Les vallees fossiles de la baie de la Vilaine; nature et evolution du prisme sedimentaire cotier du Pleistocene armoricain
Résumé
The study of a dense network of high resolution seismic profiles in the bay of Vilaine, INSU-CNRS cruise Geovill, have led to the characterization of the architecture of the sediment wedge preserved between the coast and the 50 m isobath. This wedge lies on a substratum composed of three seismic units, U1, U2 and U3 respectively attributed to metamorphic and magmatic rocks, Lutetian and Ypresian sandy carbonates and post-Eocene sediments. The coastal sediment wedge comprises three major units. A basal unit (U4), dated around 600 to 300 ky BP, interpreted as braided river sandy conglomerates. A median unit (U5) corresponding to estuarine and fluvial sandstones and clays that give way to the west to mouth bar sandstones. A sommital unit (U6) attributed to marine argillites and barrier island sandstones dated from 8110+ or -200 years at the base. These three units are bounded by two major surfaces: an unconformity between U4 and U5 and a marine (wave and tidal) ravinement surface between U5 and U6. The unconformity is interpreted as a sequence boundary between two depositional sequences: a lower one with U4 seismic unit and a topmost one with U5 and U6 seismic units. Based on the available datations, the lower sequence is attributed to the Saalian and/or Elsterian glacial cycles and, the upper sequence to the Weichselian (lowstand systems tract) and to the Holocene marine transgression (transgressive systems tract). The passage from one sequence to the other corresponds however to a drastic shift in the paleoflow directions (60 degrees ) in the Bay of Vilaine closely related to the main faults orientations. The tectonic activity in Brittany during the Pleistocene, linked to intraplate stress, seems to exert a control on sediment architecture in the coastal wedge. Indeed, the tilt of the Armorican Massif during that period has caused a complete rejuvenation of the fluvial profiles in land and the separation of the paleo-Vilaine from the Paleo-Loire river courses.