The Discovery of Conodont Fossils in Dahuoluo Area in Ganau and the Related Date Determination for Baiyunshan Formation

The Baiyunshan Formation exposed in the Dahuoluo area of Beishan, Gansu Province is a set of clastic rock assemblages of slate, sandstone, glutenite, conglomerate, and locally intercalated limestone. Due to the lack of stratigraphic basis, it is classified as the Ordovician Xilinkebo Formation (a set of shallow marine sediments dominated by siliceous rocks, intercalated with limestone, sandstone and glutenite). Through field route geological survey and profile measurement, well-preserved conodont fossils have been obtained in the limestone interlayer of the Baiyunshan Formation. They are Periodon grandis, Protopanderodus sp., Drepanoistodus sp., Spinodus sp., Scabbardella sp., and the age of the rock assemblage is limited to the Late Ordovician. Based on the analysis of rock assemblage characteristics, sedimentary sequence and sedimentary facies, it is considered that the Baiyunshan Formation was formed in the piedmont foreland basin environment, indicating that the Beishan area has entered the initial orogenic stage in the Late Ordovician, which provides an important basis for improving the stratigraphic framework of Beishan.