寒武纪芙蓉统均一石沉积组构及环境特征——以河北涞源长山组为例

均一石以隐晶质岩性、无纹层与凝块结构发育为特征,与叠层石、凝块石、树形石并列为典型的微生物岩。然而,由于均一石在1995年命名以来很少在地层记录中得到识别和描述,并且缺乏现代实例的类比物,使得关于均一石的报道极具研究价值。为研究华北地台寒武系均一石沉积组构与形成环境特征,系统性地针对河北涞源祁家峪剖面芙蓉统长山组均一石生物丘进行研究。芙蓉统长山组从下部陆棚相钙质泥岩向上变浅至浅缓坡相厚层块状泥晶灰岩,组成了一个淹没不整合型三级层序。三级层序顶部的浅缓坡相厚层块状泥晶灰岩层,代表强迫型海退过程沉积,其内部发育一系列米级均一石生物丘。研究结果表明,这些生物丘主体为致密泥晶及少量微量晶组成,其中可见到附枝菌(Epiphyton)、葛万菌(Girvanella)、肾形菌(Renalcis)等钙化微生物化石。这些钙化微生物(蓝细菌)化石的出现,代表了显生宙第一幕蓝细菌钙化作用事件的证据,同时间接的说明均一石生物丘形成于蓝细菌主导的微生物席的钙化作用过程之中。同时,生物丘内还局部集中发育底栖鲕粒与草莓状黄铁矿颗粒,表明了生物丘形成过程中复杂的微生物沉积作用机制。因此,河北涞源长山组顶部的均一石生物丘,尽管泥晶和微亮晶是其基本构成,但是各种钙化蓝细菌化石以及底栖鲕粒与草莓状黄铁矿颗粒的局部出现表明了在蓝细菌主导的微生物席中复杂的微生物活动信号,成为了解生物丘形成机制、显生宙第一幕蓝细菌钙化作用事件的典型实例。

Sedimentary Fabrics and Environmental Characteristics of Leiolite in Cambrian: A case study from the Changshan Formation in Laiyuan city,Hebei province

Leiolite is characterized by relatively structureless, aphanitic, mesostructure, lacking lamination or clots, and is classified as typical microbial rocks together with stromatolites, thrombolites and dendrites. However, because of the lack of leiolite research reports in either modern or old records since it was named in 1995, the report of a leiolite example has great research value. The Cambrian Changshan Formation at the Qijiayu section constitutes the lower part of the Furongian series. The formation is characterized by a generally shallowing-upward succession of sedimentary facies ranging from mudstone of shelf marls to micrite of shallow ramp facies, and therefore comprises a third-order carbonate depositional sequence of the drowning-unconformity type in the north of the Taihang Mountain in Laiyuan city, Heibei province. A single bed of the micrite of shallow ramp facies in the upper part of the Changshan Formation contains many dome-shaped carbonate structures distributed similar to a string of beads. These structures are described as a leiolitic bioherm, and is evidence of a forced regression. Various kinds of calcified microorganisms such as Epiphyton, Girvanella and Renalcis developed within the leiolitic bioherm. These are evidence of the first episode of a cyanobacteria calcification event in the Phanerozoic, and describe a leiolitic bioherm formed from microbial mats dominated by cyanobacteria. Furthermore, the partial concentrations of benthic ooids in the bioherm show complex microbial sedimentation during its formation. Although the bioherm basic components are micrite and microspar, the presence of calcified microorganisms and benthic ooids offers a reference for studying the complex microbial activity signals in the microbial mats, which are dominated by calcified microorganisms. It can also be regarded as an instance for further research on the mechanism of bioherm formation and the first episode of cyanobacteria calcification event during the Phanerozoic.