A seismic study of the Mekong subaqueous delta: Proximal versus distal sediment accumulation

Abstract

The Mekong River Delta is one of the largest in Asia. To understand its sediment distribution, thickness, mass budget, stratigraphic sequences and sediment-transport process, extensive geophysical and geochemical surveys were conducted on the inner portions of the adjacent continental shelf. Analyses of > 80 high-resolution Chirp-sonar profiles show the Mekong River has formed a classic sigmoidal cross-shelf clinoform in the proximal areas, up to 15m thick, with topset, foreset and bottomset facies, but constrained to water depths of < 20m. Beyond this depth, the East Sea/western South China Sea shelf is dominated by relict silt, sand and gravel with patches of early to middle Holocene mud deposits. Parallel to shore, the Mekong-derived sediment has extended > 250> 300 km southwestward to the tip of the Ca Mau Peninsula, forming a distal mud depocenter up to 22m thick, and extending into the Gulf of Thailand. A large erosional trough or channel (up to 8m deeper than the surrounding seafloor and parallel to the shore) was found on the top of the clinoform, east of the Ca Mau Peninsula.Based on the thicknesses and distribution revealed by Chirp sonar profiles, the total estimated volume of the Mekong River subaqueous clinoform on the shelf is ~120km3, which is equivalent to ~120–140 × 109 t of sediment using an average sediment dry-bulk density of 1.0–1.2g/cm3. Assuming the subaqueous deltaic deposit has formed within ~1000yr, the calculated millennial-timescale average sediment discharge to the shelf could be 120–140 × 106 t per year. Spatially, the proximal subaqueous delta has accumulated ~45 × 109 t (~33%) of sediment; the distal part around the Ca Mau Peninsula has received ~55 × 109 t (~42%) of sediment; and the remaining ~35 × 109 t (~25%) has accumulated within the central transition area, although the coastline and shoreface in this area are presently eroding. The spatially averaged 1000-yr-scale accumulate rate is up to 2cm/yr.Compared to other tide-dominated fluvial dispersal systems, the Mekong River system has a relatively young (≤1000yr) subaqueous delta, a shallow rollover at 4–6m water depth, gentle foreset gradients (0.03–0.57°), and a short cross-shelf dimension of 15–20km within 20-m water depth. Like the Amazon, Po, and Yangtze rivers, the Mekong River has developed a pervasive along-shelf deposit, which in this case extends > 250> 300km to the southwest as a result of the superimposed tidal processes, wave-induced resuspension, and a strong low-flow season coastal current.