Tectonic control for evaporite formation in Wuyang Sag,Southern North China Basin

There is a wide distribution of thick evaporites in the He21 and He1 Members of the Wuyang Sag, Southern North China Basin. Based on drilling data, previous researchers have analyzed the formation mechanism of evaporites in Wuyang Sag, and considered that their origination stems from the insufficient provenance and dry climate of the area. However, other researchers have suggested that the Qinling-Dabie Mountains, on the southern part of Wuyang Sag, acted as a stable source material throughout the deposition period of the sag. These contrasting hypotheses indicate that further investigation is needed to clearly identify the evaporite genesis mechanism in Wuyang Sag. Using both seismic and geologic data, this paper analyzes the impact of the uplift structure of the Wuyan Sag area on evaporite formation. This analysis is based on restoration of the southern boundary of the sag in the deposition period with respect to the largest strike-slip fault. Taking the erosion of the lower Tertiary strata at the southern boundary of the sag (greatest tectonic activity) as the starting point of this study, and the movement of the NW strike-slip fault at the eastern part of the sag, the following conclusions were drawn: (1) The end stage of deposition of the He22 Member of the Hetaoyuan Formation was an important period in the base level transition for the Wuyang Sag. During the depositional period from the He3 Member to the He22 Member, the sag gradually expanded as the base level was rising. In the depositional period from the He21 Member to the He1 Member, the sag gradually shrank, and the base level was lowered due to the influence of structural uplift. In the meantime, inside the sag, the left-lateral strike-slip fault at the basement became active again and caused the eastern area of the strike-slip fault to be uplifted in the depositional period from the He21 Member to deposition of the Liaozhuang Formation. (2) The structure uplift during the depositional period from the He3 Member to the He22 Member blocked part of the provenance to infill the Wuyang Sag. Therefore, this activity resulted the sag becoming closed or semi-closed, leading to the deposition of evaporites.