Different Levels of Rift Activity and Its Impact on Deposition in Offshore Area, Nanpu Sag

Key words: 
• boundary fault /
• rift activity /
• deposition /
•  

Abstract: Nanpu sag, located in Bohai Bay Basin, is a small rift lacustrine basin. In study areas, the sand distributions were effected by the fault activity in different levels and thus study of sequence stratigraphy and sedimentary should be guided by tectonic evolution as a principal axis. Through the analysis of faulting histories in different levels, the filling characteristics of sand body inside the basin can be explained. Combining 580 m core observation and 3D seismic data which covered the entire study area, sedimentary facies and fault evolution in offshore area of Nanpu Sag were defined. Periodic rifting activity in Nanpu sag could be classified into two periods from 31 Ma to 23.5 Ma, one is Shahejie Formation and the other is Donying Formation. Shahejie Formation can be divided into three sub-episodes which correspond with three third order sequences and only two sequences in Shahejie Formation would be discussed because of the overlying strata of study area were eroded in the course of the First member of Shahejie Formation and four third sub-episodes, which correspond to 4 third order sequences, were identified in Dongying Formation. Fan-delta and nearshore subaqueous fan were formed in the hanging wall of boundary fault, and the braided river delta deposits were developed in the gentle slope belt and the gravity flow deposits were found in deep lake and semideep lake area. In the same tectonic unit, steep region or gentle slope, for example, though the feature of sedimentary facies evolution is inheritance, vertical stages are obvious, by analyzing the filling sequences evolution sections. Active intensity of tectonic remains constant in each sub-episode, so vertical stages of sand bodies in the form of foreset and retrograde were controlled mainly by lake-level fluctuations which also have some impact on vertical stages between each sub-episode. After calculated and analyzed the fault activity velocity of Bogezhuang Fault, Gaoliu Fault and the faults in the transition zone coupled with the plans of sedimentary facies, to make clear the control of the fault to the deposition and filling, figured out the distributions of detrital material in catchment basin are strongly controlled by different level of faults which were classified into boundary fault and secondary fault. The activity of boundary fault with time-shared and segmentation control the migration and swing of fan delta deposit around steep slope through controlling the swing of transfer zone around the margin of basin. Some gravity flow deposits caused by earthquake or delta frontal instability occurred re-distribution along the transfer zones, which belong to secondary fault between the edge and inner of the basin. As the secondary fault movement is a multi-stage process, the inner transfer zones control the redistribution of autogenic deposits. All in all, the deposition and filling of basin were controlled by the fault evolution, activity intensity and tectonic styles and structural associations. The lake-level variation and climate exchange also play a very important role in determining the vertical stages in sub-episode. The research on fault activity in different levels and its impact for the depositional sand distribution can provide a new idea for the subsequent exploration.