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Oceanic redox condition of the Nantuo Formation at the Wuhe profile, Guizhou Province: Evidences from its geochemistry and mineralogy (Vol. 41, No.1, 2021) TEXT SIZE: A A A

FENG Yong1, GU Shang-yi1,2*, WU Zhong-yin3, SHEN Hong-juan1, ZHAO Si-fan1

(1. College of Resources and Environmental Engineering, Guizhou University, Guiyang 550025, China;
2. Key Laboratory of Karst Geological Resources and Environment, Ministry of Education, Guiyang 550025, China;
3. Guizhou Geological Environment Monitoring Institute, Guiyang 550000, China)

Abstract: The redox condition of the late Neoproterozoic ocean exerted fundamental impact on the biological evolution at the time. Various kinds of geochemical information preserved in marine sediments are mainly used to study oceanic redox conditions over geological time. However, during the Nantuo glacial period, the obvious lack of chemical sedimentary rocks such as banded iron formation and black shales resulted in the extremely limitation on the study of the redox environment during the period. In this paper, sedimentological, mineralogical, and geochemical studies on the red ferruginous diamictites of the Nantuo Formation at the Wuhe profile in the Jianhe area, Guizhou Province, have been conducted. The results indicate that contents of Si, Al, and Fe in the diamictites are similar to those of the Upper Continental Crust (UCC), but the contents of Ca, Mg, and Na in the diamictites are significantly lower than those of the UCC. These show that there could be a dominant oxidized weathering during the Nantuo glacial period. The redox sensitive elements of Mo and U are enriched at different degrees in the diamictites. Their co-variation trend in the diamictites suggests that FeMn (oxyhydr) oxides were transported through various leveled seawaters with different redox values in the ocean during the glacial period. The red color of diamictites was mainly caused by the enrichment of hematite in the rock. The irregular distribution of red ferruginous diamictites in the Nantuo Formation indicates that hematite could be formed by the dehydration of iron hydrate which was derived from the oxidized ferrous ion in the seawater at the time. The combined features of sedimentary structure of the wave ripple, the enrichment of redox sensitive elements of Mo and U, and the formation of hematite in the red ferruginous diamictites of the Natuo Formation at Wuhe profile suggest that there was an extremely anoxic reduced environment in the paleo-ocean in the South China during the Nantuo glacial period.

Keywords: Nantuo glacial period; red ferruginous diamictite; redox environment; hematite; formation mechanism

ACTA MINERALOGICA SINICA Vol. 41, No.1, 2021, page 33-44

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