STRATIGRAPHIC DIVISION AND CORRELATION OF THE MIDDLE AND UPPER PROTEROZOIC IN THE MIDDLE SECTOR OF THE NORTHERN MARGIN OF THE YANGTZE PLATFORM

The region discussed in this paper mainly involves the Yunyang area of northwestern Hubei and its neighbouring areas in Henan and Shaanxi Provinces, where there occur chiefly the middle and upper Proterozoic. Regionally corresponding strata of the Shennongjia area to the south, southern Shaanxi and northern Sichuan to the west and the Suixian-Zaoyang area to the east of the Nanyang-Xiangfan basin are properly touched. Comprehensive field and indoor studies on more than ten sections have shown the existence of a significant unconformity between the middle and upper Proterozoic in this region. The counterpart of this unconformity in Henan is the angular unconformity between the Yaoyingzhai Formation and the underlying Douling Group. This boundary was determined before in northwestern Hubei, so the stratigraphical correlation between the two provinces could not be made well. The above-mentioned unconformity is the most important boundary between the middle and upper Proterozoic in this region. The upper boundary of the Wudang Group would be limited by the unconformty. According to the isotopic age and some microfossil plant data, the age of the Wudang Group is in the range from 1600-1700 to 1000 Me, which is largely equivalent to the time range from the Changcheng System to Jixian System in North China. The strata corresponding to the Qingbaikou System are absent in this region and the Sinian System is continuous and may be divided into four stratigraphic units in ascending order, （1）the Yaoyingzhai Formation, incl- uding the Yaoyingzhai Formation of Henan and the volcanosedimentary coarse elastic association rejected from the top of the original Wudang Group; （2） the Yaolinghe Group, made up of marine basic volcanic rocks and a deep-water siliceouspelitic and fine-grained turbidite association and containing glaciofluvial deposits in the lower part; （3） the Doushantuo Formation consisting of turbidity sandstone, siltstone and deep-water pelite and micrite; （4） the Dengying Formation, represented by a dolomite association with carbonate gravity flow deposits.