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

• Main Authors: Longbo Xu, Xingzhi Wang, Mingyou Feng, Xiaohong Liu
• Format: Article
• Language: English
• Published: KeAi Communications Co., Ltd. 2021-02-01
• Series: Petroleum
• Subjects: Siliceous rocks; Late ordovician; Wufeng formation; Multi-proxy record; Pyrite framboid; Sea-level fluctuations
• Online Access: http://www.sciencedirect.com/science/article/pii/S2405656120300675

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. UEF-MoEF 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.