SUMMARY
Modern sedimentology for analyzing Cenozoic basins in deep sea areas of Việt Nam

Trần Nghi, Đinh Xuân Thành, Trần Thị Thanh Nhàn, Trần Hữu Thân,
Phạm Thị Thu Hằng, Nguyễn Thị Phương Thảo, Nguyễn Duy Tuấn, Trần Thị Dung

Modern sedimentologic study in analyzing Nam Côn Sơn, Phú Khánh, Tư Chính -Vũng Mây, Trường Sa Cenozoic basins currently situated in deep waters (500-3000 m) is a systematic approach to find out the cause-effect relationship between cyclic sedimentary evolution and cyclic thermal subsidence:

1/ According to their vertical structure, each of Cenozoic deepwater basins can be divided into 6 secondary basins corresponding with 6 sequences:

- Eocene - Early Oligocene and Late Oligocene secondary basins: linear thermal subsidence of intracontinental graben type.

- Early Miocene, Middle Miocene and Late Miocene secondary basins: cyclic expanded thermal subsidence.

- Pliocene - Quaternary secondary basins: monoclinal differentiated thermal subsidence creating the modern continental shelf and continental slope characterized by formations ranging from terrigenous-carbonate sediments in the shallow offshore area to terrigenous, calcareous-siliceous mud, pyroclastic, turbidite fan sediments in deep sea area at the foot of the continental slope.

2/ The boundaries of the secondary basins are determined based on the sediment discontinuity surfaces due to sea-level change and tectonic uplifting and erosion phases. They are equivalent to sequence boundaries and are also depositional cycle boundaries. Each sequence can be divided into 3 systems tract in relation to sea level change: lowstand systems tract or lower regression (LST), transgressive systems tract (TST) and highstand systems tract or higher regression (HST).

3/ Single-lithofacies and double-lithofacies formulas have been determined for system tracts. They are an important key to explain why the sediments at the centers of the Nam Côn Sơn and Phú Khánh basins, which used to be considered as deep sea sediments are of greatest thickness (10-12 km) and of parallel horizontal structure characterizing a shallow sea environment. This is explained that in the depositional environment there are always bottom currents which transport and redistribute terrigenous sedimentary materials brought by rivers, therefore each sediment layer contains double lithofacies consisting of alternating deltaic and marine lithofacies(amr + mr) or (amt + mt)

4/ Analyzing the deformation types of secondary basins and reconstruction of their sections before compiling lithofacies-paleographic maps by systems tracts is considered as the principal procedure in modern sedimentologic studies. It is a scientific basis for evaluating petroleum systems.

Người biên tập: PGS.TS Tạ Hòa Phương.