Characteristic Of Ooids And Giant Ooids From The Third Member Of The Mantou Formation And The Zhangxia Formation (Cambrian Series 3), Mianchi, Western Henan

The oolitic limestones superbly and continuously exposed in the Third Member of the Mantou Formation and the Zhangxia Formation(Cambrian Series 3), Mianchi Area, Western Henan. The study on oolitic limestones has the important significance for the analysis of features, distribution and evolution of ooids.The oolitic limestones from the Mantou Formation can be divided into three types:(1) a thin layer of oolitic limestones interbedded with shale,(2) oomicrudite associated with stromatolites, and as the base of the stromatolites,(3) a large segment of the thick layer of oolitic limestones. The main type of ooids include radial, radial concentric microcrystalline and crystalline ooids in which the radial and radial concentric ooids are dominant, and are the main types of ooids in oolitic limestones of the Third Member of the Mantou Formation. The features show that the oolitic limestones were formed in overall low energy environments with the alternation of high and low energy.A very special ooids—giant ooids occur in the Zhangxia Formation, Mianchi Area, Western Henan. Abundant cyanobacteria Girvanella is found in giant ooids. These Girvanella filaments are distributed in core and inner and outer cortex of the giant ooids, especially densely spreading in the outer cortex. The features show that the giant ooids are formed by the interaction of inorganic carbonate precipitation and microbial calcification with the direct participation of microbes. Above results can provide important evidences for genetic analysis of unusual giant ooids in geological time.