THE LOWER BOUNDARY OF THE SINIAN AND ITS RELATED PROBLEMS DISCUSSED IN THE LIGHT OF THE ESTABLISHMENT OF THE MACAOYUAN FORMATION AND LTS CORRESPONDING STRATIGRAPHIC POSITION

In this paper,on the basis of the data from two sections of the Huashan Formation in the Dahongshan area and the Macaoyuan Formation in the Shennongjia area of Hubei province, coupled with the data of the Precambrian of adjacent areas,the existence of the Macaoyuan Formation and its corresponding stratigraphic position is expounded and proved through an analysis of the geological contact relationships,sedimentary formations, isotopic ages and microflora.The Macaoyuan Formation is assigned to a formation at the base of the lower Sinian.The Sinian System is divisible into the upper and lower series.The base of the Doushantuo Formation represents the boundary between the upper and lower series.The lower series consists of the Macaoyuan,Liantuo and Nantuo Formations.The upper series includes the Doushantuo and Dengying Formations.The Nantuo Formation comprises the lower glacial drifts,inter-glacial drifts（corresponding to the Datangpo Member）and upper glacial drifts. The Jinning movement is discussed on the basis of an analysis of the isotopic ages and regional geological structure in southern and western China. The author considers that the unconformity between the Macaoyuan Formation and the pre-Sinian Shennongjia Group represents the Jinning movement. This unconformity,i.e.the base of the Macaoyuan Formation,is just the lower boundary of the Sinian System, which has an isotopic age of about 850 m.y.Meanwhile,the author points out that the Macaoyuan Formation should. be placed above the pre-Sinian Qingbaikou System. The Liaonan Group, Fuzhou Group and Nanfen Formation in Liaoning province may be roughly correlated with the Dengying to Macaoyuan Formations.