Sedimentary Characteristics Of Sandy Sediments In Luan Xiu Lu Section,Changxing Island

At the end of late Pleistocene,especially in the last glacial period,the climate in Bohai Sea area of China was dry and cold,the sea level dropped,the continental shelf was exposed,and the continental environment was formed.Under the influence of the cold air from Mongolia high pressure,the continental shelf gradually appears desertification phenomenon.In the past,the study on the shelf desertification in Bohai Sea mainly focused on the borehole profile,but few exposed and surface sandy sediment profiles were found.Through field research and field investigation,it is found that this type of sandy sediment profile is exposed on the surface of Changxing Island,Dalian.The geochemical element characteristics and grain size characteristics of this profile are studied,and the following conclusions are drawn:1.This section was formed in the late Pleistocene glacial stage by comparing with the Quaternary glacial and deep sea oxygen isotope stages of China through the photoluminescence test.The main climate background is cold and dry.The corresponding deep sea oxygen isotope stage is about the early stage of the last glaciation?mis4?and the late stage of the last interglacial?mis5?.2.The proportion of SiO₂ and Al₂O₃ in the major element oxides of the profile is the largest,and the SiO₂ content shows a five-stage change trend from top to bottom,which is stable-increasing-re stable-increasing-finally decreasing;while the Al₂O₃ content is slightly lower than that of the ordinary loess profile,which shows a five-stage change trend of stabledecreasing-re stable-re reducing-finally increasing as a whole.The changes of other major elements also conform to the general sediment change law.3.The overall CIA value of the profile is in the range of 50-65,which shows that the change is increasing at cby1-cby8,indicating that the climate is gradually warming;then decreasing at cby8-cby31,indicating that the climate becomes dry and cold;then increasing at cby31-cby36,indicating that the climate is once again warm;gradually decreasing and stabilizing at cby36-cby53;indicating that the climate becomes dry and cold again.Climate change is characterized by dry cold warm wet dry cold cycles.A-cn-k diagram shows that the clay minerals produced by weathering in this sediment section are mainly plagioclase and potash feldspar,but not damontite and illite.The overall weathering degree of the profile is not high,which is consistent with the change of CIA value,further indicating that the sedimentary environment of the profile is cold and dry,and the weathering degree is primary weathering.4.The grain size composition of Luan Xiu Lu section,Changxing Island is mainlycomposed of fine sand and medium sand.The mean values of average particle size and median particle size are 2.6 ? and 2.42 ?,respectively.The average grain size and median particle size of the surface from top to bottom show a trend of large small large small,and the average standard deviation is 1.41,so the sorting property is poor.The average skewness is0.27,which is positive deviation;the average kurtosis is 1.48,which belongs to sharp and narrow peak;the distribution frequency curve is multi peak distribution,and the main peak is at 3 ? whether single peak or double peak phenomenon,which indicates that there may be more material sources of sandy sediment in the profile,but the near source deposition is the main deposition mode.5.The comparison of the grain size components and parameters between the profile and the other two continental shelf sediments in the Bohai bay formed in the late Pleistocene shows that the values of the three are very similar,which further shows that the continental character of the Bohai shelf was enhanced in this period.However,the sand content in the upper part of cby8-cby11 section formed in the middle late stage of the section is obviously smaller than that in the other two sections.The double peak phenomenon of the median particle size and particle size distribution curve of about 3.9 indicates that the material in this section is a typical sand loess with near source as the main source and far source as the auxiliary source,which indicates that the section presents the characteristics of transition from sand layer to loess layer from the bottom to the top.The whole section corresponds to the typical cold and dry background in the last glacial period,and there are small fluctuations of relative warming temperature.In conclusion,the profile was formed in the last glacial period of late Pleistocene,the climate was cold and dry,the sea level dropped,almost all of the Bohai continental shelf outcropped and formed as a whole with slight desertification.With the passage of time,the temperature gradually warmed,the wind gradually weakened,the sea level rose,and the exposed continental shelf in the Bohai region was gradually covered by sea water.The sedimentary type of the region changed It has become a loess type deposit with near source as the main source and far source as the auxiliary.