关键词: 第四纪红土, 洞庭湖周边地区, 古环境, 粒度特征

Abstract: Dongting Lake Area is a semi-closed basin opening toward the north. In this area, alluvial and lacustrine sediments mainly developed in fluvial plain, low level terrace of the rivers and modern lakebed, and Quaternary red earth is widespread in low mountains, hills and hillock. Scholars paid very little attention to the red earth in this area over a long period of time in the past. In order to reveal the genesis of the Quaternary red earth in Dongting Lake Area and its preliminary climatic implications, we did a series of work. The detailed field investigation has shown that there are several types of Quaternary red earth in Dongting Lake Area, and the major type is the red earth with homogenous structure without gravel. The intact stratigraphic profile consists of the lower reticulate red earth layer or reticulate yellow brown earth layer and the upper homogeneous red earth layer or yellow brown earth layer. Ten profiles of this type of Quaternary red earth and 122 samples were collected, and their grain-size was tested by using laser particle size analyzer Malvern 2000, a product of Malvern of the UK. The results showed that: (1) Silt (4~63 μm) is the major type of particle in all samples; clay (<4 μm) is the second major type; sand accounts for the smallest portion. Aeolian particle (10~50 μm) is the dominant particle, and the mean content ranges from 33.23% to 42.23%. These features are similar to those of the loess, Xiashu loess and aeolian red earth, but quite different from fluvial red earth from Xiangjiang Rriver terrace and the residual red earth formed with the rock weathering, indicating that aeolian red earth exists on the hilly land of the west, south and east sides of Dongting Lake Area and they all belong to aggradation red earth in Mid-subtropics. (2) All the samples can be separated into four categories: vermicular red earth, homogeneous red earth, reticulate yellow-brown earth and yellow-brown earth. In terms of grain size composition, their clay contents have a ranking order of vermicular red earth >homogeneous red earth >reticulate yellow-brown earth >yellow-brown earth, while their silt contents show an opposite ranking order. Combined with other information extracted from grain size parameters, we can speculate that sedimentation significantly strengthened during the period of yellow-brown earth, so the sediments become coarser and well sorted. However, in the period of vermicular red earth, sedimentation weakens and weathering strengthens, fine particle content increases, and the red earth are poorly sorted. This probably reflects the environmental change process in which winter monsoon strengthens and weathering weakens from the mid-Pleistocene to the late Pleistocene around Dongting Lake Area. (3) Along with the direction of the winter monsoon, the average particle size and median size (φ value) of the red earth become bigger (become smaller) southwards, but the percentages of aeolian component (10~63 μm) decrease while that of the<2 μm component increase slightly. These trend, therefore, provide aeolian evidence for the Quaternary aggradation red earth sediments and suggests that the intensity of weathering enhances southwards in the Dongting Lake Area.

Key words: Dongting Lake Area, grain-size features, paleoenvironment, Quaternary red earth