Sediment dispersal and accumulation off the Ayeyarwady delta – Tectonic and oceanographic controls

Abstract

Recent sediment dispersal and accumulation on the Northern Andaman Sea continental shelf, off the Ayeyarwady (Irrawaddy) and Thanlwin (Salween) Rivers, are investigated using seabed, water column, and high-resolution seismic data collected in December 2017. 210Pb and 137Cs derived sediment accumulation rates are highest (up to 10 cm y−1) in the mid-shelf region of the Martaban Depression, a basin that has formed on the eastern side of the N-S trending Sagaing fault, where rapid progradation of a muddy subaqueous delta is occurring. Landward of the zone of highest accumulation, in the shallow Gulf of Martaban, is a highly turbid zone where the seabed is frequently mixed to depths of ~1 m below the sediment water interface. Frequent resuspension in this area may contribute to the formation of extensive fluid muds in the water column, and consequent re-oxidation of the shallow seabed likely reduces the carbon burial efficiency for an area where the rivers are supplying large amounts of terrestrial carbon to the ocean. Sediment cores from the Gulf of Martban have a distinctive reddish brown coloration, while x-radiographs show sedimentary structures of fine silt laminations in mud deposits, which indicates strong tidal influences. The seaward part of the Martaban Depression off the modern subaqueous delta is covered by relict sediment, with no apparent connection between the modern sediment deposit and the offshore Martaban canyon. On the western side of the Sagaing fault, the “Mouths of the Ayeyarwady” represent a large promontory where the subaerial delta has prograded seaward some ~200 km along the eastern flank of the Indo-Burman range since the Holocene maximum transgression. The shelf area off the “Mouths of the Ayeyarwady” presently exhibits low sediment accumulation rates (~1 cm y−1) and a relative coarse (sandy) texture. The reduced accumulation and coarse texture in the western shelf region at the present time is attributed to frequent wave resuspension, and subsequent transport of finer river-derived sediment eastward (into the Martaban Depression) during the SW Monsoon. A mud drape (accumulation rates <1 cm/yr) is present on the northwestern part of the delta, where some sediment likely escapes the shelf to the deeper Bay of Bengal. In contrast with the Gulf of Martaban, sediments in this mud drape show olive grey coloration, while sedimentary structures are dominated by mottled sandy mud with shells, and occasional sand layers. The mud drape is likely derived from a mixture of local rivers draining westward from the Indo-Burman range, and a contribution from the Ayeyarwady system delivered during the NE monsoon. Overall, the shelf offshore the Ayeyarwady and Thanlwin rivers represents a compound subaqueous delta with marked differences from east to west that are controlled by a combination of oceanographic and tectonic factors.