Étude des grands bancs de sable du Sud-est de la mer Celtique

Abstract

Two surveys (continuous seismic reflection and surface-sediment sampling) carried out in 1970 by B.R.G.M. (Bureau de recherches géologiques et minières) and B.E.I.C.I.P. (Bureau d'études industrielles et de coopération de l'Institut français du pétrole) on the southeastern part of the Celtic Sea have enabled: (1) an accurate mapping of the bathymetry; (2) the reconnaissance of several great sand banks characterizing this shelf; some new ones have been discovered; the general bank orientation is parallel to the NE-SW trend of the Western Approaches; average size: length = 50 km, width = 6 km; height = 40 m; (3) the recognition of a sand-wave field to the W of the survey area, similar to the La Chapelle Bank sand waves; and (4) the study of the superficial structure of the shelf. From these results we may assume the following sequence for the Quaternary history of the shelf: 1. (1) Post-Pliocene levelling of the shelf (Early Pleistocene). 2. (2) Gullying into this surface related to a very low eustatic sea level at −240 m at least. 3. (3) Filling-up of the submerged channels by a transgressive stage. 4. (4) Preflandrian regression down to a shore-line at about −110 to −120 m; deposition of mixed (detritic + bioclastic) sediments on the submerged (outer) part of the shelf; beginning of the accumulation of the sand banks, resulting from and parallel to NE-SW dominant tidal currents, stronger than today. 5. (5) Sea-level rise (Flandrian transgression) during which the constructional process of the sand banks is reduced. 6. (6) Present situation with decrease of the current intensity (tide, internal waves .) which remains sufficient to allow the formation of sand waves; in some parts of the shelf, the latter are organized in a field pattern (zone de l'Espérance, La Chapelle Bank), owing to the morphological effect of canyon heads and to the shelf-break shape favouring the current action.