Les sediments superficiels des Approches occidentales de la Manche

Abstract

The part of the continental terrace stretching along a 250 km axis from the Western English Channel to the shelf edge is covered by a blanket of sediments ranging from fine sands to pebbles. Three terrigenous fringing areas are of Pleistocene origin: the periarmorican pebbly strip and the quartzitic sands located off Devon—Cornwall and near the shelf edge. The remainder is composed preponderantly of bioclastic sands, deposited through the Holocene and up to the present. The tidal currents, occasionally augmented by storm-induced oscillatory water movements, exert the main control on the distribution of the sediments. An evolutionary model starting from the Würmian low sea-level environment is proposed. It takes into account the specificity of the great sand-bank area in the southwest which was submerged whilst, onshore, subaerial and periglacial conditions led to the deposition of pebbles and, to a minor extent, of detritic sands south of England. The progression of the Holocene transgression triggered the productivity of the epifauna on the submerged, hard floor zones and modified the hydrodynamic conditions which, in turn, reacted upon the distribution of the organogenic debris. The velocity of the tidal currents is maximum (over 2 knots) around Brittany, keeping the pebbly area generally free of sedimentation of smaller particles. The distribution pattern of the bioclastic fraction (concerning mainly Bryozoa) enables one to consider a general axial transportation trending towards the SW which is corroborated by previous studies dealing with small-scale sedimentary features.