Grain Size Characteristics And Optically Stimulated Luminescence Chronological Analysis Of A Banded Sand Layer Within A Sand Hill At North Of Poyang Lake

Several sand hills were developed and preserved at central shore and north of Poyang Lake.At present,many studies suggested that the sand hills are the products of aeolian process.By field investigation,it was found that several sand layers and silt layers with clear water-lain sedimentary structures in the middle-top of the sand hills around Poyang Lake.In this study,a sand layer with banded bedding from a sand hill at north of Poyang Lake were targeted and the sand hill is located at Shazhou village,Hukou county in Jiujiang City.The whole thickness of the sand layer with banded bedding is over 20 centimeters and the its average elevation is about 40 m.The bedding is characterized by alternating development of yellow sand layer and red sand layer.The thickness of each unit is in the scale of centimeter.In addition,yellow sand units are thicker than the red sand units.The samples of the sand layer were collected for three detail investigations:(1)grain size analysis;(2)the bleaching of K-feldspar IRSL signals and quartz OSL signals during their deposition;(3)age analysis by using optical dating.The following conclusions are obtained through this study:(1)The average grain size of HGDS-Y is 1.111 Φ,belonging to medium sand,with good sorting,fine skewed and very leptokurtic;the average grain size of HGDS-R is 3.129Φ,belonging to very fine sand,with poor sorting,very fine skewed and flat kurtosis.The frequency curve of the samples shows a bimodal curve;the log-probability accumulative curve can be fitted by eight distinctive populations and the sample transportation method is mainly by traction.The Y values calculated using the Sahu Discriminant Formula are 0.22 and 15.44,respectively.The above results of grain size analysis indicate that the sand layer in this area is a water-lain sediment.(2)From the methodology of optical dating,both some quartz grains and K-feldspar grains suffer from obvious insufficient bleaching problem.The insufficient bleaching phenomenon of quartz OSL signals is manifested as a relatively discrete distribution of quartz equivalent dose(De),and the over-dispersion value is 27.3%;the insufficient bleaching phenomenon of K-feldspar IRSL signals is manifested as no clear plateau in the relationship between De and stimulation temperature in the temperature range of 50℃ to250℃ when testing K-feldspar using the MET-p IRIR method.The above results show that some particles of sedimentary quartz and K-feldspar were not fully bleached during transportation and sedimentation,which is inconsistent with the characteristic of typical aeolian particles with long exposure time and complete bleaching.(3)The Minimum age model(MAM)of quartz is 38.9±5.2 ka.For K-feldspar,the improved analysis process was used to select K-feldspar slices with better bleaching for testing using the MET-p IRIR,and the K-feldspar age result is 39.9±2.3 ka.The OSL ages of quartz and K-feldspar are consistent within the error range and both indicate that the sand layer studied in this paper is a product of the MIS3 stage.The warm and humid climate characteristics of MIS3 also provide an important auxiliary factor for the water-lain origin of the sand layer.(4)Based on the analysis of the sedimentary structure features,grain size characteristics,distribution of equivalent dose of quartz,and the insufficient bleaching of K-feldspar IRSL signals,it is concluded that the banded bedding in the northern sand hills of Jiangxi Province is the water-lain sediments.(5)By field investigation,it was found that there are many such banded sand layer in the middle-top part of sand hills around Poyang Lake.It shows that there are water-line interlayers in the sand hills and they cannot be attributed solely to aeolian accumulation.This study shows that although the laminated sand layer in the northern part of Poyang Lake is higher than the highest water level since the founding of the country,it cannot be simply interpreted as aeolian deposits.Instead,the sand layer at this location is a product of water accumulation,which has certain implications for the ancient water level of Poyang Lake.In addition,the luminescence dating results of the samples indicate the MIS 3 stage,and previous studies have shown that the central and western regions of China had a large lake period during the MIS 3 stage.Further research is needed in the sand hills along the shore of Poyang Lake to reveal the paleoclimatic changes of Poyang Lake,enrich and develop the study of paleoclimatic environment and paleohydrology of the surrounding areas of Poyang Lake,and provide scientific basis for decision-making on flood control and disaster prevention in Jiangxi Province in the future.