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Surface Processes
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
When a delta is being formed the river characteristically divides so that the deltaic deposits in time come to cover a large area which may have a roughly triangular shape (like the Greek letter A, from which the name delta is derived). A section through the deposits of a typical small delta is given in Fig. 3.20; as each flood brings down its load of sediment the coarser material is dropped in front of the growing pile and comes to rest at its angle of repose in water, building up the foreset beds. The slope of these layers varies from about 12° to 32°, larger (sandy) particles standing at higher and smaller (clayey) particles at lower angles. Ahead of the foreset beds and continuous with them the finer material is deposited as the bottomset beds. As the delta is built forwards, foreset layers come to rest on earlier bottomset deposits, as shown in the figure. Foreset beds may periodically slump and slide from the upper levels of the delta slope to be re-deposited at its toe. The upper surface of the delta is composed of gently sloping topset beds of coarser material, which are a continuation of the alluvial plain of the river and cover successive foreset deposits.
Gilbert-style Pleistocene fan delta reveals tectonic development of North Island axial ranges, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2018
Callum Rees, Julie Palmer, Alan Palmer
The Gilbert-style fan delta is typically composed of fine-grained gently inclined bottomsets, steeply dipping sandy and/or gravelly foresets and flat subhorizontal sandy and/or gravelly topsets (Colella et al. 1987; Bassett and Orlowski 2004; Rohais et al. 2008). In the topsets, alluvial plain and delta plain sedimentation occur. Foreset beds form via avalanching of the fluvial bedload within the alluvial fan environment, generating high-concentration debris flows and turbidity currents (Galloway and Hobday 1996; Gobo et al. 2014). Bottomsets are influenced by the tails of debris flows and turbidity currents generated at the delta front. Fine-grained sediments are transported basinward, in suspension, where they are deposited downslope of the delta.