Chapter 17 The Myanmar continental shelf

Abstract

The Myanmar (Burma) coastline is about 2280 km long, with the continental shelf covering an area of approximately 230 000 km 2 . The Myanmar coastline may be divided into the northern Rakhine (Arakan) coast, the central Ayeyarwady coast and the southern Tanintharyi (Tenasserim) coast. The Rakhine coast lies adjacent to the Bay of Bengal, while the Ayeyarwady and Tanintharyi coast faces the Andaman Sea. The continental shelf of Myanmar receives sediments from some of the largest rivers in the world like the Ayeyarwady (Irrawaddy), the Salween (Thanlwin) and the Ganges-Brahmaputra rivers. The Myanmar shelf is characterized by a seasonal reversal of monsoon winds and coastal currents, periodic tropical cyclones and storm surges, meso-to macrotidal conditions, and neotectonic activity. The most prominent bathymetric feature on the Ayeyarwady continental shelf is the 120 km-wide Martaban Depression, at the centre of which is located the Martaban Canyon. Most of the suspended sediment discharge of the Ayeyarwady is transported eastwards by coastal and tidal currents, and trapped in the Gulf of Martaban, resulting in the formation of an extensive mud belt covering an area of more than 45 000 km 2 . Because of these features, the Myanmar shelf ranks amongst the most globally important continental shelves in the world but remains inadequately studied owing to its inaccessibility.

On the Holocene evolution of the Ayeyawady megadelta

“…1) bears the imprint of uniquely complex tectonic processes in a region of oblique subduction (Morley et al, 2007) and is a repository for unusually large sediment yields under an erosion-prone monsoon climate (e.g., Giosan et al, 2017). Sediment redistribution within the delta and on the shelf fronting is affected by strong tides amplified by the geomorphology of the region (Ramasawamy and Rao, 2014). In contrast to other Asian megadeltas, the Ayeyawady River basin is arguably less transformed by post-World War II anthropogenic impacts, although humans have probably affected delta development since at least the Iron Age as agriculture expanded along the river (Moore, 2007) and later intensified during the Pyu ( ∼ 200 BC to 1050 AD), Bagan (∼ 850 to 1300 AD) and Ava (∼ 1350 to 1550 AD) historical periods.…”

“…The shelf morphology in front of the Ayeyawady delta is complex due to its tectonic structure and the nature of Holocene sedimentation (Rodolfo, 1969a(Rodolfo, , b, 1975Ramaswamy and Rao, 2014). The width of the shelf is ∼ 170 km wide off the Ayeyawady River mouths, widening to more than 250 km in the Gulf of Mottama (Figs.…”

“…1 and 5). The shelf edge exhibits a flat, platform-like indentation in the Gulf of Mottama between 140 and 180 m deep (i.e., the Martaban depression - Ramaswamy and Rao, 2014) that features a dendritic network of channels feeding the Martaban Canyon (Rodolfo, 1975). Most of the large Ayeyawady sediment suspended load is redistributed by the strong tidal currents ( Fig.…”

“…Most of the large Ayeyawady sediment suspended load is redistributed by the strong tidal currents ( Fig. 5) and seasonally reversing wind currents to be deposited on the wide northern Andaman shelf (Ramaswamy and Rao, 2014) where it mixes with sediment from the Sittaung, Thanlwin and other smaller rivers (Damodararao et al, 2016). Semidiurnal tides vary between 2 and 3 m from the Pathein River to the Bogale River, reaching higher stages inside distributaries.…”

“…The bathymetric characteristics of the shelf and the circulation system favor deposition of fine fluvial sediments in a mud belt that widens from the western edge of the Ayeyawady coast into the Gulf of Mottama that more or less coincides in extent with the high-turbidity region (Ramaswamy and Rao, 2014). The outer shelf, including the Martaban depression, is a zone of low to non-deposition, and exhibits a relict morphology with topographic irregularities that host relict coarsely grained carbonate-rich sediment and fauna with patchy Holocene muds (Ramaswamy and Rao, 2014).…”