| As the derived sediments of the desert,the Loess sediments and the polar ice cores are called the three pillars of the paleoclimate research.The continuity of the loess deposits records the process of environmental changes.The loess of Shandong is located at the eastern margin of the loess distribution in China.The central part of the modern monsoon region in China,Sea and land transfer transition zone.It is an important part of the East Asian aeolian dust deposition system.The distribution of Shandong loess can be divided into two major regions:Coastal and island areas of Bohai Bay,In front of the Luzhong mountain.Basically east-west strip distribution.However,in this dust accumulation,from Zhangqiu in the west to Zichuan and Qingzhou in the middle,to Penglai on the east coast,all the way to the Miaodao Islands,there are one or more distinct gravel layers,The gravel layer is a prominent feature of the loess in Shandong Province distinguished from the typical loess in the Loess Plateau and the Xiashu Loess in the middle and lower reaches of the Yangtze River.Gravel layer as an important carrier to record the changes of climate and environment,many scholars at home and abroad have carried out a lot of research.However,there are few researches on sedimentary characteristics,sedimentary age and environmental significance of gravel layers in loess deposits in Shandong Province.In view of the loess in Shandong Province dust accumulation in East Asia series of important position in the study,and its mixed gravel layer has unique characteristics and scarcity,we conducted extensive field investigation and study in Shandong area,selected the most typical,well-preserved,thick accumulation of Zhangqiu Heiyu section（HY）as the research object.Studying the sedimentary characteristics of loess sediments in the upper and lower gravel layers,analysis of the structure characteristics of the gravel layer.Combined with the OSL data,we can deepen the formation and evolution of climate and environment in the monsoon region of eastern China.The results of this study show that:（1）According to the stratified characteristics of the Heikou section of Zhangqiu,Shandong,a total of 6 samples were collected during the light-release dating samples.OSL measuring instrument for Ris?TL/OSL-DA-20,the measurement method of Simplified Multiple Aliquot Regenerative-dose.The results of dating indicate that the HY profiles in Zhangqiu,Shandong are 100 cm,250 cm,370 cm,470 cm,717 cm,1260 cm at the age of 0.264±0.017 ka、15.7±0.7 ka、20.4±1.4 ka、23.6±1.4 ka、32.0±2.0 ka、39.0±2.0 ka,therefore,it is possible to know the sediments since the last glacial period of the section.The results of the dating of the upper part of the G1gravel layer and the upper part of the G2 gravel layer are 39±2.0 ka and 32±2.0 ka respectively,indicating that the G1 and G2 gravel layers formed in late Pleistocene.（2）The curve of particle size and frequency of loess samples in HY profile showed bimodal distribution,The main peaks are mainly concentrated in the range of 2060μm,and the second peak smaller than 1μm,the size distribution range is 0125μm,and the total number is positive partial-normal distribution.The content of silt in HY section is the highest,the range of change is 58.71%81.95%,the average content is73.72%,the next is the clay fraction,the variation range of the clay content is11.40%30.90%,the average value is 22.08%,the content of sand is least,and it changes between 0.72%12.34%and the average content is 4.19%.From the C-M diagram,it can be seen that the loess section C is between 100 and 200μm,M is mainly between 10 and 60μm,and the projection on the C-M diagram is more concentrated.By using Sahu discriminant formula to discriminate the sedimentary environment,the Y value of the loess sediment varied from-10.68-5.45,both less than-2.7411.According to the characteristics of grain size composition and the frequency distribution curve of grain size,it is indicated that the loess section is an aeolian deposit.（3）According to the indicators of the climatic proxy index（Md）,environmental sensitive components（3.98-25.11μm）,low frequency magnetic susceptibility(χlf),combined with the results of OSL and the deep-sea oxygen isotope,the HY profile in Zhangqiu can be divided into four types:modern farming layer（040 cm）,MIS1（40200 cm）,MIS2（200500 cm）and MIS3（5001450 cm）.Two fluvial gravel layers were formed at depths of 670710 cm（G2 gravel layer）and 12601310 cm（G1 gravel layer）.According to the results of optically stimulated luminescence dating,the accumulation age of G1 and G2 gravel beds is 38.740 ka and 2932 ka,indicating that the climate is warm and humid,and the precipitation is large.The precipitation is much larger than the water seepage and the surface runoff is formed.The flow of water carries debris in the river.Through the analysis of fabric Characteristics in G1 and G2 gravel layers,the gravel size of G1 and G2 gravel layers is dominated by fine gravel and medium gravel,and the hydrodynamic force is weak in the formation period of the gravel layer.The gravel diameter of G1 gravel layer is mainly between 1620 mm,and the gravel diameter of G2 gravel layer is mainly between 1115 mm.The gravel diameter of G1 gravel layer is larger than that of G2gravel layer,indicating that the hydrodynamic force of G1 gravel layer formation period is larger than that of G2 gravel layer.During the formation of G1 and G2gravel layers,the gravel direction showed the North-East direction（NE）and the positive east direction（E）.The paleocurrent direction changed little,showing the basic flow from east to west.The lithology components of the G1 and G2 gravel layers are dominated by limestone,and their contents are as high as 95%.The gravels are mainly secondary and sub-circular,with a total content of more than 70%,and the roundness is good.According to the stratigraphic characteristics and paleocurrent of the area,the G1 and G2 gravel layers should be near-source materials and the provenance area did not change significantly during the formation period. |
