Sedimentary architecture of the Loire River drowned valleys of the French Atlantic shelf
Résumé
The analysis of a dense grid of high resolution seismic profiles collected offshore the present day Loire River estuary indicates the presence of a thick and complex Pleistocene coastal wedge between the coast and 50 meters water depth. Most of this coastal wedge is preserved in a fossil valley network starting 10 km off the coast on the “Precontinent Breton” topographies and wedging out progressively 50 km in the offshore where the shelf flattens. This system is comprised of three main valleys 30 km long, 40-60 m deep and 0.7 to 4 km wide each, in average, with a northern valley incompletely filled by sediment. These valleys are incised into Eocene (Ypresian-Bartonian) sedimentary rocks lying unconformably on the metamorphic and magmatic rocks of the South Armorican Massif basement. The coastal wedge is comprised of six seismic units. From the base of the valleys to the seafloor, these units are successively interpreted as (1) colluvial (U1) and braided river deposits (U2), overlain by restricted marine to estuarine sediments (U3), and (2) straight to meandering fluvial deposits (U4) giving rise vertically to floodout marine sediments (U5). The whole sediment pile is capped by open marine bioturbated mudstones (U6). This succession of seismic units is organised in two depositional sequences bounded by an unconformity of regional extent, which corresponds to a drastic change in the paleovalley fill architecture. The lower sequence fills up the southern and central valleys when the upper sequence fills up the northern valley network. Both sequences are sharply truncated by a ravinement surface at the base of the offshore shales formed during the Holocene marine transgression. The correlation of the observed depth of the incisions and transgression surfaces with the global sea-level curve provides an indirect estimate of the ages of the depositional sequences. The lower sequence is probably Saalian (130-200 Ka, MIS 6) and the upper one Weischelian (110-12 Ka, MIS 2-4 and 5a-d). The ages and the infill of these valleys are very close to the ones observed in the largest paleovalleys of the inner French Atlantic shelf (Gironde, English Channel) and consistent with the sedimentary record of the continental Loire River (stepped terraces). In the later case, the Saalian period corresponds to a sharp increase on the incision of the river that shaped the morphology and determined the location of the present-day Loire valley.