Study On The Provenance Of Heavy Minerals In The Cenozoic Era Sediments Of The Chongqing-Yichang Section Of The Yangtze River And Its Implications For The Evolution Of The Yangtze River

Being the longest river in China,the Yangtze River has played a tremendous role in the geomorphology formation of the entire drainage basin by its formation,perforation,and evolution etc.The Yangtze River Valley is characterized as mountains and hills spreading all over,and the plains and lakes staggering.The topography and landforms are abundant and variable.The abundant water resource provides the important material basis for human's survival.The Yangtze Three Gorges,locating at the transition position between China's first relief stepladder and second relief stepladder,and added with the construction of the Three Gorges Water Conservancy Project in Sandouping,Yichang city,Xilin Gorge of the Three Gorges,makes this strong cutting canyon not only have the significant science value,but also have a long-term important practical research value for the use of water resources,sustainable economic planning and prevention of geological disasters.Studies on the special detrital chemical components of the planation surface and the terrace sediments on the Chongqing-Yichang line of the Yangtze River,combining with the research methods and ideas of sedimentology and rocky phase of paleogeography,and based on the comparison of planation surface and terraces,and the usage of the combined characteristics of the color,the composition,and the heavy mineral crystallization style,and provenance information studies including the backscattering,energy spectrum,and electron probe analysis of magnetite were applied to judge whether the Huangling dome,a watershed,has the West stream system,and the time when the river redirected,and figure out why the sediments have provenance differences,and even provide new confirmations and instructions for the process and timing of the perforation of the Yangtze Gorges.The horizontal comparison analysis of the east of the of the Yangtze Gorges,the Yichang area,and the Chongqing to Zigui section of the Three Gorges?the west of the Three Gorges?will be used to further study and solve the problem of the formation of the Three Gorges of the Yangtze River,and provide references for the subsequent geological safety guarantee for other water conservancy projects and geological disaster research in the Yangtze River Valley.In the first place,samples of sediments at the sites of 005,012,023,037,053 and069 from Chongqing to Yichang including the terraces on the east and west sides of Huangling dome and planation surface were studied and analyzed using the heavy mineral source provenance comparison study.Furthermore,we selected 015 site and021 site of the Yunchi group in the Yichang area?east of the watershed?,017 site and024 site of the Shanxi kiln,YC02 site of the T5,00B site of the T4,025 site of the modern riverbed etc.Panzhihua vanadium.Titanium magnetite,Emeishan basalt,and Huangling granite were used as the control group.A total of 10 samples,220magnetite particles were subjected to the energy spectrum analysis.At the same time,backscatter images were acquired.The contents of 15 sorts of oxides including FeO,SiO₂,MnO,MgO,CaO,TiO₂,Al₂O₃,Cr₂O₃,V₂O₃,SO₃,ZnO,NiO,P₂O₅,Nb₂O₅,and Ta₂O₅ etc were measured using an electron probe technique.Based on the above research results,it is concluded that the depositional periods of Yunchi group and Shanxiyao group are basically the same time as the depositional period of Fengjieyunmeng planation surface.The comparability of sediment samples suggests that the West stream system ever existed at that time.The singleness sources of the samples of the Yunchi group and the Shanxiyao group,and not coming from the west of Watershed indicated that Huangling dome had not been cut through,and there was no major river system in Yichang.This result was in consistence with the west side of Jianghan basin in the depositional environment,and mutually corroborated.From the comparative analysis of sediments at the fifth terrace and all subsequent terraces at all levels and all regions,and the modern riverbed of the Yangtze River,either east or west of the watershed,the complexity and diversity of provenances were reflected.The provenance of Panzhihua vanadium-titanium-magnetite ore,a marker of the Yangtze Three Gorges,began to appear in the sediments of the fifth terrace of Yichang area.Combined with the ESR data,it was further confirmed that after the Huangling dome was cut through,the formation of the Yangtze Gorges took place ealier than the completion of the deposition of the Fifth Grade Terrace after the end of Shanxiyao group.