The offshore Quaternary sediment bodies of the English Channel and its Western Approaches

Abstract

After a short review of present-day surficial sediments on the sea-floor of the English Channel and its Western Approaches, this paper summarises recent results on the Present-Quaternary sandy accumulations (tidal bank systems), based on sea-floor imaging, very high-resolution seismic surveys and a few piston cores. The English Channel, a typical funnel-shaped tidal shelf, is presently starved of terrigenous sediments. Most of mobile sediments at the sea-floor are autochthonous, either skeletal carbonates or siliciclastic sands reworked from old littoral deposits or incised valley fills. Three zones are considered: 1 the Central (and western Central) Channel, which is a bedload parting zone characterised by a pebbly lag making up the sole of the Quaternary sediments throughout the shelf area; 2 the Eastern Channel, a flood-dominated area characterised by medium to fine siliciclastic sands, mainly of aeolian periglacial origin; 3 the Western Approaches, an ebb-dominated area with a polygenic sediment cover characterised by coarse, carbonate-rich, gravelly to muddy sands of marine to glaciomarine origin, with a diachronous faunal content. These sediments are sourced by reworking of a former littoral barrier or incised valley fills. The seismic stratigraphy of Quaternary sediment accumulations (10-50 m) points to the predominant control of tidal dynamics, together with the general interplay between the Channel morphology and the sea-level story. The thickest deposits in the Eastern Channel and Western Approaches are built up into bank systems. Whereas the interpretation of the active Eastern Channel banks finds a general agreement, the 'moribund' banks in the Western Approaches (i.e. the southern Celtic Banks) remains under debate, owing to the lack of long cores. In our hypothesis, both systems were built up during the last post-glacial sea-level rise. The Celtic Banks would represent the tidal transgressive systems tract whereas the Eastern Channel banks correspond to the highstand systems tract. The architecture of both bank systems is studied from two examples, the Bassure de Baas bank in the Eastern Channel and the Kaiser Bank in the Western Approaches. Both examples show transgressive features. The Bassure de Baas exhibits from base to top: an estuarine mouth barrier, a wave erosion surface, a shoreface bank and finally a 'classic' tidal bank. By contrast, the Kaiser Bank consists of the offshore reworking of fully marine deposits. Its base would correspond to the amalgamation of the last glacial sea-level fall with a strong tidal ravinement surface. From base to top, the bank underwent: tidal-bar accretion, very large dune-field aggradation, perched-channels incision, storm erosion and deposition.