Preliminary Researches On Holocene Sedimentary Characters And The Environmental Changes In Pukou Area, Nanjing

The Pukou area of Nanjing is the important ligament combining the city and the harbor, this area will be the most flourish, critical and promising developmental area in Nanjing. In order to insure the stability of the industry development and city construction groundsill along the Yangtze River, it is necessary to study the sedimentary characteristics of stratum and the transference of the Yangtze River Incised-Valley from the Holocene period in the centre area of Pukou.This dissertation chose the PK35 hole columnar sample to carry on the magnetic fabrics analysis, the result showed that the F value of magnetic foliation of the sediment was between 1.005 -1.108, and on the average of 1.048: the L value of magnetic lineation was between 1.001-1.060, and on the average of 1.014: The relative growth degree A of magnetic foliation and magnetic lineation was between 0.04-67.0, on the average of 6.64, showed that the magnetic foliation grow relatively: The Q value of matrix granularity was between 0.004-1.33, on the average of 0.311: The Fs gene of flow velocity function was between 1.008-1.077, on the average of 1.038, represented that the grain deposited orderly under the slow situation of the underset energy, the separation performance of the sediment was better: Magnetic susceptibility aeolotropies P was between1.012-1.119 , on the average of 1.062 , and the magnetic susceptibility aeolotropies was bigger, which instructs that the Pukou area continuously has the stronger hydrodynamic force condition: the average inclination angle of minor axis magnetic susceptibility Kmin (K3) was 66.9°, and 76% sample's maximum host magnetic susceptibility axis inclination angle is smaller than 20°, which explains that the magnetic fabrics basically has reflected the deposit primary sediment fabrics characteristic.In order to study the characteristic of the area sediment deeply, this dissertation traversed the section of seven drill holes which was vertical to Yangtze River's, the sediment granularity was thick at the bottom of the section, but more and more thin from the bottom to the top, which demonstrated a sediment gyration of river bed-margin beach-bank-floodplain, but also appeared two or three small gyration in some drill holes. This several gyrations sediment is familiar in the drill holes on the sides of the section, but is missing in middle drill holes. This is related to river bed position and terrain altitude, so the drill holes with several gyrations was possibly to be in the flood plain at that time, the multi-times flood created several gyrations. Moreover, the thickness change characteristic of the gravel sediment on the section and the change characteristic of the river bed gravel layer demonstrated that ancient Yangtze River bed was on the southeast of the research area in the early of Holocene.The maximum host magnetic susceptibility axis angle of deflection mainly concentrates to the direction of S-N and NE-SW, the delamination statistics shows that, it is mainly on the direction of NE-SW above 28m, and S-N below 28m. According to the depositing rate of the Yangtze River delta area, we can conclude that the ancient stream direction of Yangtze Rive changed for about 8500 years (this is basically accordant to the year data of drill hole), the direction gradually changed from S-N to NE-SW, which reflected the transition history of the Yangtze River ancient stream.The Holocene is such a stage when human civilization developed rapidly, the fast change and sudden change of the regional climate appeared frequently in the Holocene influenced the development of human society greatly. This dissertation took the quality magnetic as the environment substitution index and studied the ancient climate of the Holocene, according to the change characteristic of magnetic, divided the climate of Holocene into four climate sections: 0-3,300 aB.P., the climate was relatively hot and damp: 3,300-5,500 aB.P., the climate was dry and cool, and fluctuated frequently: 5,500-9,100aB.P., the climate was warm and wet, and fluctuated less, this was the condign climate section: 9,100-10,520 aB.P., was the early Holocene, compared to the late Pleistocene the dry and cool climate had improved obviously, the division result has a very good contrast with domestic and foreign research results.Moreover, the PK35 hole sediment magnetic fabrics analysis result showed that the 11.8m and 45.5m samples showed wind deposit characteristic. But the age of 11.8m is about 3,300aB.P., which is corresponding to the III period of the new ice age presented by G.Denton, the study on section spore powder of Jiangsu Jianhu indicated that there was a low temperature event about 3200aB.P.: The age of 45.5m sample is 10520±180aB.P., which is basically accordant to the time of new female celestial wooden event. Therefore, the wind made characteristic of the sample is not created by the experimental error or other accidental factors, but decided by the climate , the environmental factor at that time. Magnetic susceptibility curve also reflected this point very well, curve 45.5m place was a valley value, the K value increased steadily upward. But the K value below 45.5 meter obviously is lower than above 45.5m.