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

机译：英吉利海峡东部长期的海床分离和沙质与砾石质的全新世海岸线

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摘要

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.
机译：英吉利海峡东部的许多英法海岸沿线分布着由边缘砾石或沙滩组成的后退悬崖。在局部，砾石在厚的细颗粒沉积物上形成了明显的屏障堆积。尽管东海峡的河床上有大量的沙质沉积物，但重要的风沙丘仅限于在Somme出海口和格里斯内斯角之间的朝西法国海岸上的浮标。这些沉积物表明，相对均匀的沙子沉积具有全新世的连续性，沙丘覆盖着海滩/冲刷物和河口沉积物。有人认为，砾石海岸线或沙丘海岸线上出现显着的风沙丘，在很大程度上反映了以下三个因素的结合：（1）沿袭的沿海岩性和形态，其特征是软岩峭壁（有或没有浮雕）可能是全新世沉积沉积物; （2）相对于全新世期间进行的大规模潮汐和风驱动海床分离过程的沿海取向，并导致海峡东角的沙子优先聚集，并且可能伴随着近岸砾石的相对富集别处; （3）相对于天气向西风场的沿海方位，这是造成此类沙尘集中的重要风沙丘建筑的原因。满足这三个条件的重要风沙丘建筑，唯一符合条件的海岸是Somme河口以北的法国西海岸，在全新世期间，海峡床层的沙向东大范围平流受益。这种沙子集中在沿由科里奥利强迫（尤其是沿海岸正交的西风西风）增强的潮流驱动的明确定义的沿海岸平行的运输路径上。随着全新世中期海平面上升减缓或稳定，这些海岸垂直风将沙子堆积在岸上，形成风沙丘，从而没有形成砾石海岸线的空间。该沿海砂运输途径仍在运行。在其他地方，沿袭的海岸形态，以及在科里奥利效应和西风强迫方面的方向都不利于在沿海水槽中进行类似的大规模集沙。结果，海岸线的特征是撤退的悬崖和海滩边缘，在局部下沉的水槽中形成了一些主要的砾石屏障，包括被海床底的砾石沉积物和悬崖撤退的暴风雨重做的岩屑。在这些屏障中，从细颗粒沉积物到上覆砾石的相貌急剧变化可能是由于东部海峡进行的大规模全新世床层分离过程引起的砂耗所致。建议如果接受东英吉利海峡现今的3-D形态的持久性以及与全新世相同的流体动力强迫（尤其是相同的风场），那么建议的沿海沉积模型，包括优先海峡床砂向索姆河口以北的南北定向法国海岸的大规模漂移，也将预测英属对岸的永久性（主要是砾石）高海平面海滩沉积。

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