Meso-Neoproterozoic Chemostratigraphic Correlation And Significance In North China And Yangtze Block

In the Meso-Neoproterozoic,there is generally lack of the help of biostratigraphy and high resolution chronostratigraphy,while the similar lithology or petrophysical properties of different regions can only establish weak correlation.In this case,chemostratigraphy highlights its important research significance.Based on the well-preserved Meso-Neoproterozoic strata and accurate chronological data in Eastern China,carbonates and black shales in the Changcheng and Jixian Systems(the Jixian area from the North China Block)and the Shennongjia Group,Cryogenian and Ediacaran Systems(the Shennongjia area from the northern margin of the Yangtze Block)were selected as the main research objects.Making full use of the theoretical knowledge of petrology,sequence stratigraphy,chronostratigraphy and elemental geochemistry,the experimental geochemical data are subject to the protogenetic test mainly by analyzing the correlation among elements,which defined the geological significance of elemental geochemical indexes and stable isotopic composition in rock strata.Then,data in this study and collected previously data were combined to form the Meso-Neoproterozoic database of the Eastern China.Finally,combining with the major sedimentary events,the spatio-temporal framework of the North China(1.65?1.4 Ga)and the Yangtze Block(1.4?0.54 Ga)are established.On the other hand,the marine environment of early Mesoproterozoic,late Mesoproterozoic and Neoproterozoic Cryogenian,and even the whole Meso-Neoproterozoic earth surface are discussed,which are based on the chemostratigraphic analyses.The purpose of this study is to provide a basis for supplementation or improvement of regional lithostratigraphic and chronostratigraphic correlation,and to provide evidence for in-depth discussion of the evolution of Meso-Neoproterozoic Marine chemical environment.The main research results of this thesis are as follows:(1)The comparable geochemical parameters were identifiedThe redox-sensitive elements(RSE),iron formation(IF),?¹³C_org,?³⁴S_CAS and?³⁴S_pyin black shale,and the rare earth elements REE,RSE,IF,?¹⁸O_carb,?¹³C_carb,?¹³C_org,?³⁴S_CASand ⁸⁷Sr/⁸⁶Sr in carbonates are effective stratigraphic correlation tools,responding to marine redox conditions,carbon and sulfur cycle,and continental weathering inputs.However,?¹³C_org and?³⁴S_CAS in black shales are susceptible to diageneses,while all indicators in the carbonates may be contaminated with late diageneses or terrigenous debris.On this basis,a large number of previous data were collected to form a database containing the information of primitive marine chemistry,which provide a basis for establishing chemical strata and analyzing marine environments.(2)The spatio-temporal frame of the chemostratigraphy(North China Block)is establishedBased on the sedimentary events(red and black beds)and marine carbonates,the spatio-temporal frame of the chemostratigraphy(1.65?1.4 Ga)is established,which is distributed in the Yanliao rift.In the Changcheng system,sediments in deep waters commonly have a lower?¹³C_org(about-32‰),which is the basis for the comparison of the black sediments in the Chuanlinggou Formation.In the Jixian system,carbonates are well preserved,and the variation trends of?¹³C_carb are similar in different regions,and the negative deviation of?¹³C_carb is a potential index to track the red beds.In the Xiamaling Formation of the Daijian System(ca.1.4 Ga),the large variation range of?³⁴S_py in the upper part of the formation can be compared from deep water to shallow water.(3)The spatio-temporal frame of the chemostratigraphy(Yangtze Block)is establishedThe global comparison of the Shennongjia Group(<1.4?1.1 Ga)shows that?¹³C_carbchanges in different regions are consistent,and the positive?¹³C_carb migration at the bottom of the Group also appears in the 1.3 Ga strata in the Siberia and Lauren Block,indicating that the base age of the Shennongjia Group should be close to 1.3 Ga owing to the lack of isotopic ages.The Gucheng and Nantuo Formations of the Nanhua System responded to the low value of CIA index related to the glacial climate change,while the low or negative value of?³⁴S_CAS-?³⁴S_py can be tracked for the manganese ore in the Datangpo Formation in the interglacial period.In the comparison of the Doushantuo and Dengying Formations of the Ediacaran System,the variated trend of different sedimentary environments(inland shelf,lagoon,continental margin,and slope/deep water basin)have consistent?¹³C_carb,which can be effectively compared.In addition,other geochemical data indicate the heterogeneity of marine redox conditions in the Ediacaran period.(4)The Meso-Neoproterozoic is characterized by redox heterogeneous oceansWhen the Yangzhuang Formation was deposited in the early Mesoproterozoic,the ocean surface had extremely shallow chemocline(the intertidal environment is characterized by anoxia,while the atmosphere is relatively oxidized).When the Shennongjia Group was deposited in the late Mesoproterozoic,the shallow environment was more oxidized.Before the Ediacaran,deep oceans were marked by anoxia,but the evolution of important life still continued.Combined with the modern marine RSE mass conservation model,the ancient marine environment is simulated.The results show that during the transition from anoxic to oxidized ocean,the RSE contents recorded in sediments varies greatly,and the frequent RSE variations in sediments responds to the heterogeneous redox environment in the ocean.