| The Danxia landform of Jingbian Wave Valley in Shaanxi province.It is located in the transition zone from the northern Loess Plateau to the Maowusu Desert.The climate is dry and belongs to the northern warm temperate semi-arid continental monsoon climate zone.The climate transition characteristics are obvious.The red layer(red sandstone)is widely distributed in the valley.It is cut by flowing water to form horizontal or inclined corrugated Danxia,which is of high ornamental value.However,whether the sandstone ripples of this landform are aquatic or aeolian,what are the environmental conditions at the time of formation,and the environmental evolution after formation have attracted the attention of the academic community,but the research results are scarce and need to be resolved urgently.Therefore,it is necessary to carry out related research on Jingbian wave landforms.In this paper,322 samples from 7 typical sections in the upper and middle parts of the wave valley were selected,field survey,sampling and indoor analysis methods were used to research the Jingbian Wave Valley chalk Grain size and chemical element composition,characteristics and vertical changes in the Middle and Upper Period of sandstone,sandstone formation genesis,climate environment,transportation dynamics,water depth and other conditions during the formation period by testing and analysis of sediment grain size,magnetic susceptibility,elemental chemical composition,and calcium carbonate,trying to reveal the formation environment of the wavy landform and its main conclusion are as follows:(1)The sandstone in the study area is coarse in particle size and poor in sortability.It is judged to belong to lake sediment by Sahu formula.The determination of the sandstone particle size composition indicates that the grain size components in the area from early to late mainly changed from medium fine sand to medium coarse sand to medium fine sand to medium coarse sand.Profiles LZ1C and LZ4C have coarser sandstones.Among them,the medium coarse sand content in the LZ4C profile is as high as 89.37%;the profiles LZ3B,LZ6B,and LZ7B are mainly medium-fine sand;LZ2 and LZ5 have more medium-thick sand and thin-layer fine sand..The particle size frequency curves of the 7 sections are mainly double peaks.The standard deviation is 0.97 except for the LZ4C section,and the other 6 sections are between 1.00 and 2.00.(2)The magnetic susceptibility in the area changes from large to smaller,and then larger to smaller from bottom to top.The change corresponds well to the granularity,indicating that the water level of the lake changed from high to low to high to lower,and then higher to lower from early to late.The average range of low-frequency magnetic susceptibility of the seven sections is 3.27×10-8 m3/kg—17.65 ×10-8m3/kg.The three thin layer sections,LZ3B,LZ6B,and LZ7B,and the fine grain layer susceptibility values of the LZ2 and LZ5 sections are higher.The LZ1C,LZ4C section,and LZ2,LZ5 coarse grain layer susceptibility are Lower.(3)The material composition of the area from bottom to top is basically the same and highly mixed.The results of geochemical elements and element ratios show that the vertical variation coefficients of the constant element oxides and trace elements in the seven sections between the formations are small.Among the constant element oxides,the SiO2 content is the most,with an average of 85.01%and Al2O3.The content is next,the average value is 6.75%,and the remaining K2O,CaO,Fe2O3,Na2O and MgO are 7.01%.Among the trace elements,Ti and Ba are more abundant,and Mn,Ni,Cu,Rb,and Sr are less.The content of SiO2 is more in the coarse-grained profile and the thickness of the thick layer,and the content of the thin layer is less.The other constant element oxides and trace elements are more in the thin layer and less in the thick layer.According to the values of K/Al,Si/Al,Rb/Sr,and Sr/Ba,the thin layers of the sections LZ1C,LZ2,and LZ5,and the values of K/Al,Si/Al,and Rb/Sr of LZ3B,LZ6B,and LZ7B are small.The Sr/Ba value is large;the thick layers of the sections LZ1C,LZ2,LZ5 and the LZ4C section are opposite,and the K/Al,Si/Al,Rb/Sr values of the LZ4C section are the largest,and the Sr/Ba value is the smallest,indicating the LZ4C section The driest.(4)As a whole,the content of CaCO3 in the study area is larger in the coarse-grained section and less in the fine-grained section,but the corresponding relationship between the CaCO3 content in the thick and thin section and the particle size is not good.Because the conditions for the formation of calcium carbonate in lakes are complex and diverse,they need to be combined with other indicators to reveal paleoenvironmental information.The average values of CaCO3 content in 7 sections ranged from 1.37%to 4.7%,and the average values of 2 coarse-grained layers were 2.17%and 4.32%,respectively.The two thick and thin layers are 4.70%and 2.45%,and the three thin layers are 2.8%,1.37%,and 2.35%,respectively.(5)Grain size,magnetic susceptibility,CaCO3,and geochemical element characteristics of the study area recorded the paleoenvironmental evolution of the upper and middle sandstones in the Jingbian Wave Valley.From the bottom up,it is divided into 7 stages.Stage 1(83?85 meters in depth):weak hydrodynamics,low lake water level,and dry climate;Stage 2(70?72 meters in depth):enhanced hydrodynamics,lake water level rising,and dry climate;stage 3(65?66.13 meters in depth):The paleoenvironment has severe fluctuations,strong hydrodynamics in thick periods,low lake water levels,warm and dry climate,weak hydrodynamics in thin periods,high water levels,and warm and humid climates;stage 4(46?47.21 meters in depth):lakes Shrinking,strong hydrodynamics,low lake water level,dry climate;stage 5(38-40 meters in depth):weak hydrodynamics,high lake water level,humid climate;stage 6(20?21.5 meters in depth):climatic fluctuations,thick The climate in the stratum is warm and dry,the hydrodynamic force is low,and the lake water level is low.In the thin stratum,the climate is warm and humid,the hydrodynamic force is weak,and the water level is high.Stage 7(10?11.93 meters in depth):The climate fluctuates,and the overall warm and dry climate causes the lake water level to decrease,Hydrodynamic enhancement. |
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