THE GRAIN SIZE EVIDENCES OF THE MIOCENE AEOLIAN SEDIMENTS AND ITS PALEOENVIRONMENTAL SIGNIFICANCE IN LINGYAN HILL, NANJING, EAST CHINA

Grain size is the main proxy to determine the aeolian sediments. Researches on the paleoclimatology and paleoenvironment recorded by the red clay have greatle progressed in the region of China's loess plateau during the past decade. The earliest red clay in the west part of the loess plateau is about 22 Ma old, while in the area east of Liupan Mountain, the bottom part of the red clay sequences is no more than 9 Ma. Red clay was regarded as the result of aeolian sedimentation, similar to the loess-paleosol sequences. Red clay records have important significance in the studies on Asian environmental evolution in the Era of Cenozoic, especially on the fields of inland desertification and the impacts of the Tibetan Uplifting. But no aeolian sediments older than the early time of Pleistocene has been found in East China.
On the basis of field investigation in the northern suburb of Nanjing, East China, the section of Lingyan Hill was discovered and the loess-like sediment layer was covered with basalt layers of the age of 12.17 Ma. Field investigation and sampling was carried out and the whole sedimentary section was about 19 m high not including the basalt layer cover. Under the basalt layer were loess-like sediments. A general analysis about the loess-like sediments was carried out on the proxies of grain size and magnetic susceptibility. According to sedimentary characteristics of the sediments and laboratory analysis results, the sediment was mainly composed of silt and clay of percentage over 85%, and no level bedding in the loess-like strata unit. The 4-m thick loess-like deposits in the section of Lingyan Hill were regarded as the aeolian sediments of the age of more than 12 Ma, which was the earliest aeolian deposit in East China. The deposit is the result of climatic event in the mid-Miocene, which was corresponding with the 15~13 Ma strong deposition of aeolian sediments in the Qin'an section in Loess Plateau and the enlargement of ice sheet in Antarctic after 15 Ma.