Sedimentary processes, bedforms and facies, associated with a coastal headland: Portland Bill, Southern UK

Bastos, A.C., Kenyon, N.H. and Collins, M. (2002) Sedimentary processes, bedforms and facies, associated with a coastal headland: Portland Bill, Southern UK. Marine Geology, 187 (3-4), 235-258. (doi:10.1016/S0025-3227(02)00380-8).

Abstract

This investigation presents new findings on the sedimentary processes and deposits associated with a coastal headland, in a mixed tide–wave setting, using data compilation (bathymetric and sediment distribution maps), collected field data (side-scan sonar and seabed sampling) and sand transport modelling. Sand transport pathways are described: (a) on the basis of coupled-system seabed morphology/sand distribution; and (b) from bed shear stress and (medium-grained) sand transport rate simulations. The presence of a sequence of sedimentary deposits, associated with a complex suite of bedforms and sedimentary facies within an overall framework of limited sediment supply, represents a gradient in shear stress and sand transport towards the headland. These sequences are observed on both sides of Portland Bill, tending towards a symmetrical distribution. The sedimentary facies distribution is combined with sand transport rates and maximum bed shear stress distribution, to suggest a conceptual model for sand dispersal and deposit formation around headlands. Sand dispersal can be explained in terms of a headland-associated eddy/bedload convergent zone concept. Maximum bed shear stress is observed at the tip of the headland and is associated with bedrock exposed on the seabed. The sequence of sedimentary deposits, away from the headland, is: sandbanks (Shambles and Portland Banks); sand/gravel flats; sand shoals (Adamant and West Shoals); and rippled sand sheets. The banks lie in an area of very high bed shear stress (u*=0.08 m s?1); this is greater than that for other tidal sandbanks, not associated with headlands. Bank formation is the result of the strong convergent (transport) component, enhanced by the development of transient headland eddies. The sandbanks are to some extent also morphologically controlled, i.e. the Portland Bank is affected by the deep bathymetry to the west side of Portland Bill. The principal characteristic of headland-associated eddy/bedload convergent zones is the development of bed shear stress convergent zones, on both sides of the headland. Two conceptually distinct regions of sedimentary processes, associated with the coastal headlands, are recognised: (a) an inner zone, with increasing gradients in sand transport and bed shear stress towards the headland; and (b) an outer zone, in which sand transport is away from the headland, associated with a decrease in the bed shear stress. These zones merge into a bed shear stress (sand bedload transport) convergent zone, which enhances the formation of sandbanks around headlands. Thus, headland-associated sedimentary deposits are a complex system, compared with the occurrence of isolated sandbanks described elsewhere. The identification of a sequence of deposits related to and extending much farther from the headland than previously supposed, demonstrates that the ‘tidal stirring concept’ of headland-associated sandbanks may explain the formation of headland-associated sandbanks; however, it ignores the presence of a suite of sedimentary deposits around such headlands. The conceptual model proposed here provides a sedimentological perspective on the formation and occurrence of headland-associated deposits, including sandbanks.

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