Age And Origin Of Ancient Gravel Beds Of Hengshan In Nanjing Liuhe And The Relationship With Old Yangtze River

For this purpose, we carry out the research to the division of sedimentary stratum, the analysis of sedimentary facies, sediment formation for the area, which reflex geomorphic evolution process in Nanjing since neogene period,in virtue of the research methods, such as Basalt U-Pb and Ar-Ar dating、sediment ESR dating、animal 、 plant fossils in layers dating and sedimentary profile survey and measurement.The field investigation result indicates that most of the sedimentary stratum is fluvial facies in Nanjing and adjacent regions. Through studying transverse section, level, chronology of river terraces and the contact relationship of sediment and basalt, we divide the sedimentary in Nanjing to 6 sets, and form the level 7 of the Yangtze River terraces together with modern Yangtze sediment.Hengshan paleogravel layer experiences a change from riverbed facies to flood land facies, and the top of the profile is covered by basalt. Predecessors dated that the age of overlying basalt K-Ar is 12.42 Ma. The result of Hengshan sedimentary sand layer ESR dating in this study is 16.7±1.6Ma. We preliminary conclude that the sedimentary age is the early late Miocene to the early Miocene, basing on dating data and mammal fossils, as Anchitherium aureilianense,ect, in comparable Dongxuan Guan,containing middle Miocene.Therefore, Hengshan sandy gravel layer is on the middle-upper of the sediment layer, formed by a river, which fanned from south to north or the north-north-east, in outlets of mountains, shifted into shallow water flood land facies because of the river diversion. From the analysis, like its sediment size and sediment composition, the sedimentary is products of paloe Yangtze.Gravel layers,covered by basalt, in Nanjing area, had constituted the watershed between Yangtze River, including the Yangtze river and its tributary Chuhe’s all levels tributaries, and Hongze lake or Gaoyou lake, maybe had crossed watershed. Gravel layers, which had complicate composition, good psephicity, further evidence that Paloe Yangtze River is the main power forming Nanjing paloegravel layer. Hengshan gravel layer, leaning towards 5°direction, instructs that the flow of the Yangtze River is north or north-north-east in the early Pleistocene, then gradually transformed to east, due to the volcanic eruption, uplift in the western regions and largely sinking in the eastern regions, on the basis of the distribution characteristics of the gravel layer and basalt eruption ages in Nanjing.