Sedimentary Characteristics And Tectonic Significance Of Paleogene-Neogene In Dingjiaergou Profile Of The Southern Ningxia Basin

The arc-shaped tectonic belt on the northeast margin of the Tibetan Plateau is the key area to study the uplift process of the Tibetan Plateau and the time limit of its tectonic geomorphology.Many scholars at home and abroad have carried out systematic research work from the aspects of structural analysis and tectonic thermal chronology.A great deal of research results have been achieved.However,due to the lack of accurate tectonic chronology,there are still many controversies about the starting time,setting time,evolution process and geodynamic background of the arc-shaped tectonic belt in the northeast margin of the Tibetan Plateau.The southern Ningxia Basin 1s located in the front of the arc tectonic belt in the northeast margin of the Tibetan Plateau.It is composed of Qingshui River Basin,Hongsibu Basin and Yantongshan Mountain between them.The tectonic belt together records the uplift process and climate change information of the late Cenozoic in the Tibetan Plateau.Taking the typical section of Dingjiaergou in the southern Ningxia Basin as the research object,this paper makes a detailed measurement,statistics and analysis of the outcrop of the section,and divides the sedimentary facies and cycle of the section by comprehensive use of various means of sedimentological research.On the basis of stratigraphic division and correlation,combined with regional paleomagnetism research results,the precise time limit of sedimentary interface transformation is determined,and the change of sedimentary source before and after unconformity is analyzed and the results of regional paleomagnetism research are combined to determine the exact time limit of sedimentary interface transformation.From the point of view of sedimentation,the uplift of Tibetan Plateau is pushed eastward to affect the initiation time and development process of the westerm margin of Ordos Basin.The main findings are as follows:(1)the northeastern margin of Tibetan Plateau experienced a complete lacustrine-depositional filling process from the Shikouzi formation of the Paleogene to the Ganhegou formation of the Neogene;The Sikouzi formation is dominated by alluvial fan conglomerate and sandstone,which represents the sedimentary characteristics of the early lacustrine basin filling,and the Qingshuiying formation is dominated by the shoreline and shallow lacustrine mudstone sandstones,which represents the sedimentary characteristics of the lacustrine basin filling peak period.The Zhangenbao formation is dominated by lacustrine mudstone,with a thick bottom layer of silty mudstone sandstones with a thin layer of horizontal gypsum,which represents the sedimentary characteristics of the early stage of lake basin decline,while the Ganhegou formation is dominated by fluvial sandstone and mudstone deposition,which represents the decline of the lake basin.(2)there are obvious depositional discontinuities between the Zhangenbao formation and the Ganhegou formation,and the Ganhegou formation is superimposed on the paleo-erosive surface after the Zhangenbao formation deposition.(3)the sedimentary discontinuity between the Zhangenbao formation and the Ganhegou formation indicates that the eastern margin of the Tibetan Plateau will initially affect the western margin of the Ordos Basin,and that the eastern margin of the Tibetan Plateau will initially push eastward and affect the western margin of the Ordos Basin.Specific manifestations:the provenance changed from the early NE direction to the SW direction;The clastic zircon peak value of the Ganhegou formation increases by 190～210 Ma than that of the Sikouzi formation,and the heavy mineral assemblage of the Sikouzi formation is dominated by hematite,while the Ganhegou formation is characterized by its heavy mineral assemblage.Garnet is dominant in the Gou formation,and the tectonic uplift in the late sedimentary stage of the Zhangenbao formation resulted in sudden death of the concentric fauna.Through the comparison with the magnetic stratigraphic profile of Hejiakouzi,it is established that the time limit of sedimentation discontinuity between the Zhangenbao formation and the Ganhegou fornation is about 7.8 Ma.(4)about 2.5 Ma.The eastward push of the Tibetan Plateau has affected the western margin of Ordos Basin to reach its peak,which is manifested in the following aspects:the Ganhegou formation in the region was completely involved in the tectonic deformation.There is a regional angular unconformity contact between the Ganhegou formation and the overlying early Pleistocene Yumen conglomerate.