Paleoclimatic Significance of Grain-size Composition of Neogene Red Earth at Xunyi, Shaanxi Province

Key words: 
• Shaanxi Xunyi /
• grain-size composition /
• paleoclimate

Abstract: An approximately continuous section of Late Cenozoic earthy deposits, developed at Xiaqiang Village, Xunyi County, Shaanxi Province, is 230 m in thickness and consists of the complete Quaternary loess-paleosol sequence and its underlying Neogene red earth beds. The loess-paleosol deposits of Quaternary as the aolian genesis has been accepted broadly by geologists. The red earth of Neogene has also been considered to have the same origin as that of loess in recent years. Therefore, the characteristics of the grain-size analysis of the red earth, similar to that of loess, could be used as a substitute index for the paleoclimate study. The climatic evolution of Neogene is discussed in this paper by studying on the grain-size analysis. We choose four groups of grain size data: the mean, 30μm, 63μm and 2μm, which are shown in four curves. Then, we could see (Fig. 2) that these four grain-size curves are all relatively stable and show the features of high-frequency and low-amptitude. Each of these four curves of Neogene red beds can be subdivided into four parts ranging from the bottom up to the top. The first part composed of the 20th bed group has high content of mean, 30μm and 63μm, but low content of 2μm. The second part formed by the bed groups of 19～18th is between the first and third parts, whose content of the mean, 30μm and 63μm is gradually decreasing all the way till the bottom of the third part, while that of the 2μm is increasing till to the highest of the third one. The third part, composed of 17～15th bed groups, has high content of 2μm and low content of mean, 30μm and 63μm; The forth part consisting 14～13th bed groups is relatively stable without obvious undulation, although the content of the mean, 30μm and 63μm is slightly higher, while the 2μm slightly lower than those of the third one. These differences of four parts can also be seen in Fig. 3 the age/depth relations of the different sedimentary ratio of each part. Each curve of Neogene, especially that of 2μm, can further be subdivided into four million-grade cycles, each of these cycles containing many smaller cycles of ten thousand grade. The sharp changes of the curves near the top reveal the different features of grain-size composition of Quaternary from those of Neogene. The curves of coarse particle of Neogene show little changes, which probably means the small uplift of Qin-Tibet Plateau. Winter and also summer monsoon had been primarily formed though rather weak. The average content of 2μm particles is remarkably higher in Neogene red beds than that in Quaternary loess-paleosol, but with a certain fluctuation, suggesting that the Neogene climate was mainly moist and warm and that the strength of summer monsoon had a slight greater change. The grain-size curve of 2μm particles can be divided into four stages. Stage 3, consisting of 17～15th bed-groups, shows the strongest summer monsoon, being the warmest and wettest. However, stages 4, 1 and 2 indicate that the summer monsoon of these stages is relatively weaker than that of stage 3. In other words, the feature of the Late Neogene climate of lLoess Plateau was generally warm and moist. With this background climate there were alterations of warm-wet and cool-dry climate. A distinct change in the curve can be seen since Quaternary, which is indicated by the increase of coarse particles and decrease of 2μm particles. And the curve changed from high-frequency, low-amptitude pattern of Neogene to that of low-frequency, high-amptitude. This obvious change is mainly related to the strong uplift of Qin-Tibet Plateau since Quaternary and also to the formation and expansion of Antarctic ice sheet. The grain-size curves are in agreement with susceptibility curve and strata analysis.