Carnets de Géologie [Notebooks on Geology]: Letter 3 (CG2012_L03)

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Contents

[1. introduction] [2. The Croagnes section]
[3. Micropaleontology] [4. Carbon isotope stratigraphy]
[5. Conclusion] [Bibliographic references]


A new upper Bedoulian section
in the Aptian stratotypic area:
Croagnes (5 km NW of Gargas, Vaucluse, SE France)

Michel Moullade

Centre de Recherches Micropaléontologiques, Muséum d'Histoire naturelle, 60 Bd Risso, 06000 Nice (France);
Laboratoire de Géologie des Systèmes et des Réservoirs Carbonatés, Aix-Marseille Université, Campus St-Charles, Case 67, 3 Pl. Victor Hugo, 13331 Marseille Cedex 03 (France)

Guy Tronchetti

Laboratoire de Géologie des Systèmes et des Réservoirs Carbonatés, Aix-Marseille Université, Campus St-Charles, Case 67, 3 Pl. Victor Hugo, 13331 Marseille Cedex 03 (France)

Christine Balme

Parc naturel régional du Luberon, 60 place Jean Jaurès, 84400 Apt (France)

Pascal Mauroux

Mairie de Saint-Saturnin-lès-Apt, BP 37, 84490 Saint-Saturnin-lès-Apt (France)

Manuscript online since October 2, 2012
[Editor: Bruno Granier; copy editor: Christian C. Emig; language editor: Donald E. Owen]

Click here to download the PDF version!

Abstract

The discovery and exploitation of a new outcrop of the "A1 Unit of yellow marls and marly limestones" of Leenhardt (1883), comprised between the Urgonian limestones and the "Aptian marls" of the authors, leads to question the generalized attribution of this formation to the Deshayesites grandis ammonite Subzone. Data from this section at Croagnes rather lead to place A1, at least locally in the stratotypic area of the surroundings of Apt, at the level of the onset of the anoxic event OAE1a, just below the Roloboceras hambrovi Subzone. The assumed isochroneity of this Unit is also questioned.

Key Words

Aptian; Bedoulian; stratotype; microbiostratigraphy; carbon isotope stratigraphy; OAE1a.

Citation

Moullade M., Tronchetti G., Balme C. & Mauroux P. (2012).- A new upper Bedoulian section in the Aptian stratotypic area: Croagnes (5 km NW of Gargas, Vaucluse, SE France).- Carnets de Géologie [Notebooks on Geology], Brest, Letter 2012/03 (CG2012_L03), p. 193-199.

Résumé

Une coupe inédite du Bédoulien supérieur dans l'aire stratotypique aptienne : Croagnes (5 km au NW de Gargas, Vaucluse, SE de la France).- La découverte et l'exploitation d'un nouvel affleurement de l' "Unité A1 des marnes et calcaires marneux jaunes" de Leenhardt (1883), comprise entre l'Urgonien et les "marnes aptiennes" des auteurs, conduit à mettre en question l'attribution généralisée de cette formation à la Sous-Zone d'ammonites à Deshayesites grandis. Les données obtenues à Croagnes permettent plutôt de la situer, au moins localement dans le secteur stratotypique des environs d'Apt, au niveau du début de l'événement anoxique OAE1a, juste avant la Sous-Zone à Roloboceras hambrovi, et remettent en question le postulat de son isochronie.

Mots-Clefs

Aptien ; Bédoulien ; stratotype ; microbiostratigraphie ; stratigraphie isotopique du carbone ; OAE1a.


1. Introduction

Leenhardt (1883) was the first to describe in the vicinity of Apt a thin decametric Unit of yellowish marlstones overlying the Urgonian limestones and underlying the grey-blue Aptian marls. This Unit was named "A1" and, on the basis of few ammonites, given an Early Aptian age by the author.

Subsequent studies (Goguel, 1933; Aboussouan, 1963; Roch, 1971; Masse, 1976) of these beds from several outcrops close to localities surrounding the Apt area (Gordes, Murs, Villes, Le Chêne, Joucas) (Fig. 1 ) confirmed their Bedoulian and even more precisely late Bedoulian age. This age assignment was particularly based on the occurrence of the species Deshayesites deshayesi among the ammonites collected in A1.

Fig. 1
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Figure 1: Geographic location of the area of study. Croagnes is located at 43° 93' N, 5° 33' E.

This Unit A1 has recently been briefly mentioned by Masse & Fenerci-Masse (2011) in a paper more extensively devoted to stratigraphic correlations in the upper Barremian-lower Aptian platform carbonates of southeast France. In this work these supra-Urgonian beds are given a still more precise latest Bedoulian age, since they are assigned to the Deshayesites grandis ammonite Subzone (Fig. 2 ).

Micropaleontological and geochemical results that we have obtained by studying a deeply hidden outcrop discovered by one of us (P.M.) close to Croagnes shed a new light on this Unit and in particular led us to question its recent assignment to the D. grandis Subzone. The purpose of this paper is thus to provide a short description of this new outcrop and to present an overview of the first results from its study.

Fig. 2
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Figure 2: Ammonite zonation of the Bedoulian and Aptian (lower part) stages (modified from Dutour, 2005; Ropolo et al., 2006, 2008; Reboulet et al., 2011; Moullade et al., 2011).

2. The Croagnes section

The studied section (Fig. 3 ) is located within the forest of Coste Granière, 500 m NE of the small village of Croagnes (Fig. 1 ). A 13 m thick sequence of marls, superficially yellowish/brownish or, locally, bluish (Fig. 4 ) is exposed there, in a kind of sloped clearing. In depth marls of the first part of the section consist of dark grey, moderately indurated, thin platelets, which become progressively softer and darker up in the series, where they include plant debris. Echinids (Miotoxaster collegnoi) also occur, mostly from the middle part of the section.

This facies and its fossil content are consistent with what is usually described as and ascribed to the A1 Unit. However, direct contacts with the supposedly enclosing beds, respectively the Urgonian limestones, which locally crop out nearby, and the Aptian marls, seen W of Croagnes, cannot be found there. Thus we analyzed samples from this series for micropaleontology and geochemistry, in order to clarify its stratigraphic status.

Fig. 3
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Figure 3: Schematic log of the Croagnes sections.

Fig. 4
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Figure 4: Outcrop of the Croagnes (Coste Granière) section.

3. Micropaleontology

The foraminiferal content of the samples taken from the Croagnes section differs significantly from that (Moullade et al., 2008) of the typical Aptian marls from the same area. Planktonic forms are very rare and low diversified, benthic foraminifers and ostracods are much more predominant. Fragments of echinids, bivalves and other invertebrates are abundant among the >250 µm washed residues. On the other hand, benthic foraminifers include there a significant proportion of very shallow water forms, such as small orbitolinids, miliolids, Arenobulimina, Nezzazzata, Trocholina, Conorboides, Patellina. These taxa are most often absent or, for some of them (Patellina), extremely rare in the typical Aptian marls. Such assemblages lead us to suggest that the deposition of the marls of Croagnes either occurred soon after the demise of the Urgonian platform or, preferably, as indicated by obvious signs of transportation, that these shallow-dwellers came from the close Urgonian platform and were redeposited in slightly deeper periplatform environments.

Fig. 5
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Figure 5: A.- δ13C measurements and first occurrences of microfossil markers in the Croagnes and Clavaillan (base) sections. B.- Isotopic (δ13C) and biostratigraphic records at La Bédoule (LB1 borehole) (modified from Lorenzen et al., 2012).

Among the remaining part of the microfauna from Croagnes, it is of interest to note that three species (Fig. 5 ), which are known as well-calibrated biostratigraphic markers in the stratotypic area of Cassis-La Bédoule (Moullade et al., 1998, 2005), follow a similar global pattern of near simultaneous first appearances in both localities:

In Cassis-La Bédoule the sequence from bed 137 to bed 147 is located just below the lower boundary (bed 148) of the ammonite Roloboceras hambrovi Subzone (Figs. 2 & 5 ) of the D. deshayesi Zone (Ropolo et al., 2006). On the other hand, in a well at La Bédoule (Lorenzen et al., 2012) bed 146 has been correlated with the C3/C4 Menegatti et al. (1998) isotopic stages boundary (Fig. 5 ). Thus, on the basis of these three microfaunal markers, the Croagnes section appears to straddle both the lower boundary of the Roloboceras hambrovi ammonite Subzone and that of the Schackoina (Leupoldina) cabri foraminiferal Zone, as well as the C3/C4 transition.

4. Carbon isotope stratigraphy

In view to better constrain these stratigraphic hypotheses based on correlations with the stratotypic area of Cassis-La Bédoule, isotopic measurements for δ13C were performed on 9 samples from Croagnes at the Leibniz-Laboratory for radiometric Dating and Isotope Research in Kiel (Germany). This set of samples included 8 samples from the Coste Granière section, and one taken from a small isolated outcrop located across the Cemetery of Croagnes; the geometric position, the facies (grey-blue yellowish platy marly limestone) and the composition of the microfaunal content of this outcrop led us to place it slightly below the base of the Coste Granière section, i.e., still closer to the top of the Urgonian.

The measured values and the signature of the plotted curve at Croagnes (Fig. 5 ) appear to be consistent with the middle/upper part of the C3 isotopic stage and its transition towards C4.

As a kind of counterproof, δ13C measurements were also performed on three samples of upper Bedoulian marlstones from the base of the section of Clavaillan (Figs. 1 & 6 ), 2 km WSW of Croagnes, where numerous ammonites of the Deshayesites grandis Subzone have been found (Dutour, 2005; plus our own record). The δ13C values obtained there (Fig. 5 ) are significantly higher than those from Croagnes and very close to those of the C6/C7 transition measured in Cassis (Kuhnt et al., 1998) and La Bédoule (Kuhnt et al., 2011; Lorenzen et al., 2012) at the level of the base of the Deshayesites grandis Subzone.

Fig. 6
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Figure 6: Outcrop of the base of the Clavaillan section, where ammonites of the D. grandis Subzone have been found.

5. Conclusion

Preliminary micropaleontological and isotopic data obtained from a newly discovered "A1 like" section at Croagnes point to ascribe this outcrop to the middle part of the late Bedoulian, around the onset of the OAE1a event. These beds cannot be attributed there to the Deshayesites grandis Subzone, an age recently assigned by Masse & Fenerci-Masse (2011) to all the other known outcrops of the A1 Unit. Our results led also to assume that in the area of Croagnes there is a gap of observation between A1 and younger beds indisputably dated of the D. grandis Subzone, which show the lithological facies and microfaunal content of the typical "Aptian marls". This gap must cover at least the most part of the isotopic stage C4, the entire C5 and most of C6.

However, before reconsidering the global age assignment of Masse & Fenerci-Masse (2011) quoted above, further micropaleontological and isotopic researches ought to be carried out on all the other known outcrops of this Unit, the isochroneity of which is not fully demonstrated yet.

Acknowledgements

We thank Wolfgang Kuhnt and Janne Lorenzen (University of Kiel, Germany) for having taken care of the isotopic measurements proceeded at the Leibniz-Laboratory for radiometric Dating and Isotope Research, and Pierre Ropolo (Marseille) for the determination of the ammonites collected in the Clavaillan section. We also thank Bernard Clavel (Messery), Yves Dutour (Museum of Aix en Provence) and Zdeněk Vašíček (VSB-Technical University Ostrava, Czech Republic) for their constructive reviews of a first version of the manuscript, as well as Donald E. Owen (Lamar University, USA), for his assistance in revising the English text.

Bibliographic references

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