The sedimentary evolution of distributive fluvial system in arid and low accommodation space sag: A case study of the lower fourth sub-member of Shahejie Formation from Bonan Sag in Zhanhua Depression, Bohai Bay Basin

Abstract The distributive fluvial system (DFS) mainly dominates sedimentary process of arid zones and is quite different from fans of mild climate zones. recent researchers focused on DFSs and try to reveal development law of DFSs in arid zones. However, different time scales of research from modern DFSs, ancient DFSs and physical simulation experiment induced debates on the sedimentary character and evolution of ancient DFSs. This paper constructs sedimentary model of ancient arid DFSs in constrain with sequence stratigraphy and sag structure for providing a new way to understand the ancient DFSs in arid zones. During the period of lower fourth sub-member of Shahejie Formation (Es4 x ), arid climate and low accommodation space developed in Bonan Sag. That provide well natural conditions for investigating the ancient DFSs in arid zones. The unique evolution of sequence stratigraphy and distinctive sag structure constructed two type distributive fluvial system in different topography in Bonan Sag during the period of Es4 x . In the gentle slope belt, the DFS transported sediments in a long distance and wide range. The DFS developed in large scale and thin sand body during the transgressive system tract (TST). As the sediment supply decreased, retrogradation and distal fan developed in the DFS during the regressive system tract (RST). The DFS of gentle slope belts shortened and narrowed during the RST stage. In the steep slope belt, more accommodation spaces generated with the high gradient. The concentration of accommodation constructed thick DFS with restricted distribution during the TST stage. During the RST stage, the sediment supply decreased and accommodation spaces became low and migrated with the evolution of sequence stratigraphy and sag structure. In response to the evolution of sediment supply and accommodation spaces, the DFS developed progradation in space with thinner sand body.