黄土高原最老红粘土的发现及其地质意义

黄土高原红粘土的形成演化与青藏高原隆起、亚洲季风形成演化等有着十分密切的关系。黄土高原中部厚达303m的朝那黄土-红粘土剖面的古地磁年代学表明红粘土形成于8．1Ma，是目前黄土高原上发现的最老的红粘土沉积。这个事实表明大约在8．1Ma：本区构造活动频繁，断裂十分发育，使得鄂尔多斯夷平面解体，六盘山地区呈断块上升，而六盘山以东地区下沉接受红粘土沉积。这可能指示了毗邻的青藏高原有一次显著的构造隆升。

The Oldest Red Clay in the Chinese Loess Plateau and Its Geological Significance

The widely distributed red clay sediments underlying the ChineseLoess Plateau record truly Neogene environmental evolution, and the initiation and development of red clay sedimentation are intrinsically related to the uplift processes of the Tibetan Plateau and the evolution of East Asia monsoon system. The red clay is regarded as a focal proxy on past global changes. In this paper, the magnetostratigraphy of a new loess_red clay section, which was found recently, and its geological significance are reported. The new section is located at Chaona of Lingtai county(107.2°E， 35.1°N) in the central Loess Plateau. The loess_red clay deposit at Chaona is about 303 m thick.It consists of a complete Quaternary Loess_soil sequence with a thickness of about 175m and 128 m of Late Tertiary red clay deposits. About 400 orientated block samples and 3600 bag samples were collected from this section. The magnetic remanence was measured at the Paleomagnetism Laboratory, both Lanzhou institute of Geology, CAS and institute of Geophysics and Geology, CAS. Based on the correlation with the standard geomagnetic polarity time scale, the magnetostratigraphic results indicate that the age of Chaona red clay extends to 8.1 Ma, which is at least 0.5 Ma older than previously reported, and it is the oldest red clay deposition in the Loess Plateau. The commencement of red clay at ～8.1 Ma may imply that the Ordos planation surface was broken by the movement of Haiyuan_Liupanshan Faults. And the western part adjacent to Tibetan Plateau to be uplifted to form embryo Mt. Liupanshan and the eastern part to be down_faulted to receive red clay deposition. We link this faulting to an initial uplift of the Tibetan Plateau. The undulating nature of the broken Ordos planation surface may explain the chronological and depth discrepancies among various cross_sections of red_clay. While gravels and sands are found in the lowermost red clay deposits in different regions during field morphostratigraphical survey. The phenomena suggest that the running water processes played an important role on the early red clay deposition