The Oolitic Shoal-microbial Mound Associations And Genetic Controls From The Mantou Formation Member ?-?(Cambrian Series 3),Yiyang Area,western Henan

Oolitic limestones and microbialites(stromatolites,onlollites and sthrombolites)widely occur in the Mantou Formation Member II to III(Cambrian series 3),Yiyang Area of western Henan.Oolitic limestonesdisplay thick-bedded and multilayer structureand are formed in shallow oolitic shoal environment.different microbialites dispay discontinuous distributionand overlap verticallyas microbial mound.The alternation of oolitic shoals and microbial moundsindicates that there are close relations on the formation and development between these two kinds of carbonates.Four kinds of oolitic shoal and microbial mound associations occur in the study sections on the basis of field macroscopic observation and indoor microscopic analysis,including respectively littoral oolitic shoal-back shoal microbial mound association and littoral oolitic shoal-inter-shoal restricted sea(back shoal)microbial mound associationin Member II and littoral oolitic shoal-back shoal microbial mound association and littoral oolitic shoal-inter-shoal intertidalmicrobial mound associationin Member III.The littoral oolitic shoals in Member II mainly consist of radial concentric ooids and radial ooids with micritic or microsparry calcite cementationand are formed in littoral shallow oolitic shoal with the alternation of low and high energy.The microbialites in microbial mounds include oncolite,stromatolite and thrombolite with abundant calcified microbes such as Girvanella and Epiphyton.The microbial moundsare dominated by a singletype or the alternation of different types of microbialites and formed in low energy back-shoal depressions or inter-shoal restricted sea.The littoral oolitic shoals in Member III mainly consist of concentric radial ooids with micritic or microsparry calcite cementationand have the similar genetic enronment with the littoral oolitic shoals in Member II.The microbial mounds occur no longer with the development of one kind of microbialite but in the alternations of different kinds of microbialites and are formed in low energy inter-shoal intertidal environment.The microbialites develop upward on the basement ofoolitic shoal and consist of calcified microbes such as Girvanella and Epiphyton.The hydrodynamic conditionsand sea levelchanges control the development of oolitic shoal-microbial mound associations and the occurrence of different microbialites in microbial mound.High-energy is in favour of the development of oolitic shoal,but low-energy is in favour of the development of microbial mound and various microbialites.Oolitic shoal-microbial moundcarbonate association can evolve into clastic supratidal musflat in regression.Clastic supratidal musflat,on the other hand,can evolve intooolitic shoal-microbial moundcarbonate association in transgression.