Giant ooids are commonly seen in the Precambrian and Lower Triassic,while they are hardly reported in the Cambrian. Recently,we found giant ooid limestone with about 35 cm thick in the Lower Cambrian Tianheban Formation at the Shiliu section in Shizhu area,Chongqing. The diameter of these giant ooids is generally 5-7mm,with a maximum diameter of 9mm. The giant ooids make up 55％ of rock components. Meanwhile,the recrystallized normal ooids and calcareous shells are relatively common in the limestone. The cements of sparry calcites can be found between the grains. Abundant Cyanobacteria fossils found in the outermost laminae of a giant ooid,form a sort of protruberance-shaped laminae named as “abortive laminae”,with which the microbial cause of giant ooids is clearly revealed. The recrystallized inner dark laminae of giant ooids similarly contain messy girvanell fossils. So it is thought that the active involvement of microorganisms and frequent strong storms are the necessary conditions for the formation of giant ooids.
Duan Xiong,Shi Zhiqiang,Jin Xin et al. Microbial cause for giant ooids: Evidence from the Lower Cambrian in Shizhu area,Chongqing[J]. JOPC, 2015, 17(2): 241-248.
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