福建北部闽江二级阶地第四纪红土粒度特征及其成因研究

The Origin of Quaternary Red Earth on the Second Terrace of the Min River in Northern Fujian: Evidence from Particle Size Characteristics

以福建闽江大横红土剖面为例,通过系统的粒度特征分析,探讨闽江二级阶地上红土沉积的成因及其古环境响应.研究结果表明：大横剖面红土的粒度组成以粉砂为主,分选较差,峰度平缓,表现出近源物质混杂堆积的特点.剖面不同层位之间的粒度组成表现出明显不同的特征,0～6.5 m层位红土以粘质粉砂颗粒为主,粒径变化随深度波动缓和,推测为近源高处古土壤或碎屑物质在降雨或坡面径流的侵蚀作用下向地势平缓的地区搬运沉积形成;6.5～8.9 m红土随地层深度增加中值粒径迅速增大,并在7.5 m以下出现剧烈波动,这样的粒级变化特征结合沉积层内存在多量植物根孔和碳屑的现象,推测该层可能是较大的古洪水爆发携带大量山地古土壤和风化壳碎屑物质在阶地上快速沉积的结果;8.9～11.3 m红土粒级波动无规律,分选性差,表现出河漫滩相沉积的特征.

In this study,the origin of the red earth in northern Fujian and its response to the environmental change were discussed through the grain size analyses of Daheng section on the second terrace of Min River. The results show that the red earth in Daheng section are mainly composed of silt particles,with poor sorting and gentle kurtosis,which indicates the mélange of material from proximal area. The characteristics of the grain size change obviously in different horizons,the main compositions of red earth during 0 ~ 6. 5 m are silt and clay,with little fluctuation with depth. It is inferred that they are deposits that denuded from paleosoil or weathering crust on the higher altitude by rain or overland flow. The median diameter of red earth during 6. 5 ~ 8. 9 m increased rapidly with depth,and they show great fluctuations below 7. 5 m. Combining with the presence of multiple plant root holes and carbon debris in this layer,it is speculated that the deposits in this layer are rapid sedimentations of erosion of palesoil and weathering crust carried by paleoflood. The formations during 8. 9 ~ 11. 3 m are flood plain deposits with disordered particle size fluctuation and poor sorting.