SUMMARY

On the age and forming conditions of the Thiên Nhẫn Formation
in North Trung Bộ, Việt Nam

Shigeki Hada, Trần Văn Trị, Đoàn Nhật Trưởng

The Thiên Nhẫn Formation, distributed in North Trung Bộ, Việt Nam, was established by Mareitchev and Zhamoida A.I. [1] and referred to Lower-Middle Triassic, but without the stratotype.

The lectostratotype described in this paper by the authors extends from the Thiên Nhẫn Mt to Thịnh Hương, Hương Sơn District, Hà Tĩnh Province. The formation is characterized mainly by cherty shale, bedded chert and chert bearing manganese, over 800 m thick. These sediments are parallely thin-bedded, thin-banded and rhythmic of turbidite-type, containing deformed and fragmented fossils of radiolarians and conodonts. However, they can be attributed to the families Entactiniidae and Albaillellidae of Middle-Late Paleozoic and Ozarkodina (Spathognathodus) sp., especially Palmatolepis sp., close to P. glabra lepta of Late Devonian age.

The above sediments and fossils characterize the offshore deep-water environment of the continental slope and ocean of the Viet-Lao Paleotethys basin extending to Thailand.

Ngày nhận bài: 7-8-2004

Người biên tập: Vũ Khúc