Résumé
La succession cambrienne du versant sud de la Montagne noire (France) dépasse 2 000 m d’épaisseur. Elle est composée de quatre grands lithosomes : le premier et le troisième à caractère silicoclastique, le deuxième et le quatrième à caractère mixte carbonaté-silicoclastique. Le premier lithosome (épais de plus de 1 000 m) correspond à la Formation de Marcory, ensemble gréso-pélitique dans lequel s’intercalent des niveaux micro-conglomératiques et plus rarement conglomératiques. Le deuxième lithosome (épais de 400 m) comprend, de bas en haut, les Formations de Pardailhan, de Lastours, du Pont de Poussarou, de la Tanque et de Coulouma. Il est composé de calcaires et dolomies interstratifiés dans des grès (à la base) et de pélites vertes et violacées. La base est datée du Botomen par les archéocyathes et de l’Ovetien supérieur par les trilobites (Cambrien inférieur). La Formation de Coulouma, riche en trilobites, échinodermes et brachiopodes inarticulés, a permis de définir sept niveaux paléontologiques (A1-F) qui appartiennent à l’intervalle Caesaraugustien inférieur-Languedocien moyen (Cambrien moyen). Le troisième lithosome (épais de 400 à 650 m) est composé d’une alternance de grès et de pélites. Il est constitué des Formations de Ferrals et de La Gardie. Pauvre en fossiles, la partie sommitale a permis de définir, cependant, trois niveaux paléontologiques (G-H2) qui appartiennent au Languedocien supérieur (Cambrien moyen terminal). Enfin, le quatrième lithosome correspond à la Formation du Val d’Homs. Cette unité est composée de couches carbonatées fossilifères interstratifiées dans des pélites. Les trilobites ont permis de lui attribuer un âge Cambrien supérieur.
Abstract
The Cambrian succession of the southern Montagne Noire (France) is more than 2000 m thick and comprises four lithosomes. The first and third lithosomes are siliciclastic, whereas the second and fourth lithosomes have a mixed (carbonate-siliciclastic) character. The purpose of this paper is to propose an updated review of the fossil record and of the Cambrian litho- and biostratigraphic units in the southern Montagne Noire. Three biostratigraphic scales are used in this work: (1) the Siberian scale based on archaeocyaths for the Lower Cambrian (Rozanov and Sokolov, 1984), (2) the French paleontological levels based on trilobite, brachiopod and echinoderm assemblages for the Middle Cambrian (Courtessole, 1973; Courtessole et al., 1988), and (3) the Middle Cambrian stratigraphic chart, based on trilobite stratigraphic ranges, recently proposed for south-western Europe (Álvaro and Vizcaïno, 1998). The first lithosome (Marcory Formation) consists of alternating sandstone and shale beds (more than 1000 m thick) with isolated conglomeratic layers. Some centimetre-thick layers of limestone located at the upper part of its lectostratotype (in the Orbiel valley) have yielded hyoliths and Chancelloriid sclerites. The second lithosome (around 400 m thick) is divided into the Pardailhan, Lastours, Pont de Poussarou, La Tanque and Coulouma formations. The Pardailhan Formation is composed of limestone and dolostone, with intercalations of sandstone and green shale; this formation has been dated on the basis of archaeocyaths and trilobites as respectively Botoman and late Ovetian (Early Cambrian). The Lastours Formation, in which three members are recognized in this work, consists of massive and stratified limestones and dolostones with rare shaly intercalations; based on a trilobite assemblage, a latest Marianian to early Bilbilian age (Early Cambrian) is proposed for the upper member of the formation. The Pont de Poussarou Formation is characterized by a massive white limestone unit, rich in bioclastic debris; locally it grades into the overlying La Tanque Formation, which is composed of alternating bioclastic limestone and shale and is characterized by reddish and purple colours. Finally, the Coulouma Formation is a monotonous succession composed of green (and rarer purple) shale with carbonate nodules paralleling the stratification; the abundance of trilobites, echinoderms and inarticulate brachiopods in this last formation has enabled seven paleontological levels (A1-F) to be recognized from latest early Caesaraugustian to middle Languedocian, i.e. Middle Cambrian. The third lithosome (400 to 650 m thick), comprising the Ferrals and La Gardie formations, is composed of alternating sandstone and shale. In spite of the scarcity of the fossil record, the upper part of this lithosome has been divided into three paleontological levels (G-H2) belonging to the late Languedocian (latest Middle Cambrian). Finally, the fourth lithosome (Val d’Homs Formation) shows a re-apparition of limestone interstratified among shale beds, and is dated by trilobites as Late Cambrian.
Dernière mise à jour le 02.07.2015