黄河上游官亭盆地古洪积扇前沿全新世地层序列研究

Sediment Stratigraphy of the Paleo-pluvial Fans in the Guanting Basin along the Upper Yellow River

青海官亭盆地喇家遗址史前巨大灾难事件,是我国史前环境变化与文明演变研究领域的热点之一。通过对官亭盆地喇家遗址北侧大红山前古洪积扇前沿地带的野外考察,进行土壤沉积物地层划分、结合理化指标测试分析,发现该地带土壤与沉积物的基本层序,自下而上依次为晚更新世马兰黄土(L1)—全新世早期过渡性风成黄土(Lt)—全新世中期古土壤(S0)—全新世晚期风成黄土(L0)—现代耕作土壤(MS)。这个层序特点与黄河北岸第二级阶地喇家遗址区域相同,也与黄土高原地区全新世土壤沉积物的层序相同。在全新世中期黑垆土古土壤(S0)中夹有三层红色泥流沉积层(RC1、RC2、RC3),是全新世中期后段气候恶化条件之下,暴雨山洪泥流溢出沟槽在地面扩散沉积形成。该地带以RC1为代表第一期山洪泥流事件早于喇家遗址毁灭的年代。以RC2为代表的第二期山洪泥流层卷入密集的木炭屑和杂物,表明在山洪泥流发生的时候有火灾发生,这对应着3850 a B.P.毁灭喇家遗址的大地震和山洪泥流群发生灾难事件。以RC3为代表的第三期山洪泥流事件,与喇家遗址区域相同发生在3600 a B.P.。其后在古洪积扇前沿到黄河第二级阶地的范围,再没有发生山洪泥流溢出沟槽扩散覆盖土地并且毁灭聚落成灾的现象。在喇家遗址北侧古洪积扇区域建立了全新世土壤地层序列,分析了该地带山洪泥流与喇家遗址灾难事件的联系。这对于研究揭示史前时期以来官亭盆地人地关系演变具有科学意义。

One of the main focuses of research in the field of prehistoric environmental change and civilization evolution in China is the prehistoric calamities that occurred in the area of the Lajia Ruins in the Guanting Basin along the Upper Yellow River,between the Qinghai-Tibet Plateau and the Loess Plateau.Sediment deposition relationships were studied across the Guanting Basin by detailed field observations and sedimentological and stratigraphic analysis.It was established that the sequence of Holocene aeolian loess paleosols is basically the same in the central and eastern parts of the Loess Plateau.A complete and continuous stratigraphic section was discovered on the western bank of the Ganggou Gully,at the northeastern corner of the Lajia Ruins.Three layers of red clay mudflow sediment(RC1/RC2/RC3)within the paleosol(S0)were formed over the Guanting Basin and surrounding areas,mainly due to disastrous climatic decline in the middle and late Holocene.Multiple flash floods and mudflows transported a large amount of loose material from the Tertiary gully to the Guanting Basin.Large-scale flash flooding and mudflows frequently overflowed into the shallow region of the gully at the front of an ancient alluvial fan and were spread and deposited over the Yellow River terrace,destroying the prehistoric settlement of the Qijia people(the Lajia Ruins)located in the low-lying area.Moreover,dense particles of charcoal in the middle and lower parts of the red clay mudflow(RC2)indicate a significant fire at 3850 a B.P.in the Guanting Basin,at which time a great earthquake and flash flooding and mudflows occurred.This study established a pedostratigraphic sequence at the periphery of the Guanting Basin and analyzed the causes of the events.It is of scientific importance,and reveals the evolution of human-land interconnection in the region in prehistoric times.