Genesis and sedimentary environments of the ferruginous ooids from the Majiaoba Formation in northern Longmen Mountains,Sichuan

The ferruginous ooids from the Majiaoba Formation in northern Longmen Mountains,Sichuan consist of nuclei and concentric circles on the basis of the microstructures indicated by microscope and SEM examination.The nuclri are composed of hematite and quartz in colloids,whereas the concentric circles comprise hematite with the thickness of 3-5 μm in densed tabular parallel arrangements.The ferruginous ooids cited above are interpreted as the products formed in the favourable pH and Eh conditions in the intertidal to the subtidal shallow-water environments during the initial rising of sea level after large-scale regressions.