The South China and Indochina Peninsula Deltas——A Comparison

The five deltas of South China and Indochina Peninsula are all deposited in downfaulted basins. The downfault structure is mainly shown by the red weathering crust on the basement of the Quaternary,island hills on plain,islands off estuary,seismic activity and others.The thickness of Quaternary of Chao Phraya Delta is over 650m but that of Zhujiang Delta is only 63m, showing the different amplitude of downfaulting. The deposits of Early and Middle Pleistocene can be found in the three deltas of Indochina Peninsula but the kown oldest datings are only 52 138 a B P and 45 120 a B P in Hanjiang and Zhujiang Delta,respectively. Since middle stage of the Latest Glacial Period the five deltas have undergone three times of deltaic deposition and two times of discontinuity occurred.The maximum of Holocene transgression is mostly dated at 6 500-5 000 a B P.The advance rate of plain during the past hundred years is 63-120m/a in Zhujiang Delta but that of Chao Phraya Delta is only 4.0m/a. The similarity of the deltas can be enumerated as follows:abundant water and less silt content,composite deposits of several rivers,braided stream,several river mouths,smaller channel curvature,extensive flood plain and wetland,south-westward extending of delta plain,and sedimentary of mangrove. The deltas have following differentiation.The areas of the deltas are quite different.The altitude of plain is larger in the delta inside of estuary.The submerged delta developed in the delta with smaller shelf gradient.The hydrodynamics of the deltas can be recognized as follows:runoff-tidal type of Zhujiang Delta and Chao Phraya Delta,runoff-wave type of Hanjiang Delta and Red River Delta,and runoff-tidal-wave type of Mekong Delta.