中宁贺家口子砾石研究对青藏高原东北缘隆升提供的证据

Evidence of tectonic uplift in northeastern margin of Tibetan Plateau:constrained by gravel of Hejiakouzi section in Zhongning County

对中宁贺家口子出露的新生代各层位砾石进行统计。结果显示,干河沟组、萨拉乌苏组和全新世上部冲积层砾石成分以浅变质砂岩为主,玉门组砾石以脉石英为主,而萨拉乌苏组、全新世上部冲积层中砾石成分出现花岗岩、片岩及石膏。砾石主要呈次圆状和次棱角状,长短轴比主要为1~2,为近圆状,砾石主要集中在粗砾类别,分选差至中等。砾石粒径分布曲线主要显示为近对称分布的特征,呈宽峰正态分布。各层位砾石特征结合沉积环境演化分析表明:寺口子组沉积时,本地区已具有一定的高程。清水营组稳定沉积至彰恩堡组沉积结束。随后香山地区、南华山地区发生隆升,而本地区仍为相对稳定的滨湖相、河流相沉积,一直持续至干河沟组沉积结束。之后为一段时期的沉积间断,直至玉门组沉积。此时本地区沉积环境发生显著变化,即本地区发生隆升。而此次的隆升范围相比之前中新世的隆升有所扩大。至萨拉乌苏组沉积时,持续的构造作用使天景山断裂发生逆冲,水系水流方向发生改变,故萨拉乌苏组及全新统上部冲积层出现清水营组的石膏及南华山的花岗岩和片岩砾石。说明青藏高原隆升在宁夏中南部地区表现为多期隆升,且影响范围有所不同。

The authors selected the Hejiakouzi section in Zhongning County and statistically analyzed outcrops of gravel in various Cenozoic layers.The results indicate that the compositions of the gravel in the Ganhegou Formation,Salawusu Formation,and upper Holocene alluvium are mainly low-grade metamorphic rocks and those in the Yumen Formation are mainly vein-type quartz.Moreover,granite,schist and gypsum clasts are also present in the Salawusu Formation and upper Holocene alluvium.The gravel are mainly subrounded to subangular.The subrounded gravel have axial ratios mainly in the range of 1 to 2,indicating a nearly round shape.The gravel are mostly coarse and exhibit moderate-to-poor sorting.The distribution curve of particle sizes is nearly symmetric and exhibits a broad-peak normal distribution.By analyzing the characteristics of gravel in different layers and assessing the evolution of the sedimentary environment,the authors conclude that this region has been at a constant elevation since the deposition of the Sikouzi Formation.The deposition rate was steady from the deposition of the Qingshuiying Formation until the deposition of the complete Zhangenpu Formation.Subsequently,uplift occurred in the Xiangshan and Nanhuashan areas,while the study region continued to experience relatively steady deposition of near-shore lacustrine and fluvial facies until the deposition of the complete Ganhegou Formation.Then,a sedimentary discontinuity occurred until the deposition of the Yumen Formation.At that time,significant changes of the sedimentary environment took place in the region;specifically,uplift occurred in this area.The magnitude of this uplift was larger than that of the previous Miocene uplift.During deposition of the Salawusu Formation,persistent tectonism caused thrusting along the Tianjingshan fault and changed the flow directions of the region s hydrologic systems.Therefore,gypsum from the Qingshuiying Formation and granite and schist gravel from the Nanhuashan area appeared in the Salawusu Formation and upper Holocene alluvium,which indicates that the uplift of the Tibetan Plateau was a multistage process that impacted different regions in the central and southern Ningxia.