洛阳二里头遗址南沉积剖面的粒度和磁化率分析

洛阳盆地内二里头遗址南沉积剖面位于遗址所在二级阶地的前缘,属于河流堆积。对剖面的沉积物样品做了光释光测年及磁化率、粒度分析。结果表明:剖面沉积年代大致在2000~5000 aBP之间。剖面下部(113~123 cm)沉积物粒度较细,磁化率偏高,粒度曲线反映水流较弱的泛滥平原沉积环境,属河漫滩堆积;中部(90~113 cm)粒度较粗,磁化率较低,粒度曲线反映水动力较强的边滩–漫滩沉积环境,属河床边滩堆积;上部(0~90 cm)粒度变细,磁化率升高,粒度曲线反映水动力条件较弱的泛滥平原沉积环境,属河漫滩堆积。其中沿水平方向可见剖面中部的河床边滩堆积直接覆盖在二级阶地顶面的龙山时期灰坑之上,由此可以确认剖面中部地层记录了一场漫上了河流阶地的特大洪水事件,这一记录与孢粉分析得到的气候变化过程具有明显的相关性。4000 aBP前后异常洪水的出现和大洪水前后河流过程的变化,可能是对4200 aBP全球性气候异常事件的区域响应,对我国最早的都邑——二里头城址的选址有重要的影响。

Analysis on Grain Size and Magnetic Susceptibility of the Sediment Profile in the South of Erlitou Site, Luoyang

A fluvial sediment profile was found in the south of Erlitou Site,Luoyang Basin,locating in the frontier lines of terraceⅡ.The OSL dating,magnetic susceptibility analysis and grain size analysis were taken.It is showed that the depositional age of the profile is between 2000–5000 aBP.The profile can be divided into three sections: the lower section(113–123 cm) is with fine grain size and high magnetic susceptibility,and the grain size curves indicated flood plain environment,showing floodplain deposits;the middle section(90–113 cm) is with coarse grain size and low magnetic susceptibility,and the grain size curves indicated boundary beach-flood plain environment,showing riverbed-boundary beach deposits;the middle section(0–90 cm) is with fine grain size and high magnetic susceptibility,and the grain size curves indicated flood plain environment,showing floodplain deposits.It is found that the middle riverbed-boundary beach deposits covered on the Longshan ash pit of the terraceⅡ,which records a great flood spreading over the river terrace.This flood is significantly correlated with the pollen analysis result.The 4000 aBP abnormal flood and the changes of the river might be regional responses of the 4200 aBP global abnormal climate event.The location of Erlitou city site might be greatly influenced by the flood event.