| Jingbian of Shaanxi is located at the junction of Loess Plateau in Northern Shaanxi and the southern margin of Mu Us Sandy Land.22 km southeast of Jingbian,there are Danxia landform areas of wave valley,which is in the transition section of positive negative landform.The cross bedding is developed with complete sequence and obvious texture.In recent years,the research of Danxia in Northern Shaanxi is mainly in the aspects of tourism resources development,geomorphic characteristics and landscape analysis.However,there is no systematic study on the physical and chemical properties,sedimentary environment characteristics and paleoclimatic significance of Danxia red sandstone in Jingbian.The red sandstone contains a lot of paleoclimate and environmental information,and it is helpful to analyze its physical and chemical properties,which is helpful to the further development.The sedimentary genesis and climate evolution of the red sandstone in Jingbian are studied,which provides scientific basis for protecting Danxia landform.This article takes the red sandstone in Danxia geomorphic area of Jingbian County as the main research object,and selects 8 profiles of three typical sedimentary types for research.The first is the cross profile of thick sandstone and thin layer sandstone,the second is all the cross profile of thin layer sandstone,and the third is the cross profile of thick sandstone.241 samples were collected from 8 profiles for experimental analysis.The results show that the main conclusions are as follows:(1)The results of grain size and magnetic susceptibility show that the red sandstone in this area is deposited in non aeolian desert facies.The particle size of red sandstone in each profile is mainly medium sand and coarse sand,with little clay content and coarse particle size,which is not consistent with aeolian desert sediments.The particle size of the cross profile between thin layer and thin layer is smaller,the size of thick layer and thick layer cross profile is larger,and as for the thick layer and thin layer interbedding profile,particle size is large in thick layer and smaller in the thin layer;the distribution curve of particle size frequency is mainly single peak,and it is peak and mostly extremely positive.There is a good relationship between magnetic susceptibility and grain size.The high value of low frequency magnetic susceptibility corresponds to fine grain,and the low value corresponds to coarse grain.The mean value of low frequency susceptibility is(5.15?17.44)×10-8m3·kg-1,the average range of frequency suscepti-bility is 3.66%to 10.39%,the maximum value of ?lf and minimum value of ?fd are in LZ2 profile,and the minimum value of ?lf andmaximum value of ?fd are in LZ7a profile.(2)The color of the profile is mostly purplish red and red.Combined with the determination results of elements,minerals and CaCO3,it is revealed that the profile is in a warm and dry environment as a whole.In the thick layer and thick layer interbedding profile,the content of CaO,Mn,Sr is higher,the content of Fe2O3 and SiO2 is lower.In the thin layer and thin layer interbedding profile,the content of Fe2O3,Al2O3 Na2O and V is more,the CaO content is less.In the thick layer and thin layer interbedding profile,the content of elements in the thick layer and thin layer is significantly different,which is generally shown that the content of Al2O3,CaO,V,Ni and Zn in thin layer is more than that of thick layer.The detrital minerals are mainly quartz and feldspar,the content of quartz and calcite in the thick and thick interbedded profile is more,the content of calcite in the thin layer and thin interbedding profile is less,in the thick layer and thin interbedded profile,the thin layer quartz content in LZ4 profile is more,and the content of plagioclase and calcite in the two profiles of LZ7a and LZ7b is more.The climate index(the ratio of feldspar to quartz,F/Q)reflects the dry climate in the thick layer of thick layer and thin interbedded profile.The general rule of CaCO3 is that the content of CaCO3 in the thick layer and thick layer interbedding profile is relatively high,and the content of CaCO3 in the thin layer and thin layer interbedding profile is relatively low.The Na/K ratio changes between 0.55 and 0.73,the average value of chemical alteration index(CIA)in each profile is 48.57%to 51.02%,which shows that the weathering effect before deposition is weak.(3)The cross bedding and horizontal bedding show that the sandstone in the study area has the bedding characteristics of fluvial and lacustrine sediments.The average particle size of sandstone is between 1.04? to 2.05?,which is obviously larger than that of wind-based sediment,and its sorting is worse than that of wind-formed sediment.The content of calcium is high,the average content of calcium carbonate is between 1.92%to 7.03%,which is inconsistent with the characteristics of desert wind-based sediments.The Sr/Ba ratio is low,which reflects the low salinity of sedimentary environment.Combined with Sahu formula and element ratio,the sedimentary environment in this area is not wind-formed deposition,but fresh-water lake deposit.(4)The climate change during the formation period of the profile in the study area is fluctuant.According to the depth of the profile,it can be divided into three stages:warm and dry,wet,warm and dry.During the formation period of thick and thick interbedding profile,the precipitation is relatively less and the climate is relatively dry;during the formation period of thin and thin interbedding profile,the precipitation is more and the climate is hot and humid;during the formation period of thick and thin interbedding profile,the climate changes fluctuate greatly,the climate is relatively dry during the formation period of thick layer and hot and humid during the formation period of thin layer.(5)Through the variation of physical and chemical properties of sandstone in the whole profile,the changes of hydrodynamic force and water level fluctuation during the deposition period are revealed.Corresponding to the stage change of climate,the change law of hydrodynamic is "overall weak,overall strong,overall weak",and the change law of water depth is "shallow,deep,shallow".During the formation period of the coarse-grained sedimentary profile with thick interbedding,the lake water shrinks,water level drops,the overall hydrodynamic force is weak and the lake water fluctuation is small;during the formation period of the fine-grained sedimentary profile with thin interbedding,the lake water expands,water level rises,the overall hydrodynamic force is strong and the lake water fluctuation is large;during the formation of the cross profile of the thick and thin interbedding,the water level of the lake rises and falls frequently,the thick layer and coarse grain are formed in the relatively stable period of the lake water,the thin layer and fine grain are formed in the relatively unstable period. |
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