准噶尔盆地西北缘玛北地区百口泉组扇三角洲沉积模式

准噶尔盆地西北缘二叠纪-三叠纪强烈的同生断裂活动和逆冲推覆构造使西北缘三叠系为大型内陆坳陷湖盆沉积, 发育多级坡折, 大型坳陷湖盆坡折带控制了玛北地区扇体的发育。通过岩芯、铸体薄片及测井资料, 结合地层超覆叠置、岩性纵向组合及砂体的展布分析可知玛北地区三叠系百口泉组发育典型的扇三角洲沉积, 根据沉积相标志和扇三角洲内部成因沉积单元的精细刻画, 识别出3种亚相和11种微相:即扇三角洲平原亚相的水上泥石流砾岩微相、辫状河道砂砾岩微相、平原河道间砂泥岩微相;扇三角洲前缘亚相的水下主河道砾岩微相、水下河道砂砾岩微相、水下泥石流砂砾岩微相、水下河道间砂泥岩微相、水下河道末端砂岩微相、河口坝-远砂坝砂岩微相;前扇三角洲亚相的前扇三角洲粉砂岩微相和前扇三角洲泥岩微相。玛北地区三叠系是在湖侵背景下多级坡折控制的扇体, 扇三角洲是重力流与牵引流综合沉积作用的结果, 向北东物源方向呈退积沉积, 是一种典型的湖侵退积型扇三角洲。

Fan Delta Depositional Model of Triassic Baikouquan Formation in Mabei Area,NW Junggar Basin

The strong intensely syn-sedimentary faulting and thrust-nappe structures resulted in the northwestern margin of Junggar Basin belongs to the large-scale landlocked depression basin in Late Permian-Triassic Period, within multistage slope-break belt, which controled the development of fan body in Mabei region. Based on the core observation, rock thin sections and logging,combined with the stratigraphic overlap, the longitudinal lithologic combination and the distribution of sand body concluded that the typical fan delta deposits were developed in Triassic Baikouquan Formation of Mabei area. According to sedimentary microfacies markers and detailed depiction of internal sedimentation unit of fan delta can identify 3 subphases and 11 microfacies:three kinds of submicrofacies named fan delta plain, fan delta front and prodelta. The fan delta plain included super-aqueous mudslides conglomerate microfacies, braided channel glutenite microfacies, plain interchannel sand-mudstone microfacies. The fan delta front can be divided into sub-aqueous distributary channel conglomerate microfacies, underwater channel glutenite microfacies, sub-aqueous mudslides glutenite microfacies, underwater inter-channel sand-mudstone microfacies, the end of the underwater channel sandstone microfacies, mouth bar-distal sandbar sandstone microfacies, and as well as the prodelta can be include pro-fan delta siltstone microfacies, pro-fan delta mudstone microfacies. The fan delta system which formed sudden flood alternating with normal water flow obviously controlled by the slope breaks under the background of lake invasion in Mabei area, and also characterized by retrogradation deposition toward to the NE source direction. The fan body is a kind of a lake invasion retrogradtional fan delta in the study area.