渭河下游全新世古洪水滞流沉积物研究

Holocene Paleoflood Slackwater Deposit in Lower Reaches of Weihe River

通过广泛的野外考察,在渭河下游发现了全新世古洪水滞流沉积物,对其进行了深入观察、地层划分和系统采样。在实验室内进行磁化率、烧失量、碳酸钙和粒度成分等多指标的分析测试。结果表明,这些古洪水沉积物以细粉沙为主,性质为粉沙质亚黏土,致密坚硬,具有水平或波状层理,与覆盖在河流阶地面的风成黄土和古土壤有着显著的区别。通过OSL测年断代,发现它们与渭河上游固川镇剖面的古洪水SWD为同一期特大洪水的沉积物。这表明渭河流域在3200—3000aBP发生了一组3次特大洪水事件。这个时期正是全新世大暖期结束之际,也就是中国历史上商代末期。渭河流域特大洪水事件的发生与全球性气候恶化转折密切相关。

The Holocene palaeoflood slackwater deposit（SWD） was found in the lower reaches of the Weihe river by field investigations.Sedimentary samples were taken after detailed observations and pedo-stratigraphic subdivisions.Grain-size distribution,magnetic susceptibility,loss-on-ignition and CaCO3 content were measured in the laboratory.The results show that these bedded palaeoflood slackwater deposits are silty clay,very dense and hard,with blocky structure and low magnetic susceptibility,which are very different from the aeolian loess and soils overlying on the river terraces.Optically stimulated luminescence（OSL） dating shows that these palaeoflood events occurred between 3200 and 3000 a B.P.,exactly the same time as that documented in the upper reaches of the Weihe river.This time was the ending of the mid-Holocene climatic optimum,and also the ending of the Shang Dynasty in Chinese history.These palaeofloods are closely related with the global climatic decline at about 3100 a B.P.The results are of great significance in understanding the response of hydro-climatic system to global change.