The tectonic environment of Mahu sag in Junggar basin was relatively stable in Badaowan stage of Early Jurassic,during which grooves and slopes were developed and the sediment supply was sufficient,providing conditions for development of braided rive delta.The sediments of the braided river delta that are characterized by coarse grain,low maturity,developed tractive-current depositional structure,incision and superimposition of multi-stage underwater distributary channel vertically and normal cycle of sedimentary sequence and so on,can be divided into 2 subfacies such as braided-river delta plain and braided-river delta front.On the plane,different slope gradients in different areas result in the differences of sizes of braided river deltas whose sizes are relatively large in steep slopes and relative small in gentle slopes.Vertically,different subfacies associations and sandbody configuration were developed at different stages of lake level rising.Braided-river delta plain was mainly found at the early stage of lake level rising;braided-river delta front was the dominant subfacies at the middle stage of lake level rising;mud-coated sand type of braided-river delta front was developed at the late stage of lake level rising.%早侏罗世八道湾期,准噶尔盆地玛湖凹陷构造环境相对稳定,沟槽和坡折发育,物源供给充足,为辫状河三角洲发育提供了条件.辫状河三角洲沉积具有粒度粗、成熟度低、牵引流沉积构造发育、垂向上多期水下分流河道切割叠置和呈正旋回沉积序列等特征,可细分为辫状河三角洲平原和辫状河三角洲前缘2种亚相.在平面上,由于不同地区坡度不同,而导致辫状河三角洲规模具有差异,陡坡带辫状河三角洲规模较大,缓坡带辫状河三角洲规模相对较小.在垂向上,在湖平面上升的不同阶段,辫状河三角洲发育不同的亚相组合和砂体构型,湖平面上升早期,发育辫状河三角洲平原亚相;湖平面上升中期,辫状河三角洲前缘亚相占据主导地位;湖平面上升晚期,发育“泥包砂”型辫状河三角洲前缘沉积.
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