Analyses of the Significances of Jurassic Coal Layers in Sequence Stratigraphy in Junggar Basin

Mainly three sets of coal layer assembles developed in Junggar Basin Jurassic strata, respectively lying in the lower section of Badaowan group,the down part of the upper section of Badaowan group and the bottom of Xishanyao group.In the division proposal of the Jurassic sequence stratigraphy of Junggar Basin,sb1,sb2,sb4 boundaries lay in the lower section of Badaowan group,the down part of the upper section of Badaowan group and close to the three coal layer assembles at the bottom of Xishanyao group. The accumulation of coal varies greatly due to the differences in required accommodation which supplies space for coal accumulation in different system tracts of different sequences as well as in different periods of the identical system tract,and the differences in coal layer developments which accommodate quick accumulation,with the differences in sedimentary environments.Thus there are time-penetrating phenomena in the Jurassic coal layers, and they may develop in each system tract of the sequence.Therefore,the Jurassic coal layers mainly developed in (1)the lowstand and highstand system tracts of sequence Ⅰ.(2)the lowstand system tract of sequence Ⅱ.(3)the highstand system tract of sequence Ⅲ.(4)the lowstand system tract of sequence Ⅳ locally.