Sedimentary facies and chronological study for a Late Pleistocene sand layer with swash cross bedding within a sand hill at north of Poyang Lake, as inferred from sedimentary structure and optical dating of K-feldspar and quartz grains

Abstract

Poyang Lake is the largest freshwater lake in China. Currently, there are several sand hills around Poyang Lake and they were mainly interpreted to be formed by aeolian process. However, by field investigation, it is found that several sand/silt layers with parallel bedding, horizontal bedding and swash cross bedding were preserved within sand hills around Poyang Lake. Compared with recent highest water level in Poyang Lake since 1949, the sand/silt layers with parallel bedding, horizontal bedding and swash cross bedding are higher than 22.63m from several meters to more than 10 m. From the view of sedimentary structure, the sand/silt layers mentioned above should be treated as water-lain sediments. In this study, one sand layer with swash cross bedding at elevation of ∼47.5m within a sand hill at north of Poyang Lake was targeted. The bleaching of K-feldspar IRSL signals and quartz OSL signals during their deposition were further investigated by using optical dating techniques. It is found that both K-feldspar IRSL signals and quartz OSL signals were partially bleached and such phenomenon is very different from typical aeolian sediments. The partially bleaching of both K-feldspar IRSL signals and quartz OSL signals is consistent with interpretations by sedimentary structure of the targeted sand layer, i.e. the sand layer with the swash cross bedding within the sand hill should be interpreted as hydraulic sediments and they were not formed by aeolian process. The OSL ages of targeted sand layer were then analyzed by three different methods, i.e. single aliquot MET-pIRIR dating of K-feldspar, single aliquot OSL dating of quartz as well as single grain OSL dating of quartz. The OSL ages of three methods were all consistent with each other and the age results show that the targeted sand layer was formed during MIS 3a. Although previous studies suggested that previous Poyang Lake (Pengli Ze) was initially formed during mid-Holocene, our study indicates that previous Poyang Lake (Pengli Ze) already existed during MIS 3a and it might experience extremely high water level phase at that time.