青藏高原东北缘喇家遗址覆盖沉积物性质及史前灾难成因研究

【作者】 王海燕；

【导师】 庞奖励； 黄春长；

【作者基本信息】 陕西师范大学 ， 自然地理学， 2021， 博士

【摘要】 灾难性地表过程常会对人类生存和生活环境带来重大威胁,因而其长期以来是地理学的重要研究内容,其中有关灾难性地表过程的发生机制和发生规律及其准确预测已是当前研究的重点。近几十年来,因全球变化引起的区域环境变化及相关的灾难性地表过程对人类生活和社会发展产生了重大影响,现实案例不胜枚举。青藏高原东北缘是早期人类活动的主要区域和进军高原的主要扩散通道,区域内史前文化高度发达,同时其自然环境又高度脆弱、极具过渡性,灾害性地表过程频发,是研究史前人类活动和环境变化的典型区域。有关青藏高原东北缘地带史前人类发展与土地利用、史前环境演变与灾难及其对人类古文明的影响等问题受到越来越多的关注,而“喇家遗址”因其灾难性特征又被称“东方庞贝”,受到考古学家、人类学家、历史学家、地质学家的重点关注,因而成为目前研究的热点问题。本文以环境与人类文明演变为切入点,选择官亭盆地为研究区域,以喇家遗址为研究重点区域,对喇家遗址及周围地带(主要是黄河北岸)的地质背景及演变、地貌特征及发育过程、地表水文过程、沉积物类型及空间分布等问题进行深入调查研究,对不同沉积物的沉积学特征、矿物学组成、物理化学性质(磁化率、烧失量、TOC、色度、粒度、元素)、形成年代等进行详细解析,在获得大量第一手实验数据的基础上,利用矿物示踪技术、水文学和沉积学及地球化学的方法和技术,对研究区气候变化轨迹、地表过程、灾难过程及与史前文化的关系等问题进行了深入的研究等,成果可为目前区域研究中的难点问题和争议问题的解决提供第一手可靠证据、为准确理解人地关系提供案例。通过综合分析,论文获得了下列主要认识:(1)明确了官亭盆地内主要沉积物的类型及空间分布规律。对盆地内的多种沉积物进行实地调研和甄别研究,发现区内沉积物除过古洪积扇砾石层和风成黄土和古土壤之外,主要有山洪砂土沉积物、红色粘土沉积物、沟谷现代碎屑沉积物、黄河河漫滩沉积物等四种类型,不同沉积物明显分布在不同的地貌位置。山洪砂土沉积物在黄河两岸广大的古老洪积扇均有分布,常和红色粘土沉积物伴生出现,呈现混杂和交错的沉积特征;红色粘土沉积物主要出现在古洪积扇前沿及其与黄河阶地之间的洼地,沿着吕家沟、岗沟、王石沟沟道两侧剖面,可见沉积厚度较大,其致密坚硬,干燥时十分坚硬,夏季降雨则会二次液化流动;沟谷现代碎屑沉积物主要分布在吕家沟、岗沟和马家沟的沟槽内;黄河河漫滩沉积物沿黄河两岸呈现断续分布。(2)覆盖喇家遗址废墟的山洪砂土(所谓“黑砂”)和红色粘土沉积物属于典型的山洪泥流作用的产物。覆盖喇家遗址西部和西南部的山洪砂土沉积物内部常具有斜层理构造,颗粒大小变化巨大,分选性极差,形态棱角十分明显,内部常混入有不规则的黄土团块和红色粘土泥球。而遗址东部东北部的红色粘土沉积物致密坚硬,可见风化假层理,内部常见扭曲、翻卷等构造,夹有黄土团块、石子、文化遗物等,夏季降雨时则会二次液化。这些重要的沉积学特征表明它们的形成过程具有搬运距离比较短、堆积速度快,堆积作用出现和结束突然的特征,即发生具有明显的间歇性和突发性,是受到暴雨山洪较大的沉积动力触发所形成,这与现代山洪泥流沉积物的特征可进行良好对比,可以确定它们是山洪泥流沉积物。(3)覆盖喇家遗址废墟的山洪砂土(所谓“黑砂”)和红色粘土沉积物源自于官亭盆地北侧的吕家沟、岗沟等支流沟谷区域。沉积物的矿物组合、矿物标型特征(特别是锆石)、磁化率、色度参数、粒度等特征显示,覆盖于喇家遗址废墟的山洪砂土和红色粘土沉积物与黄河北侧沟谷现代碎屑沉积物相同,与黄河河漫滩沉积物区别显著。结合沉积物的重矿物分析结果、空间分布及区内地貌特征,发现覆盖遗址西部西南部的山洪砂土中黑色矿物与岩石碎屑主要成分是角闪石,而红色岩土碎屑富含赤褐铁矿。可以肯定地说,覆盖喇家遗址废墟的山洪砂土沉积物,是来自于吕家沟沟源拉脊山东坡的古老变质岩基岩地区砂砾,与源自于第三系红层丘陵沟壑区的沙土与粘土物质的混合物。而遗址东部东北部的红色粘土沉积物,则主要源自于官亭盆地北坡吕家沟和岗沟出山口第三系红层丘陵巨大的古滑坡带。这些覆盖沉积物是黄河北侧拉脊山东坡和第三系红层丘陵沟壑区的暴雨山洪泥流过程中,将拉脊山山区和丘陵沟壑区大量地表的粘土、沙土、砂砾等松散物质沿沟谷搬运到官亭盆地,在洪积扇表面及其前沿黄河阶地面不同地貌位置快速堆积的结果,这些并非黄河洪水的沉积物。(4)综合分析基础上建立了官亭盆地二级阶地上覆沉积物的地层序列和年代框架。在野外详细调研基础上,结合室内理化性质指标和测年数据,将黄河二级阶地上覆沉积物自下而上依次划分为:河流相沉积物(T2-al)→马兰黄土(L1-1,32.00～11.50 ka BP)→过渡性风成黄土(Lt,11.50～8.50 ka BP)→全新世中期古土壤(S0,8.50～3.10 ka BP)→全新世晚期风成黄土(L0,3.10～1.50 ka BP)→现代土壤(MS,1.50～0 ka BP)。其中古土壤(S0)发育不连续,其被厚度不等(100～400 cm)的山洪砂土(FFD)或红色粘土(RC)沉积物多次分割,将S0分为多个亚层。(5)根据第一手野外和室内实验数据,重建了官亭盆地末次冰期结束以来的气候变化过程。利用磁化率、烧失量、TOC、色度、粒度、元素等气候替代指标并结合测年数据,分析认为研究区末次冰期结束以来的气候变化过程可划分为:晚更新世末期(32.00～11.50 ka BP),气候干冷,风成堆积作用强烈,形成了马兰黄土,而成壤作用十分微弱;全新世早期(11.50～8.50 ka BP),气候逐渐向暖湿方向转变,风成堆积减弱,而成壤作用逐渐增强;全新世中期(8.50～3.10 ka BP),区域气候温暖湿润,植被生长茂盛,成壤作用强烈,普遍发育黑垆土;全新世晚期(3.10～1.50 ka BP),气候再次向干冷转化,成壤作用减弱,黄土的堆积作用增强;现代土壤(1.50～0 ka BP),受人类活动影响明显。(6)气候不稳定而诱发的地表灾难过程及集群性灾难事件频发对官亭盆地人类史前文化的兴盛衰退及发展演变有重要影响。古土壤(S0)中夹有的山洪砂土和红色粘土沉积物指示古土壤的成土过程被多期灾害事件沉积物中断,在3.85～3.60 ka BP(14C年龄)官亭盆地是一个山洪泥流灾害多发期,是一个气候不稳定时段。气候不稳定与多次大地震共同诱发的灾害性地表过程频繁出现,明显影响了人类文化的演进,导致齐家文化衰落,直到3.60 ka BP前后,自然环境趋于稳定并逐渐好转,古土壤(S0上)在恢复发育,灾害事件减少,卡约/辛店文化在区内兴起并发展,转变为畜牧业和谷物种植并存的混合农业类型。更多还原

【Abstract】 Catastrophic surface processes often threaten the human survival and living environment,and have long been an important research content of geography.Researches on the mechanism,regularity and accurate prediction of catastrophic surface processes are topics of great interest to many scholars.The global change induced regional environmental variation and related catastrophic surface processes have greatly affected human life and social development during the last decades,and there are numerous practical cases.The northeast margin of the Tibetan Plateau is the main area of prehistoric humans activities and the main diffusion channel for entering the plateau,and the prehistoric culture is highly developed.At the same time,the natural environment of this region is highly fragile and extremely transitional,and disasters occur frequently.Therefore,it is a typical area for studying prehistoric human activities and environmental changes.Along with the continuous advancement of studies,more and more attention has been paid in academia to the development and evolution of prehistoric humans and their land use at the northeast margin of the Tibetan Plateau,as well as to the prehistoric environmental changes and disasters and their influence on the evolution of ancient civilization and ancient culture of mankind.Like Pompeii in Italy,the Lajia Ruins is a rare example of an archaeological site preserved by a natural disaster.It has attracted extensive attention from archaeologists,anthropologists,historians and geologists,and has therefore become a hot topic in current research.Taking the evolution of environment and human civilization as the breakthrough point,the Guanting Basin as the study area,and Lajia Ruins as the key research area,this paper focuses on the investigations of the geological background and evolution,geomorphological characteristics and developmentent,hydrological characteristics,sediment types and spatial distribution of the Lajia Ruins and its surroundings(mainly on the north bank of the Yellow River).The sedimentological characteristics,mineralogical composition,physic-chemical properties(magnetic susceptibility,loss-on-ignition,TOC,color parameters,grain-size,chemical elements)and the formation ages are analyzed in detail.On the basis of a large amount of first-hand experimental data,the climatic change,surface processes,disaster processes and the relationship with prehistoric culture in the study area are comprehensive studied by using minerals tracing,hydrology,sedimentology and geochemistry.These results can provide reliable first-hand evidence for the resolution of difficult and controversial issues in current regional studies,and provide cases for accurately understanding of man-land relationships.Based on these systematical studies,we can draw a series of new understandings.The specific conclusions are as follows:(1)Definition of types and spatial distribution of the main sediments in Guanting Basin.The field investigations and screening studies of various sediments in the basin reveal that there are mainly four types of sediments,i.e.flashflood sediments,red clay sediments,modern gully debris sediments,and floodplain sediments of the Yellow River,besides the gravel layer of palaeo-pluvial fans and aeolian loess and palaeosol.Different sediments are obviously distributed in different geomorphic positions.The flashflood sediments are widely distributed in the region,and are distributed in enormous palaeo-pluvial fans on both banks of the Yellow River.They often coexist with red clay sediments,showing mixed and staggered deposition.The red clay sediments are mainly distributed in the front part of the palaeo-pluvial fans and the low-lying areas between the palaeo-pluvial fans and the terrace of the Yellow River.Along the profiles on both sides of Lüjiagou,Ganggou and Wangshigou gullies,it can be seen that the sediments are thick,dense and rigid,and very rigid in dry state,and will liquefy again in summer rainfall.The modern gully debris sediments are mainly distributed in the grooves of Lüjiagou,Ganggou and Majiagou gullies.The floodplain sediments of the Yellow River are distributed intermittently along the banks of the Yellow River.(2)The flashflood(so-called "blackish sand")and red clay sediments covering the Lajia Ruins are typical flashflood and mudflow deposits.The flashflood sediments covering the west and southwest parts of the Lajia Ruins often present oblique bedding structure,the grain size varies greatly,the sorting is extremely poor,the grain shape is sharp and angular,and loess masses and red clay balls are often mixed.The red clay sediments in the east and northeast parts of the Lajia Ruins are very dense and rigid,have weathered pseudo-bedding,are often twisted and rolled inside,and are often involved in loess lumps,stones and cultural remains,etc.,which will liquefy again in summer rainfall.These important sedimentological characteristics indicate that their formation process is characterized by short transportation distance,fast accumulation speed,sudden appearance and end of accumulation,obvious intermittence and suddenness,that is,triggered by great sedimentary power of heavy rainfall and flashfloods.It can be compared well with the characteristics of modern flashflood and mudflow deposits.These characteristics indicate that these flashflood and red clay sediments are clearly identified as flashflood and mudflow deposits.(3)The flashflood(so-called "blackish sand")and red clay sediments covering the Lajia Ruins come from the tributary valleys such as Lüjiagou and Ganggou gullies on the north side of Guanting Basin.The characteristics of mineral composition,mineral typomorphism(especially zircon),magnetic susceptibility,color parameters,grain-size of sediments show that the flashflood and red clay sediments covering the ruins are the same as the modern gully debris sediments on the north bank of the Yellow River,and are significantly different from the floodplain sediments of the Yellow River.Combined with the analysis results of heavy minerals,spatial distribution and geomorphological characteristics,it is found that the main component of black minerals and rock debris in the flashflood sediments covering the west and southwest parts of the Lajia Ruins is amphibole,and the red rock debris is rich in hematite-limonite.To be sure,the flashflood sediments covering the ruins are the mixture of gravel from ancient metamorphic rock bedrock area on east slope of the Lajishan Mountains,the headwaters of the Lüjiagou gully and sand and clay from the gully-hilly regions of the Tertiary Red Formation.The red clay sediments in the east and northeast parts of the Lajia Ruins are mainly originated from the enormous palaeo-landslide belts of Tertiary Red Formation in the exit of Lüjiagou and Ganggou gullies on the north slope of Guanting Basin.These overlying sediments are the result of carrying a large amount of loose materials such as clay,sand,gravel on the surface of Lajishan Mountains and gully-hilly regions along the valley to Guanting Basin in the proesses of flashfloods and mudflows,and quickly accumulating on the surface of the pluvial fan and the terrace of the Yellow River at different geomorphic positions.That is to say,they are not flood sediments of the Yellow River.(4)Establishment of a complete pedo-stratigraphic sequence and chronology of the sediments overlying the second terrace of the Yellow River in Guanting Basin.On the basis of detailed field pedo-sedimentological observations and descriptions,as well as physic-chemical analysis and OSL dating,the pedo-stratigraphic sequence includes fluvial sediments(T2-al)→ Malan loess(L1-1,32.00～11.50 ka BP)transitional loess(Lt,11.50～8.50 ka BP)→palaeosol(S0,8.50～3.10 ka BP)→ recent loess(L0,3.10～1.50 ka BP)→modern soil(MS,1.50～0 ka BP).Among them,the palaeosol(S0)developed discontinuously,which was divided into several sub-layers by different thickness(100～400 cm)of flashflood(FFD)or red clay(RC)sediments.(5)Based on the first-hand field and laboratory experimental data,the climate change since the end of last glacial period in Guanting Basin has been reconstructed.Based on the results of climatic indicators such as magnetic susceptibility,loss-on-ignition,TOC,color parameters,grain-size and chemical elements,combined with dating data,the climate change of the basin since the end of the last glacial period can be divided into:the late Pleistocene(32.00～11.50 ka BP),dry and cold climate,aeolian loess accumulation,Malan loess formation,weak weatering and pedogenic transformation;the early Holocene(11.50～8.50 ka BP),the climate gradually changed to warm and humid,the aeolian accumulation weakened,the weathering and pedogenic transformation was increased;the mid Holocene(8.50～3.10 ka BP),the climate was the warmest and wettest,the vegetation was flourishing,the weathering and pedogenic transformation was strong,and the Chernozem developed generally;the late Holocene(3.10～1.50 ka BP),the climate changed to dry and cold again,with weakened pedogenic transformation and enhanced loess accumulation;Modern soil(1.50～0 ka BP)is obviously affected by human activities.(6)The catastrophic surface processes and cluster disaster events induced by climate instability have an Important impacts on the prosperity,decline and evolution of human prehistoric culture in the Guanting Basin.The flashflood and red clay sediments inserted into palaeosol(S0)indicate the soil forming process of palaeosol is interrupted by the sediments of multi-stage disaster events.There was a prevailing period of flashfloods and mudflows at 3.85～3.60 ka BP(14C ages)in the Guanting Basin,and its climate was unstable.The catastrophic surface processes induced by climate instability and many major earthquakes frequently occurred and affected the evolution of prehistoric culture.The natural environment has stabilized and gradually improved at approximately 3.60 ka BP,and the palaeosol began to develop over the land surface,and the disaster events decrease,and the Kayue/Xindian culture rose and developed in the region,and a mixed agriculture of livestock herding and millet cultivation was developed.更多还原