黄土高原晚第三纪红土的形态学和地球化学特征及其环境指示意义

Geochemical and Morphological Character of the Late Tertiary Red-earth at the Loess Plateau and Its Environmental Implications

对甘肃西峰地区晚第三纪红土和第四纪风成典型黄土的化学元素、矿物组成和矿物颗粒形态特征研究揭示出 ,该区红土下部 (6 .2MaB .P .前 )是红化的土壤物质与其它物质一起经流水搬运再堆积的产物 ;中部 (6 .2～ 3.4MaB .P .)未发现再搬运的痕迹 ,是风尘堆积经风化和间歇性地下水位波动作用所形成的 ;上部 (3.4～ 2 .6MaB .P .)是典型的风尘堆积 -土壤序列。由于干旱区的存在是黄土高原风尘堆积的前提条件 ,该区大规模风尘物质的堆积起始于 6 .2MaB .P .前后 ,由此我们认为早在晚第三纪作为源区的中国西北和中亚地区干旱化发展趋势已经相当明显 ,而且这种变化特征对黄土高原沉积环境产生的影响是深远的。

The Xifeng red earth sequence,a type of section for loess plateau region,is studied based on morphology and geochemistry.The result shows that the lower part (6.2 Ma B.P.)was a fluvial alluvial deposition;its middle part (6.2～3.4 Ma B.P.)was derived from eolian dust depositin,as stated in earlier study,but significantly affected by weathering porcesses and groundwater oscillation.The upper part (3.4～2.6 Ma B.P.)was a typical eolian formation,similar to the Quaternary loess soil sequence.Since eloian dust deposits at the loess plateau was mainly transported from the deserts in northwestern China,we interpret the origin of the red earth formation started at 6.2 Ma B.P. as an indication of the strengthening of the aridification on the Asian continent.