The English Channel and its Western Approaches constitute a 700 km long epicontinental sea located in a temperate environment and in a tectonic setting where subsidence is minimal.A sedimentological survey of this region reveals quite distinct provinces: a central sector (Western Channel) where predominantly bioclastic sediments are widely represented, bounded by two mainly terrigenous‐rich zones, the outer terrace of the Celtic Sea and the Eastern Channel. Lateral sedimentary variations of decreasing grain size are interpreted in terms of current velocity patterns. The various types of such sequences may relate to the degree of mixing of older lithoclastic sediments with the Holocene bioclastic supply.A scenario for the evolution of the recent sedimentation of this epicontinental sea is presented. Starting from a permanent marine zone—at least since the Würm period—that bounds the Bay of Biscay, the Holocene transgression progressed over the English Channel. In the Western Channel earlier terrigenous deposits were gradually overlapped by bioclastic sediments that originated on the pre‐Mesozoic rocky substratum and which were particularly extensive off the Armorican Massif. In the subsequently submerged Eastern Channel the pre‐Holocene clastic source has undergone comparatively less modification and still crops out in most of the area. With the opening of the Straits of Dover, at about 9000 yr BP, submersion was complete and new hydrodynamic conditions developed as the eustatic level stabilized. The present sediment distribution is in harmony with the hydrodynamic setting except on the southern rise of the Celtic Sea where both morphology and sedimentary patterns still largely reflect pre‐Holocene conditions.

Based on the interpretation of geophysical data (very high resolution seismic data combined with EM1000 swath bathymetry), this paper reviews the history of the Channel River throughout the late Cenozoic. New evidence does illustrate how the interplay of tectonics, eustacy and climate have influenced this northwest European drainage system.The concepts of sequence stratigraphy allow the subdivision of the sedimentary record into depositional sequences bounded by unconformities, resulting from globally synchronous sea-level changes. However, the recognition of eustatic sea-level changes in a cold climate environment is very difficult, because of the interplay of additional phenomena such as glacio-isostatic sealevel changes with glacio-eustatic changes. For our interpretation, it was necessary to adjust these concepts with the relative importance of geomorphological processes determining the behaviour of rivers and steering their evolution.The foundations of the modern drainage system were laid after the Oligocene Channel inversion. In general these early rivers occupied shallow valleys. The Pleistocene cold climates resulted in fluvial incision. Through time, the Channel River has adopted a drainage system that can be divided into three parts: (i) the drainage basin comprising the Southern Bight, the eastern Channel and the London and Paris basins, (ii) the river zone itself, which begins at the Cotentin peninsula and passes through the Hurd Deep, before reaching the continental shelf-break in the Celtic Sea, (iii) the depositional basin represented by the Celtic Banks complex and by the deep sea fans, located at the foot of Shamrock and Black Mud canyons. During important lowstands (marine oxygen isotope stages 22, 16, 10, 6 and 2), the Channel River seems to have reached the shelf-break, as indicated by the internal structure of the banks.

No abstract

Bedforms of the Surtainville area, off the Cherbourg peninsula, include subtidal sandwaves, which are a good example of mega‐structures associated with the strong tidal currents prevailing in the English Channel. A fine‐scale study using a high‐accuracy echo‐sounder and side‐scan sonar shows that some of these sandwaves have a crescentic shape and a strong asymmetry indicating a sand movement toward the north. The sandwaves range in height from about 3.5 m to 7.5 m, in width from 100 m to 500 m and in length from 70 m to 200 m; their internal structure, revealed by the simultaneous use of a high‐resolution seismic source, is characterized by large ‘foreset’ beds dipping in the same direction as the lee sides of the sandwaves. Groups of foresets are limited by reactivation surfaces which we interpret as erosional surfaces created by subordinate tides. The presence of horizontal erosional reflectors inside the sandwaves and the truncation of the present‐day profiles may reflect the effects of storms. The asymmetry of the tide in the area studied, shown by long‐term current measurements, indicates that these sandwaves belong to classes III or IV of Allen's (1980a) classification; the observed structures correspond very well to the prediction of Allen's conceptual model, but we suggest that long term phenomena like equinox cyclicity, associated with storms, may be responsible for their origin rather than the neap‐spring‐neap tidal cycles responsible for the internal structure of intertidal bedforms.

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.This interpretation still needs to be modelled. For instance, the channels perched at the top of the Kaiser Bank point to an erosional peak, which could be related to a tidal resonance episode within the Western Approaches basin.

No abstract

No abstract

No abstract