豫西渑池地区寒武系第三统张夏组的巨鲕及其成因

巨鲕是指那些直径超过2mm、海相成因的大型鲕粒,它们在结构上与鲕粒相似,但却远没有鲕粒分布广泛,成因也与鲕粒不尽相同。豫西渑池地区寒武系第三统张夏组发育了大量的巨鲕,其核心由粒径小于2mm的放射状鲕粒或泥晶球粒组成,形成于弱搅动的水体环境中;圈层以泥晶或由Girvanella丝状体组成的暗色纹层与由微亮晶方解石组成的浅色纹层交替发育为特征。巨鲕是在低、中等能量交替的滩间海环境中由Girvanella丝状体的生长、微生物诱导的钙化作用和无机碳酸钙沉淀而成的。Girvanella丝状体在巨鲕的内、外圈层均有分布,尤以外圈层分布更为密集,显示了微生物在巨鲕形成过程中具有重要作用,这为探讨巨鲕的成因提供了一个重要的实例。

Giant ooids and their genetic analysis from the Zhangxia Formation of Cambrian Series 3 in Mianchi area,western Henan Province

Giant ooids are those large ooids with diameters greater than 2mm formed in marine environments. They are similar with the normal ooids in structure but have a different origin and limited distribution. Abundant giant ooids occur in the Zhangxia Formation of Cambrian Series 3 in Mianchi area,western Henan Province. Their cores consist of micritic pellets or radial ooids with diameters less than 2mm and are formed in weak-agitating environments. Their concentric layers are characterized by the alternations of the dark laminations of micritic calcites or Girvanella filaments and the light laminations of microspar calcites. Giant ooids are formed by the growth of Girvanella filaments,microbially-induced calcification and chemical precipitation of calcium carbonate in the oolitic inter beach banks environments with the alternation from low to medium energy conditions. Girvanella filaments distribute both in inner or outer concentric layers of the giant ooids, especially dense in the latter. It clearly shows that the microbes play an important role in the formation of giant ooids, and provides an important evidence for the study of origin of giant ooids.