Evolution Of The Cenozoic Basins In The Western Margin Of The Ordos Block

The western margin of Ordos Block is located in the west of North China Plate,which is in direct connected with the Alxa Block to the west and the Tibet Plateau Block to the southwest,respectively.The western margin of Ordos Block is subject to the northeastward expansion of the Tibaten Plateau and is also affected by the post-arc extension driven by the westward subduction of the Pacific Plate.Therefore,the tectonic and sedimentary evolution of the western margin of the Ordos Block is actually dominated by the Tibet Plateau tectonic systems and the Pacific tectonic tectonic systems.The conversion of those two tectonic systems is a major tectonic event happened in the central and eastern China during the Cenozoic.However,it is unclear about how and when did this conversion of the two tectonic systems take place on the western margin of the Ordos Block,which seriously affects the debate on the tectonic evolution process in the western margin of the North China caused by the northeastward expansion of the Tibaten Plateau,and affected the deployment of national oil and gas resources exploration in and around the Ordos Block.Two important Cenozoic basins,namely the Yinchuan Basin and the South Ningxia Basin,are developed in the western margin of Ordos Block and have accumulated thick Cenozoic strata.The sedimentation filling process in the basin shows that they formed a unified lake system in the late Oligocene-Middle Miocene,and were separated into two independent basins in the late Miocene to the Pliocene due to the activity of the Niushoushan-Guyuan fault.Research on the stratigraphic framework,filling sequences,deposition-controlling structures and prototype of the Cenozoic basins in the western margin of Ordos Block are conducted in this paper,especially a high-resolution magnetostratigraphic age of the Cenozoic strata in the Yinchuan Basin is provided.The sedimentary and tectonic evolution process of the Cenozoic basins in the western margin of Ordos Block and the important tectonic events are discussed in this paper based on the combination study of sedimentology and tectonic deformation.Based on the results of the magnetostratigraphy study of the XL15-1-01 borehole in Yinchuan Basin together with the previous magnetostratigraphy and paleontology research,the Cenozoic stratigraphic interface ages of each formation in the west margin of the Ordos Block are redefined.The main conclusion and are as follows:?1?The precise age of the Paleogene-Neogene strata in the Yinchuan Basin was first proposed based on the study of magnetostratigraphy of the XL15-1-01 borehole in the southwest Yinchuan Basin.The upper age limit of the Qingshuiying Forrmtion is 21.32 Ma,the Zhangnengbu Formation extends from21.32 to 10.17 Ma,and the Ganhegou Formation spans the interval of 9.58 to 2.77 Ma.?2?Two important tectonic-sedimentary interfaces on the western margin of the Ordos Block are discovered,which represent two important tectonic events in the area.A regional tectonic uplift event occurred in the late Miocene?¹0 Ma?,resulting in the formation of unconformity between the Zhangenbu Formation and the Ganhegou Formation.It is indicated that the northeastward expansion of the Tibaten Plateau led to this event since the distribution of this unconformity interface is limited to the western margin of the Ordos Block and its western area,and the transition of the Tibet Plateau tectonic systems and the Pacific tectonic systems in the western margin of Ordos Block occurred in¹0 Ma.The unconformity between Yumen Formation and Ganhegou Formation was formed in the late Pliocene-early Pleistocene?2.77-1.04 Ma?.The Yumen Formation conglomerate is basically free of deformation and its underlying strata were strongly deformed to folds,indicating the strong northeastward expansion of theTibet Plateau started in².77 Ma,and the deposition of the Yumen Formation conglomerate marked the end of a relatively stable planation.?3?It is proposed that the Cenozoic basins in the western margin of Ordos Block has experienced the initial rifting-depression in the Eocene,basin connectivity in the Oligocene-late Middle Miocene,basin division in the late Miocene-Pliocene,and the demise of the South Ningxia Basin and intensified rifting-depression of the Yinchuan Basin since the early Pleistocene.The coupling relationship between the evolution of the Cenozoic basins in the western margin of the Ordos Block and the northeastward expansion of the Tibet Plateau is described in detail.Initial rifting in Eocene:The western margin of the Ordos Block in the Eocene is characterized by the small catchment basin filling above the paleo-topography controlled by the activity of the negative-reversal normal fault.The Sikouzi Formation?E₂s?is basically near-source coarse debris deposition in the whole area,except for the Sikouzi area in the South Ningxia Basin.The Oligocene-middle Miocene rifting driven by NW-SE extension stress:In the late Oligocene-Middle Miocene,the deposition of the Qingshuiying Formation?E₃q?in the early stage was in shallow lakes that connected to each other,and was in the stage of thermal subsidence driven by the subduction of the Pacific Plate in the late stage.These basins were all connected in the study area in the late stage,and a thick layer of mudstone with a thickness of 80-100 m was deposited.The sediments of Zhangenbu Formation?N₁z?are coarsening upward.The shallow lake sediments in the bottom of Zhangenbu Formation conformably overlain on the mudstone at the top the Qingshuiying Formation deposited in the deep-lake environment.Late Miocene-Pliocene basin division:In the period of late Miocene?¹0 Ma?,the Niushoushan Mountain began to uplift along the Niushoushan-Luoshan fault and separated the Yinchuan Basin and South Ningxia Basin.These two basins began to evolve independently since the late Miocene.The South Ningxia Basin was divided by four arc-shaped thrust-slipping faults.After a short-term uplift the area of Yinchuan Basin experienced re-rifting drivien by NW-SE extension comeing from NE-SW squeeze in this area.The development of the Yinchuan Basin since the late Miocene is similar to the“Impactogen”model.Disappearance of the South Ningxia Basin and intensified rifting of the Yinchuan Basin since the early Pleistocene:In the early Pleistocene,a layer of conglomerate deposited throughout the study area.Its cosmic nuclide buried age is 1.04-0.12 Ma.Therefore,the most intense tectonic shortening event on the northeastern margin of the Tibaten is defined in 2.77-1.04 Ma,combined with the magnetostratigraphic age of the underlying Ganhegou Formation.The South Ningxia Basin has died in this period,in contrast,the rifting-depression in the Yinchuan Basin has intensified.In addition,the late Pleistocene Shuidonggou Formation bearing Paleolithic is widely developed in the western margin of Ordos Block,with an age of³1,000 years.We named the lake in this time developed in the western margin of the Ordos Block as“Ningxia Paleo-megalake”.