The Microbialites And Their Evolution From The Second And Third Member Of The Mantou Formation (Cambrian Series 3), Weihui, Henan

The second and third Member of the Mantou Formation(Cambrian series 3)occur superbly in Weihui Area of Henan Province and characterized by purple and isabelline thick-bedded mudstone interbedded with oolitic limestones.The microbialites such as stomatolite,oncolite and thrombolite are widely distributed in carbonates.Abundant oncoids occur in the lower part of the second member of the Mantou Formation(Cambrian Series 3),Weihui area,Henan.The ellipsoidal oncoids are dominated by the small nuclei and irregular dark-light cortices.They are usually interbedded with irregular mud bands and formed in intertidal channel with the alternation of low to high water energy.The cap-like and round oncoids are characterized by the big nuclei and regular dark-light cortices.They are interbedded with wavy stromatolites and formed in intertidal –subtidal channel with the alternation of low to high water energy.The vertical sequence of oncoid-bearing limestone evolves upward from supratidal-intertidal lime-mud flat via intertidal –subtidal channel to nearshore oolitic shoal,which reflect the rising sea level and increasing sea water energy gradually upward.The bvious thrombolites occur in the upper part of the second member of the Mantou Formation.They display irregular massive or dentritic mesoclots and are dominated by Girvanella filaments and micritic calcites.Some radial ooids are scattered in mesoclots.Girvanella filaments mainly display as microbial clusters and are densely winding and intertwined each other or irregular distribution.The thrombolites are often interbedded with columnar stromatolites and overlie oolitic limestones and underlie purple mudstones,which reflect that the thrombolites are formed in low water energy and intermittently agitated environment.The carbonate oolitic shoals and microbial bioherms were developed in the lower part of the third member of Mantou formation.By the field outcrop and thin section observation,two evolution stages are recognised: 1st,the stage of oolitic shoals bearing giant ooids,which subdivide into three substages of normal ooids,normal ooids bearing giant ooids and giant ooids according to the proportion change of normal ooids and giant ooids.The normal ooids are mainly radial ooids and concentric radial ooids.The giant ooids often take normal ooids as their nuclei and display the thin and multi coatings reflecting the high energy and frequent changes of sea water.2nd,the stage of microbial bioherms and can be divided into four substages of oncoids,laminites,stromatolites and thrombolites according to the different types of microbial carbonates.Cyanobacteria fossil Girvanella were found in different type of microbialite.They are often distributed in the dark laminations in oncoids and stromatolites and exhibit winding and intertwined each other and bedding-arrangement presenting middle and high hydrodynamic condition.The Girvanella in thrombolites have unclear arrangement reflecting low hydrodynamic condition.The fall of sea level have controlled the evolution from oolitic shoal facies to microbial bioherm backshoal facies.