泾河下游古洪水滞流沉积物地球化学特征研究

Geochemical Characteristics of the Holocene Flood Slackwater Deposits in the Lower Reaches of the Jinghe River

通过沿泾河河谷广泛的野外考察,在高陵县杨官寨段发现典型的全新世黄土—土壤剖面。经过详细观测、系统采样和化学元素、粒度成分和磁化率等多指标测定,从剖面鉴别出两组古洪水滞流沉积层(SWD),分别记录了发生在4 200～4 000 a B.P.和3200～2800 a B.P.期间的特大洪水事件。该研究着重对古洪水滞流沉积层与黄土层和古土壤层的元素地球化学特征进行对比研究。结果表明:(1)在风成沉积物中,Fe2O3、Al2O3、K2O、MgO等元素含量在古土壤层高于黄土层,而CaO和Na2O元素含量则表现出相反的变化。表明风化成壤过程中,这些元素的迁移变化受到全新世以来气候变化和成壤作用的影响。而Zr、Hf、Ti、Cr、Y、Th等微量元素含量与地壳平均丰度相当,且变化甚微。这是由于这些微量元素主要包含在粗颗粒和重矿物之中,其化学性质比较稳定,具有强抗风化性,很少受到风化成壤作用的影响。(2)单个古洪水滞流沉积层,是洪水悬移质泥沙在高水位滞流环境中沉积形成的,分选性很好,故其底部多粗颗粒和重矿物沉积,Zr、Hf、Ti、Cr、Y、Th等微量元素含量呈现突出的尖峰;自下向上随着颗粒逐渐变细,粘粒成分增加,Fe2O3、Al2O3、K2O、MgO等元素含量出现高峰。这两类元素由下向上的相互消长关系,与沙级颗粒含量和磁化率值变化规律对应,反映出古洪水滞流沉积层的基本理化特性。这个研究成果对于完善古洪水沉积学和水文学的方法和理论具有重要的科学价值。

Through widespread field survey along the Jinghe River valley, a Holocene loess-soil profile with interbed- ded food slackwater deposits （SWD） was found at the YGZ site at the riverbank near the Yangguanzhai village. Sam- ples were collected systematically and the chemical elements, grain-size distribution and magnetic susceptibility were measured in the laboratory. All of these indexes showed that there are two bedsets of palaeoflood slackwater deposits in the profile. They have recorded the extraordinary palaeoflood events occurred during 4200 -4000aB. P. and 3200 - 2800aB. P. , respectively. The comparative study of the geochemical difference between palaeoflood slackwater depos- its and the eolian loess and soil showed that： （1） The concentrations of major elements FezO3, AIEO3. K20, MgO are higher in the palaeosol （S0） and lower in the loess （L~, Lt, L0 ）. The abundances of elements CaO and Na： O showed a reversed change, indicating that these elements were affected by weathering and pedogenesis along with the Holocene climatic change. The abundances of trace elements including Zr, I-If, Ti, Cr, Y and they are very stable and close to the averages in the Earth~ crust. This is because these trace elements are connected with the coarse frac- tion and heavy minerals. They have stable chemical properties and are not much affected by weathering and pedogene- sis. （2） In a single palaeoflood slackwater deposit （SWD） bed, the sorted deposition in floodwater resulted in a con- centration of the coarse fraction and heavy minerals at the bottom of the SWD bed. Thus, Zr, Hf, Ti, Cr, Y and oth- er trace elements show a prominent peak. However, the increased abundance of clay minerals resulted in an increase in the major elements including Fe203, A1203, K20, MgO in the top of a SWD bed. This change is well consistent with the change of sand-sized particles and magnetic susceptibility in a SWD bed. These physico-chemical features are very useful in identifying palaeoflood slackwater deposits in the eolian sediment profiles. These findings are of great importance in palaeoflood hydrology.