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[HCG28-03] Sediment facies and environment in distributary channels of the Mekong River delta, Vietnam
Keywords:distributary channel, tide-dominated delta, sediment facies, tidal river, backwater, tidal rhythmite
The result shows clearly two main tracts in the river channels in Vietnam; an upstream, fluvial-dominated tract and a downstream, tide-dominated tract, and these tracts are divided into two subzones respectively. From upstream to downstream, the four subzones are identified: fluvial-dominated, tide-affected; fluvial-dominated, tide-influenced; tide-dominated, fluvial-influenced; and tide-dominated, fluvial-affected.
Tidally induced water-level changes affect the entire study area and extend into Cambodia. Salinity intrudes ~15 km upstream of the river mouth during the wet season, and ~50 km upstream during the dry season. Brackish water species of mangroves, mollusks, and diatoms, however, occur landward of these limits, suggesting that highly diluted brackish water may reach ~160 km upstream of the river mouth during the dry season. In the fluvial-dominated tract, channels are sinuous and show a seaward-deepening trend, whereas width is relatively constant. In the tide-dominated tract, channels are straight, and show seaward-widening and seaward-shallowing trends. Natural levees are present in the fluvial-dominated, tide-affected subzone, but are replaced by mangroves. In the fluvial-dominated tract, mud content is low, sand grain size fines seaward, and gravelly sand and sand are dominant facies. In the tide-dominated tract, mud content is high, sand grain size is constant, recycled sand is common, and tidal rhythmites are the dominant facies. Mud pebbles are common in sediments of a large part of the study area.