| 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. |
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