The sequence stratigraphy and depositional characteristics of fan-delta complexes in the Upper Bayingebi Member (Lower Cretaceous) in Chagan Sag, Inner Mongolia, China

The Chagan Sag is a small Early Cretaceous intracontinental passive rift subbasin in the Yingen-Ejinaqi Basin Group generated on the Tianshan-Xingmeng Orogenic Belt. The Lower Cretaceous strata contain the Bayingebi, Suhongtu, and Yingen formations deposited during the syn-rift phase. The Wuliji Structural Zone bounded by a high-dip syn-sedimentary fault is an intensively studied area with an extent of similar to 300km(2) having abundant available subsurface dataset and great hydrocarbon potential. According to well- and seismic-based sequence stratigraphic study, the Upper Bayingebi Member generally corresponds to a complete third-order transgressive-regressive sequence in the Wuliji Structural Zone. The transgressive system tract gradually thins towards the north, but the regressive system tract covers the entire study area in this third-order sequence. Further investigations reveal that tectonic subsidence and climate change are two predominant allogenic factors for controlling stratigraphic development in the Wuliji Structural Zone. During the deposition of the Upper Bayingebi Member, transverse gravity flow dominated fan-delta complexes sourced from the Maodun Subuplift are well developed in the Wuliji Structural Zone. The retrogradational conical fan deltas are mainly located in the southern sector during the transgressive system tract stage, but the progradational Gilbert-type fan deltas are widely developed during the regressive system tract stage. The sedimentary evolution and stratigraphic architecture are directly linked with preexisting topography, accommodation, and sediment supply. In addition, climate change and vegetation types are thought to be critical controls for fluid properties originated from feeder drainage systems and depositional processes of the construction of fan-delta complexes in the distinct system tracts.