Grain size analysis and optical dating for a red sand layer at the top of Shaling sand hill in Xingzi Town,Jiangxi Province
Background,aim,and scope As the largest freshwater lake in China,there are several sand hills around Poyang Lake.In previous studies,these sand hills were thought to be mainly formed by aeolian process.However,during our field investigation,it was found that some middle-top layers of sand hills exhibit very clear water-lain sedimentary structures.Thus,the accurate interpretation for the sedimentary layers within the sand hills is important not only for understanding the environment evolution of Poyang Lake during the Quaternary,but also for the assessment of flood risk in surrounding areas.In this study,a red sand layer(elevation at about 58.3 m)containing obvious mud at the top of Shaling sand hill in Xingzi Town was targeted.The red sand layer was further studied by the grain size analysis and quartz single grain OSL dating technique.Combining with the sedimentary characteristics of red sand layer,its environmental significance and depositional age were further determined.Materials and methods Two grain size samples(SL1-GS-29 and SL1-GS-30)were collected from the targeted red sand layer.For the grain size analysis,the grain size composition,sorting,frequency distribution curve,probability cumulative curve and Sahu discriminant function were further applied.For optical dating,the targeted red sand layer and its adjacent two sedimentary layers were dated by using the quartz single grain regeneration dating protocol,which is followed by simplified single-grain pulse annealing test involving two annealing temperatures(240℃and 280℃).The simplified single-grain pulse annealing test was added to further help screen the quartz grains with thermally stable OSL signals.In this study,three OSL samples(SL1-4,SL1-3 and SL1-2)were collected.Among them,SL1-4 was collected from the adjacent mud layer,which is directly below the red sand layer.SL1-3 is collected from targeted red sand layer and SL1-2 was collected from the adjacent yellow sand layer,which is directly above the red sand layer.Results The grain size analysis shows that:(1)the composition of the coarse sand and medium sand for SL1-GS-29 are 50.8%and 32.4%,respectively,while those for SL1-GS-30 are 55.9%and 26.2%.Thus,the targeted sand layer is mainly composed of coarse sand and medium sand.(2)The sorting(standard deviation)of SL1-GS-29 and SL1-GS-30 were calculated at 1.75 and 1.61,respectively.Thus,the targeted red sand layer is poorly sorted.(3)The frequency distribution curve of SL1-GS-29 shows one obvious peak at 502 µm and several other small peaks at finer grain size.Similar case is for the SL1-GS-30,i.e.its frequency distribution curve shows one obvious peak at 564 µm and several other small peaks at finer grain sizes.(4)The probability accumulative curves of SL1-GS-29 and SL1-GS-30 both show that the red sands contain one group of traction,two groups of saltation and four groups of suspension.Especially,the percentages for the group of traction can both reach about 25%for two grain size samples.(5)If the Sahu discriminant function is applied,Y values of SL1-GS-29 and SL1-GS-30 are calculated at 20.73 and 18.25,respectively,both of which are significantly greater than-2.74.For optical dating,the single grain quartz OSL ages of SL1-4,SL1-3 and SL1-2 were(24.7±2.1)ka,(24.0±2.4)ka and(22.4±3.9)ka,respectively.Discussion By carrying out grain size analysis and single grain quartz OSL dating for a red sand layer at the top of Shaling sand hill,some new insights were obtained:(1)the Shaling sand hill was thought to be formed by four stages,i.e.the red sand accumulation stage at 95 ka BP,the red sand accumulation stage at 46 ka BP,the yellow sand accumulation stage during 25—15 ka BP and modern sand accumulation stage since 250 a.Our study suggests that the formation of Shaling sand hill can be much more complex and the red sand layer can also be formed during MIS2 stage.(2)The sands at the top of Shaling sand hill were thought to be transported by wind and they were from local low flood plain of Yangtze River and its tributary Ganjiang River to relative high elevation.However,such assumed process was challenged by our study,i.e.the grain size composition,sorting,frequency distribution curve,probability cumulative curve and Sahu discriminant function for SL1-GS-29 and SL1-GS-30 all indicate that the targeted red sand layer was not formed by aeolian process.(3)Some sand layers at middle-top of sand hills around Poyang Lake indicate relative humid environment,rather than arid environment,i.e.during the MIS2,the Poyang Lake region was not all under dry climate condition,there were still some humid phases,leading to not only mud depositions at the top of Shaling sand hill,but also the red sand layer containing obvious mud.Conclusions For the targeted red sand layer with obvious mud at the top of Shaling sand hill,the grain size composition,sorting,frequency distribution curve,probability cumulative curve and Sahu discriminant function all indicated that it was not formed by aeolian process and it can't be used as indicator for arid environment,even though single grain quartz OSL dating shows that it was formed during MIS2 stage.Recommendations and perspectives In addition to aeolian sediments,some water-lain deposits were also preserved within sand hills around Poyang Lake.In the future,more studies should be carried out on the sand hills around Poyang Lake,in order to further understand the hydrological process of Poyang Lake during the Late Quaternary.
万方数据
维普
