The Definition of Chengjisihanshan Formation and Its Significances in Baijiantan Region,West Junggar,Xinjiang

There exists a normal succession of volcanic-sedimentary strata in Baijiantan region, West Junggar. Based on the 1:50000 regional geological survey and measured 1:5000 section, the rock assemblage is found consisted of gravel sandstone, sandstone, tuffaceous sandstone, siltstone and tuffaceous siltstone in the lower part. Interbeds of basalt and siliceous rock coupled with local flagstone constitute the upper part, which is more than 570 m thick and shows angular unconformity overlapping the Early Carboniferous Baogutu Formation and Xibeikulasi Formation. The fossil and volcanite zircon U-Pb isotope dating indicates that this formation occurred in a period later than Late Carboniferous Moscovain. It is distributed from north of Da'erbu'te river to Sa'ertuohai and can be easily recognized on the outcrop. The rock and fossil assemblages are different from the Upper Carboniferous Hala'alate Formation. According to the definition and establishment requirements about "formation" in Stratigraphic Guide of China, the Chengjisihanshan section in Baijiantan region is assigned as stratotype and defined as Chengjisihanshan Formation.