Ningxia In Central And Southern Tertiary Sedimentary Facies Analysis Of The Relationship Between And The Tibetan Plateau Evolution

The uplift and evolution of Qinghai-Tibetan Plateau is one of the greatest geological events on the Earth. The average height of 5000m above sea level of the plateau has resulted in the very thick Cenozoic sediments accumulating in and around the margin of the plateau, in these sediments there is a lot of information about the evolutional process of Qinghai-Tibetan Plateau, many studies have been carried out in the foreland basins around the Plateau, such as the Indian river foreland basin, the Ganges river foreland basin to the south, the southwestern Tarim basin to the northwest, the Hexi Corridor foreland basin to the north and the Linxia foreland to the east.The central and southern parts of Ning Xia is located to the northeast margin of the Qinghai-Tibetan Plateau, which has been affected by the tectonic process of the plateau directly, however, to these days few studies on the relationship between deposition of sediments and tectonic environments have been carried out. Based on the former researches this work want to fill the blankness in this region and try to find the relationship between the tectonic processes of Qinghai-Tibetan Plateau and the characteristics of Cenozoic sediments especially the Tertiary sediments, and to combine the Cenozoic deposition into the whole development of the Qinghai-Tibetan Plateau. By the analysis on sedimentary facies and regional to outcrop scale structures, the work on the development of the northeastern margin of plateau since Cenozoic time and its causal relationship with the uplift of Qinghai-Tibetan Plateau was carried out, in the study the author finds that:As the first deposit in the Ning Xia since Cenozoic time, the Sikouzi Formation has special signification, this formation in the southern and central parts of Ning xia was deposited in the forebulge area facing the Zou Lang Nan Shan mountain thrust belt which formed the foreland basin between 40Ma to 30Ma. Shortly after the collision between Indian Plate and Eurasian Plate 55Ma to 45Ma ago, the collision effect was transmitted to the region of Zou Lang Nan Shan mountain, strong thrusting occurred in this region during that time, the Zou Lang Nan Shan mountain was uplifted by thrust faults, which resulted in the formation of the earliest foreland basin to the north and northeast regions, the foredeep of this foreland basin was located in theparts of Gan Su Province, the paleocurrent pointed to the southeast. While the forebulge part of this foreland basin was located along the western margin of Ordos Plateau and Xiangshan area in Ning Xia, the back-bulge part of foreland basin was to the east of western margin of Ordos Plateau. Because of the flexing and weakness of forebulge, some normal faults would occur on the forebulge, and most of them dipping to the orogenic belt were distributed in the area facing the orogenic belt. It was these normal faults that controlled the distribution of the Sikouzi Formation. The Sikouzi Formation is proximal sediments depositing rapidly, which formed alluvial fans around the forebulge, all these coarse deposits changed into fluvial facies to the west, the paleocurrent pointed to the west or southwest, the provenances were the uplift of western margin of Ordos Plateau, when all these rivers went into foreland basin, they fed into longitudinal river of foreland basin, in that time the Yellow River was not formed, the regional large river flowed to the southeast.Based on the study of sedimentary facies and transverse change of thickness, the author thinks that there was no large united lake developing in the southern and central parts of Ning Xia, but several smaller ones occurred in Oligocene. The Qingshuiying Formation was deposited in the stage when the thrusting activity became weaker in the Zou Lang Nan Shan mountain, in this period the foreland basin and the thrust belt were uplifted because of isostatic effect which led to the change of underfilled basin to overfilled one, the depositional system gradually changed according to this shift, i.e. from the longitudinal river system during Sikouzi period to the transverse river system and salt lakes in the distal regions during the qingshuiying period. Moreover, syn-depositional tectonic activity has been found in the Qingshuiying Formation in Longde County, these thrust faults means that the southern part of Ning Xia has been affected by the compression from Qinghai-Tibetan Plateau during Oligocene at least, this is one of proofs that show during the early period of development the Qinghai-Tibetan Plateau had effected the studied area already.Moreover, the author has found the thrusting occurring later middle Miocene in Jijingzi basin located to the west of southern part of Helanshan mountain, which resulted in the thick syn-thrusting sediments deposited. The compression came from the west. All these means that during the later middle Miocene the western part of studied area had been deformed by tectonic activity, which is different from the viewpoints of former workers that the deformation of the Tertiary sediments in the Ning Xia had occurred only after later Pliocene or Pleistocene. And the angular unconformity between Qingshuiying Formation and Hongliugou Formation are found along the Chahanbulage-Sanguankou-Qingtongxia-Guyuan fault, this angular unconformity means that during later middle Miocene the deformation front of Qinghai-Tibetan Plateau had already reached the Chahanbulage-Sanguankou-Qingtongxia-Guyuan fault, i.e. the western margin of Ordos Plateau, along the fault some regions began to uplift, which became the local provenances to the basin, the early foreland basin was deformed according to this activity, the studied area was incorporated into the thrust wedge of northeastern part of Qinghai-Tibetan Plateau. The possible reason for the expanding of thrust wedge was that the strong uplifting of northern part of Qinghai-Tibetan Platrau in the same time, which resulted in the critical angle of wedge exceeded its critical value. The Chahanbulage-Sanguankou-Qingtongxia-Guyuan fault was just like the Haiyuan fault, the northeast marginal fault of Qinghai-Tibetan Plateau nowadays as the marginal fault during middle Miocene, and the expending of northeastern part of plateau is not a typical overstep sequence, but belong to out-of-sequence deformation, this accords with the deformationrule of thrust wedge. Furthermore, In the basin the upper Hongliugou Formation became coarser to the top, which also shows the important effect of strong tectonic activity occurring during later middle Miocene. By analyzing the paleocurrents shown by conglomerate and its composition, the author found the Xiangshan area began to uplift during later middle Miocene, and the some uplifts like Xiangshan area broke up the former foreland basin, made it be some piggy-back basins, which deny the viewpoints of former workers that the deformation of the Tertiary sediments in the Ning Xia had occurred only after later Pliocene or Pleistocene. By analyzing the paleocurrents and sedimentary facies, the author believes that today's drainage system in studied area was not established until later middle Miocene to Pliocene.In the field the proofs that only disconformity occurred between Sikouzi Formation and Qingshuiying Formation were found, there is one layer of paleosol between them, which means that there was depositional break. This disagrees to the viewpoint that the Sikouzi Formation and Qingshuiying Formation is continuous. Moreover, the similar situation also occurred between Hongliugou Formation and Ganhegou Formation, there also was depositional break between them. And there are no conformity occurring among the Tertiary strata.On the basis of the analysis on the historical movements occurring in Yinchuan region, the author believes that although the formation of Yinchuan graben was the result of the collision between Indian plate and Eurasian plate, the direct reason for it was the compression from the thrusting from the south, so the Cenozoic Yinchuan graben is impactogens.