Deltaic Environments

Deltas occur where a river flows into a sea or lake and deposits its load of sediment. There must be little wave or tidal action to allow the sediment to build up at the mouth of the river and not be washed away. It is the decrease in energy that causes deposition of sediments, with the coarsest grains settling out first.The build up of sediment causes the river channels to become blocked so that the river switches its course and splits into distributary channels. Deltas are just above sea level and in between the distributary channels there are usually swamps. In equatorial and humid tropical climates grows very rapidly vegetation and is abundant. Plant remains require anaerobic conditions if they are to be preserved, and swamps provide these conditions. This is why ancient plant remains are preserved as coal in equatorial deltaic sequences.The Delta TopThe delta top is dominated by distributary channels with areas of swamps, bays or flood plains in between. The channels change course frequently, a process known as delta switching that leads to lateral changes in the pattern of sedimentation. Sediments deposited on the delta top are called the topsets. Coarse grained sands and gravels are deposited in the channels. They make up the bulk of the sediments in the delta top and form channel sandstones, which may contain cross bedding.Clays are deposited in areas between the channels. In the swamps, anaerobic conditions allow peat formation. Diagenetic processes lead to the formation of coal. Soils in which the trees grew may be preserved as seat earth.The Delta SlopeAs the river meets the sea, the coarsest sand grains are deposited first and the finest silt last. Deposition occurs on the delta slope and as the front of the delta advances into the sea, a vertical succession forms.Sediments deposited on the delta slope are called foresets. Coarse sand, often cross bedded, is deposited at the top. Lower down there are likely to be finer sands and silts. Marine fossils may be found.Offshore DepositionThis takes place in the area of low energy deeper water at the bottom of the delta slope. These sediments are called bottomsets. They consist mainly of clay and silts, which are thinly bedded and lack sedimentary structures. They lithify to form shales, which may contain marine fossils.Deltaic SequencesThe sequence produced in a delta is a coarsening upwards sequence. It is repeated many times and each cycle is known as a cyclothem. It starts with marine shales and silts then fine to coarse sandstones, and finally coals, sandstones,seat earth and clay. Limestones are sometimes found at the base of the succession, representing normal marine conditions before the deltaic sequences begin.Bioturbation and trace fossils are common, with marine fossils in the bottomsets and non-marine fossils in the higher parts of the deltaic sequence. Plant fossils are often found and sometimes, if the surface of the delta is exposed, soils and coals may form. There is often very rapid deposition of mud covering the plant material before it decays. Many of the swamps are anaerobic, which increases the chances of plant material being preserved to form coal. The cyclothem or part of it may be repeated many times in succession, due to subsidence or emergence, leading to marine transgressions and regressions. These are caused by: changes in sea level local isostatic changes, due to the mass of sediment causing the sea floor to sink changes in position of the delta lobes, due to migration of the channels changes in the rate of sedimentation, allowing the delta to build out or be inundated by the sea.

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