The Nậm Mặn Formation and equivalent units in the Suối Bng Formation in West Bắc Bộ

Bi Ph Mỹ, Đon Nhật Trưởng, Nguyễn Đnh Hữu, Nguyễn Văn Honh,

Nguyễn Văn Lồng, Nguyễn Vĩnh, Phan Sơn, Trần Đăng Tuyết

The Nậm Mặn Formation was established in 2001 by authors of the Mường T Geological and Mineral Map sheet group at 1:50,000 scale with the editorship of L Hng. According to these authors, this formation (Fm) covers directly Upper Paleozoic granite and grades upward into the Suối Bng Fm (T3n-r sb). The Nậm Mặn Fm yields Carnian and Norian bivalves, therefore, it was dated as Late Carnian-Norian.

Authors of the article New materials on the Nậm Mặn Fm, Mường T area [2] organized a survey on the stratotype of this Fm in March and April, 2011, and collected two fossil collections of: Late Carnian corals and Norian bivalves. Results of this survey have been confirming that in this area there are terrigenous-carbonate beds described in the Nậm Mặn Fm [5, 6]; this is just the lower part of the Suối Bng coal-bearing Fm [4].

After the examination of sections of the Suối Bng Fm, in the present article the authors would like to present the following ideas:

1/ Sections of the Nậm Mặn Fm is equivalent to the lowest part of the Suối Bng Fm in the whole West Bắc Bộ region;

2/ Existing related materials in the whole West Bắc Bộ region allow us to define:

- The Nậm Mặn Fm and the lowest part of the Suối Bng Fm have the youngest age as Early Norian. The Suối Bng Fm began to be formed in this age, but it is not excluded the possibility of the Late Carnian age, very Late.

- The stratigraphic gap between the Suối Bng Fm and older beds must exist in Late Carnian, or between Late Carnian and Early Norian.

Người bin tập: GS.TSKH Vũ Khc.