Abstract

Pưa Lai, Nậm Mặn sections, Pác Ma, Tắt Héo limestones and some stratigraphic equivalents are considered as the lowermost part of the Suối Bàng formation in West Bắc Bộ. Among them, Pưa Lai, Nậm Mặn sections are characteristic. According to the new data, they  are Late Carnian –  Early Norian in age, formed not at the same time and overlying unconformably the older rocks, such as Permian limestones or Paleozoic granites.... The deposits in Pưa Lai and Nậm Mặn sections yield fossils but they are different in rock facies and elements of faunas. At Pưa Lai occur black gray low-carbonaceous terrigenous rocks, containing Pelecypoda, Ammonoidea. At Nậm Mặn occur highly carbonaceous rocks, lens of multi-colored limestone, containing in addition Corallia, Brachiopoda. According to the above mentioned data and the last literatures we consider that the lower part of the Suối Bàng formation is of two types of section: Pưa Lai and Nậm Mặn.