晋陕峡谷北部地区新近纪砾石层的成因不同学者有不同的解读,本文以晋陕峡谷北部山西河曲地区为重点,对分布于河曲地区的新近纪砾石层进行详细的野外考察和磁性地层采样工作.野外考察发现,自河曲县大埝墕村开始向东至黄柏村,新近纪砾石层受黄河及其支沟切割广泛出露.在该区自东向西依次选取的4个地点,即黄柏村、桃山村、巡镇和榆岭村北,GPS测得的砾石层的海拔高度分别为1012m、998m、1002m和998m,海拔高度均在1000m左右,表明该组砾石层基本上分布在一个水平面上.对4个地点砾石层之上的河漫滩相沉积和红粘土地层以10～20cm的间隔系统地采集了古地磁定向样品,共获得定向样品1481块.古地磁测年结果显示,黄柏村、桃山村、巡镇和榆岭村4个剖面底部的形成年代分别为8Ma、8Ma或稍年轻、5.8Ma和4.8Ma,形成年代自东向西逐渐变晚.年代数据、砾石特征和海拔高程数据揭示了该区砾石层的形成是一个大致南北流向的新近纪河流逐渐西侧向侵蚀并逐渐废弃原先河道的过程.%Although there are quite different views on the formation and evolution of the Yellow River,the interpretation of the Neogene conglomerates lying on bedrock of modern Yellow River banks in northern Jinshaan Gorge is a key point to understand whether the ancient Yellow River formed in Neogene in this area.Based on detailed field investigation,Hequ area (bend),which locates in northern Jinshaan Gorge,is carefully selected to study the continuously exposed Neogene conglomerate and its relationship with the evolution of the Yellow River for its huge thickness of overlapped datable red clay deposits.Four sections in the southern margin of Hequ bend (39°20′N,111°15′E) from east to west,namely,Huangbai section,Taoshan section,Xunzhen section and Yuling section,were measured their thicknesses of conglomerate,floodplain deposits and red clay deposits.The thickness of the conglomerate in Huangbai section is about 15m and declines to 5m to Yuling section and the conglomerate is mainly composed of limestone pebbles and sandstone pebbles.The thickness of the floodplain deposits and red clay has the same declination trend from Huangbai section to Yuling section,that is,from 91m to about 35m.The altitudes of the conglomerate in four sections measured with portable GPS are about 1012m,998m,1002m and 998m separately from Huangbai section to Yuling section.These altitude data show that the Neogene conglomerate practically tiles on a slightly slant plane in eastern Hequ area.Oriented samples were collected at intervals of 10～20cm from the floodplain deposits and red clay on the conglomerate,and 1481 samples were collected in total.These samples were treated in two operation steps.Firstly,all samples were performed anisotropy of magnetic susceptibility measurement to obtain magnetic susceptibility data.Secondly,demagnetization was carried out in a low field room in Bergen University and remnant magnetization was measured using 2G Enterprises made 755 type cryogenic magnetometer.Magnetic susceptibilities of Jingle Formation in different sections can be easily correlated and include five magnetic susceptibility peaks,while magnetic susceptibilities of Baode formation are a bit complicate and difficult to correlate among sections.Results of magnetostratigraphy show that the subchrons corresponding to bottom strata of four sections are C4n.2n,C4n.2n,C3r and C3n.2r correlated to Geomagnetic Polarity Timescales respectively.Inception ages of the sections,i.e.,formation ages of the conglomerate in four sites,calculated according to average sedimentary rates of adjacent subchrons are about 8Ma,8Ma or slightly younger,5.8Ma and 4.8Ma in turn from Huangbai section to Yuling section.The formation ages of four sections indicate a distribution rule of the Neogene conglomerate,that is,the conglomerate in eastern part of Hequ bend formed firstly and becomes younger and younger to west.The formation ages and the altitude data indicate that the formation of the conglomerate in Hequ area was a lateral migration result of an almost south-north direction Neogene river.The river located in Huangbai section in about 8Ma B.P.and migrated to Yuling in about 4.8Ma B.P.
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