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目前国内对于我国新元古代南华系划分的分歧很大,主要原因之一在于对板溪群及其相当层位地层的构造属性及其上限年龄的认识存在较大差异,为此,在详细的沉积学调查的基础上,对湘黔桂地区典型丹州群和下江群(与板溪群相当)有关剖面上顶部地层沉凝灰岩开展了锆石年代学研究。沉积学调查表明,板溪期沉积地层与上覆南华冰期沉积地层之间的转换界面清晰,界面上下岩性、结构、构造、沉积相特征迥异,显示板溪群和上覆南华纪冰期沉积期间存在明显的沉积—构造转换过程。而板溪晚期沉凝灰岩的锆石U-Pb同位素年代学研究则表明,板溪群及其相当层位的上限年龄均趋向于720 Ma,这与Sturtian冰期的启动年龄718 Ma接近。因此,基于沉积盆地演化的阶段性、全球性重要古气候演化的区域可对比性以及同位素年代学分析,将南华冰期启动年龄设定为720 Ma是恰当的,将我国南华系底界置于江口冰期沉积层系之底也是合理的,这为我国南华系底界GSSP的选定提供了重要的年代学和沉积学约束。
At present, one of the main reasons for the great difference in the division of the Neoproterozoic South China System in China is that there is a big difference between the understanding of the structural properties of the Banxi Group and its equivalent stratigraphy and the upper limit of age. For this reason, Based on the sedimentary investigation, the zircon geochronology was carried out on the top tuff-like tuff in the section of the typical Danzhou and Xiajiang groups (equivalent to Banxi group) in Xiang, Qian and Gui areas. Sedimentological investigation shows that the conversion interface between the Banxi period sedimentary strata and the overlying South China glacial depositional stratum is clear. The features of the lithology, structure, structure and sedimentary facies of the upper and lower interfaces are very different, indicating that the Banxi Group and the overlying Nanhua Ice Age sedimentary period There is a clear process of sedimentary-structural transformation. The zircon U-Pb isotopic dating of the Late Tuhui limestone in Banxi shows that the upper age of Banxi Group and its equivalent strata tend to 720 Ma, which is close to the start-up age of 718 Ma in the Sturtian glacial period. Therefore, based on the stages of sedimentary basin evolution, regional comparability of important global paleoclimate evolutions and isotopic chronological analysis, it is appropriate to set the starting age of the South China Ice Age to 720 Ma. It is appropriate to place the bottom boundary of the South China System in the estuary The bottom of the sedimentary system in the glacial period is also reasonable, which provides important chronology and sedimentology constraints for the selection of GSSP at the bottom of the South China System.