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贵阳乌当下奥陶统湄潭组之上,中奥陶统黄花冲组之下的泥质斑状灰岩中,盛产Yangtyeella kueiyangensis Yoh俗称“扬子贝层”。贵州地层古生物工作队地层组(1977)及贵州区域地质调查大队编图组(1982)把这一套地层命名为“十字铺组”,时代定为中奥陶世;而南京地质古生物研究所穆恩之等(1979)、贵州区域调查大队杨绳武等(1980)则将这一套地层命名为牯牛潭组,时代为早奥陶世晚期。这样,对贵阳乌当这段地层的划分产生了截然相反的意见,这两种意见不仅出现在上述不同作者的专著或论文中,甚至还出现在同一专著中(赖才根等1982),可见意见分歧之大。因此,对贵阳乌当“扬子贝层”的划分、对比、生物组合及有关的命名问题,进行系统的总结,深入的讨论,很有必要。
The lower Ordovician Meitan Formation in Wudang, Guiyang is rich in Yangtyeella kueiyangensis Yoh, commonly known as “Yangtze shell layer” in the muddy limestone below the Huanghuakao Formation in the middle Ordovician. The stratigraphic group of the Paleontology Task Force of Guizhou Province (1977) and the Guizhou Geological Survey Brigade (2002) named the formation “Cross Shop” and the age was defined as the Middle Ordovician; while the Nanjing Institute of Geology and Palaeontology Grace et al. (1979), and Yang Shengwu et al. (1980) of the Guizhou Regional Investigation Team named this set of formations as the Yanniutan Formation, which was dated to the Early Ordovician. Thus, a diametrically opposed view was given to the division of the strata in Wudang, Guiyang, both of which appeared not only in monographs or essays by the various authors described above, but even in the same monograph (Lai et al 1982) Great differences of opinion. Therefore, it is necessary to systematically summarize and discuss the division, comparison, biological assemblage and related naming of the “Wudang” layer in Wudang, Guiyang.