中国科学院南京地质古生物研究所机构知识库

Middle 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.