The discovery of manifold kind fossils in the Bayan Obo Group in Inner Mongolia and the establishment of the Kuangou Group

There are the largest rare-earth ore deposit in the world and the extremely complicated earth crust structure in the Bayan Obo area. Through three years' systematic investigation in the study area, the following progress has been made: the stratigraphic sequence of Bayan Obo Group is amended and reduced from 18 members to 15 members. They are as follows: Dulahala Formation (H_1~3), Jianshan Formation (H_4~5), Halahuogete Formation (h_6~8), Bilute Formation (H_9~10), Baiyinbulage Formation (H_11~12), and Hujiertu Formation (H_13~15). The Bayan Obo Group is unconformably underlain by the Lower Proterozoic metamorphosed rock-basement rocks, and in turn it is covered by Boligemiao Formation of upper Carboniferous Series. The microflora, predominated by Micrhystridium, Baltisphaeridium, are firstly discovered in the black slate and limestones of Dulahala Formation and Jianshan Formation. Their assemblage features may be correlated with the Lower Cambrian microflora found in Kunming region of Yunnan Province as well as with the fossil found in some regions in Europe. These microflora possesses obvious Early Cambrian character. Many species and genera of microanimal fossils have been firstly found in the siliceous rocks, intercalated with the black slate of Jianshan Formation. The well preserved subclass Tabulatomoepha fossils of species Lichenaria sp. and Rhabdotetradium sp. have been firstly collected from Halahuogete Formation (H₈), together with Porifera Archaeocuphia sp. and trace fossils. The assemblage of Lichenaria-Rhabdotetradium-Archaeocyphia was found from Early-Middle Ordovician strata in North China Platform. Based on the palae-ontological animals and flora fossils, the Bayan Obo Group may be subdivided into two parts: Lower Cambrian and Lower-Middle Ordovician. The lower part of Bayan Obo Group is assigned to the narrow Bayan Obo Group (H_1~5), and the upper part of Bayan Obo Group is named after the Kuangou Group (H_6~15). So, the discovery of manifold kind fossils of the Bayan Obo Group in Inner Mongolia and the establishment of the Kuangou Group are of great significance for the study of the stratigraphy and geological structure in this region.