汉江上游郧县段全新世古洪水滞流沉积物特征

Characteristics of Palaeoflood Slackwater Deposits of Yunxian County in the Upper Reaches of the Hanjiang River

对汉江上游郧县段一级阶地上典型古洪水滞流沉积物、古土壤和黄土3种不同沉积物的宏观特征、粒度成分、理化性质、微观形态及地球化学元素进行了综合分析对比。结果表明,研究剖面中的古洪水滞流沉积物呈灰黄色(2.5YR7/2),与上下相邻地层往往呈突变关系且界线清晰;粒度组成总体较粗,粉砂质细砂土,分选较好;磁化率稍高于马兰黄土,但显著低于古土壤;烧失量平均值低于黄土和古土壤;石英颗粒表面分布有水下环境机械相撞形成的V形坑;元素的含量及UCC标准化曲线均与黄土和古土壤差异明显。与渭河谷地相比,汉江上游谷地古洪水滞流沉积物具有粒度组成偏粗、分选性稍差的特点。该研究成果对汉江上游谷地古洪水滞流沉积物的鉴别和古洪水水文参数的重建具有重要意义。

Palaeoflood slack water deposits（SWD） are very useful for hydrological reconstruction.Search for SWD has become a key step in the study of palaeoflood hydrology.Three types of sediments（palaeoflood SWD,paleosol and loess） are surveyed over the first level river terraces of the upper Hanjiang valley.Comprehensive analyses of macroscopic characteristics,grain-size distribution,magnetic susceptibility,loss-on-ignition,micromorphology,and major-elementcomposition are conducted in the laboratory.Results are shown as follows.（1） The color of palaeoflood slackwater deposits in the upper Hanjiang valley is gray-yellow（2.5 YR 7/2）,with abrupt vertical changes and clear boundaries in the adjacent stratigraphies.（2） The palaeoflood SWD has coarse texture with silty fine sand,and is better sorted than other types of sediments,indicating that this type of sediment was formed by the suspended sediment load of floodwater.However,compared to palaeoflood SWD in the Weihe river valley,the latter is even better sorted because the majority of it is silty fine sand with even smaller grain size.（3） The magnetic susceptibility of the palaeoflood SWD is slightly higher than that of loess,but significantly lower than that of luvisols.（4） Loss-on-ignition of palaeoflood SWD is lower than that of loess and paleosol,which suggests that the palaeoflood SWD was fresh sediment without being affected by weathering and pedogenesis after deposition.（5） In micromorphology,the surface of the palaeoflood SWD is distributed with V-shaped pits,typically caused by physical collision and friction in running water.（6） The palaeoflood SWD is also different from paleosol and loess in major-element compositions and UCC（upper continental crust）-normalized patterns.These results are of great importance to the identification of the palaeoflood slackwater deposits in the upper Hanjiang valley and the reconstruction of the palaeoflood hydrological parameters.