Formation of the Mekong Subaqueous Delta: proximal versus distal accumulation
Abstract
Extensive geophysical and geochemical surveys in 2006-2015 conducted off the Mekong Delta on the inner and portions of the adjacent continental shelf show the Mekong River has formed a classic sigmoidal cross-shelf clinoform, up to 15 m thick, with topset, foreset and bottomset facies, but constrained to water depths of <20 m. Beyond this depth, the East Sea/western South China Sea shelf is dominated by relict sand and gravel with patches of early to middle Holocene mud deposits. Parallel to shore, the Mekong-derived sediment has extended >250 km southwestward to the tip of the Ca Mau Peninsula, forming a distal mud depocenter up to 22 m thick, and extending into the Gulf of Thailand. A large erosional trough or channel (up to 8 m deeper than the surrounding seafloor and parallel to the shore) was found on the top of the clinoform, east of the Ca Mau Peninsula. Compared to other tide-dominated fluvial dispersal systems, the Mekong River system has a relatively young (≤1000 yr) subaqueous delta, a shallow rollover at 4-6 m water depth, gentle foreset gradients (0.03-0.57o), and a short cross-shelf dimension of 15-20 km within 20-m water depths. The total estimated volume of the Mekong River subaqueous clinoform on the shelf is estimated as ∼120 km3, which is equivalent to ∼120-140×109tons of sediment using an average sediment dry-bulk density of 1.0-1.2 g/cm3. The calculated millennial-timescale average sediment discharge to the shelf is ∼120-140×106 tons per year. Spatially, the proximal subaqueous delta has accumulated 33% of sediment; the distal part around the Ca Mau Peninsula has received ∼42% of sediment; and the remaining 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 2 cm/yr.
- Publication:
-
EGU General Assembly Conference Abstracts
- Pub Date:
- April 2018
- Bibcode:
- 2018EGUGA..20.5344L