SUMMARY

The meso-cenozoic geological and tectonic character of Tràng An, Ninh Bình

Đỗ Tuyết, Nguyễn Đại Trung, Trần Tân Văn, Nguyễn Đình Hữu,
Đàm Ngọc, Đinh Tiến Dũng, Trần Minh Thuận, Trịnh Thị Thúy

The Tràng An Landscape Complex in Ninh Bình is admired by Việt Nam and foreign people for its outstanding landscape. Almost the whole area of Tràng An is composed of limestone. The limestone is pure with a high percentage of calcite, horizontal or gently dipping attitude, thin to medium-bedded. Especially, it is strongly affected by the Indochina folding and the tectonic movements of the Indian and Eurasian plates as well as the collision between the Western Pacific plate and the Asian plate creating a subduction contact that led to the formation of Cenozoic depressions on the Red River Delta and Red River basin in the Gulf of Bắc Bộ . The pre-Tertiary rigid foundation was subsided to the depth of over 5,000 m in the Hà Nội plain and nearly 20,000 m in the Gulf of Bắc Bộ. This event led to the inclination of the rigid foundation, including that in Tràng An area. Thus, in Tràng An, a system of faults trending in NW-SE, NE-SW, meridian and sub-meridian directions cuts through the limestone to create a cellular structure. Long-term karstification processes in a tropical monsoon climate with high precipitation has created favorable conditions for the outstanding landscape to develop here.

Tràng An is located on the shoreline of the Gulf of Bắc Bộ, and was several times reworked by sea transgressions in the Pleistocene. But the effects were expressed most clearly in the Holocene with the Flandrian sea transgression 5,000 years ago, of which the maximum sea level reached 6 m, when Tràng An was transformed into an archipelago. Then, the sea gradually regressed, isolating Tràng An on land leaving erosion notches on the bedrock at elevations of 3-4 m and 1.5-2 m. Three levels of sea level notches and stone roofs have formed with sea shells embedded at the elevations of 5-6 m, 3.5-4 m, 1.5-2 m. At a depth of 2 m, the sea regressed again to its present state. The lowest surface of Tràng An is the local base erosion level, and is also the present water table causing many karst valleys and closed depressions to become wetlands. Horizontal caves penetrate the mountains, connecting the valleys and closed depressions together into a continuous network of passages that carries tourists from the dreary, tranquil caves to the romantic realm of bright sky and sparkling water - a fairy land on Earth.

Người biên tập: GS.TS Trần Văn Trị.