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

Key words: 
• Junggar Basin /
• Mabei area /
• Triassic /
• Baikouquan Formation /
• fan delta /
• depositional model

Abstract: 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.