THE DEFINITI ON OF THE LONGTAN FORMATION AND THE PRINCIPLE OF ITS DIVISION——A DISCUSSION

The Longtan Formation is the most important coal-bearing strata in southern China.The type locality is Longtan,Nanjing.The Longtan Formation was originally called the Longtan coal measures established by Liu Jichen et al. （1924）.Later,Sheng Jinzhang（1962）changed the Gufeng bed and the Longtan coal measures into formations according to the demands of the stratigraphic code.But a late Maokou-aged typical fusulinid fauna dominated by Neomise- llina was found in sandy limestone at the top of a coal-barren member in the lower part of the so-called Longtan Formation.So according to that com- blued with the lithology and contact relationships between these strata,the present author（1962）suggests that this member should be separated from the formation and named formally the Yanqiao Formation so as to represent a special sedimentary type dominated by late Maokou-aged clastic rocks loca- lly with coal in southern China.By doing so,the original connotations of the Longtan Formation and the Gufeng Formation can be restored and retained, and the traditional ideas that no coal was formed in the Maokouian stage and that there cannot exist a facies change relation between carbonate rocks and clastic rocks have been challenged. Those that advocate multiple stratigraphic division suggest that the Long- tan Formation should be restricted to the member of coal-bearing clastic rocks and assigned to the Lower Permian or extend to include the strata between the Gufeng Formation and the Lower Triassic Qinglong Group.Both were de- posited continuously. The existence of different opinions and disputes about the definition and age of the Longtan Formation is due to the use of two different concepts of stratigraphic division besides that there are different understandings as to whe- ther Altudoceras and Paraceltites are typical enough. At present,the determintion of the ages of the strata and stratigraphic division and correlation mainly depend on the paleontological method combined with various stratigraphical methods.In this paper a unified concept of stra- tigraphic division is used and the natural boundary of the“events that occur- red”is selected as the method and criteria for defining the Longtan Forma- tion and its age.