北京西山下苇甸剖面寒武系徐庄组鲕粒研究

A study on ooids in limestones of the Cambrian Xuzhuang Formation at Xiaweidian outcrop in Western Hill of Beijing

对北京西山下苇甸剖面寒武系徐庄组鲕粒灰岩进行了观察和研究,在其中不仅发现了有核及无核同心—放射状鲕、单(或多)晶鲕、泥晶鲕和复合鲕,而且还发现了暗色颗粒包壳、凝聚颗粒和黄铁矿聚集体等特殊组构。暗色颗粒包壳的发现尚属首次,其可能是丝状蓝细菌菌体的残留物。凝聚颗粒由方解石或白云石构成,可能代表了微生物席内的含氧气泡。生物碎屑上的附着体可能是微生物群落的痕迹,在其内部发现有类似鲕粒的球状颗粒。复合鲕胶结松散,包壳不发育,表明其可能形成于相对稳定的水体环境。研究表明,徐庄组鲕粒可能形成于水动力条件相对较弱的浅海环境中,微生物在鲕粒的形成过程中发挥了较大的作用。

This paper systematically studied the characteristics of oolitic limestone of the Cambrian Xuzhuang Formation at Xiaweidian outcrop in Western Hills of Beijing. Core-beard and core-missed con- centric-radial ooids, monocrystalline/polycrystalline ooids, micritic ooids and compound ooids have been found in this study. At the same time, we also found some special textures, such as dark-colored cortices, compounded granules and pyrite agglomerates. Dark-colored cortices are found for the first time, which are probably the remains of filamentous cyanobacteria bodies. Compounded granules, which may represent the oxygen-containing bubbles in microbial mats, are composed of crystals of calcite or dolomite. An attach- ment is found on the surface of a bioclast and ooid-like particles are found inside the attachment. Compoun- ded ooids, which are loosely compacted and lack of outside cortices, may indicate that they are generated in a relatively quiet shallow water environment. Our research shows that ooids in the Xuzhuang Formation were probably generated in a quiet shallow water environment and microbes played a key role in this process.