Clast assembling, bed-forms and structure in gravel beaches

Both textural maturity and structure acquired by gravels on beaches are lar gely a response to the interaction between beach surface and wave-backwash energy. The turbulent energy driving the processes of particle rejection an d selection at the surface of growing gravel sheets is partly controlled by the grain size of the sheet itself, so the process is to a large extent se lf-regulating. Beach surfaces are seen to comprise many discrete sheets of gravel competing for growth at their boundaries, but each characterised by a uniform or uniformly gradational texture. There are two main types of gravel sheet: (1) selection pavements which occ ur on low beach slopes, showing little areal grain-size or grain-shape vari ation, and (2) turbulence shadows which occur on steeper slopes and produce assemblages of clasts which may show perfect lateral shape and size gradat ion. The clasts which make up these various gravel sheets are termed 'clast asse mblages', and such assemblages are the fundamental units from which beaches are constructed. Clast assemblages are classified in terms of their textur al maturity-the degree to which they exhibit uniformity in clast size and s hape. In beach sections they are, either singly or in combination, bounded by planes of discontinuity (bedding planes) to form beds. Repeated combinations of either clast assemblages or beds in a genetic asso ciation are regarded as sedimentary structures, many of which are diagnosti c of the gravel beach environment. Growth of beaches involves the stacking of sedimentary structures, and four growth patterns have been identified. T he beach structure is, therefore, classified in a hierarchy comprising clas t assemblage, bed, structures and growth form, and it is the growth pattern which may be related to tidal range. Recognition of the processes which generate beach gravels through the struc ture of the gravels permits an analysis of the internal structure of major gravel bodies such as barrier beaches. It adds another set of criteria whic h may further lead to an understanding of the processes responsible for the generation and evolution of these large gravel forms. Three types of gravel lithosomes have been identified. (1) Regressive barri er bars which form a series of gravel ridges separated by lagoonal deposits . Barriers are built initially by swash bars which grow in size and coarsen through time to become wave-resistant forms. They form as a response to ti mes when the sediment, unable to be evenly distributed and sorted on the be ach surface, forms a discrete bar seaward of the active beach. This is the result of a punctuated or continuously high sediment supply compared with t he wave energy available to disperse the sediment, or a falling sea level w hich shifts the locus of sediment accretion. (2) In contrast, regressive (p rograding) gravel sheets form as a response to a continuous supply of sedim ent to the beach surface, allowing it to build seaward by the uniform accre tion of sediment which is sorted and retained on its surface. (3) Gravel sh eets produced in transgression are characterised by an abundance of spheric al clasts and are often overlapped by the sand beaches which occur seaward of them.