Abstract: The aeolian origin of red clays in the Loess Plateau has been unanimously recognized. The earliest red clay was found in the late Oligocene and early Miocene, indicating that the arid environment had been formed in the inland northwest of China at the end of Paleogene. The grain size characteristics of red clay in the loess plateau show that the average grain size gradually decreases from the north to the south, indicatin g that the initial monsoon system has been established, and the winter wind played an important role in the transport of aeolian dust. The grain size distribution of red clay indiactes that it is composed of two components. The coarse grain (>20 μm) represents those transported by the near surface wind (winter wind), while the fine grain (<5 μm) represents those transported by the upper westerly wind. The grain size composition characteristics of red clays well record the development and change of atmospheric circulation. Since the end of Oligocene, the average grain size and sedimentation rate of red clays in the west of Liupanshan Mountain increased at 21.3-20.2 Ma, 16.0-13.3 Ma and 8.7-6.9 Ma, which may be related to global cooling and the Qinghai-Tibet Plateau uplift, and the 8.7- 6.9 Ma increase was mainly controlled by global cooling. In the red clay section to the east of Liupanshan, the grain size of variation characteristics shows that the grains deposited during 7.6 Ma to 6.2 Ma or to 5.4 Ma is coarse, indicating that the winter monsoon was strong. After that, the average particle size was fine up to 3.6 Ma, and the deposition rate was low, indicating that the winter monsoon was weak climate environment. From 3.6 Ma to 2.6 Ma, most profiles show an increase in mean grain size, an increase in sedimentation rates, and an increase in both winter and summer monsoon, which indicates to the transition to Quaternary glacial period. The development of the Arctic ice sheet and the uplift of the Qinghai-Tibet Plateau may have contributed to these changes.

Key words: grainsize, red clay, Loess Plateau, winter monsoon, environmental implication