Sandwaves on the Southeast Vietnam Shelf recorded by high resolution seismic profiles: formation and mechanism

Abstract

The application of high resolution seismic data using boomer sound source has revealed a wide distribution of large-scale bedforms (sandwaves) on the Southeast Vietnam continental shelf. Bedforms that are a few meters high in wave height and hundreds of meters long in wavelength are primarily developed in the inner shelf (20-40 m) and considered to be formed under the presentday marine hydrodynamic conditions. Those bedforms developed in the deeper water (120 m) of the northernmost part of the continent can be interpreted as the relict morphological features formed during the latest sea-level lowstand of the late Pleistocene period. Two sediment transport paths have been identified on the basis of the bedform's leeward orientation: northeast-southwest (alongshore) and north-south (cross-shore). A quantitative bottom current map is constructed from sandwave dimensions, surface sediments and measurement data. The strongest current velocities that gradually decrease toward the southwest are indicated by large sandwaves in the north (field B). Water depth, surficial sediment composition and bottom current are three factors that control the development of bedforms.

Internal solitary wave-induced flow over a corrugated bed

“…In the ocean, the lower end of such a range is not typical of ISWs propagating over shelf topography, so the cases in which a/a corr = O(1) will not often be seen (oceanic ISWs typically have amplitudes in the range of 10 to 100 m and large amplitude sand ripples are typically 1 m to 2 m in amplitude (for example, see Carle andHill (2009), Schwab et al (2000)). However, note that there are observations of sand waves of 5 to 8 m in amplitude (Boggs et al 1974), reaching up to 12m (Bui et al 2009) in the Northern South China Sea). 1 Nevertheless, the fluid dynamical processes elucidated by the laboratory experiments, in combination with the numerical computations over an extended parametric scale are expected to be relevant to ISW-bottom topography encounters in shelf areas, where bathymetric effects are significant.…”

Surface sediment grain-size distribution and sediment transport in the subaqueous Mekong Delta, Vietnam

“…The monsoon consists of northeast winter monsoon (November to April), blows from the North and the Northeast at an average speed of 8 m/s while the Summer monsoon blows from the South and the Southwest at an average speed of 6 m/s (Ninh 2003;Dung et al, 2009). The Holocene sediments unconformably overlie on the late undivided Pleistocene facies (Ta et al, 2002a). The change of Holocene sediment facies reflected a rapid sea-level rise since the last glacial episode (Ta et al, 2001).…”

“…The Holocene formation consists of incised valley infilling and overlying deltaic sediments. The incised valley infilling sediments range from estuarine channel/tidal river sandy silt, muddy tidal flat/salt marsh, to estuarine sand and finally to the open bay muddy facies (Ta et al, 2001;2002a). The deltaic sediments have been formed since around 5.3 cal ka.BP in association with the decrease of sea level (Ta et al, 2001).…”

“…The deltaic sediments have been formed since around 5.3 cal ka.BP in association with the decrease of sea level (Ta et al, 2001). The deltaic facies includes bottom set, foreset, sub-to intertidal flat, foreshore, beach ridge/marsh and finally deltaflood plain (Ta et al, 2002a).…”

“…The progradation of the Mekong River delta has led to form a great flat plain of ~62,520 km 2 during the regression of relative sea level over the last 4550 yr BP (Nguyen et al, 2000), after the maximum highstand sea level was reached at ~ 2.0-2.5 m above the present sea level (Stattegger et al, 2013). The Mekong River delta was changed from tide-dominated delta to tide-wave-dominated delta during the Late Holocene (Ta et al, 2002a), and the delta coast has migrated ~200 km seaward since 8000 yr BP (Tamuara et al 2007 and. The rapid progradation of the delta system at a rate of 26 m/yr around the Ca Mau Cape (~200 km downstream from the Mekong River mouths) demonstrates that the wave influence has caused resuspended sediments to be bypassed into the Gulf of Thailand by the longshore current (Xue et al, 2010).…”