cHAPTER 6 Deltas

Deltas are discrete shoreline protuberances formed where rivers enter oceans, semi-enclosed seas, lakes or lagoons and supply sediment more rapidly than it can be redistributed by basinal processes. Generally, deltas are served by well-defined drainage systems which culminate in a trunk stream and supply sediment to a restricted area of the shoreline, thus resulting in the formation of a depocentre. Less organized and immature drainage systems produce numerous closely spaced rivers which induce uniform progradation of the entire coastal plain rather than point-concentrated progradation and depocentres. At the river mouth, sediment-laden fluvial currents which have previously been confined between channel banks suddenly expand and decelerate on entering the receiving water body. As a result the sediment load is dispersed and deposited, with coarsegrained bedload accumulating near the river mouth, whilst finer grained, suspended sediment is transported offshore and deposited in deeper water. Basinal processes such as waves, tides and oceanic currents may assist in the dispersal of sediment and also rework sediment deposited by the fluvial currents. Many of the characteristics of deltas stem from the results of this interplay between fluvial and basinal processes. The tectonic settings of major deltas are varied (Audley-Charles, Curray and Evans, 1977), but the most common are passive continental margins (e.g. the Niger delta), marginal basins or foreland basins associated with mountain belts (the Irrawaddy and Ganges-Brahmaputra deltas) and cratonic basins (the Rhine delta). As deltas are often major depocentres, they produce exceptionally thick sequences which have been recognized throughout the geological column. In addition to their palaeogeographic significance, ancient deltaic successions are also important as sites of oil, gas, and coal reserves in many parts of the world.