Relationship between phosphogenesis and sequence architecture: Sequence stratigraphy and biostratigraphy in the Middle Ordovician of the Armorican Massif (NW France)
Résumé
In the Armorican Massif (NW France), the Middle Ordovician formations contain abundant phosphatic (P) beds associated with terrigenous sediments laid down on a storm-dominated shelf. Facies analysis and sequence stratigraphy carried out on sections located in the Crozon Peninsula (W Armorican Massif) allow us to distinguish four P-facies: (1) beds with P-nodules and crusts, contained within sediments laid down on the distal and median parts of the inner shelf; (2) amalgamated beds with P-nodules and crusts and (3) beds with lingulid brachiopods, heavy minerals and P-clasts associated with retrogradation phases; (4) P-clastbearing conglomerates, associated with progradation phases. The genesis of these facies and their location in the depositional sequences are controlled by the fluctuation of terrigenous inputs during sea-level changes ("sediment volume partitioning"). We propose a genetic model integrating the superposition of very-high-frequency (VHF) and high-frequency (HF) eustatic signals. In this model, the amalgamated beds and beds with lingulid brachiopods, heavy minerals and P-clasts result from the superposition of several sea-level rise episodes of VHF cycles, occurring within the retrogradation phase of a HF cycle. The lateral extension of P-facies was investigated in the eastern part of the Armorican Massif (Bénioc, Domfront/Les Tanneries and St Aubin-du-Cormier sections). Several early Darriwilian transgressive events associated with phosphogenesis episodes are dated through high-resolution chitinozoan studies. Some of these transgressive events are at least of regional scale (e.g., ornensis and protocalix transgressive events). The topmost calix transgressive event is clearly identified over the northern Gondwanan and Avalonian domains, and may reflect a global eustatic signature.