宁夏中南部第三系沉积相分析及与青藏高原演化之间的关系

【作者】 张进；

【导师】 何国琦； 马宗晋；

【作者基本信息】 北京大学 ， 构造地质学， 2004， 博士

【摘要】 青藏高原的隆起及发展演化是新生代以来地球上最重大的地质事件之一。平均海拔5000m左右的青藏高原在其周缘以及内部的盆地中堆积了很厚的新生代沉积,在这些沉积物中蕴涵了大量有关青藏高原演化的信息,许多研究已经围绕着喜马拉雅山前前陆盆地(包括印度河与恒河前陆盆地)、高原的北部和东北部边缘而展开。 宁夏中南部地区位于青藏高原的东北缘地区,直接受到青藏高原构造运动的影响,但在本区开展的分析沉积与构造之间关系的工作目前还比较少,本次工作就是在前人的基础上填补该地区的空白,试图找寻该地区新生代尤其是第三纪陆相沉积的分布和特征与青藏高原构造运动之间的关系,将该地区新生代的沉积纳入到青藏高原整体发展的过程之中。主要通过沉积相分析的方法,结合少量的区域以及露头尺度的构造分析,判断青藏高原东北缘新生代以来的变化及其与高原隆升之间的因果关系,研究发现: 做为新生代宁夏境内的第一套沉积,寺口子组具有其特殊的意义,分析表明宁夏中南部始新统寺口子组是走廊南山前陆盆地中前隆位置靠前陆盆地一侧的沉积。当55—45Ma印度板块与欧亚板块碰撞后很短时间内,碰撞的效应就在走廊南出体现,出现了一期比较强烈的逆冲推覆构造,走廊南山逆冲隆起,形成最早一期前陆盆地,盆地的前渊部分位于现今的甘肃境内,古流向为南东向。而该前陆盆地的前隆沉积单元则位于宁夏境内的鄂尔多斯西缘以及香山等地区,而隆后沉积单元则位于鄂尔多斯西缘以东地区,由于挠曲作用以及薄弱的性质,在该前隆上产生了正断层,这些正断层基本上分布在前隆向造山带的一侧,并倾向造山带方向,少部分分布在朝向克拉通(隆后)的一侧,正是这些正断层控制了宁夏南部寺口子组的分布,基本上是一种近源的快速堆积,形成一系列围绕前隆的冲积扇或洪积扇沉积,这些粗碎屑沉积向西逐渐转变为辫状河沉积,古水流向西或南西,物源为鄂尔多斯西缘隆起,当进入前陆盆地土体后与平行造山带的径向河流汇合,当时黄河还没有发育,区域性的大河向南东流。 根据宁夏中南部清水营组沉积相以及厚度明显的横向变化,论文认为在渐新世宁夏中南部不存在一个范围广泛的大湖泊,而是几个彼此独立的湖泊。宁夏中南部清水营组沉积的构造背景是处在走廊南山逆冲活动减弱的阶段,前陆盆地以及造山带均衡隆起,使得整个前陆盆地逐渐由非补偿型盆地转变为过补偿型盆地,沉积体系也逐渐产生了相应的转变,由寺口子期的径向河流体系转变为清水营期的横向河流体系以及远端的盐湖沉积。此外在隆德清水营组内发现了同构造沉积,这些逆冲构造的发育说明至少在渐新世早期宁夏南部就已经受到了来自青藏高原的挤压作用,这也是青藏高原早期直接影响本地区的证据之一。 在贺兰山中南段西侧的吉井子盆地内发现了晚中新世中期的逆冲推覆活动,动力源来自西部,并堆积了厚度较大的同构造沉积,说明晚中新世中期本区西部已经直接遭受到了构造活动的改造。因此否定了前人认为宁夏地区第三系的变形是发生在上新世末期—更新世以后的观点。而且在查汗布拉格—三关口—青铜峡—固原断裂沿线发现了晚中新世中期由于构造活动形成的红柳沟组与清水营组之间的角度不整合。该角度不整合的出现说明在晚中新世中期,青藏高原的变形前锋已经到达查汗布拉格—三关口—青铜峡—固原断裂,即鄂尔多斯西缘地区,沿断裂一些地区开始隆起,成为盆地内外的局部物源供应地,早新生代前陆盆地因此解体,宁夏中南部地区并入到青藏高原东北缘逆冲构造楔之中,该逆冲构造楔中新世向北东扩展的原因可能是由于青藏高原北部在同期一次显著的隆升运动使得逆冲楔临界角超过临界值,而向前扩展的,查汗布拉格—三关口—青铜峡—固原断裂是早期类似于目前海原断裂的青藏高原变形前缘,而整个青藏高原东北部的扩展并不是标准的前展式,而是一种没有特定顺序的变形,这也符合逆冲构造楔的变形规律。在晚中新世中期,在盆地内部也发现红更多还原

【Abstract】 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.更多还原