Depositional environments, hematite–chamosite differentiation and origins of Middle Ordovician iron ooids in the Upper Yangtze region, South China
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Published:2024-06-14
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ISSN:0037-0746
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Container-title:Sedimentology
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language:en
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Short-container-title:Sedimentology
Author:
Luan Xiaocong12ORCID,
Sproat Colin D.2,
Jin Jisuo3,
Zhan Renbin14
Affiliation:
1. State Key Laboratory of Palaeobiology and Stratigraphy, and Center for Excellence in Life and Paleoenvironment, Nanjing Institute of Geology and Palaeontology Chinese Academy of Sciences Nanjing 210008 China
2. Department of Geological Sciences University of Saskatchewan Saskatoon SK S7N 5A5 Canada
3. Department of Earth Sciences Western University London ON N6A 5B7 Canada
4. University of Chinese Academy of Sciences Beijing 100049 China
Abstract
ABSTRACTMiddle Ordovician iron ooids in the Upper Yangtze region of South China are composed mainly of hematite and/or chamosite, found in mixed siliciclastic and carbonate successions, with hematitic ooids occurring in the west, and predominantly chamositic ooids in the east of the study area. In the three iron ooid‐bearing Middle–Upper Ordovician successions, 19 microfacies are recognized and grouped into eight facies associations, representing a shallow‐water mosaic comprising restricted and semi‐restricted lagoons, and open marine subtidal deposits interfingering with tidal flat and shoal facies. Hematitic ooids with well‐sorted and well‐rounded quartz grains formed in the transgressive shoal setting when the depositional environments changed from restricted lagoon to bioclast–quartz shoal and open marine subtidal. Episodic stasis and erosional intervals during transgression controlled the formation of hematite‐rich and mixed hematite–chamosite laminae within the cortices of hematitic ooids. In contrast, chamositic ooids formed in a semi‐restricted lagoonal environment, under long‐term transgressive condensation. Alternating episodes of relatively oxic conditions with thriving organisms and eutrophication‐driven anoxia resulted in the alternation of porous and dense laminae consisting mainly of chamosite in chamositic ooids. The stromatolite‐like cauliflower structures associated with chamositic ooids suggest microbial activity that promoted iron concentration, similar to the origin of approximately coeval iron‐rich oncoids in South China. Various iron ooid types demonstrate that these coated grains could form in a range of depositional setting and palaeooceanographic conditions on a generally shallow‐marine platform during the Ordovician.
Funder
Chinese Academy of Sciences
National Natural Science Foundation of China
China Scholarship Council
State Key Laboratory of Palaeobiology and Stratigraphy
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