华北地台东北缘寒武系芙蓉统叠层石生物丘中的钙化蓝细菌

Calcified cyanobacterias within the stromatolotic bioherm for the Cambrian Furongian Series in the northeastern margin of the North-China Platform

华北地台东北缘的芙蓉统,大致为长山组和凤山组所组成,可以进一步划分为3个三级沉积层序;层序划分主要基于沉积相序列的旋回性所代表的沉积趋势,较深水的陆棚相钙质泥岩和深缓坡相条带状泥晶灰岩和泥灰岩组成的凝缩作用序列、与高水位体系域和强迫型海退体系域的中至浅缓坡相碳酸盐岩组成的总体向上变浅序列,是这些三级沉积层序的基本构成,从而形成了较为典型的淹没不整合型层序。那些典型的叠层石生物丘,类似于微生物礁,主要发育在长山组和凤山组下部构成的三级层序的强迫型海退体系域之中,代表了缓坡型台地中相对海平面下降阶段的沉积记录。这些叠层石生物丘中的叠层石,泥晶和微亮晶是其基本构成,最为特征的是发育着一些典型的钙化蓝细菌化石,表明了这些寒武纪芙蓉世的叠层石生长于蓝细菌主导的微生物席的钙化作用之中。最为重要的是,在构成叠层石生物丘的粗糙纹层柱状和穹窿状叠层石中,较为普遍地发育着"石松藻(Lithocodium)";这种谜一样的钙化蓝细菌化石,与其他的钙化蓝细菌化石一起,表明了寒武纪叠层石形成过程中复杂的微生物沉淀作用,成为窥视叠层石生长和石化过程中重要的微生物作用信号。就像其名称所蕴含着的高级绿藻中的松藻(Codium)的涵义一样,"石松藻(Lithocodium)"状的钙化蓝细菌,多描述于中生代的微生物碳酸盐岩中,而且还常常被解释为结壳状有孔虫或"海绵骨针的网状物",其生物亲和性还存在着剧烈的争论。因此,华北地台东北缘寒武系芙蓉统中的叠层石生物丘,特殊的层序地层位置代表了较为典型的强迫型海退沉积记录,特别的钙化微生物构成代表着叠层石生长和石化过程中复杂的微生物作用信号,成为深入了解"寒武纪-早奥陶世微生物碳酸盐岩复苏期"和"显生宙早期第一幕蓝细菌钙化作用事件"中的微生物造礁和成丘作用的典型实例。

The Furongian Series in the northestern margin of the North-China Platform can be subdivided into the Changshan and Fengshan formations, in which three third-order depositional sequences can be discerned on the basis of the depositional trend reflected by the cyclicity of sedimentary-facies successions. These third-order sequences make up of the depositional sequence belonging to the type of a drowned-unconformity, which are constituted by a generally upward shoaling succession from a condensational succession of deeper-water fine deposits made up by both calcareous mudstones of the shallow-shelf facies and marls of the deep-ramp facies in the lower part to a shallow-water carbonate succession belonging to the mid- and shallow-ramp facies in the upper part. Stromatolitic bioherms that are similar to the microbial reef are typically developed in the forced-regressive system tract of third-order sequence made up of both the Changshan Formation and the lower part of the Fengshan Formation, which represent the typical depositional record that is the response to the third-order relative sea-level falling. Furthermore, micrites and microspars are the fundamental composition for stromatolites making up the stromatolitic bioherm, and some calcified cyanobacteria fossils within these stromatolites are resulted from the calcification of microbial mats that are dominated by cyanobacterias. Importantly, there are lots of ＂Lithocodium＂ within those column and domical stromatolites composed by rough lamina, which become an important singles of microbial activities and demonstrate that these stromatolites might be resulted from a sophisticated microbial precipitation together with other calcified cyanobacteria fossils. Lithocodium, one enigmatic cyanobacteria fossil that had been frequently described in Mesozoic microbial carbonates and had been grouped into the Codium belonging to a high-grade green algal, or had been interpreted as a kind of encrust foraminiferas as well as the sponge spicule network, which reflect that its biological affinity have strongly been debated. Therefore, stromatolitic bioherms of the Furongian in the northwestern margin of the North-China Platform provide an important example for the further understand of the building action of microbial reefs or bioherms during ＂a resuscitate period of microbial carbonate from Cambrian to the Early Ordovician＂ or ＂the first episode of the Cyanobacteria Calcification Events in the Phanerozoic＂, since their particular sequence-stratigraphic position might delegate a record of the forced-regression and their composition of specially calcified microbes might represent an important signal of complex microbial activities during both the growth and forming process of stromatolites.