Deciphering the upper ordovician Wufeng siliceous shale depositional environments (Wuxi, NE Chongqing) based on multi-proxy record

Abstract

Based on stratigraphic variations of petrology, geochemistry (major, trace elements), and pyrite framboids, we study the sedimentary environment of siliceous rocks of the Wufeng Formation in Wuxi, Northeastern Chongqing, China. Coupled Al₂O₃/(Al₂O₃+Fe₂O₃), SiO₂/Al₂O₃, with Al-Fe-Mn values indicate that Wufeng Formation are deposited in a continental margin and influenced by detrital input. U_EF-Mo_EF and V/Cr-U/Th cross-plots suggest that the siliceous shale was formed in dysoxic to anoxic conditions. Redox proxies imply that the lower, the middle to upper, and the top part of the Wufeng sediments were deposited in an anoxic, dysoxic and oxic environment, respectively. Accordingly, the average size of pyrite framboids are gradually increased from bottom to top, showing that increased oxidisability. A complete vibratory third-order cycle of sea level fluctuations during the Wufeng deposition can be identified. During Wufeng to Longmaxi transition, the sedimentary environment exhibits a short-scale oscillatory pattern and was probably transformed from an outer shelf to inner shelf. Coupled multi-proxies are considered more reliable proxies for deciphering redox conditions in fine-grained sediment.

Keywords

Siliceous rocks / Late ordovician / Wufeng formation / Multi-proxy record / Pyrite framboid / Sea-level fluctuations

Declaration of competing interests

The authors declare that they have no conflict of interests.

Acknowledgements

This research was funded by the National Major projects of Science and Technology (Grant No. 2017ZX05001001-002) and the Project of Sichuan Key Laboratory of Natural Gas Geology of China (Grant No. 2015trqdz01) and Western plan project of State Scholarship Fund. Sincere thanks go to our colleagues involved in unconventional oil and gas researching in the Sichuan Basin. We are particularly grateful to Professor Guang Hu and the anonymous reviewer for their time and valuable comments.

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