Provenance Study Of The Miocene Sediments Near Nanjing And Their Palaeocliamte Implications

The Yangtze River originates from the Tibetan Plateau and is the longest river in Asia. As the product of both tectonics and climate, the evolution history of the Yangtze River has been of great interest to many geologists and geomorphologists. Much work has been carried out, but the history of Yangtze River is still a mysterious problem owing to the complex of the river.In this thesis, the author focused on the provenance change of sediments near Nanjing in the lower Yangtze valley using combined methods of detrital zircon U-Pb ages and heavy minerals in order to discover some clues for the evolution of the Yangtze River. In order to investigate the palaeoclimate, the proxy of grain-size, susceptibility and the value of Rb/Sr was applied to study the sediments near Nanjing, and the X-ray diffraction analysis (XRD) was applied to study the clay minerals in the sediments in the Yangtze Delta and the red clay-loess sequences in the Chinese Loess Plateau (CLP)1) Late Cenozoic fluvial sediments are widely distributed near Nanjing. Most of them are overlain by basalts, from which preventing the underlain to be eroded. Lingyanshan section (LYS),Xiaopanshan section (XPS),Tanggongshan section (TGS),Guizishan section (GZS),Jiangpu sections (JP) and Jiangning Fangshan section (FS) around Nanjing are all covered by basalts with the age of 12.2-8.6 Ma. Because of the terrain and geographical location of JP and FS, we think that the sediments in JP and FS were not formed from the ancient Yangtze River. According to the typical sedimentary strata and sedimentary facies of large river, we believe that the Miocene fluvial sediments of the XPS,GZS and TGS should be the product of the ancient Yangtze River at that time.2) In order to uncover the characteristics and provenance of these sediments, heavy mineral composition and detrital zircon U-Pb ages were measured. Compared with the modern deposits of the Yangtze River, we found the detrital zircon U-Pb age patterns, especially XPS,GZS and TGS, are similar to the modern Yangtze sediments, with slightly difference.The content of heavy minerals in the Miocene fluvial sediments was lower than the modern sediments of the Yangtze River. At the same time no hornblende and pyroxene had been found in the Miocene fluvial sediments, which is contrary to the modern sediments of the Yangtze River;As hornblende, pyroxene and other heavy minerals are unstable and easy to be reconstructed by post-diagenesis and weathering, it is hard to justify the difference of provenance between ancient and modern Yangtze River. As the resistance of detrital zircon in the process of physical and chemical weathering, its U/Pb ages are a common tool in provenance studies for fluvial sediments. The patterns of detrital zircon U-Pb age of XPS,GZS and TGS are similar to the modern sediment, which indicated that the Three Gorge must have been cut through and sediments originated from the upper Yangtze Block could be carried to the Yangtze delta through the Three Gorges.3) On the basis of field investigation near Nanjing, the section of Lingyanshan (LYS) Hill was discovered and the loose sediment layer was covered with basalt layers of the age of 10.32Ma. Field investigation and sampling were carried out on the whole sedimentary section. Under the basalt layer were loose sediments. A general analysis about the loose-like sediments was carried on the proxies of grain size, magnetic susceptibility and the value of Rb/Sr.According to the sedimentary characteristics of the loose sediments and laboratory analysis results, the loose sediment was mainly composed of silt and clay, and no level bedding in the upper part of loose sediment. The 1.1m thick deposits in the section of LYS Hill were regarded as the Aeolian sediments of the age more than 10.32Ma, which was the earliest Aeolian deposits in east China. The deposit was corresponding with the 15-13Ma strong deposition of Acolian sediments in the Qin'an section in Loess Plateau.4) The X-ray diffraction analysis (XRD) was applied to study the clay minerals in the Yangtze Delta and the red clay-loess sequences in the Chinese Loess Plateau (CLP). The results show that the clay mineral compositions of the Yangtze and Yellow River drainages are mainly consisted of illite, chlorite, kaolinite and smectite. During each geological time, the sediments of different deposition profiles along the Yangtze River and Yellow River have different combinations of clay minerals. In each continuous deposition profile, there exists a phenomenon that illite trends to gradually increase which reflects the gradual cooling of climate. The ratios of illite to kaolinite (â…¢/Kao) and illite to smectite plus kaolinite(â…¢/(Sm+Kao)) may serve as a proxy to evaluate the paleoclimate.