The Identification of the braided fluvial delta in the 3rd Member of Badaowan Formation in Shixi area and a study of its sedimentary characteristics

The sedimentary facies of 3rd Member of Lower Jurassic Badaowan Formation in Shixi area of Junggar Basin was previously regarded as meandering river delta.The authors consider it to be the product of tractive current under fairly strong hydrodynamic conditions,as evidenced by the texture,sedimentary structure and distribution characteristics of the sandstone.Its sedimentary environment belongs to the braided fluvial delta.Based on a synthetic study of its sedimentary characteristics,the authors identified 2 subfacies and 5 microfacies in the study area,which include the braided delta distributary channel,braided delta flood plain swamp,braided delta subaqueous distributary channel,tributary bay,and mouth bar.The distribution characteristics and evolution processes of sedimentary facies are inferred as follows: during the early stage the water level fell,the study area developed braided delta front;in the middle stage the water level continued falling,the study area developed braided delta plain;in the late stage the water level rose,and the sediments became thinning.The northwest Halaalate Mountain,Delun Mountain and Zhayier Mountain located in the margin of the study area constituted the main material sources.