THE EVOLUTION OF DELTAS: EXAMPLES FROM HOLOCENE ASIAN DELTAS

At present, the Asian coasts have many large deltas, which give us good examples for understanding deltas generally, including their evolution, sediment facies, and response to sea-level changes. The major reasons that so many large deltas are found in Asia are the existence of large rivers with high sediment discharges, which have their sources in the high peaks of the Himalayas and the Tibetan Plateau with high precipitation due to the monsoonal climate; and the stable or slightly falling sea level over the last 6 ky. During the Holocene, some Asian deltas have prograded more than 200 km seaward, creating wide delta plains. Hence, the Asian coastal zone is the depocenter and major sediment sink of the world today. Most Asian deltas are located along tide-dominated coasts. The exceptions are the Huanghe delta, which is on a wave-dominated coast, and the Chaophraya delta, which is on a low-energy coast. Mean tidal ranges are more than 2 m, with a maximum range exceeding 4 m. These coastal settings have also been changed by Holocene delta progradation. For example, the Mekong and Song Hong deltas evolved from tide-dominated deltas into more wave-influenced tide-dominated deltas as wave energy increased as a result of delta migration from inundated bays to open-shelf environments during the Holocene. The increasing wave influence has affected deltaic sediment facies, subaerial and subaqueous topography, progradation rates, and sediment dispersal patterns, resulting in beach-ridge formation, steepened delta fronts, decreased progradation rates, and coarser sediment facies. Today's wide delta plains, which formed mostly over the last 6 ky, display a variety of evolutionary styles. Multirivers have created single/combined wide delta plains, e.g., the Ganges-Brahmaputra and Chaophraya-Maeklong river deltas. Delta-lobe switching has resulted in lateral variation of delta progradation, sediment facies and erosional surfaces in delta systems, e.g., the Huanghe and Song Hong deltas. Tectonic subsidence has caused local sea level to rise on a millennial scale in the region of the Ganges-Brahmaputra delta, resulting in a concave shoreline topography with features such as transgressive estuaries, in contrast to other mature Asian deltas, which exhibit convex topography.