Redefinition of the Yangtianba Formation-complex, Wenxian, Gansu and its geological significanceo

Since the Bikou Group has been set up, it has gone through disintegration many times. However, the disintegrated stratigraphic units are distributed mainly near Lueyang in the east of the West Qinling and it is not yet very clear about how they extend westward and how they correlate with other regional stratigraphic units. 1∶50000 regional geological surveys indicate that the Yangtianba Formation-complex-one of the main component units of the Bikou Group-should be disintegrated again. A whole-rock Rb-Sr age of 376±40 Ma, coral fossils and a wealth of microfloras have been obtained from different geological bodies in the area of the original Yangtianba Formation-complex. On that basis, combined with the differences in rock association, metamorphism and deformation, regional distribution and contact relationships, the original Yangtianba Formation-complex is further disintegrated and the new Yangtianba Formation-complex and Late Paleozoic structural stratigraphic blocks (Caoheba Formation, Zhaojiazui clastic rocks and Cambrian stratigraphic block) are mapped. It suggests that the original Yangtianba Formation-complex is a non-Smith stratigraphic unit, including stratigraphic units of different ages. It also proves the structural origin of the Bikou massif.