Abstract：Giant ooids are those large ooids with diameters greater than 2mm formed in marine environments. They are similar with the normal ooids in structure but have a different origin and limited distribution. Abundant giant ooids occur in the Zhangxia Formation of Cambrian Series 3 in Mianchi area,western Henan Province. Their cores consist of micritic pellets or radial ooids with diameters less than 2mm and are formed in weak-agitating environments. Their concentric layers are characterized by the alternations of the dark laminations of micritic calcites or Girvanella filaments and the light laminations of microspar calcites. Giant ooids are formed by the growth of Girvanella filaments,microbially-induced calcification and chemical precipitation of calcium carbonate in the oolitic inter beach banks environments with the alternation from low to medium energy conditions. Girvanella filaments distribute both in inner or outer concentric layers of the giant ooids, especially dense in the latter. It clearly shows that the microbes play an important role in the formation of giant ooids, and provides an important evidence for the study of origin of giant ooids.
Dai Mingyue,Qi Yong′an,Chen Yao et al. Giant ooids and their genetic analysis from the Zhangxia Formationof Cambrian Series 3 in Mianchi area, western Henan Province[J]. JOPC, 2014, 16(5): 726-734.
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