New Recognition of Carbonate Nodules Genesis in Sinian Doushantuo Formation in Zigui Area and Its Geological Implication
Zhang Mingzheng
Peng Songbai
Zhang Li
Fang Jiasong
Zhang Xianjin
Han Qingsen
Abstract：The carbonate nodules with strongly negativeδ13 C anomalies are widely developed in the black muddy shale in the forth member of the Doushantuo Formation from the Sinian System in the Zigui area,Yangtze Craton.Whether they were re-lated to leakage of gas hydrate deserves further studies.Studies of sedimentary structure,petrography and geochemistry of these carbonate nodules show that these nodules have typical oscillatory zoning,sparry sphere structure,framboidal pyrite and agglomerate fabrics,which are related to venting system.The carbonate nodules display obvious negativeδ13 C anomalies (-5.65‰ to-6.76‰),strong enrichment of U and Mo elements (UEF=8-26,MoEF=99-320)and Y/Ho ratios between 31.05 and 37.31.Their REE distributions display gentle left-sloping patterns and their major and trace elements such as K, Sc,V,Cr,Co,Ni,Rb,Sr,Ba,Th,U and Mo indicate a hypoxia and sulfidic setting,which is consistent with characteristics and formation environment of cold seep carbonates.SiO2 ,MgO,CaO and CO2 contents of oscillatory zoning within the nodules show continuously increase or decrease changes,respectively.They reflect three stages of continuous evolution processes inclu-ding primary formation,subsequent diagenesis-metasomatism and final diagenetic reworking.Accordingly,it can be proposed that these carbonate nodules were ultimately developed as decomposition,diagenesis and metasomatism of the low temperature stored solid gas hydrate sediment in the black muddy shale when the temperature became higher after the Neoproterozoic Gaski-ers glaciation (582-551 Ma).This study provides an important record that confirms existence of the ancient natural gas hydrate and offers an important geological evidence for exploring shale gas (methane gas)reservoirs in the Sinian System and Lower Paleozoic sedimentary covers in the Yangtze Craton of South China.
Keywords：Yangtze CratonZigui areafourth member of Doushantuo Formationancient natural gas hydratecold seep car-bonate nodulegeochemistry
Publication Date：2016-01-01
Online Publishing Date：2025-08-15(First online date of this platform, not the publication date of the document)
Pages：18( 1977-1994 )