神木邱家鄢最高阶地的年代及对中游黄河演化的指示

通过详细的野外调查，发现陕西神木邱家鄢地区存在一古河流阶地，根据该阶地上覆晚第三系红粘土推测该阶地为晚第三纪形成的。对该阶地上覆地层进行调查表明，该区晚第三纪红粘土之下存在约53米的风成砂层和河流相砂层。对红粘土和粉砂层进行古地磁测年样品的采集和测试。古地磁研究结果表明，该剖面底部的年代约在6．8 Ma左右，而红粘土与粉砂层之间的界限在6．27 Ma。根据深度－年代之间的关系，推算得出底部粉砂层的沉积速率比上部红粘土的沉积速率要快10倍以上，结合区域气候变化和构造运动历史的研究，邱家鄢剖面在（6．8－6．27）Ma之间快速沉积粉砂层的来源可能是中游黄河的河漫滩，指示了山陕峡谷段南北流向的古黄河可能在中新世晚期的6．8 Ma以前已经在该区出现。

Magnetostratigraphy of Red Clay and Silt Layer of Qiujiayan Section,Shenmu County and Its Implications to the Formation of the Yellow River

Based on detailed field investigation, one oldest river terrace is found at Qiujiayan Village, Shenmu County.Ac-cording to the Late Tertiary Red Clay on the terrace, the formation age of the terrace could be conjectured.Furthered investigation suggested the bottom of the section is composed of 53-meter wind-formed sand and lake-lacustrine sand.Magnetostratigraphy sam-ples were collected and measured.The result of the measurement shows the formation age of the section bottom is 6.8Ma B.P.and the boundary of Red Clay and sand formed before 6.27Ma.On the basis of the relation of section depth and age, conclusion is made that the sedimentary rate of the sand layer is ten times faster than that of the red clay .Combined with regional climate change and tectonic movement history, the rapid sand sedimentary rate between 6.27Ma to 6.8Ma could contribute to the flood plain sedi-ment.This conclusion indicate that the north-south direction of Yellow River formed in the Loess Plateau before 6.8Ma.