Stratigraphic Formation of the Mekong River Delta and Its Recent Shoreline Changes

Where the Mekong River discharges into the East Sea ( also known as the South China Sea), it has formed the world's third largest delta plain with an area of similar to 50,000 km(2). Numerous cores recovered from the subaerial delta reveal that it has prograded similar to 220 km southeastward within the past 7,500 years. Recent extensive seismic and geochemical surveys of the adjacent subaqueous delta indicate that the Mekong River forms a classic sigmoidal, cross-shelf clinoform immediately off its distributaries that is up to 15 m thick, with topset, foreset, and bottomset facies. These deposits are constrained within water depths of 20 m. Mekong-derived sediment packages extend similar to 300 km along shelf in the southwestward direction to the tip of the Ca Mau Peninsula, where they form a distal mud depocenter up to 22 m thick. These sediment packages can also be traced into the Gulf of Thailand to water depths of 25 m. The proximal and distal deposits cover similar to 11,000 km(2) of the shelf. Historically, the Mekong Delta has prograded seaward at a mean rate of > 30 m yr(-1), or 7 km(2) yr(-1); however, study of the past 43 years of Landsat images indicates that the mode of sedimentation in the delta shifted starting in 2005. From 1973 to 2005, the Mekong Delta's seaward shoreline growth decreased gradually from a mean of 7.8 m yr(-1) to 2.8 m yr(-1) and after 2005 it became negative, with a retreat rate of -1.4 m yr(-1). The net deltaic land area gain has also been slowing, with the mean rate decreasing from 4.3 km(2) yr(-1) (1973-1979) to 1.0 km(2) yr(-1) (1995-2005), and then to -0.05 km(2) yr(-1) (2005-2015). Thus, in about 2005, the subaerial Mekong Delta transitioned from a constructive mode to an erosional (or destructive) mode. Furthermore, not only is the subaerial Mekong Delta land area gradually diminishing, but high-resolution CHIRP sonar profiling surveys off the east-central Ca Mau Peninsula reveal that this portion of the subaqueous delta is also eroding. With the construction of more dams, sand mining, delta subsidence, increasing storms, and sea level rise, the Mekong Delta will likely face more destructive changes, with erosion both of coastlines and underwater deposits.