Girvanella in Oncoids from the Mantou Formation (Cambrian Series 3), Mianchi, Western Henan
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摘要: 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20 μm，最大延伸长度约为400 μm.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.Abstract: Girvanella filaments 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 Girvanella filaments 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 Girvanella filaments 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 20 μm in diameter and 400 μm 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. Girvanella filaments share the similar growth pattern no matter where they occur in oncoids, showing that they have similar growth condition. The mass occurrence of Girvanella filaments in oncoids has much to do with their growth condition of low-energy coastal oolitic inter-shoal or back-shoal facies. This kind of low-energy condition faclitates the growth and preservation of Girvanella filaments. Further studies on the distribution characteristics and micro-fabrics of the Girvanella filaments 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 interaction between microorganism and environment.
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Key words:
- Mianchi area /
- Cambrian Series 3 /
- oncoids /
- Girvanella /
- Mantou Formation /
- stratigraphy
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图 1 地质背景、地层序列及核形石特征
a.华北地台寒武纪第三世毛庄期沉积相分布，据冯增昭等(1990)修改；b.渑池地区寒武系第三统馒头组二段含核形石地层序列；c~f馒头组二段下部含核形石层；c.野外露头剖面；d.野外露头平面；e.独立的核形石个体；f.核形石中的明暗纹层
Fig. 1. Geological background, stratigraphic succession and the features of oncoids
图 2 Girvanella微结构特征及丝状体在核形石核心中的分布
a和b.核形石中的Girvanella丝状体；c.以生物介壳为核心的核形石；d和e为c的细节显微照片，其中黄色箭头部分为c中核形石核心内生物介壳及其边缘的微生物钻孔；f.以碎屑团块为核心的核形石；g和h为f的细节显微照片，其中黄色箭头所指为f中核心中内碎屑团块中的石英颗粒、生物碎屑、丝状体碎片等；i.以丝状体团块为核心的核形石; j和k为i核心内相互缠绕的Girvanella丝状体(G:Girvanella缩写)
Fig. 2. The microfabrics and distribution of Girvanella filaments in the core of oncoids
图 3 Girvanella丝状体在核形石包壳中的分布
a~d为核形石纹层中丝状体的4种分布方式；a.缠绕叠覆方式, 多见于内圈层；b.穿层方式, 多见于圈层中部；c.凸起方式, 多见于圈层中部；d.匍匐或水平方式, 多见于外圈层；a’~d’分别为4种方式的素描
Fig. 3. The distribution of Girvanella filaments in the cortex of oncoids
图 4 Girvanella丝状体在核形石间围岩中的分布
a, b.核形石之间分布的包绕生物介壳的丝状体，表现为不连续，无规律；c, d.核形石之间的丝状体团块，表现为小而有一定磨圆的团块状
Fig. 4. The distribution of Girvanella filaments in the sediments among oncoids
图 5 核形石中的Girvanella丝状体SEM微观形态
a.SEM低倍下密集分布的丝状体集合；b为a的高倍照片；c, e, g.SEM下Girvanella管状纵切面；d, f, h.SEM下Girvanella管状横截面；b, e, f中可见管壁为自形程度较高的细小的方解石颗粒
Fig. 5. SEM microstructure of Girvanella filaments in the oncoids
图 6 Girvanella丝状体管壁、管心及围岩矿物的EDS结果
Fig. 6. EDS spectrum of minerals in the wall and core of Girvanella filamentous tube and the matrix
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