Sedimentary facies of distributary channels of the whole Mekong River delta
Yoshiki Saito1, Van Lap Nguyen2, Thi Kim Oanh Ta2, Toru Tamura3, Yutaka Kanai4, Rei Nakashima4 and Katsuto Uehara5, (1)Estuary Research Center, Shimane University, Matsue, Japan, (2)HCMC Institute of Resources Geography, VAST, Ho Chi Minh City, Vietnam, (3)AIST - National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan, (4)AIST, Geol Survey of Japan, Tsukuba, Japan, (5)Kyushu University, Fukuoka, Japan
Abstract:
The Mekong River delta, one of the world’s largest deltas, has extended from Phnom Penh in Cambodia (apex) to the Vietnam coast from the Saigon River mouth in its eastern end to Cape Camau in its western end, and has the delta plain area of > 50,000 km
2. The delta has prograded more than 200 km for the last 6–7 ka. The river-mouth area of the delta is meso-tidal with the mean tidal range of 2.5 ± 0.1 m and the maximum tidal range is 3.2–3.8 m. Sea level in winter (dry season) is higher than summer by 30-40 cm due to setup in the South China Sea basin by northerly. Mean wave height is 0.9 m. The water discharge of the Mekong is 470 km
3/y and its sediment discharge is 160 million t/y, or tenth and ninth largest in the world, respectively. The water discharge varies by season, controlled by a monsoonal tropical-subtropical regime. The flow at Phnom Penh, Cambodia, reaches a maximum in October and a minimum in May. SSC has also a seasonal change from more than 300 mg/L in a wet season to less than 50 mg/L in a dry season at the Cambodia-Vietnam border. ~1m tidal water-level changes are observed at the border in a dry season.
To understand the combined influences of rivers and tides on river-bottom sediments, we have collected ~210 surface samples from distributary channels of the whole Mekong River delta in Vietnam from the Cambodia border to five river mouths during a dry season from January to May 2015. Additional sampling expedition in a wet season will be conducted in October 2015 for one distributary channel from the border to its river mouth. The characteristics of channel bottom sediments in a dry season show clearly river- and tide-dominated areas spatially, based on sedimentary facies (grain size and sedimentary structures). Tidal rhythmites are well recognized within ~100km from the river mouths and mud balls are well found in the middle reach of the survey area. The spatial distribution of river- and tide-dominated areas is closely linked with the morphology and system of distributary channels; e.g., a deepening trend to a shallowing trend in distributary channels towards the river mouths, respectively.
