LATE-EOCENE TECTONIC EVENT IN QIKOU SAG OF BOHAI BASIN

Qikou Sag, located in the west-central of Bohai Basin, has experienced Eocene-Oligocene rifting and Neogene depression, and there are two main unconformity in sag, previous research emphasis on the tectonic movement between Paleogene and Neogene and ignore the late-Eocene tectonic event, however, from the perspective of reformation strength of the Sha-3 (Eocene) original appearance, the latter is more important.

Based on the latest 3-D seismic and drilling data, it is showed that most uplifts express truncation erosion in Sha-3 member and the main boundary faults don’t control the Sha-3 member deposition which all mean a late development, and it is resulted that there exists a relatively vast lake in Sha-3 period in area, the sedimentary boundary exceeds the residue strata range, which is strongly reformed by the late-Eocene tectonic event.

The event not only change the original sedimentary appearance of Sha-3 period, but also have impact on tectonic, magmatic and other aspects. Through statistics on the fault system of Cenozoic structural layers, the result shows that the fault activity strength migrates from southwest to northeast and the strike of faults changes from northwestward to nearly east-westward since late Eocene. Affected by structure, in Eocene sedimentation and magmatism are more controlled by northwestward faults and develop in southwestern, and volcanic rocks are mainly tholeiitic and calc-alkaline basalts; after the Eocene sedimentation and magmatism are controlled by nearly east-westward faults and develop in northeastern, and the volcanic rocks gradually transform to alkaline. Meanwhile, tectonic stress change from extension in Eocene to extension superposing strike-slip effect in Oligocene.

In addition, basins in the eastern of China are mostly affected by late-Eocene tectonic event, most of them show unconformity between Eocene strata and Oligocene strata. Analyzing the manifestation, reformation characteristics and occurrence time of this tectonic event is not only important to the restoration of Eocene original appearance, but also has a significant impact on resource exploration and evaluation of Bohai Basin and Cenozoic basin in China eastern.

This study was supported by NSFC (Grant No. 41402093,41330315) and Scientific Research Found of Northwest University (No. 13nw08).