The Features And Evolution Of Ooids And Oolitic Limestones From The Mantou Formation(Cambrian) Of Westren Henan

The Middle Cambrian Mantou Formation Member II and III are widely exposed in Yiyang area of western Henan.They are mainly composed of thickbedded purple red and brown yellow shales,oolitic limestones and various microbialites.The paper deals with the macroscopic characteristics and cortex texture of ooids,the formation mechanism of micrite calcites and sedimentary environmental features of oolitic limestones.The study can provide evidences for Middle Cambrian paleoenvironmental reconstruction of study area and the microbial genesis of ooids.Ooids are spherical particles with core and cortex and occur in sediments from the Late Precambrian to the Quaternary.There are 4 types of ooids in study area: micrite irregular radial ooids,containing micrite envelope bright crystalline epidermis ooids,micrite tangential-radial ooids and microsparry regular radial ooids on the basis of their morphology,texture and calcite composition.The ooids can change from irregular radial ooids to concentric radial and regular radial ooids from bottom to top of the Mantou Formation,which indicates that there is the minor change on ooidal texture but the morphology of ooids become more regular and round.Based on the lithology analysis of the Cambrian Mantou Formation,the study area has gradually deepened from the two stage to the three section of the Mantou Formation,and the change of the morphology and structure of the oolitic grains.It represents a trend of multiple cycles in the sedimentary environment of this area,which is generally a trend of water depth and hydrodynamic enhancement.Abundant micrite calcites are found in the different positions of ooids.Under the SEM,they are mainly composed of calcium.Besides,they also contain some silicon,aluminum and iron.They display hexagonal honeycomb texture and flake wall,which indicate that the micrite calcites in Ooids are closely related to the microbial activities.Cyanobacteria fossils Girvanella are widely found in ooids of Middle Cambrian Mantou Formation.They appear as flexuous,unsegmented,and unbranched tubular objects which are densely winding,being intertwined with each other and mainly occur in core and dark cortex of ooids.Abundant microbial borings are also found in ooids.These borings display unsegmented and branched tabular pores which are distributed in different positions of ooids.They appear as dendritic or net distribution on the outside of ooids and can co-occur in ooids with Girvanella filaments.Above genetic analysis of micrite calcites and the occurrence of Girvanella filaments and microbial borings in ooids are of great significance to elucidate the microbial genesis of ooids.