祁连山党河盆地二道泉剖面新近纪沉积物磁性矿物与古水流及其对盆地水系演化的启示

Magnetic minerals and paleocurrent of the Neogene sediments on the Erdaoquan section in the Danghe River basin of Qilian Mountains,and implications to the basin drainage evolution

二道泉剖面(39°01'34″N,95°52'49″E)位于党河盆地腹地,呈北南向展布,长790m,党河盆地位于祁连山腹地,是研究青藏高原北部构造沉积与地貌演变的理想地区。本研究在野外地层观测、古流水重建及砾石组分分析的基础上,对党河盆地中西部二道泉剖面新近系白杨河组顶部-玉门砾岩底部河湖相沉积物进行磁化率测量与等温剩磁实验分析。结果显示,剖面沉积物的磁性矿物主要有磁铁矿和赤铁矿,其中赤铁矿相对含量较高。通过对比发现剖面中黄色调粗粒沉积物中的赤铁矿含量显著高于红色调细粒沉积物。砾石成分和向南的古流向结果显示,白杨河组晚期至玉门砾岩早期地层沉积物主要源自于二道泉剖面北部的野马南山地区。磁化率及沉积相揭示出源区物理风化作用逐渐增强,赤铁矿是从源区直接搬运沉积的,而非自生形成;这一过程未受外界气候环境的影响,赤铁矿多为碎屑,岩石颜色发黄。综合分析认为,二道泉剖面疏勒河组到玉门砾岩组早期沉积时党河盆地发育一条规模逐渐变大、水动力逐渐变强的由北向南流动的河流,现今流向为北西向的党河水系可能最早在新近纪玉门砾岩沉积中期以后才形成。

The Danghe River basin is located in the hinterland of Qilian Mountains,which is the ideal area to understand the tectono-sedimentation and geomorphic evolution of the northern Tibetan Plateau.In this paper,we present results of magnetic susceptibility and isothermal remnant magnetization(IRM)combined with field stratigraphic observation,paleocurrent direction and the gravel compositions in the Erdaoquan section(39°01'34″N,95°52'49″E)in 790m long from the top of Baiyanghe Formation to the bottom of Yumen Conglomerate Formation in the Neogene.Our analysis show that the magnetic minerals in the sediments are composed primarily of magnetite and hematite,and the content of hematite is high.By comparison,the hematite content in yellow coarse-grained sediments is significantly higher than that in red fine-grained sediments.The southward paleocurrent directions and gravel compositions indicate that the sediments are mainly derived from the Yemanan Shan in the northern Erdaoquan section during the late Baiyanghe Formation to early Yumen Conglomerate Formation period.Magnetic susceptibility and sedimentary facies reveal that the hematite probably transported from the source region as clast rather than autogenetic cement in depositional process;this process is not affected by the external climate environment,so the rock is mostly detrital and yellow in color.We propose that there was a southward unidirectional paleoflow system with increasing magnitude and hydrodynamic energy in the Danghe River basin during the deposition of the Shulehe Formation to early Yumen Conglomerate.The present Danghe River with northwest flows was formed after the middle of the Yumen Conglomerate deposits in the Neogene.