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Sediments to the Sea

Collectively, the modern Ayeyarwady (Irrawaddy) and Thanlwin (Salween) rivers deliver >600 Mt/yr of sediment to the sea. Most recent studies show: 1) There is little modern sediment accumulating on the shelf immediately off the Ayeyarwady River mouths. In contrast, a major mud wedge with a distal depocenter, up to 60 metres (200 ft) in thickness, has been deposited seaward in the Gulf of Martaban, extending to approximately 130 metres (430 ft) water depth into the Martaban Depression. Further, 2) There is no evidence showing that modern sediment has accumulated or is transported into the Martaban Canyon; 3) a mud drape/blanket is wrapping around the narrow western Myanmar Shelf in the eastern Bay of Bengal. The thickness of the mud deposit is up to 20 metres (66 ft) nearshore and gradually thins to the slope at −300 metres (−980 ft) water depth, and likely escapes into the deep Andaman Trench; 4) The estimated total amount of Holocene sediments deposited offshore is approximately 1,290 by 109 tonnes (1,270 by 107 long tons; 1,422 by 120 short tons). If we assume this has mainly accumulated since the middle Holocene highstand (~6000 yr BP) like other major deltas, the historical annual mean depositional flux on the shelf would be 215 megatonnes (212,000,000 long tons; 237,000,000 short tons) per year, which is equivalent to ~35% of the modern Ayeyarwady-Thanlwin rivers derived sediments; 5) Unlike other large river systems in Asia, such as the Yangtze and Mekong, this study indicates a bi-directional transport and depositional pattern controlled by the local currents that are influenced by tides, and seasonally varying monsoons winds and waves.