New insights about the Mesoproterozoic sedimentary framework of North China Craton

Abstract

At the south‐western margin of the North China Craton (NCC), there existed a lot of Mesoproterozoic strata. However, it is hardly known about the Mesoproterozoic stratigraphic sequence and deep structures of the south‐western part of NCC (most is the Ordos Block) due to the thick Phanerozoic sedimentary cover above the Precambrian basement and strata. Combining the drilling well data, seismic profiles, and outcrops around the whole Ordos Block, it becomes possible to reveal the deep structure and distribution of Mesoproterozoic strata. The distribution and sedimentation of Mesoproterozoic at the southwestern margin of NCC were controlled by a series of NE–SW extension deep faults just existed in Mesoproterozoic Changcheng System and Jixian System. According to the drilling well data, the Changchang System includes several various thickness volcanic rocks, and the stratigraphic sequence and lithological combination are similar to the Yanliao Rift. During the sedimentation from Longjiayuan Formation to Duguan Formation, the water got deeper revealed by the content of Ce element gradually decreasing. However, the content of Ce element unusually increases during the steady subsidence of the Mesoproterozoic basin, while the Xunjiansi Formation sedimented. The deposition of tephra or hydrothermal might contribute a lot to the increase of the Ce element's content. Besides that, numerous soft‐sediment deformation structures developed in carbonate rocks of Mesoproterozoic Xunjiansi Formation at the south‐western margin of NCC. What's more, the Xunjiansi Formation with soft‐sediment deformation structures can be regarded as a marker bed for the comparison with the Wumishan Formation in Yanliao Rift. Besides that, there is a set of residual sandstone conformably covered the dolomite of Mesoproterozoic Fengjiawan Formation. It corresponds with Xiamaling Formation of Building System in Yanliao Rift System based on the stratigraphic sequence and relative sedimentary time limited by the youngest zircon U–Pb age peak 1,511.1 ± 10 Ma (n = 7) and single zircon U–Pb age 1,436 ± 28 Ma. Therefore, the south‐western part of NCC have high similarity of stratigraphic sequence and sedimentary framework with Yanliao Rift. Moreover, the development of volcanic rocks in Changcheng System, soft‐sediment deformation structures, and Ce abnormal increasing in Xunjiansi Formation might suggest that there developed an active Mesoproterozoic Rift in the south‐western part of the NCC similar to Yanliao Rift. Generally, the south‐western part of NCC share similar tectonic background with Yanliao Rift.