江西九江地区第四纪红粘土上的“黄帽子”

A thin layer of yellow-brown-colored earth was generally found on Quaternary red clay in Jiujiang, Jiangxi Province. A typical profile was established. Both particle size distribution and REE (rare earth elements) characteristics of the yellow-brown-colored earth of the profile fully suggested its aeolian origin and close similarity to Nanjing Xiashu loess. The study also implied aeolian origin of the underlying Quaternary red clay. Compared with the red clay, the yellow-brown-colored earth was less weathered because of its lower content of free iron and higher mole ratios of SiO₂/Al₂O₃ and SiO₂/(Fe₂O₃+Al₂O₃) as well as its less developed chemical microtextures of quartz grains. In order to study the ages of the two deposits comparatively, the thermoluminescent dating method was used. As a result, the bottom of the yellow-brown-colored earth was dated to 60±5 ka B. P. and the upper part of the red clay 388±54 ka B. P. It was suggested that the yellow-brown-colored earth was formed in the Late Pleistocene and was probably the aeolian deposit of the Last Glacial, which corresponded with the Malan loess in the Loess Plateau of the northwestern part of China; while the underlying red clay was formed in the Middle Pleistocene. A "yellow cap" on Quaternary red clay in Jiujiang implied a great climatic and environmental variation in the beginning of the Late Pleistocene in the southern part of China, especially in the middle and lower reaches of the Yangtze River. The event not only halted the rubification, once dominating the region, but also produced a widespread covering of aeolian deposit, as only occurred in the cold and dry environment.