甘肃临夏盆地新近纪红粘土粒度组成的古环境意义①

Paleoenvironment Significance of Grain-size Composition of Neogene Red Clay in Linxia Basin, Gansu Province

临夏盆地十里墩剖面新近纪红粘土粒度组成特征、磁化率值分析表明:8.2Ma以来,临夏盆地经历了干旱化的逐级发展和东亚冬季风的逐渐加强过程。8.2～7.4Ma,东亚冬季风形成,干旱化已经发生,风成红粘土开始在盆地沉积;7.4～6.8Ma,亚洲冬季风逐渐加强,开始影响黄土高原地区,形成大面积的红粘土堆积,盆地为半干旱～半湿润的气候环境;6.8～6.0Ma气候与构造稳定,基本维持了前期的气候特点;5.9～5.3Ma,盆地冬季风强度略有增加,为干湿与干凉交替的气候环境;5.3～4.8Ma盆地冬季风盛行、干旱化程度强烈,气候特征为干凉;4.8～4.0Ma,夏季风开始盛行,气候变的湿润。4.0Ma后,青藏高原开始发生又一次强烈隆升。

The Linxia basin is situated in the northeastern part of the Qinghai-Tibet Plateau, and it is a flexural basin bounded by mountains. The weathering and erosion materials that come from Plateau can be deposited in the Basin in the first place, the sediments of the basin completely record the history, of the climatic changes brought by the uplift of the Qinghai-Tibet Plateau. Therefore, the sediments of the Linxia basin can sensitively indicate the uplifting processes of the Qinghai-Tibet Plateau and the climatic changes in a short period of time. Analytic consequences of the grain-size composition and the magnetic susceptibility of the Neogene Red Clay in Linxia basin show： Since 8.2 Ma, climatic changes undergo Asian aridification periodic evolution and east Asian winter monsoon secular reinforcement processes. During 8.2 - 7.4Ma, east Asian winter monsoon begins to form in this area, early stage Asian aridification results in the formation of eolian dust source, Neogene Red Clay begins to deposit in the basin, especially, aridity or semi aridity grassland environment and/or grassland environment with less forest offer favorable background for the Red Clay deposits. The east Asian winter monsoon that is gradually strengthened begins to influence the Loess Plateau in the east part of the Liupan mountain（7.4 -6.8Ma）, then, mass accumulations of the Red Clay start to deposit in the loess plateau. Meanwhile, the climate of the basin transforms semiaridification into semihumidity. From 6.8 Ma to 6.0Ma, the climate and the tectonics are fairly stable. During 5.9 - 5.3Ma, climatic changes undergo a period of small scope changes between the winter and summer monsoon intensity in Linxia basin, it indicates a dry wet and dry cool climatic environment. Between5.3 Ma and 4. 8 Ma, the east Asia winter monsoon system prevails, aridification is the strongest in the basin evolution history from 8.2 Ma to 4.0 Ma, climatic characteristics is cool and dry. The summer monsoon begins to strengthen and the climate becomes warm and humid（4.8 -4.0Ma）. After 4.0Ma B. P, the Qinghai-Tibet Plateau begins to strongly uplift again, which results in the deposits of massive gravel conglomeration named Jishi Formation in the Linxia Basin.