豫西渑池寒武系第三统馒头组核形石中的Girvanella

Girvanellain Oncoids from the Mantou Formation(Cambrian Series 3),Mianchi,Western Henan

寒武纪微生物岩中的Girvanella化石在地层中广泛分布,这为微生物参与微生物岩的形成提供了更直观的证据.豫西渑池地区寒武系第三统馒头组二段碳酸盐岩中发育了数量丰富的微生物岩类型——核形石,其核心、纹层及围岩中保存的Girvanella丝状体化石具有密度大、完整度高的特点.对核形石的微观分析表明,其核心、包壳等各个部位均大量保存有Girvanella丝状体.这些丝状体为任意弯曲、不分节、不分枝的管状集合体,单管直径约为20μm,最大延伸长度约为400μm.核形石核心以丝状体包绕的灰泥团块或砾屑为特征,局部可见丝状体包绕生物碎屑成核;包壳内的丝状体多出现在暗色纹层中,其分布表现为缠绕叠覆、穿层、凸起和匍匐(或水平状)这4种方式.在核形石的任意部位,Girvanella丝状体均具有相似的分布形式,表明它们的生长条件是相似的.核形石中Girvanella丝状体的大量出现与其形成于低能的滨岸鲕粒滩滩间或滩后低洼地带有关,这种低能条件对Girvanella的生长及保存十分有利.对研究区核形石中的Girvanella化石在微生物岩中的分布特征、微组构等的进一步研究将为寒武纪其他微生物岩的成因及微生物与环境的相互作用提供重要实证.

Girvanellafilaments in the microbialite are widely distributed in Cambrian,which provide more visual evidences for the participation of microorganism in the formation of the microbialite.Abundant oncoids,a kind of microbialite,occur in the carbonates from the Second Member of the Mantou Formation（Cambrian Series 3）,Mianchi area,western Henan.The Girvanellafilaments which are preserved in the nuclei,cortices and surrounding rocks exhibit large density and high integrity.The microscopic analysis on the oncoids shows that the Girvanellafilaments are preserved in both of the nucleus and cortex of oncoids.The filaments appear as a flexuous,unsegmented and unbranched tubular colony.Each individual is about 20μm in diameter and 400μm in maximum extended length.They are mainly densely winding,being intertwined with each other with irregular distribution.The nucleus of oncoids is dominated by filaments surrounding lime clumps and debris,occasionally the bioclasts.The filaments in the cortex of oncoids mainly occur in dark laminae and exhibit four kinds of growth patterns including winding and intertwined,trans-laminar,convex and bedding-arrangement.Girvanellafilaments share the similar growth pattern no matter where they occur in oncoids,showing that they have similar growth condition.The mass occurrence of Girvanellafilaments in oncoids has much to do with their growth condition of low-energy coastal oolitic inter-shoal or backshoal facies.This kind of low-energy condition faclitates the growth and preservation of Girvanellafilaments.Further studies on the distribution characteristics and micro-fabrics of the Girvanellafilaments which were well preserved in the oncoids in the study area are expected to offer significant evidences for the genesis analysis of other microbialite in Cambrian and for the inter-action between microorganism and environment.