| Traditionally,it is believed that the Cryogenian in the East Yangtze Gorges area only stands for the record of Nantuo Formation,and lacks the Gucheng Formation and the Datangpo Formation.It is found that a set of 6m thick Shale intercalated with Manganesebearing Limestone strata is developed among the Cryogenian in Qinglinkou section of East Yangtze Gorges area,which Corresponds to the Datangpo Formation.This means that the Cryogenian of East Yangtze Gorges area should be a complete Cryogenian stratigraphic record.Based on the sections measurement and testing data,the rock-stratigraphy characteristics of the Cryogenian in the Guchengcun,Qinglinkou and Xilianping sections,the vertical evolution characteristics of the chemical indexes such as CIA and Rb/Sr,and the U-Pb geochronology records are analyzed in order to confirm this finding.On this basis,the new scheme for division of the Cryogenian was proposed.Then,the types and characteristics of the lithofacies and facies developed in the Cryogenian were identified,and finally,the evolution model of Cryogenian was established.This thesis mainly obtains the following results or understanding:(1)Based on the characteristics of rock combination and the principle of division and comparison of rock stratigraphic units,compared with the Cryogenian of Changyang area,the Cryogenian in the East Yangtze Gorges area is divided into three units from bottom to top: Gucheng Formation,Datangpo Formation and Nantuo Formation.(2)Geochemical indexes such as CIA and Rb/Sr show that the Cryogenian in the East Yangtze Gorges area is composed of two sets of glacial strata of cold climate origin and a set of interglacial strata reflecting warm and humid climate,which is consistent with the Cryogenian in Changyang area,confirming the rationality of the new division scheme.(3)The youngest detrital zircon age of 651.2 ± 14.0 Ma obtained from the top of the Datangpo Formation in the Qinglinkou section,affirm that the Datangpo Formation of the East Yangtze Gorges area belongs to the interglacial deposition of Cryogenian.(4)12 types of lithofacies can be divided from the Cryogenian in the East Yangtze Gorges.And 7 types of facies also can be distinguished,they including Subglacial till facies,Melting-out till facies,Periglacial-marine fan facies,Proximal reformed till facies,Distal reformed till facies,Inner shallow sea facies and Outer shallow sea facies.(5)The Gucheng Formation and Nantuo Formation main develop the Subglacial till facies,Melting-out till facies,Periglacial-marine fan facies,Proximal reformed till facies and Distal reformed till facies.And the Datangpo Formation main develops the Proximal reformed till facies,Distal reformed till facies,Inner shallow sea facies and Outer shallow sea facies.(6)Glaciation still exists during the interglacial period of Datangpo.And it is a continuous phase change process from the coastal moraine sedimentary facies of Xilianping to the Inner shallow sea facies of Qinglinkou,and then to the Outer shallow sea facies of Guchengcun.The innovation point of this thesis is to propose a new scheme for the division of Cryogenian in the East Yangtze Gorges area,that is,the Cryogenian in the East Yangtze Gorges area can be divided into Gucheng Formation,Datangpo Formation and Nantuo Formation from bottom to top,while only Nantuo Formation is divided in the traditional scheme.At the same time,the types and development characteristics of glacial lithofacies and facies of the Cryogenian were determined,and the sedimentary evolution model was established.These understandings provide a basis for the study of glacial evolution and sedimentary paleogeography of the Cryogenian in the East Yangtze Gorges area,and also provide a new direction for the exploration of Datangpo manganese deposit. |
