THE MEGA-DELTAS OF ASIA: A DIFFERENT MODEL FOR DELTAIC SYSTEMS?

Seven of the world’s ten largest rivers originate on the Himalayan/Tibetan uplift and form massive deltas along the Asian margin. Research on these mega-deltas over the past decade has revealed many common traits that distinguish them from most other deltas. One distinction is that the young, elevated source areas are relatively proximal and yield an immense and relatively coarse sediment load. Downstream, this load is distributed widely across the margin under high-energy fluvial and coastal regimes. The resulting wedge of deltaic deposits is compound in nature, comprising distinct subaerial (coarse) and subaqueous (fine) components. Both of these clinoforms prograde, but they are separated by a zone of little or no deposition maintained by strong bed shear on the inner shelf. Over millennial timescales, the Asian mega-deltas are sensitive to variations in monsoon circulation, which drives 90% of the precipitation and river discharge. During the Late Quaternary, riverine sediment loads varied an order of magnitude in response to changing strength of the wet summer monsoon. Thus, delta stratigraphy has been controlled as much by climate as by the rivers and sea level. Furthermore, the synchronicity of post-glacial transgression and peak summer monsoons has also been a critical factor in sequence development, causing contemporaneous increases in sediment supply and accommodation. In many of the Asian mega-deltas, tectonics also play an important role in the distribution and preservation of deltaic deposits, yielding distinct sub-basin stratigraphies within the system. Overall, regional controls such as the monsoon-driven climate, catchment geology, and high-energy coastal setting contribute to the unique character of Asia’s mega-deltas as compared with better-documented systems in the West.