Sedimentary system and palaeogeographic evolution of Ordos Basin, northern China

Extensive work on outcrops and drilling wells in the Ordos Basin, northern China, has been carried out over the past decades to study its depositional systems. Investigation of depositional environments, lithology, sedimentary structures, and fossils in the basin reveals that during the Changchengian and Jixianian periods, the depositional system transformed from the continental-nearshore environment to carbonate tidal flat. From the deposition period of Xinji to the Zhushadong formations, the western and southern margin of the basin evolved from sand flat to dolomite flat. The open platform dominated the basin during the deposition period from Mantou to the Sanshanzi formations. From the deposition period of Yeli to the Liangjiashan formations, the eastern and southern margins of the basin evolved from argillaceous dolomitic flat to dolomitic flat. The large-scale transgression in the deposition period of Majiagou Formation resulted in extensive shallow-water epicontinental deposits. In the deposition period of Fengfeng Formation, the western margin of the basin was dominated by platform margin slope and continental slope-trough deposits, while the southeastern and southern parts were mainly open-platform deposits. During the deposition period of Pingliang Formation, shoal deposits occurred on the platform margin in the southern part of the basin. During the deposition period of Beiguoshan Formation, the dominant open platform and platform foreslope deposits were only distributed in the southwestern corner of the basin. The Ordovician and Carboniferous strata were separated by a major unconformity and the Silurian to Devonian were missing in the basin. In the deposition period of Benxi Formation, a filling process and the deposition of tidal flat–lagoon–barrier island–shallow sea shelf occurred in the basin. During the deposition period of Taiyuan Formation, the range of the shallow sea-shelf sedimentation in the eastern part reduced. During the deposition period from Shanxi to the Zhifang formations, the delta and shallow lacustrine deposits developed. The depocenters migrated in different periods. From the deposition period of the Yanchang Formation to the Cretaceous, lacustrine, meandering rivers, braided rivers, and deltas dominated the basin. The maximum lacustrine transgression occurred in the deposition period of Chang 7 Member of the Yanchang Formation, which can be divided into 10 members with Chang 1 at the top and Chang 10 at the bottom. During the deposition period from Fuxian to the Anding formations, the meandering river, delta, and shallow lake deposits developed, and the depocenter of shallow lake varied. Deep lacustrine sediments developed in the deposition period of the Anding Formation. During the deposition period of the Fenfanghe Formation, alluvial fan deposits dominated, then transformed into a desert and shallow lake facies in the Cretaceous, which resulted in the shrinking of the depocenter.