Seismic stratigraphy of the Vendean-Armorican platform of the French Atlantic shelf: new insights into the history of the North Atlantic ocean

Abstract

AbstractThe evolution of the North-Atlantic Ocean from its rifting stage during the Upper Jurassic until the present-day passive margin is recorded by the sedimentary wedge of eastern French-Atlantic platform. The study of a dense network of high resolution seismic profiles on the Vendean-Armorican platform (VAP) obtained during INSU-CNRS cruise “Geovend”, led to the characterization of the architecture of the sediment wedge preserved between the coast and Armorican margin shelf edge. This sediment wedge lies on a substratum composed of metamorphic and magmatic rocks of Palaeozoic age (Ub). The sediment wedge comprises six seismic units (U1-U6) bounded by regional unconformities: Jurassic marine succession (U1), Upper Cretaceous marine rocks (U2), Eocene-Oligocene marine deposits of the incipient VAP (U3), Miocene (U4) and Plio-Quaternary (U5) marine deposits overlain by the last sea-level rise ravinement deposits (U6). Above the basal unconformity at the top of Ub, the units are bounded by angular unconformities (top of U1, U2, U3), truncation with channel incision (top U4) or planar marine ravinement (top of U5) surfaces.Most of these unconformities are due to the tectonic activity of the bay of Biscay during the Mesozoic including (1) the North Atlantic rifting during the Jurassic to Early Cretaceous, (2) the propagation of the ocean crust and counterclockwise rotation of the Iberian block during the Aptian-Albian to Coniacian (magnetic anomaly 33–34) producing troughs at the top of U1 filled by downlapping U2 sediment wedges, (3) the Alpine compression at the origin of folding and faulting and the unconformable deposition of U3, and (4) the late compressive deformation during the Miocene that affected U4. The VAP acquires its actual configuration during U4. Sedimentation on the platform was then affected by climatically-controlled relative sea-level changes (U5 to U6) that forced U5 shelf margin sediment deposition above an incised unconformity and subsequently overlain by U6 transgressive sediment blanketing.One of the main interest of the VAP area is the existence of pre- to post-rift units that helps to decipher with high resolution seismics the long-lived evolution of the Armorican margin. Such units are preserved because of the specific characters of this area located on the flank of the former Aquitaine basin (near the “celtaquitaine” flexure) and the presence of the Rochebonne basement high.The VAP thus displays most of the tectonosedimentary evolution of the West Atlantic margins. This paper would however constitute a basis for comparisons to other examples around the Atlantic ocean and then contribute to strengthen the running models of passive margin evolution.