is located about 600m N-E from Thuoux (close to Aspremont and Saint-Pierre-d'Argençon), 5km west of Aspres and 7km north of Serres (Département des Hautes-Alpes, France). Coordinates: 44°30'55"E; 5°42'25"N (Figs. 1 - 2 ).

Fig. 01
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Figure 1: Location of the Thuoux section proposed as GSSP Oxfordian.
© www.geoportail.fr & IGN - Institut Géographique National, 73 avenue de Paris, F-94165 Saint-Mandé Cedex (France)

The Thuoux section, in contrast to Savournon, is not disturbed by faults and provides readier recognition of the units described below. Moreover, the gentler slope of the Thuoux section precludes strong mixing of faunas as well as pollution of the section by elements coming from the upper layers.

Fig. 02
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Figure 2: The Thuoux section with the position of the Callovian / Oxfordian boundary (C: Callovian and O: Oxfordian).

Meléndez (2004, 2006) expressed doubts about choosing Thuoux "because of preservation of ammonites as pyritic nuclei" or "the poor state of preservation of ammonites, as small pyritic nuclei, making them difficult to interpret". As a consequence, in the table p. 18 of Meléndez (2006), ammonites from Redcliff are assessed (5) against Thuoux (1). This is surprising as Thuoux and Savournon ammonites are preserved in a virtually identical way (Fortwengler & Marchand, 1994c, d). At both Thuoux and Savournon, the fossils are carbonate internal moulds corresponding either to the phragmocone alone, or including part or all of the body chamber. Never are the ammonites pyritic. In both sections, the average diameter of ammonites is about 5 cm.

2. Description of the Thuoux section proposed as the GSSP of 0xfordian stage

In the South-Eastern Basin of France a very thick marly limestone series bearing white and carbonate nodules crops out. The succession is known under the name of "Terres Noires", a term which also denotes its facies. This unit ranges from upper Bajocian to middle Oxfordian (Antecedens Subzone). At the Thuoux section, the stratigraphic interval from the upper Callovian (Lamberti Zone, Lamberti Subzone) to the lower Oxfordian (Mariae Zone, uppermost Praecordatum Subzone) is well preserved and almost 200m in thickness (Fig. 3 ).

Fig. 03
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Figure 3: Log (after Emilia Huret) and biostratigraphy (D. Fortwengler and D. Marchand) of the "Thuoux" section.

This study was initiated by Fortwengler during the 1980s, resulting in 1989 in the first publication in which detailed subdivision of this section was established. That biostratigraphical framework was later updated (Fortwengler & Marchand, 1994a; Fortwengler et al., 1997) and officially accepted by the Groupe Français d'Étude du Jurassique (Cariou et al., 1997). At present, it is accepted by all Western European colleagues and notably by Chapman in Weymouth (1996, 1997, 1999) and Jardat (2010) in the French Jura. After more than 25 years' research, the Callovian/Oxfordian transition has been observed in the South-Eastern Basin in 61 sections (Fig. 4 ), and in thirty of them exhaustive in situ collection of ammonites has been carried out, in order to determine precisely the limits of every horizon to analyse the ammonite populations (Fortwengler, 1989; Marchand et al., 1990; Fortwengler et al., 1997). More than 5,000 ammonites were collected close to the upper Callovian/lower Oxfordian boundary. It is particularly important to note that in all these collections, from all levels, Phylloceratidae and especially Sowerbyceras, are always present and abundant.

Fig. 04
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Figure 4: Locality map of the 61 outcrops in the "Terres Noires" Formation of the Southern-Eastern Basin of France where the Callovian-Oxfordian boundary is recognized.

At the levels 5B, 6A-6B, 7 and 8A (field numbers) which correspond to the Callovian/Oxfordian transition (Fortwengler, 1989), ammonitids are more abundant than phylloceratids, whereas from levels 8B/9 (uppermost Scarburgense biohorizon and lowermost Woodhamense biohorizon) we record the reverse. The latter is interpreted as due to the occurrence of extensional tectonics correlated with deepening and so with increase of phylloceratids. This phenomenon is known from numerous regions of Western Europe (Marchand, 1986).

Faunal comparisons were effected between the Boreal and Tethyan provinces, thanks to very precise collections made on the Isle of Skye (Turner, 1963; Sykes, 1975; Morton & Hudson, 1995) and around Nice (Dardeau & Marchand, 1979), and in between in Yorkshire, Dorset, the margins and centre of Paris Basin (Agence nationale pour la gestion des déchets radioactifs, ANDRA drillings), the Jura, the South-Eastern Basin of France, and several European countries (Germany, Bulgaria, Poland and Rumania with the help of colleagues in these countries).

Among all the Western European sections (see references), the Thuoux section is clearly the most accessible, the most interesting and the most reliable. It begins with upper Callovian (Lamberti Zone) and ends with lower Oxfordian (slightly below the Mariae Zone/Cordatum Zone boundary) deposits. This stratigraphic interval is continuous and, quite rarely for the "Terres Noires", free of faults. Moreover, sedimentary layers are sub-vertical, which makes thickness measurements easy. The occurrence of thin, slightly more carbonate-rich levels leads, through differential erosion, to smooth recesses between carbonate-rich intervals, which facilitates the collecting of ammonites in situ. There are also reddish carbonate intervals and concentrations of aligned grey to ochre nodules that constitute highly visible and very reliable markers. In these sections, ammonites are frequent, and commonly of greater size than in Savournon, with shell diameter above 10 cm. Counts made by D.F. and D.M on their collections from Thuoux are as follows (Fortwengler & Marchand, 1994d):

Collected macro-invertebrates are low in diversity as ammonites frequently reach 100% of the macrofauna. Other cephalopods are present but belemnite rostra and rhyncholithes are scarce, and nautilids extremely rare. Bivalves are not frequent although sometimes they are concentrated inside ammonite body chambers (levels 7B and 9). Brachiopods and echinoderms are totally absent at Thuoux. And what is true at Thuoux is true within all the "Terres Noires" of the Dauphinois Basin. 

3. The Callovian-Oxfordian boundary and the fossil record (Figs. 4 - 5 )

a. Ammonites (Marchand, 1979; Fortwengler & Marchand, 1994a, b, c, d)

Uppermost Callovian ammonites (Lamberti Zone, Lamberti Subzone, Lamberti and Paucicostatum biohorizons).

Plate 1
Scale 1

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Plate 1: 1. Alligaticeras aff. alligatum (Leckenby) (M). Level 5B, Lamberti Zone, Lamberti Subzone, Lamberti horizon, n° DF 1741: Thuoux. 2. Poculisphinctes poculum (Leckenby) (M). Level 5B, Lamberti Zone, Lamberti Subzone, Lamberti horizon, n° DF 4588: Thuoux. 3. Quenstedtoceras lamberti (Sowerby) (M). Level 5B, Lamberti Zone, Lamberti Subzone, Lamberti horizon, n° DF 2584: Thuoux. 4. Kosmoceras duncani (Sowerby) in Badaluta, 1976 (fig. 6-2) (M). Level 5B, Lamberti Zone, Lamberti Subzone, Lamberti horizon, n° DF 18698: Thuoux. 5. Hecticoceras (Orbignyceras) paulowi (de Tsytovitch) (M). Level 5B, Lamberti Zone, Lamberti Subzone, Lamberti horizon, n° DF 2467: Thuoux. 6. Hecticoceras (Putealiceras) punctatum Lahusen (m). Level 6A, Lamberti Zone, Lamberti Subzone, Paucicostatum horizon, n° DF 1376: Thuoux. 7. Hecticoceras (Putealiceras) punctatum Lahusen (M). Level 6A, Lamberti Zone, Lamberti Subzone, Paucicostatum horizon, n° DF 2503: Thuoux. 8. Cardioceras paucicostatum Lange (M ?). Level 6A, Lamberti Zone, Lamberti Subzone, Paucicostatum horizon, n° DF 2469: Thuoux. 9. Hecticoceras (Lunuloceras) pseudopunctatum Lahusen (M ?). Level 6A, Lamberti Zone, Lamberti Subzone, Paucicostatum horizon, n° DF 2482: Thuoux. 10. Poculisphinctes sp. aff. poculum (Leckenby) (m). Level 6A, Lamberti Zone, Lamberti Subzone, Paucicostatum horizon, n° DF 18082: Thuoux. 11. Hecticoceras (Lunuloceras) pseudopunctatum Lahusen (m). Level 6B, Lamberti Zone, Lamberti Subzone, Paucicostatum horizon, n° DF 2558: Thuoux. 12. Cardioceras aff. paucicostatum Lange (m ?). Level 6B, Lamberti Zone, Lamberti Subzone, Paucicostatum horizon, n° DF 2530, Thuoux. 13. Peltoceratoides eugenii (Raspail) (m). Level 6B, Lamberti Zone, Lamberti Subzone, Paucicostatum horizon, n° DF 18098, Thuoux. 14. Peltoceratoides eugenii (Raspail) (M). Level 6B, Lamberti Zone, Lamberti Subzone, Paucicostatum horizon, n° DF 2573: Thuoux.
All specimens are from D. Fortwengler's collection. Photos: D. Fortwengler; preparation of plate: Simone Dutour. Red point: end of phragmocone.

Note that the genera Kosmoceras and Distichoceras, clearly present in the last horizon of the Lamberti Subzone (5B), have never been collected in the Paucicostatum horizon (level 6) of Western Europe. Hecticoceratins (about 50% of fauna) abound but still display "Callovian" morphology (Pl. 1 : 6, 7, 9, 11). A quite rare species, Hecticoceras coelatum Coquand, is characterized by ventral ribs that are continuous around the ventral axis. In contrast, in the specimens of H. coelatum Coquand, from the Lamberti biohorizon (5B), these ribs never cross the ventral axis. In every French record of this species, transition from one morphology to the other quite reliably indicates the Callovian / Oxfordian boundary.

Within this biohorizon the history of the two important genera also diverges:

The combination of these two genus lineages (Cardioceras and Peltoceratoides) permit a very precise and reliable biostratigraphy.

Basal Oxfordian ammonites (Mariae Zone, Scarburgense Subzone, Thuouxensis, Scarburgense and Woodhamense biohorizons) (Pls. 2 - 3 ).

Plate 2
Scale 2

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Plate 2: 1. Peltoceratoides aff. schrœderi (Prieser) (M). Level 6A, Lamberti Zone, Lamberti Subzone, Paucicostatum horizon, n° DF 2655: Thuoux. 2. Cardioceras aff. paucicostatum (Young & Bird) (M). Level 7, Mariae Zone, Scarburgense Subzone, Thuouxensis horizon, n° DF 1365: Thuoux. 3. Hecticoceras (Brightia) thuouxensis Fortwengler & Marchand (m). Level 7, Mariae Zone, Scarburgense Subzone, Thuouxensis horizon, n° DF 2605: Thuoux. 4. Hecticoceras (Brightia) thuouxensis Fortwengler & Marchand (M). Level 7, Mariae Zone, Scarburgense Subzone, Thuouxensis horizon, n° DF 2311: Thuoux. 5. Hecticoceras (Brightia) thuouxensis Fortwengler & Marchand (m). Level 7, Mariae Zone, Scarburgense Subzone, Thuouxensis horizon, n° DF 18156: Thuoux. 6. Euaspidoceras armatum (de Loriol) (M). Level 7, Mariae Zone, Scarburgense Subzone, Thuouxensis horizon, n° DF 1457: Thuoux. 7 a, b. Peltoceratoides eugenii (Raspail) morphe eugenii Raspail (M). Level 7, Mariae Zone, Scarburgense Subzone, Thuouxensis horizon, n° DF 1666: Thuoux. 8 a, b, c. Peltoceratoides eugenii (Raspail) morphe eugenii Raspail (M). Level 7, Mariae Zone, Scarburgense Subzone, Thuouxensis horizon, n° DF 2655: Thuoux. 9 a, b, c. Hecticoceras coelatum (Coquand). Level 8A, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, n° DF 1617: Thuoux. 10. Cardioceras aff. scarburgense Young & Bird (M). Level 8A, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, n° DF 18751: Thuoux. 11. Cardioceras aff. scarburgense Young & Bird (m?). Level 8A, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, n° DF 18752: Thuoux.
All specimens are from D. Fortwengler's collection. Photos: D. Fortwengler; preparation of plate: Simone Dutour. Magnification: black scale bar = 1 cm, except fig. 9c: red scale bar = 1.5 cm. Red point: end of phragmocone.

"Les Richiers" section, ammonites are characterized by occurrence (up to 30% of fauna), of a distinctive Hecticoceratinae taxon: Hecticoceras (Brightia) thuouxensis Fortwengler et al. (1997) (Pl. 2 : 3, 4, 5). This index species is unknown below (in Paucicostatum horizon) and becomes rare at the base of the Scarburgense biohorizon.

The Thuouxensis biohorizon was first described in the South-Eastern Basin, where it was identified in more than 30 sections. Recently it was also described in the Paris Basin from ANDRA drillings (Thierry et al., 2006). Throughout the Thuouxensis biohorizon, the bifurcation points of Peltoceratoides always lie in the upper half of the lateral flank (Bonnot et al., 1997, 2002) (Pl. 2 : 7, 8). Further, among Cardioceratinae, the paucicostatum morph remains present and the scarburgense morph (Pl. 2 : 2) makes its first appearance.

This level corresponds to the Scarburgense biohorizon. The index species is easy to recognize (Pl. 2 : 10-11; Pl. 3 : 1-3) even though some individuals, mainly among the biggest, might be confused with C. paucicostatum Lange. This stratigraphic level is also characterized by individuals belonging to Hecticoceras coelatum species (Pl. 2 : 9), whose ribs pass through the venter, Eochetoceras villersensis (M) and E. hersilia (m) (d'Orbigny) (Pl. 3 : 12).

In the sub-Tethyan Province, this Scarburgense biohorizon is also characterized by Hecticoceras with less pronounced sculpture such as H. chatillonense (Pl. 3 : 7, 11) mainly in 8A (lower part), and H. socini (in Gygi, 1990), mainly in 8B (upper part).

Peltoceratins from level 8B (Pl. 3 : 8, 9) display abrupt bifurcation points at or near the umbilical seam (Bonnot et al., 1997). At this level, Taramelliceras is slightly more frequent (Pl. 3 : 14a, 14b).

Plate 3
Scale 3

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Plate 3: 1 a, b. Cardioceras aff. scarburgense Young & Bird (M). Level 8A, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, lower part, n° DF 3880: Thuoux. 2. Cardioceras aff. scarburgense Young & Bird (m). Level 8A, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, lower part, n° DF 3883: Thuoux. 3 a, b. Cardioceras scarburgense Young & Bird (M). Level 8A, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, lower part, n° DF 3881: Thuoux. 4. Hecticoceras (Brightia) sp. (m). Level 8A, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, lower part, n° DF 1608: Thuoux. 5. Cardioceras scarburgense Young & Bird (M). Level 8A, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, lower part, n° DF 18419: Thuoux. 6. Hecticoceras (Brightia) cf. thuouxensis Fortwengler & Marchand (M?). Level 8A, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, lower part, n° DF 18493: Thuoux. 7. Hecticoceras (Brightia) chatillonense de Loriol (M). Level 8A, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, lower part, n° DF 18767: Thuoux 8. Peltoceratoides eugenii (Raspail) morphe eugenii Raspail (M). Level 8A, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, lower part, n° DF 3717: Thuoux. 9. Peltoceratoides athletoides (Lahusen) morphe athletoides Lahusen (M). Level 8B, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, middle part, n° DF 1944: Thuoux. 10. Peltoceratoides athletoides (Lahusen) morphe athletoides Lahusen (M). Level 8B, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, middle part, n° DF 18563: Thuoux. 11. Hecticoceras (Brightia) chatillonense de Loriol (M). Level 8B, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, middle part, n° DF 1875: Thuoux. 12. Eochetoceras villersensis (d'Orbigny) (m). Level 8B, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, middle part, n° DF 1385: Thuoux. 13. Cardioceras scarburgense (Young & Bird) (M?). Level 8B, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, middle part, n° DF 1659: Thuoux. 14 a, b. Taramelliceras episcopalis (de Loriol). Level 8B, Mariae Zone, Scarburgense Subzone, Scarburgense horizon, middle part, n° DF 19536: Thuoux.
All specimens are from D. Fortwengler's collection. Photos: D. Fortwengler; preparation of plate: Simone Dutour. Red point: end of phragmocone.

At Thuoux, this level corresponds to the Scarburgense biohorizon (upper part). Abruptly, ammonites are less frequent and Cardioceras is very scarce. Records of Eochetoceras villersensis (d'Orbigny) and E. hersilia (d'Orbigny) at the base of that level, as in the French Jura (Jardat, 2010), and the disappearance of Hecticoceras coelatum Coquand are typical of that level. From this stratigraphic level, Phylloceratina become more abundant than Ammonitina, due to a well-known deepening in numerous sections in Western Europe (Dardeau et al., 1994). In other sections, very similar to that investigated at Thuoux, the top of this stratigraphic level has yielded some Cardioceras close to the species C. woodhamense Arkell (Fortwengler & Marchand, 1994a; Marchand et al., nearing completion). C. woodhamense Arkell definitively appears above the Scarburgense horizon and not below, as claimed by Callomon (1993) and Page et al. (2006, 2009a, b).

Though ammonites are very rare, faunal assemblages indicate the Woodhamense horizon and the upper part of the basal Praecordatum Subzone.

In conclusion, the correlation potential of ammonites collected from the Thuoux section is extremely precise (Fig. 5 ).

Fig. 05
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Figure 5: Vertical range of the major species of ammonites found in the "Terres Noires" Formation at the Callovian-Oxfordian boundary (NB: 5B to 10A are field subdivisions).

In this paper, subdivisions proposed for the Thuoux GSSP are the same proposed in 1997 by Thierry et al. for the Callovian and Cariou et al. for the Oxfordian. They are known in all Western Europe and have been used without difficulty in the United Kingdom, Germany, Switzerland and Poland (Marchand & Tarkowski, 1990), and only on the basis of Peltoceratins in Bulgaria and Rumania where Cardioceratins are unknown. Correlations may be also possible:

b. Foraminifera

On certain samples, Jutson (in Poulsen & Jutson, 1996) wrote: "Thuoux foraminifers and nannoplankton in the studied interval were low in diversity but moderate in abundance. Preservation was, in general, poor. Only two species recorded were stratigraphically significant". These two species are Ophthalmidium compressum Ostenfeld and O strumosum (Gümbel); O. compressum disappears at the Thuouxensis biohorizon where it is replaced by O. strumosum, the latter appearing in the Paucicostatum biohorizon and persisting into the Scarburgense biohorizon.

c. Dinoflagellates

This study was carried out by N.E. Poulsen from sampling performed in 1994 during the 4th Oxfordian and Kimmeridgian Working Group Meeting. Results can be summarized as follows. "Most of the stratigraphical important species are present in the assemblages" (citations from Poulsen in Poulsen & Jutson, 1996). Comparison of the range charts for the Savournon and Thuoux sections shows that important species occur in both sections. Moreover, Poulsen (in Poulsen & Jutson, 1996) confirms that Durotrigia filapicata Gocht disappears at the top of the Callovian whereas Wanaea fimbriata Sargeant appears at the base of the Oxfordian, but is very rare. These results are in accordance with what is known "in North Sea Region and in East Greenland". Poulsen (in Poulsen & Jutson, 1996) also noted that "the preservation of the palynomorphs in the Thuoux section is poor and the number of cysts is low" and that "in the Savournon section dinoflagellate cysts are excellently preserved and present in large numbers".

d. Ostracodes

Preliminary research shows they seem to be rare but present at Savournon (Tesakova, 2008).

e. Isotope stratigraphy

This may be feasible from rare belemnite rostra.

f. Magnetostratigraphy

All samples are negatively remagnetized (B. Galbrun, oral communication).

4. Satisfaction of geological requirements for Thuoux GSSP proposal

The Thuoux section fulfills numerous requirements proposed in the Guidelines of the ICS, i.e., International Commission on Stratigraphy (Remane et al., 1996).

a. Geological requirements

NB: On the basis of biostratigraphic data, the sedimentation rate is 6 to 20 times higher than in Weymouth, UK.

b. Biostratigraphic requirements

c. Other methods

d. Faunal populations

e. Other requirements

5. Correlations and comparisons with the alternative candidate GSSPs

a. Redcliff Point, Weymouth, Dorset (UK)

This section, well analysed by Page et al. (2006, 2009a), has six shortcomings, here addressed in order of decreasing importance. 

* The first disadvantage is clearly acknowledged by the British authors themselves: the section is not reliably exposed. "For some time, the exposure (Redcliff Point) has been covered by talus and vegetation, but recent coastal erosion has now re-exposed a Callovian-Oxfordian boundary sequence" (in Page, 1994a, b). "The extent and quality of the exposure at Ham Cliff varies from season to season and from year to year, depending on the height of the shingle storm beach and cliff erosion" (in Meléndez, 2003: 30, p. 17). "The section has been obscured by slumping in recent years" (in Meléndez, 2004: 31, p. 9). "Correlation between these descriptions by different authors has been difficult due to the variable condition of the exposure and the non persistence of various beds taken as marker horizons" (in Page et al., 2006). "The soft nature of the mudrock outcrop makes the placing of a permanent marker problematic" (in Page et al., 2009b, p. 107). But Page et al. (2009b, p. 107) wrote in conclusion that "although the area is subject to contemporary landslipping, continuous cross-boundary sequences remain available."

We therefore spent many hours at this section with Chapman in 2003 and we (Fortwengler & Marchand) confirm that it is very difficult, even for Callovian / Oxfordian boundary specialists, to recognise the lithological succession described by Page et al. (2009a, b: fig. 2, p. 105). Callomon (in Callomon & Coope, 1993) had said about the Ham Cliff section -close to Redcliff Point- (quoted by G. Meléndez: 2003) "The reason why it had been neglected since Arkell's days was that it was usually obscured by slumping and shingle. I remember looking at it in 1956 and actually finding the Lamberti Zone there, in a small outcrop above the shingle. But all I learned was that it confirmed Arkell's report of an occurrence which had been ignored previously. Exceptional storm conditions in the winter of 1991-1992 swept it all clear, giving a fine exposure". And Callomon (in Callomon & Coope, 1993) described an exceptionally clean section at Redcliff Point where C. woodhamense Arkell occurs below C. scarburgense (Young & Bird). Unfortunately, Page et al. (2009a) have accepted that mistaken succession.

* The second drawback is palaeontological: the faunal assemblage is poorly preserved, with rather low diversity. The assemblage has not been illustrated, except for the Cardioceratins (1 plate). It should be emphasized that Cardioceratins predominate at Redcliff Point, but are "typically crushed" (though not always the inner whorls). This makes observation of the venter very difficult so that no fully reliable determination is possible. In addition, Hecticoceratinae are rare and the absence (in Page et al., 2009a) of Hecticoceras (Brightia) thuouxensis Fortwengler & Marchand does not allow recognition of the Thuouxensis biohorizon. However, this species is present close to Redcliff Point: it has been figured by Chapman (1999: Pl. 7, fig. F) in association with H. (B.) chatillonense de Loriol (Chapman, 1999: Pl. 7, figs. G-H). H. (B.) thuouxensis was also collected in Paris Basin boreholes (Thierry et al., 2006) and in the Jura (Jardat, 2010). It is not a rare species at that level. Finally, individuals belonging to the genus Peltoceratoides, which is present though not abundant, were not used by Page and collaborators as biostratigraphic markers even though they are chronologically as precise as Cardioceras at the Callovian / Oxfordian boundary in the South-Eastern Basin of France (Fortwengler et al., 1997; Bonnot et al., 1997, 2002).

* The third disadvantage concerns ammonite systematics. The Page et al. (2009a, b) proposal relies on a dubious determination of Cardioceratinae species as well as an erroneous phylogeny.

(i) About scarburgense (Young & Bird) and woodhamense Arkell (Fortwengler & Marchand, 1994b; Marchand et al., nearing completion).

  1. C. scarburgense (Young & Bird) does not yet have an individualized keel, and shows no proverse ribs making a chevron (V-shape) on the venter; 
  2. In contrast, the species C. woodhamense, erected by Arkell in 1939, incorporates an apomorphy that characterizes every later Cardioceras species: the presence of a keel or, more precisely, of primary and secondary ribs elevated on the ventral axis, forming a fairly prominent chevron. In lateral view, C. woodhamense shows a rather crenulated venter, which never occurs in C. scarburgense.

(ii) About Woodham Brick Pit Cardioceratinae and about C. redcliffense (Marchand et al., nearing completion).

  1. Cardioceratinae from the Woodham Brick Pit, pictured by Arkell under the following names: Q. mariae, Q. omphaloides, Q. aff. williamsoni (all with undulating ribs in inner whorls like the holotype of C. woodhamense Arkell), correspond to the "thick" and "medium thick" morphotypes of C. woodhamense Arkell.
  2. As consequence: the 157 C. scarburgense not pictured by Arkell are simply, from a biological point of view, the "thin morph" of C. woodhamense Arkell.

So our conclusion is that, at Woodham Brick Pit, there is a lacuna in place of the Scarburgense biohorizon because the Woodhamense biohorizon (levels A and B of Arkell, 1939) lies directly on the Upper Callovian (stratigraphic level C, Lamberti biohorizon, Arkell, 1939).
Remark: The same lacuna at the Scarburgense biohorizon is also known in Normandy (Raynaud, oral communication).

In summary, Cardioceras redcliffense (Page et al., 2009a) looks like C. paucicostatum Lange except for some individuals which are similar to C. scarburgense (Young & Bird), such as the holotype (Page et al., 2009a: Pl. 1D). No variants are close to C. woodhamense Arkell.

(iii) About the succession of ammonite assemblages in Western Europe (Marchand, 1986; Marchand et al., nearing completion).

Arkell (1939) exhaustively collected 899 ammonites from levels A and B at Woodham Brick Pit. The collection consists of Cardioceratinae (n = 297; 33% of the total assemblage), Hecticoceratinae (N = 102; 11.3 %), Perisphinctinae (n = 73; 8.1 %), Euaspidoceratinae (n = 44; 4.9 %) and also g. Taramelliceras (n= 281; 31.2 %) and Creniceras (n = 102; 11.3%).

In the French Jura (Jardat, 2010), as well as in the Swiss Jura (Gygi, 1990), Taramelliceras and Creniceras are always scarce in association with C. scarburgense, but always abundant with C. woodhamense, collected above. A similar succession occurs in Weymouth at Redcliff Point. Chapman (1999: Pl. 9) has pictured individuals that belong to C. woodhamense Arkell, in particular, the individual of his fig. U, which is very similar to Arkell's holotype. These C. woodhamense are associated with Taramelliceras sp. and C. renggeri, as at Woodham Brick Pit. These results are very clearly synthesized by Chapman (1999) in his fig. 10, p. 95.

In conclusion, the succession of biohorizons at Redcliff Point is actually: C. paucicostatum -> C. scarburgense -> C. woodhamense, instead of C. paucicostatum -> C. redcliffense -> C. woodhamense -> C. scarburgense.

* The fourth drawback is connected with sedimentation.

At Redcliff point, clays representing the youngest Callovian biohorizon and the oldest Oxfordian biohorizon are no thicker than 50 cm (Page et al., 2009a: fig. 1). Thus, the following sentence is surprising: "Although the biohorizons are thinner, the Redcliff Point sections currently offer the potential for higher resolution than has been described for SE France" (Page et al., 2009a).

It is particularly surprising since, at Thuoux, the thickness of the same horizons (levels 6B and 7) reaches about 20m, which is 40 times thicker than at Redcliff. However, the three palaeontologists that have worked at Thuoux were unable to identify within those 20 m more than two geographically widespread biohorizons, although the ammonite assemblages there are more diverse.

* The fifth drawback concerns the thickness of the section.

The small thickness observed at Redcliff Point makes any cyclostratigraphic analysis dubious. By contrast, such an analysis may be made in twenty sections of the South-Eastern Basin where precise biostratigraphic control is available. The Thuoux and Saint-Pierre-d'Argençon sections, for instance, could be sampled every 8 cm. 

* The sixth drawback concerns correlations with the Boreal Domain and the Pacific Province.

In the Redcliff Point GSSP proposal (p. 90 - 91), Page et al. (2009a) have written about the "bioprovincial context" in general, without any attempt to correlate the various biostratigraphic schemes with each other.

For us, this is problematic because every GSSP must be, by definition, a reference point for any available biostratigraphic schemes. That is the true aim of a GSSP.

* Appendix: The case of Peyrale (Savournon, near Serres, Hautes-Alpes) proposed as GSSP auxiliary section.

Since 2003, Meléndez has considered that the Peyrale section located close to Savournon "displays some particular features making it a suitable candidate section for Oxfordian GSSP". In 2007, in a new paper by Meléndez, Atrops and Page (2007: fig. 3), these authors proposed this section as an "auxiliary section" of the Redcliff Point GSSP. The first 10m are attributed to the "Lamberti biozone" (levels 10 to 21) and the last 4-5 m (levels 19-21) yielded C. paucicostatum Lange (= Q. paucicostatum in Page et al., 2009a). As in the Redcliff section, Meléndez, Atrops and Page recognized three levels in the Paucicostatum horizon, which are from lower to upper:

These authors place the "base of the Oxfordian stage at the base of the level 23 coinciding with the first recording of some specimens of Cardioceras of the group C. redcliffense Page et al., a new form displaying clear intermediate features between C. paucicostatum-gamma and C. scarburgense (Young & Bird)".

At this juncture, two points should be made. In our study of the Peyrale/Savournon section (Fortwengler & Marchand, 1994d) we were able to state that:

Final remark: the Woodhamense biohorizon, which at Redcliff Point (Page et al., 2009a) lies between the Redcliffense and Scarburgense biohorizons, vanishes in the Peyrale section. Since we know the Peyrale / Savournon section very well (Fortwengler & Marchand, 1994b), we are aware that the Woodhamense biohorizon is present at Savournon. However we collected that species more than 25 m above the Paucicostatum horizon. As everywhere else, the Woodhamense biohorizon lies above the Scarburgense horizon, and not below.

b. Dubki section: Russia

Interesting information on the Callovian / Oxfordian boundary comes from our Russian colleagues and a short summary follows. According to Kiselev and Rogov (2004, 2005) and Kiselev et al. (2006), the Callovian / Oxfordian boundary is readily identifiable at Dubki, Saratov (52nd parallel). Thus, in 2006, they proposed the Dubki section as a candidate GSSP for the Lower Oxfordian boundary. We wish to address certain points related to this proposal. The Lamberti Zone (Upper Callovian), 4.60m thick, is subdivided into 2 subzones (Henrici and Lamberti) and 5 biohorizons (Henrici, Praelamberti, Lamberti, Mojarowskii and Paucicostatum).

The Mariae Zone, 2.60m thick, is subdivided into two subzones (Scarburgense, Praecordatum) and 3 biohorizons (Scarburgense, Alphacordatum and Praecordatum). In Russia, the genus Taramelliceras and the species Creniceras renggeri occur quite late (Bukowskii Subzone). Latest Kosmoceras disappears at the top of the Mojarowskii biohorizon (with the species K. mojarowskii), whereas the last occurrence of the genus Quenstedtoceras is registered slightly higher, at the Callovian / Oxfordian boundary, consistent with Western Europe.

Concerning ostracods, the species Infacythere dulcis disappears at the Callovian / Oxfordian boundary whereas Nophrecythere oxfordiana first occurs slightly above that boundary (Tesakova, 2008). The same level is readily recognized from foraminifera which are represented by diverse and abundant benthic taxa. The Callovian / Oxfordian boundary coincides with the boundary between the Lenticulina tunida and Epistomina elschankensis zones and the Ophthalmidium sagittum and Epistomina volgensis zones. This section has yielded good palaeomagnetic results based on reversals, except at the Callovian / Oxfordian boundary.

In 2004, Kiselev and Rogov had proposed an identical zonation for the Lamberti Subzone. In contrast, their proposed subdivision of the Mariae Zone differs slightly from our proposal.

At the base is the Scarburgense biohorizon from which they record two species, Vertumniceras luppovi (Amannijasov) and Sublunoceras deperditum (Rollier), that are absent from assemblages in France, the United Kingdom and Switzerland. However, they have accepted the Woodhamense biohorizon as proposed by Fortwengler and Marchand in 1994b. This is reasonable as we regard the individual of Pl. 2: 17 of the former authors (Orenbourg area: in Kiselev & Rogov, 2004) as being very close to C. woodhamense var. normandiana. Kiselev and Rogov (2004) consider that the Praecordatum Subzone begins with an unnamed horizon that yields Cardioceras praecordatum and Peltoceras sp., overlain by the Renggeri horizon that yields Taramelliceras (Richeiceras), Creniceras, Peltoceras sp. and Cardioceras cf. alphacordatum. Their pictures of Q. henrici, Q. praelamberti, Q. lamberti, C. scarburgense, C. praemartini, C. alphacordatum, C. praecordatum and C. bukowski seem to us to be correctly identified except for C. costicardia (their fig. 11/12), C. cordatum (their fig. 13/14) and C. paucicostatum (their fig. 15/16). Species found in France and the United Kingdom have been shown to be present in Russia also, which makes correlation possible, as Kniazev (1975) had already demonstrated.

The importance of the Dubki section is thus well established, in particular, due to mixing of "Boreal" and "Tethyan" faunas, which make long-distance correlations possible. Noticeable is that this faunal mixing, as frequently in Western Europe, occurs near the boundary between the Scarburgense and Praecordatum biohorizons whenever the palaeoenvironment was sufficiently open.

It is important to note that some species recorded at Dubki have been identified from other deposits by Rogov and Egorov (2003) and Kiselev and Rogov (2005). We are in agreement with almost all the proposed names. Accurate correlation with Western Europe is now possible with the help of these excellent papers.

c. Our proposals for correlation outside of Europe

* Argentina (biochronological interpretation: A. Bonnot)

In "Biostratigraphy of the transect Chacay Melehue-Sierra de Reyes, Argentina", Parent (1998) considered the dimorph pair Peltoceras gr. constantii (M) / arduennense (m) typical of the Cordatum Zone (lower part). The proposed occurrence of Prososphinctes gr. mazuricus / claramontanus and Prososphinctes gr. mairei / matheyi confirms that interpretation. By contrast Mirosphinctes gr. syriacus / regularis and Euaspidoceras cf. kobyi would rather suggest the Mariae Zone, and some individuals collected at Rahueco are similar to those described from Mount Hermon (Syria) for the lowermost Oxfordian. In our opinion, many specimens from Syria are very close to Eochetoceras villersensis / hersilia, a dimorph species that characterizes the top of the Lamberti Subzone up to the base of the Woodhamense biohorizon (Raynaud et al., nearing completion) in France and the United Kingdom. 

Later, in 2006, Parent compared ammonite successions of the Middle Callovian to Upper Oxfordian in both Neuquén-Mendoza and Tarapaca basins. Near to the Callovian-Oxfordian boundary he recognized:

* Chile (biochronological interpretation: A. Bonnot).

Hillebrandt and Groschke (1995) have described a biostratigraphic succession based on different species of the genus Peltoceratoides. They considered the Primus Zone to be equivalent to the Lamberti Zone, and Dimorphus and Eugenii zones equivalent to the Mariae and Cordatum zones.

As has been pointed out by Bonnot et al. (2002), "if we roll out the hypothesis of pronounced endemism … the Chilean forms described … can all be ascribed either to Peltoceratoides eugenii (d'Orbigny) or to Peltoceratoides athletoides (Lahusen)". As a consequence: their "Eugenii Zone" must be correlated with the Athletoides Subzone of the Mediterranean province = Scarburgense Subzone of Western Europe.

* North America (biochronological proposition: D. Marchand & D. Fortwengler).

Reeside (1919) figured ammonites from Wyoming (Sundance Formation) and from Montana (Ellis Formation). In these regions, he recognized the genus Quenstedtoceras and 5 species. In our opinion, only Q. collieri (from the Ellis Formation) can be a true Questedtoceras. The other species (of the Sundance Formation) seem to have a small keel and are, probably, true Cardioceras. Reeside (1919) distinguished three groups of cardioceratids, for us, from the Mariae Zone:

  1. Cardioceras albanense, C. crassum, C. latum, and C. russelli, Quenstedtoceras hoveyi, Q. subtumidum and Q. suspectum. These "species" have morphological affinities with European fauna of the Scarburgense Subzone;
  2. Cardioceras americanum, C. auraroense, C. bellefourchense, C. crookense, C. plattense, C. obtusum and C. wyomingense. These "species" have morphological affinities with the European fauna from the base of the Praecordatum Subzone;
  3. Cardioceras alaskense (but not Pl. 7, figs. 1-3), C. cordiforme, C. distans, C. haresi, C. hyatti, C. lilloetense, C. martini, C. schuberti, C. spiniferum, C. stantoni and C. whitfieldi have morphological affinities with the European fauna from the upper part of the Praecordatum Subzone.

* "Pacific area part" (Bonnot et al., 2002)

Peltoceratoides williamsoni (Phillips) and allied species are found almost worldwide, and correlations can be envisaged between Western Europe and the Indo-Madagascar domain + Indonesia + Japan, where the genus Peltoceratoides is known.

Acknowledgements

We acknowledge the valuable comments on the initial draft by Raymond Enay, the careful linguistic improvement provided by the Language Editor, Stephen Carey, and the detailed remarks of the Referees of this paper, Nicol Morton, Federico Oloriz and Mikhail Rogov, who contributed to improve the scientific impact of this publication. Last but not least our thanks also go to the Copy Editor, Christian C. Emig, and the Editor, Bruno Granier.

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