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

Chagan sag is a small characteristic Lower Cretaceous intracontinental rift basin in Yingen-Ejinaqi Basin, China-Mongolia border area. The Wuliji Structural Zone, which is restricted by a high-gradient syn-sedimentary fault,... [ view full abstract ]

Chagan sag is a small characteristic Lower Cretaceous intracontinental rift basin in Yingen-Ejinaqi Basin, China-Mongolia border area. The Wuliji Structural Zone, which is restricted by a high-gradient syn-sedimentary fault, has been the predominant study area with an extent of ~300 km2 due to abundant available subsurface dataset and great hydrocarbon significance. According to well and seismic-based sequence stratigraphic study, the Upper Bayingebi Member generally corresponds to a complete 3rd-order and three 4th-order T-R sequences in the Wuliji Structural Zone. The TST section gradually thins out towards the north, while the RST section covers the entire study area in this 3rd-order sequence. Further investigations reveal that tectonic subsidence and climate change are two predominated allogeneic factors for controlling stratigraphic development in the Wuliji Structural Zone. In general, asymmetric and episodic tectonic subsidence with different scales contributes to the formation of different-order sequence boundaries, and climate change from palynological and geochemical evidences would exert more influence on stacked patterns as well as system tracts with the changing ratio of evaporation to precipitation (E/P).
During the deposition of the Upper Bayingebi Member, transverse gravity-flow dominated fan- delta complexes sourced from the Maodun Sub-uplift are well distributed in the Wuliji Structural Zone based on comprehensive sedimentlogical analysis. The retrogradational conical-type fan deltas (i.e. nearshore subaqueous fans) are mainly located in the southern sector during the TST stage, while the progradational Gilbert-type fan deltas widely developed during the RST stage. Thesedimentary evolution and stratigraphy architecture are directly linked with pre-existing topography, accommodation and sediment supply. Besides, climate change and vegetation types are also considered to be critical controls for fluid properties and depositional processes of associated deposits originated from drainage systems in the distinct system tracts.