Study Of A Sand Layer With Swash Cross Bedding Within The Sand Hill At North Of Poyang Lake,by Using Grain Size Analysis And Optical Dating

There are several sand hills around Poyang Lake,such as sand hills from Hukou county to Pengze county as well as Songmen Island in Wucheng Town.Previous studies suggest that these sand hills were formed by aeolian process,especially for the middle-top part of the sand hills.However,by field investigation,it was found that several sand/silt layers in the middle-top part of sand hills around Poyang Lake exhibit clear water-lain sedimentary structures.It is interesting that the elevation of these sand/silt layers was significantly higher than the highest water level of Poyang Lake since 1949.In this study,one sand layer with swash cross bedding at elevation of～47.5 m in sand hill at north of Poyang lake were targeted.The samples were collected for grain size analysis,the bleaching of K-feldspar IRSL signals and quartz OSL signals analysis and depositional age analysis by using optical dating.The following conclusions are obtained through this study:(1)From the view of grain size,the targeted sample is mainly composed by medium sand and it is only moderately sorted;the frequency curve of PZTZ-9A exhibits one main peak and a small peak;the log-probability accumulative curve can be fitted by seven distinctive populations and Y value is calculated at 2.13 if the Sahu discriminant function is applied.The above grain size results are consistent with the interpretations by sedimentary structure,i.e.the targeted sand layers with swash cross bedding should be interpreted as water-lain sediments,even though the targeted sand layer was significantly higher than the highest water level of Poyang Lake since 1949.(2)From the methodology of optical dating,both K-feldspar grains and quartz grains suffer from obvious insufficient bleaching problem.The insufficient bleaching of K-feldspar IRSL signals are manifested by the lack of D_e-temperature plateau in the multi-elevated-temperature post-IR IRSL(MET-p IRIR)method when the average age model is used.This very different from typical Aeolian sediments.The insufficient bleaching of quartz OSL signals is manifested by the following three clues:(1)The quartz aliquots exhibit very scattered De distribution and the over-dispersion value is larger than 20%for single-aliquot distribution when single-aliquot regenerative-dose(SAR)protocol is used.This is very different from typical Aeolian sediments,i.e.the quartz OSL signals of typical Aeolian sediments generally produce homogeneous De distribution.(2)The single aliquots of quartz of targeted samples exhibit statistically positively skewed dose distributions,while the dose of quartz aliquots of typical Aeolian sediments exhibit normal distribution.(3)For typical Aeolian sediments,the quartz De should be significantly lower than K-feldspar 250°C IRSL De,if average age model is applied.However,the targeted samples in this study exhibit abnormal relationship between the quartz De and K-feldspar 250°C IRSL De,when the average age model is applied,i.e.the quartz De of targeted samples is abnormally higher than the K-feldspar 250°C IRSL De,when the average age model is applied.In summary,the OSL signals of some quartz grains suffer from obvious insufficient bleaching problem.Such phenomenon,again,support that the targeted sand layers are deposited in a water-lain process,rather than the aeolian process.(3)To obtain reliable depositional ages of the targeted sand layer,the parameter of water content is assigned with a large range,i.e.the water content is assigned as 1.6±0.3%,10±2%and 20±4%,respectively,and the corresponding K-feldspar OSL age of the targeted sand layers is constrained between 33.4±0.4 ka and 39.3±1.1 ka,which are all belong to MIS3a stage.Correspondingly,the OSL age of quartz and K-feldspar are consistent with each other within error.The optical dating results show that the target sedimentary within the sand hills is formed during MIS 3a stage.This study shows that the middle and upper parts of sand hills can not be fully interpreted as aeolian sediments,at least some of the interlayers are the water-lain sediments even though the targeted sand layers are much higher than the highest water level of Poyang Lake since 1949.The interlayers of water-lain sediments within the sand hills might might contain important information on past hydrolocial process of Poyang Lake.On the other hand,our study results show the Pengli Ze(the previous name of Poyang Lake)was formed at least at MIS 3a stage,rather than the Holocene Optimum.In addition,the the water level of Poyang Lake(Pengli Ze)during MIS 3a stage may be significantly higher than the highest water level of Poyang Lake since 1949.In the future,more research should be carried out at different sand hills around Poyang Lake,to further understand the formation of the sand hills.It not only helps us better understand the evolution of Poyang Lake,but also be very useful for government policy on sustainable water supply,flood control and ecological safety on Poyang Lake.