陕西旬邑新近纪红粘土微形态特征及其意义

Characteristics and Significance of the Micromorphology of Neogene Red Clay of Xunyi,Shaanxi Province

研究证实 ,新近系红土的土壤微形态特征也是判断古气候 ,划分对比地层的可信指标之一。用此方法研究陕西旬邑下墙晚新生代剖面的新近系及第四系最底部的岩石微形态特征及古气候 ,可将其分为 4个大段 ,自下而上第1大段 ,约相当于 6 .8～ 5 .3Ma ,成壤作用以弱为主 ,气候是在新近纪总体暖湿背景下的凉干阶段 ;第 2大段 ,相当于5 .3～ 4 .2Ma ,成壤作用以强为主 ,是新近纪最为暖湿的阶段 ;第 3大段 ,约相当于 4 .2～ 2 .6Ma ,成壤作用较弱 ,气候由本阶段前半期较凉干变为后半期的较暖湿 ;第 4大段为进入 2 .6Ma的第四纪初期 ,成壤作用极弱 ,气候由新近纪的总体暖湿突然变得冷干 ,在四大段的基础上还可看出次一级的微形态及气候变化与波动。此外还可根据土壤微形态特征划分中、上新统及上新统、更新统的界线。

This study has certified that investigating the characteristics of mic romorphology of Neogene red clay is also one of the effective methods to analyze the paleoclimate, and correlate and subdivide the strata. According to the ch aracteristics of micromorphology of Neogene red clay developed at Xiaqiang Villa ge, Xunyi County, Shaanxi Province, the Neogene paleoclimate of that area could be subdivided into four stages ranging from the bottom to the top, The first sta ge is around 6.8～5.3 Ma B. P. In this stage, the pedogenesis is generally weak; most of the grains are without directional arrangement; and the clay film is not cle ar. From these facts, we imagine the climate of that time was in a situation of cool-dry under the background of Neogene general climate of warm-humid. The s eco nd one is around 5.3～4.2 Ma B. P., with strong pedogenesis, directional arrange me nt of grains and well-developed drape-type clay film. The climate of t his stage might be the warmest and the most humid in all four stages. The third on e is almost in 4.2～2.6 Ma B.P.,and the pedogenesis in this stage is generally w eak, wi th only a little strong one appeared in this late period .We consider that the m ain climate condition of this stage was firsthg semiarid, then changed to semi- hum id. The fourth one is after 2.6 Ma B .P. ,which had entered the Quaternary. In t his stage, the pedogenesis is extremely weak,with very few remained optical cla y film. These characteristics reflect the climate of this stage was very cold an d dry quite different from its previous Neogene general Climate. Within each of these four Paleoclimatic stages, the micromorphology also clearly shows the clim atic sub-changes and sub-fluctuations. Moreover, the boundary between the Mio cen e and the Pliocene, and also that between the Pliocene and the Quaternary could be determined at the beginning (or bottom)of the second stage (or part),and begi nning (or bottom)of the fourth stage (or part),respectively.