Magnetostratigraphy And Paleoenvironment Of The Red Clay Deposits In The Western Piedmont Of Lüliang Mountains

Neogene red clay deposits in the Chinese Loess Plateau (CLP) contain much paleoclimate information. They detailedly recorded the evolution of the East Asian monsoon and the history of aridity of the East Asian inland area, and furtherly provided the information of uplift of Qinghai-Xizang Tibetan and global climate change. The eolian deposits in the eastern CLP are of great significance to understand the tectonic movements of Ordos block in the Miocene and Pliocene because they directly covered unconformabally on Mesozoic sandstone basement and had significant different basal ages compared with the eolian dust in the western CLP. The red clay deposits in the CLP are considered to be eolian origin, so older red clay deposits contain information of early paleomonsoon evolution and paleoenvironment change. In the piedmont of Luliang Mountains, the red clay deposits always interbedded with sand-gravel layers, lacustrine strata and fossil layers. These layers are extremely important to find out the evolution of the piedmout of Luliang Mountains, ecological environment and climate change events of CLP in the Neogene.Based on carefully investigate of the Neogene strata in the western piedmont of Luliang Mountains, Shilou, Fuxing and Weijiawa three red clay profile were chose, and other sections in Baode and Fugu were also chose for comparison. The rock magnetism, magnetic polarity stratigraphy, particle-size, magnetic susceptibility, sedimentation rate, biological fossil etc. were studied to discuss the age, sedimentary environment, formation processes of red clay deposits in the western piedmont of Luliang Mountains, and further explore the tectonic movements of Luliang Mountains and Ordos block in the late Cenozoic, and the environmental changes of the CLP in the Miocene and Pliocene.The upper part of red clay in western piedmont of Luliang Mountains show redder color, which demonstrate deep reddish-brown or purplish red, stronger soil character and purer texture than the lower part. The lower part which display light reddish-brown, light red and yellow color, always interbedded with grayish white sands, sand-gravel layers and grayish-green lacustrine strata. The basal age of the red clay in western piedmont of Luliang Mountains were centralized at 7-11 Ma. Field deposit characteristics, particle-size, magnetic susceptibility, quartz particle micromorphology, and comparison with other sections revealed that the Shilou red clay are eolian origin, and this is the oldest red clay deposit found so far on the eastern CLP. However, the lower part of Fuxing and Weijiawa red clay exhibit water transportation or transformation evidence with coarse particle size, lower susceptibility etc. Furthermore, several layers display obvious characteristics of water flow action, such as bedding or burying of fossils. By comparing lithologic features, particle size distribution, magnetic susceptibility, deposition rate and biological fossils of red clay sequence in western piedmont of Liiliang Mountains, the paleomonsoon evolution from 11 to 2.6 Ma can be divided into four stages:largely fluctuated stage (11.0-8.3 Ma), relatively strengthen period (8.3-6.5 Ma), extremely changed stage (6.5-4.9 Ma) and further intensified period (4.9-2.6 Ma).