A laminated microbial ecosystem at the summit of the Cambrian Explosion

The Guojiaba Formation (Cambrian Stage 3), composed of laminated sandstones, siltstones, and mudstones mainly, is characterized by two units of MISS (biolamination) in the north margin of Yangzi plate. Biolamination arises from the microbial growth and it is characterized by alternation of light layers and dark layers in polished slides and thin sections. The light layers represent the normal background deposition where the development of microbial community is inhibited; whereas, the dark layers are rich in organic materials. SEM examination on the dark layers identified seven types of microorganisms in morphology confirmed the development of microbial community. The occurrence of the biolamination in Guojiaba Formation indicates a complex microbial ecosystem within epibenthic microbial mats in a relatively semi-closed to closed sedimentary environments at the summit of the Cambrian Explosion.寒武统第三阶郭家坝组（Guojiaba Formation）主要由层纹状砂岩、粉砂岩与泥岩构成，在扬子板块北缘发育有两套MISS（生物层理）。生物层理由微生物生长作用形成，在抛光薄片与显微薄片中表现为浅色层与暗色层的交替分布。浅色层代表正常背景沉积环境，此时微生物群落的发育受到抑制；而暗色层则富含有机质。对暗色层开展的扫描电子显微镜（SEM）形貌学分析，共识别出7类微生物，证实了该区域微生物群落的发育。郭家坝组中生物层理的产出，表明在寒武纪大爆发的巅峰时期，该区域处于相对半封闭至封闭的沉积环境，其表栖微生物垫内部存在复杂的微生物生态系统。