Microbe-mediated, marine authigenic formation of ooidal chamosite: Insights from upper Ordovician carbonates of the South-Western Yangtze platform (China)

Si Chen, Min Zeng, Jingchun Tian, Kefa Ren, Bo Wang, Zhifang Zhao, Xi Chen, Frank R. Ettensohn, Thierry Adatte

Research output: Contribution to journalArticlepeer-review


Chamosite is a common marine authigenic mineral and microbial involvement has been often assumed during its formation. However, the fossil record of such microbial activity is ambiguous. Chamosite oolitic ironstones widely occur in the Upper Ordovician of the Yangtze Platform, but their origin remains unclear. In this study, detailed sedimentological and mineralogical investigations on ironstones from the Haima section (south-western Yangtze Platform) helped unravel the critical role of cyanobacterial activity during marine chamosite authigenesis. Stratigraphically, deposition of the Haima oolitic ironstones corresponded with an early Katian transgression, temporally equivalent to many other early Palaeozoic counterparts worldwide. Petrographic observations revealed that different chamosite forms (ooids, peloids and matrix) all developed in close association with calcified cyanobacteria, suggesting that cyanobacteria-involved chamosite formation existed across a wide range of hydrodynamic conditions from above fair-weather wave base to below storm wave base. Further mineralogical characterization (backscatter imaging and electron-microprobe analyses) at single-grain scale revealed clear mineral zonation from the matrix (dispersed iron oxide and microquartz) through the calcified cyanobacterial sheaths (dispersed chamosite) to the ooid (chamosite). This zonation is best explained by cyanobacterial microbial mats that created benthic reducing and high-pH conditions. These conditions drove near-seafloor, dispersed iron oxide and microquartz (primary precipitates formed in oxic seawater) to form chamosite. The authigenic chamosite, originally dispersed within and surrounding the microbial mats, could be reworked to develop other chamositic grains. It is proposed here that the early Katian transgression was responsible for the chamosite–oolitic–ironstone development. The marine transgression submerged earlier-exposed lands and likely supplied Fe-Al-rich colloidal materials to an open marine setting for the chamosite authigenesis. Moreover, transgression likely triggered upwelling, delivered ferruginous deeper water as an extra Fe source and provided nutrients for the cyanobacterial microbial-mat development, which was essential for the widespread chamosite authigenesis across the south-western Yangtze Platform.

Original languageEnglish
Pages (from-to)1655-1678
Number of pages24
Issue number5
StatePublished - Aug 2023

Bibliographical note

Publisher Copyright:
© 2023 International Association of Sedimentologists.


  • Calcified cyanobacteria
  • Fe cycle
  • Upper Ordovician Pagoda Formation
  • Yangtze Platform
  • chamosite oolitic ironstones
  • sea-level change

ASJC Scopus subject areas

  • Geology
  • Stratigraphy


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