Long-term marine bedload segregation, and sandy versus gravelly Holocene shorelines in the eastern English Channel

Retreating cliffs comprising fringing gravel or sandy beaches line much of the English and French coasts in the eastern English Channel. Locally, gravel has formed significant barrier accumulations overlying thick finer-grained deposits. Although sandy deposits are abundant on the bed of the eastern Channel, significant aeolian dunes are restricted to embayments on the west-facing French coast between the Somme Estuary and Cape Gris Nez. These embayments show Holocene continuity of deposition of relatively homogenous sand, with the dunes overlying beach/washover and estuarine sediments. It is argued that the occurrence of gravelly shorelines or of sandy shorelines exhibiting significant aeolian dunes largely reflects a combination of three factors: (1) the inherited coastal lithology and morphology, characterised by soft-rock cliffs with or without embayments that were potential Holocene sediment sinks; (2) coastal orientation relative to large-scale tide- and wind-driven marine bedload segregation processes that operated during the Holocene and have led to preferential sand concentrations in the eastern corner of the Channel, and, possibly, concomitant relative enrichment in nearshore gravel elsewhere; (3) coastal orientation relative to the synoptic westerly wind field responsible for significant aeolian dune building where such sand concentrations occurred. The only coast where these three conditions were met favourably for significant aeolian dune building is the west-facing French coast north of the Somme Estuary, which benefitted during the Holocene from large-scale eastward advection of sand from the bed of the Channel. This sand was concentrated along a well defined coast-parallel transport pathway driven by tidal currents enhanced by Coriolis forcing and especially by the coast-orthogonal synoptic westerly winds. As sea-level rise slowed or stabilised following the middle Holocene, the sand was piled onshore in embayments to form aeolian dunes by these coast-orthogonal winds, leaving no scope for the formation of gravel shorelines. This coastal sand transport pathway is still operational. Elsewhere, the inherited coastal morphology, and orientation in terms of both the Coriolis effect and westerly wind forcing, have not favoured similar large-scale sand trapping in coastal sinks. As a result, the shorelines are characterised by retreating cliffs and fringing beaches, with a few major gravel barriers formed in local downdrift sinks, and comprising clasts reworked by storms from offshore gravel deposits on the bed of the Channel and from cliff retreat. The sharp change in facies from fine-grained sediments to overlying gravel within these barriers may have been enhanced by sand depletion caused by the large-scale Holocene bedload segregation processes that operated in the eastern Channel. It is suggested that if the permanence of the present-day 3-D morphology of the eastern English Channel and the same hydrodynamic forcing (notably the same wind field) as during the Holocene are accepted, then the suggested model of coastal sedimentation, involving preferential large-scale drift of Channel bed sand towards the north-south-oriented French coast north of the Somme Estuary, would also predict permanent, dominantly gravelly, high sea-level, beach sedimentation on the opposite English coast. (C) 2002 Elsevier Science B.V. All rights reserved.