Characteristics of alkaline layer cycles and origin of the Lower Permian Fengcheng Formation in Mahu sag,Junggar Basin

Ancient Mahu lake was a typical carbonate saline lake in the Early Permian where abundant carbonate minerals( mostly with positive ion of sodium) were deposited. Based on the data of major elements,oxygen and carbon isotope,petrology and mineralogy characters of systematic samples of study core section,rhythm characteristics and their mechanism were discussed. The study core section alternated with soda layers and sedimentary tuff layers. Light soda layers are relatively pure nahcolite and sodium,and have major elements characteristics as follows: High content of Na_2O,low content of CaO,MgO,SiO_2 and Al_2O_3. Dark sedimentary tuff layers developed white spot shaped minerals of shortite,and have major elements characteristics as follows: Lower content of Na_2O,high content of CaO,MgO,SiO_2 and Al_2O_3. Both soda layers and sedimentary tuff layers have high values of [delta]~(18)O and [delta]~(13)C,which indicate that sedimentary water has high salinity during the depositional period of whole layers. Light sod