摘要
Global abrupt climate change from Marinoan snowball Earth to greenhouse Earth, recorded as cap carbonate overlain on diamictite, had shed the first light on Cambrian bio-radiation. The most documented cap carbonate sections are typical with comprehensive d13 C negative values and ubiquitous sedimentary structures, such as tepee-like, sheet-crack etc., which are associated with successive glacial eustatic variation caused by isostatic rebound in shallow-water facies. Here we report a deepwater basinal cap carbonate section with strong negative δ^(13) C values in the southern margin of the Qinling Orogen,Heyu, Chengkou County, Chongqing in China, which consists of massive dolostone with abundant carbonaceous laminae. However, it lacks the sedimentary structure as mentioned above and is overlain by thin-bedded silicious shales and cherts. A K-bentonite bed was discovered within the base of cap carbonates, about 0.7 m above the top of the Marinoan diamictite. Magmatic zircons that were separated from the K-bentonite bed yield a SIMS concordia U–Pb age of 634.1 ± 1.9 Ma(1 r, MSWDCE= 0.31,ProbabilityCE= 1.000, n = 20). The age is in good agreement with previously reported TIMS U–Pb ages for the termination of Marinoan glaciation and provides ageochronological constraint for the Ediacaran successions in the Qinling Orogen.
Global abrupt climate change from Marinoan snowball Earth to greenhouse Earth, recorded as cap carbonate overlain on diamictite, had shed the first light on Cambrian bio-radiation. The most documented cap carbonate sections are typical with comprehensive d13 C negative values and ubiquitous sedimentary structures, such as tepee-like, sheet-crack etc., which are associated with successive glacial eustatic variation caused by isostatic rebound in shallow-water facies. Here we report a deepwater basinal cap carbonate section with strong negative δ^(13) C values in the southern margin of the Qinling Orogen,Heyu, Chengkou County, Chongqing in China, which consists of massive dolostone with abundant carbonaceous laminae. However, it lacks the sedimentary structure as mentioned above and is overlain by thin-bedded silicious shales and cherts. A K-bentonite bed was discovered within the base of cap carbonates, about 0.7 m above the top of the Marinoan diamictite. Magmatic zircons that were separated from the K-bentonite bed yield a SIMS concordia U–Pb age of 634.1 ± 1.9 Ma(1 r, MSWDCE= 0.31,ProbabilityCE= 1.000, n = 20). The age is in good agreement with previously reported TIMS U–Pb ages for the termination of Marinoan glaciation and provides ageochronological constraint for the Ediacaran successions in the Qinling Orogen.
基金
supported by the National Natural Science Foundation of China(Grant Nos.41072054,40672053,41462001)
