SUMMARY

Characteristics of the sediments and sea level fluctuations during Holocene

in the Sông Cửu Long Plain

Đỗ Văn Lĩnh, Nguyễn Huy Dũng, Nguyễn Huỳnh Thiên Hương, Phạm Thế Tài,

Nguyễn Đan Vũ, Trịnh Minh Phuông, Trần Văn Toàn, Bùi Thế Vinh, Huỳnh Hồ

Holocene sediments in the Mekong delta are characterized by 5 stratigraphic units: Q₂¹; Q₂²₁; Q₂²₂ ; Q₂³₁; Q₂³₂, and correspond to thickness intervals of 0.5-40 m, 1-33 m, 0.8-25 m, 0.5-22.5 m, 0.5-10 m respectively. The stratigraphic units are mainly found in boreholes (Q₂¹, Q₂²₁) and exposed to surface (Q₂²₂ , Q₂³₁, Q₂³₂); section in coarse to fine grained interbedding type, fine to coarse grained interbedding type to which contains much organic matter. Some mainly sedimentary structures consist of bedding types as horizontal/cross bedding (Q₂¹), parallel/cross bedding (Q₂²₁), discontinous parallel wavy bedding or chaotic structure (Q₂²₂), discontinous parallel wavy bedding (Q₂³₁), irregular parallel bedding and chaotic structure (Q₂³₂); colors of sediments are white grey, greenish grey, yellow, brown and black. Sedimentary depositional environments are fluvial (a), fluvial - marine (am) and marine (m). The average rate of sedimentary accumulation changes from 0.11 mm/year minimum to 11.5 mm/year maximum, and average rate is 5.8 mm/year with decreasing trend from Middle Holocene to present.

The sea level fluctuation in the Mekong Delta included 2 great periods: From Late Pleistocene-Early Holocen to 6.0 Ka BP was the trangressive great period and 6.0 Ka to present has been the regressive great period. The maximum sea level of transgression was 4.0-4.5 m higher than the present sea level at 6.0 Ka BP. The regressive great period with 04 periods of time: 1) 5,7-4,5 Ka; 2) 3,5-2,5 Ka; 3) 2,5-1,5 Ka; 4) 1,5 Ka to present. The altitude of traces of sea level of concave limestone corresponds to period 1 and 2 is 2.8-3.3 m and 0.7-1.3 m, respectively. The period 1 and 4 are still unclear the traces of sea level of long concave limestone. At present, sea level is rising trend, this may forecat that the sea level rise in the future with maximum of 4,0-4,5 m and it will take more 600 years in future time if a repeat of velocity of 7,5 mm/year as in the past. This means that, in the next 100 years, the sea level may rise by 0,75 m in the Mekong Delta.

Người biên tập: TS. Nguyễn Tiến Hải