中文摘要：

扬子地台东南缘较深水相区的前寒武纪-寒武纪过渡地层为一套以硅质岩和硅质碎屑岩为主的黑色岩系沉积。过去对这套硅质岩的沉积模式有多种解释,而且由于生物化石的匮乏而很难确定前寒武纪-寒武纪界线的位置。桂东北地区三江剖面前寒武纪—寒武纪过渡地层老堡组硅质岩中保存有良好的微生物岩。在根据有机碳同位素和微体化石确定的前寒武纪-寒武纪界线之下,微生物岩以叠层石-凝块石组合为标志,具有与台地相同期地层类似的特征;在界线之上,则以集群出现的球形至椭球形石英质微球体组合和典型的微生物膜沉积结构为主要特征。微生物席/膜的硅化可能是前寒武纪-寒武纪之交较深水相硅质岩沉积的重要模式。

英文摘要：

The strata of Precamhrian-Cambrian transitional interval period at the deep water facies along the southeast margin of Yangtze Platform is composed of black rock series which dominated by the chert and some silicislastic sedi ments. There is a long term debate about the depositional model of the chert sediment, as well as the definition of the Precambrian-Cambrian boundary because of few index fossils reported. In this paper, we report the microbolite preserved in the chert of the Laobao Formation and demarcate of the Precambrian Cambrian boundary by the organic car bon isotopic variation and the microfossil biostratigraphy. The microbolite in the chert are composed of stromatolites and thrombolites under the Precambrian-Cambrian boundary which is similar to those in the contemporary sediments on the carbonate plarforms. After the Precambrian Cambrian transition, the cherty microbolite are dominated by miero-spherites and biofilm structures. The well preserved microbial sedimentary structures in the chert of the Laobao Formation imply the important roles of the microbial mat on the chert formation, together with an hydrothermal activity.