北京西山下苇甸剖面寒武系徐庄组鲕粒研究

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。

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

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 concentric-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 attachment is found on the surface of a bioclast and ooid-like particles are found inside the attachment. Compounded 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.