MOBILITY OF PALEOGENE BASIN-CONTROLLING FAULTS IN THE BOZHONG SAG AND DEPOSITIONAL RESPONSES

The Bozhong sag, the center of tectonic subsidence and deposition in the Bohai Bay Basin, was surrounded by the Shijiutuo uplift, Shaleitian uplift, Bonan uplift and Bodong uplift of Paleogene. Complicated tectonic belts and sedimentary systems were developed between uplifts and sags due to differentiated movement of basin-controlling faults which were under different mobility rate. Based on the detailed interpretation of 3D seismic profiles, we calculated the mobility rate of main boundary fault in different depositional stages, and compared the distribution patterns of the deposits. The results suggest that the peak of fault mobilization occurred in the deposition stages of Es₃ and Ed₃ when the Bozhong Sag was intensely rifted. In a plane view, the Bozhong sag did not exhibited in a typical pan-like due to the difference of fault mobilization at different locations. The sedimentary facies at basin edge were controlled obviously by the fault activity rate. Progradation of the clastic deposits alternated with retrogradation in accordance with the change in fault activity rate. Compared with others, the tectono-depositional evolution of the Bozhong sag in Paleogene was characterized by strongly rifting at the deposition stages of Ed₃. The rift then became a depocenter of a remote delta at the Bohai Bay basin after the rift turned into a depression during the depositional stages of upper Ed₂ and Ed₁.