Les formations carbonatées de la séquence ptérocérienne (Kimméridgien pars) dans le Jura francais et les régions voisines

Abstract : Detailed analysis of Kimmeridgian carbonate formations that have been carried out by P. BERNIER (1985) in southern Jura Mountains is completed through this present work. This study allows to establish a new nomenclature of Kimmeridgian formations of French Jura and to insert them into a rather well defined stratigraphic frame in spite of the scarcity of paleontological evidences. Three major sequences have been recognized through successive formations. They are still distinguishable off Jurassian platform in the North Tethysian basin. Sequanian sequence stops at the top of Platynota Zone (Early Kimmeridgian, French acceptation) with the unconformity U0. Top of the sequence is characterized to the North by lime mud deposit (Calcaires de Besançon) on a harbored inner shelf and to the South by sediments enriched with oncolites (Calcaires d’Aranc) which are coincident with submarine ridge located close to the Northwestern edge of the Haute-Chaine. At once Pterocerian sequence corresponds to lime muds flooring a subtidal moat limited to the South by oncolitic deposits. This sequence is marked by argillaceous limy facies, enriched with macrofauna (Pelecypods and Gastropods) then by back-reef type carbonate deposits enclosing Stromatoporids (Calcaires de Matafelon). Maximal extension of this moat, coincident with period of marl deposits (Marnes de Rang), can be correlated to the shale recurrences recorded through Divisum zone in South-East basin. The diversity of the facies which overlie stromaporids bearing limestones gives evidence for the structuration of the platform through Divisum and Acanthicum zones. Tidal depositional environment developed upon a structural high (faisceau salinois) which splitted the former moat into two subbasins characterized by light fine limestone accumulations: Calcaires des Fins to the North and Calcaires de Chougeat to the South. At the beginning of the Virgulian sequence, the Northern Jura Mountains were overlapped by marly facies : Calcaires et Marnes de Savoyeux. In opposite, to the South, this major sequence is not distinguishable everywhere because of facies monotony. They are enriched with skeletal algae: Calcaires de Foncine-le-Bas. Upon the Haute-Chaine, in the vicinity of Nantua and Oyonnax, former ridge with oncolites was covered by bioclastic and oolitic sediments (Calcaires d’Oyonnax, Couches de Prapont inférieures) during Kimmeridgian time (French acceptation). Sporadic marine ingressions (documented by presence of ammonites) occurred over this area.
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Thierry Chevallier. Les formations carbonatées de la séquence ptérocérienne (Kimméridgien pars) dans le Jura francais et les régions voisines. Stratigraphie. Université Claude Bernard - Lyon 1, 1986. Français. ⟨tel-02276580⟩

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