埃迪卡拉纪—寒武纪之交微生物岩特征对比及古海洋学意义: 以汉南—米仓山地区为例*

寒武纪初期不仅发生了宏体生物大爆发,而且也出现了地质历史时期少见的蓝细菌鞘体大规模钙化事件。埃迪卡拉纪—寒武纪之交海水化学性质的转变对真核生物的演化起到了重要作用,但是这种转变对微生物岩发育特征以及蓝细菌钙化事件的产生有无影响,目前尚不明确。鉴于此,对华南上扬子北缘汉南—米仓山地区上埃迪卡拉统—寒武系第二统多个典型微生物岩发育剖面进行了系统野外调查和室内岩石学分析,结果表明:上埃迪卡拉统灯影组叠层石广泛发育,以平铺状、缓波状特征为主,而凝块石既可以呈补丁状分布于叠层石纹层间,又可以呈细小的凝絮状、粘结状特征构成厚层岩层;寒武系第二统仙女洞组叠层石丰度显著降低,以高大、坚硬的丘状隆起为特点,包括单独的凝块石丘,以及微生物与古杯的联合建丘。虽然寒武系第二统微生物岩的层状结构和凝块结构与埃迪卡拉系相比并无太大差异,但是寒武系微生物岩内部保存有大量的钙化微生物化石,已识别出附枝菌(Epiphyton)、肾形菌(Renalcis)和葛万菌(Girvanella)等多种类型。在收集、整理前人有关微生物岩特征和发育资料的基础上,本次研究初步整理出华南寒武系第二统微生物岩的时空分布特点,发现寒武纪第二世第三期是钙化微生物大量发育的一个时期,在随后的第四期达到一个小的高峰。对于此次蓝细菌钙化作用幕的启动机制,除前人提出的海水高钙离子浓度和蓝细菌体内二氧化碳浓缩机制等认识外,寒武纪早期海水性质的转变(方解石质原生矿物受成岩改造程度较低)、适度的陆源碎屑输入(黏土组分保护作用)也有利于钙化微生物结构的保存,应引起重视。

Characteristics and palaeoceanographic significances of microbialite development in the Ediacaran-Cambrian transition: a case study from Hannan-Micangshan area

Early Cambrian not only witnessed the explosion of animals,but also experienced a large-scale cyanobacterial calcification event that was rare in Precambrian. The transition of seawater chemistry from Ediacaran to Cambrian played a crucial role in the evolution of eukaryotes,but it is still unclear whether this transition also had impacts on the development and compositions of microbialites and on the start of cyanobacterial calcification event. This study conducted a systematic field investigation and petrological examination on the microbialite structures from the Upper Ediacaran to Cambrian Series 2 in the Hannan-Micangshan area that is located in the northern part of the Upper Yangtze area of the South China Block. The stromatolites of the Dengying Formation show regularly and irregularly laminated structures; the thrombolites are generally distributed between stromatolite layers or shown as individual,thick-bedded rocks,with fine-grained clotted and lump structures. In the the Xiannüdong Formation of Cambrian Series 2,the abundance of stromatolites is significantly reduced in the study area. The composition of microbialites is characterized by rigid,mound-like structures,including individual microbial mounds,and calcimicrobe-archaeocyathan bioherms. Although the laminated and clotted structures of microbialites of Cambiran Series 2 are similar to those of the Ediacaran,the Cambrian forms contain a large number of sheath-calcified microbial microfossils in microstructures,including Epiphyton,Renalcis, Girvanella, etc. Based on the literature review results of the characteristics and occurrences of the microbialites of Cambiran Series 2,this study has preliminarily sorted out the temporal and spatial distributions of the microbialites in the Cambrian Series 2 of South China. This study agrees with previous understandings that the high Ca²⁺ concentration in seawater and the reactive of CO₂ concentration mechanism in cyanobacteria may have played a key role in the initiation of Cambrian cyanobacterial calcification event. In addition,the seawater chemical compositions in the Cambrian Series 2 that were changed from “aragonite-dolomite sea” to “calcite sea”,as well as the exceptional burial mechanism of clay coating within calcified microstructures may also be seen as two favorable conditions for the preservation of early Cambrian calcified microorganisms,which should be paid attention to.