铜陵红土石英颗粒特征及其环境意义

Characteristics of quartz grains in the red clay of Tongling City and their environmental implications

目前我国南方网纹红土的成因、物源存在争议,通过分析铜陵红土剖面石英粒度和石英颗粒表面形态特征,并结合已发表年代数据探讨了红土沉积物的搬运动力和物质来源。结果表明,铜陵剖面石英粒度表现出明显的风力搬运特征,粒径整体偏细,剖面自下而上粒径变粗,粗粉砂(10~50μm)组分为众数粒组,粒度频率分布曲线总体呈现出双峰且主峰明显的特征,并在粗粒端含有隐峰,颗粒总体分选较差,呈正偏态,峰态尖锐;石英颗粒大多呈次棱角状或次圆状,颗粒表面既出现水下磨光面、V型坑、三角型坑等水成特征,也出现碟型坑、新月型坑、麻坑等风成特征,还具有两种特征叠加的现象。剖面自上而下颗粒磨圆度逐渐变好,风成特征更加明显。综合分析推测铜陵网纹红土是风力搬运近源与远源物质共同沉积,并随着东亚季风的变化,不同物源区物质的贡献程度也发生转变。

Hot debate has been occurred for long concerning the origin and provenance of the vermicular red clay in South China.In this paper,transportation mechanism and provenance of the vermicular red clay were studied by means of grain size and surface textures of the quartz grains from the red clay in Tongling city in the south of Anhui province.The quartz grains from the Tongling section show some obvious characteristics of wind transportation.The grains are fine in general and gradually coarsen up from bottom to top.The vermicular red clay is dominated by silt(10~50μm),ranging from 40.62%to 60.37%.The frequency curves of the quartz grains are essentially bimodal with a low hidden peak in the coarse fraction.And the frequency curves of different layers in the profile shows good consistency.Microscopic images of quartz grains show that they are,in fact,the mixture of rounded and poorly rounded grains.Some particles have obvious hydrodynamic effects remained on the surface,such as underwater polished surfaces,V-shaped pits,etc.,some have obvious traces of wind action,such as dish-shaped pits,crescent-shaped pits,etc.,while the others show some surface texture jointly formed by hydrodynamic and wind actions,suggesting a mixture of distant and nearby sources.The near-source sediments may come from the floodplain of surrounding rivers,where the hydrological characteristics are retained due to the short-distance of wind transport.The surface of the quartz particles at the bottom of the profile has obvious features of hydrogenic origin,and the aeolian features increase upwards.Finally,it is confirmed that the red vermicular clay in Tongling city is a kind of mixed deposits composed of the particles from near and distant wind sources,and the contribution of the materials of different sources depends upon the change of East Asian monsoon.