关键词: 网纹, 沙山, 光释光, 鄱阳湖

Abstract:

Sand hills distributed along shores of the Poyang Lake are composed of alternate eolian sand layers and silt layers. The Shaling sand hill, about 2 km in width and 5 km in length, is located in Liaohua Town of Xingzi County. On its north and west margins, three sections (SS1, SS2 and SS3) were investigated. Vermiculated mottles in this hill can be divided into three types. Vermiculated mottles of type Ⅰ are thick and dense and only occur in the lower part (layer 1) of section SS1. Vermiculated mottles of type Ⅱ are relatively thin and sparsely scattered and occur in the beds (layer 2) above type Ⅰ. Vermiculated mottles of type Ⅲ are indistinctly shaped and occur in the upper part (layer 5) of the section SS1and the upper part (layer 2) of the section SS2. A sand bed (layer 4) separates the vermiculated mottles of type Ⅲ from that of type I and type Ⅱ in section SS1. Vermiculated mottles of type Ⅲ cover a sand bed (layer 1) in section SS2. The section SS3 only comprises one sand bed (layer 1) and develops no vermiculated mottle. Vermiculated mottles of types I and type II are mature vermiculated mottles, whereas type Ⅲ are immature. OSL samples were collected from each sand bed. 125-250 μm quartz grains were separated from sand samples following the sequences of the standard pretreatment. The equivalent dose was measured using the Single Aliquot Regeneration protocol in the Laboratory of Surface Process of Nanjing University. The annual dose was determined by contents of U, Th and K measured using the neutron activation method in China Institute of Atomic Energy. In OSL dating of sand samples, no evidence indicates that the samples show OSL saturation signals and the annual dose of the samples was affected by weathering. So, the OSL ages can be interpreted as the deposition ages. The sand layers of Section SS1 and SS2 were deposited about 80 ka and 71 ka ago respectively and the sand layer of Section SS3 has been accumulated since 29 ka. From these OSL ages, it can be thought that the age of the layers with mature vermiculated mottles is older than 80 ka and the age of the layers with immature vermiculated mottles ranges from 71 ka to 29 ka. The top boundary age of vermiculated beds can be inferred to be 80-29 ka if the layers with immature vermiculated mottles are taken as the vermiculated beds. The top boundary age of vermiculated beds is older than 80 ka if the layers with mature vermiculated mottles are taken as the vermiculated beds. Both inferred top boundary ages are much younger than 400 ka, which was previously reported as the formation age of the vermiculated beds. However, this conclusion is drawn on three OSL ages in Shaling area and should be validated by other studies in the future.

Key words: vermiculated mottle, sand hill, OSL, Poyang Lake