Abstract:

Poyang Lake is the largest freshwater lake in China. At present， 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， two representative sand layers with parallel bedding were targeted and two bulk samples were collected for particle size analysis. One sand sample （XZ-1） is collected from a sand layer with the parallel bedding within Shaling （sand hill） in Lushan city. Currently， the Wusong elevation of sand sample （XZ-1） is ~36.5 m. The other sand sample （HGDS-7） is collected from another sand layer with parallel bedding within a sand hill at Shazhou village in Hukou county. Currently， the Wusong elevation of sand sample （HGDS-7） is ~40.3 m. The results are as following： （1） Sand sample XZ-1 is moderately sorted， while sand sample HGDS-7 is poorly sorted. （2） The frequency curve of XZ-1 exhibits one main peak and a small peak， while that of HGDS-7 exhibits bimodal characteristics. （3） The log-probability accumulative curve of XZ-1 can be fitted by six distinctive populations， while the log-probability accumulative curve of HGDS-7 can be fitted by eight distinctive populations. （4） If the Sahu discriminant function is applied， Y value of XZ-1 is calculated at 3.71， while Y value of HGDS-7 is calculated at 8.89. The above particle size results are consistent with the interpretations by sedimentary structure， i.e. the targeted sand layers within the sand hills should be interpreted as water-lain sediments， rather than aeolian sediments， even though the targeted sand layers are much 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.

Key words: Poyang Lake, sand hill, parallel bedding, grain-size analysis, water-lain sediments