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下—中三叠统界线的划分一直是国际地层学研究中一个有争议的问题。中国西南地区从台地相到盆地相不同类型的下—中三叠统地层均较发育,为奥伦尼克阶—安尼阶界线层型候选剖面的研究创造了有利条件。根据贵州望谟甘河桥剖面和贵州省贵阳市青岩剖面牙形石的研究,可识别出6个可与国内外进行对比的牙形石带,自下而上分别为:①Neospathdus cristagalli 间隔带,②Neospathodus pakistanensis 间隔带,③ Neospathodus waageni 间隔带,④Neospathodus homeri-N.triangularis 组合带,⑤Chiosella timorensis 间隔带,⑥ Neogongdolella regalis 延限带。在贵阳青岩剖面和望谟甘河桥剖面,早—中三叠世存在一个 Neospathodus- Chiosella-Neogongdolella 的牙形石演化系列。在这个演化序列中,在 Neospathodus homeri-N.triangularis 带上部出现 Neospathodus homeri 和 Chiosella timorensis 之间的过渡类型 Neospathodus qingyanensis,表明在下—中三叠统附近牙形石存在较好的演化关系。在贵阳青岩剖面,下—中三叠统界线位于紫云组顶部之下1.5 m 处 Chiosella timorensis 带首现的位置,在望谟甘河桥剖面,下—中三叠统界线位于紫云组顶部之下0.5 m 处 Chiosella timo- rensis 带首现的位置。依据上述情况,本文建议贵阳青岩剖面可作为下—中三叠统界线候选层型剖面,望谟甘河桥剖面可作为下—中三叠统界线候选层型的辅助剖面加以深入研究。
The division of the Lower-Middle Triassic boundary has been a controversial issue in the study of international stratigraphy. The different types of Lower-Middle Triassic strata in the southwestern China from the platform to the basin are well developed, which creates favorable conditions for the study of Orennik-Ani boundary line strata candidate profiles. According to the study of Ganmo Bridge in Wangmo, Guizhou Province and the conodont in Qingyan Section, Guiyang, Guizhou Province, six conodont belts that can be compared with those at home and abroad can be identified, which are: ①Neospathdus cristagalli interval, ② Neospathodus pakistanensis, ③ Neospathodus waageni, ④ Neospathodus homeri-N.triangularis, ⑤ Chiosella timorensis, ⑥ Neogongdolella regalis. In the Qingyan section of Guiyang and the Wangmeiqiao section of Wangmo, there is a series of evolutionary conodont of Neospathodus-Chiosella-Neogongdolella in the Early-Middle Triassic. In this evolutionary sequence, Neospathodus qingyanensis, a transitional type between Neospathodus homeri and Chiosella timorensis, appears in the upper Neospathodus homeri-N.triangularis zone, indicating a good evolution of the conodont near Lower-Middle Triassic. In the Qingyan section of Guiyang, the Lower-Middle Triassic boundary is located at the first spot of the Chiosella timorensis at 1.5 m below the top of the Ziyun Formation. At the Wangmengqiao section of Wangmo, the boundary between the Lower-Middle Triassic is located at the top of the Ziyun Formation Chiosella timo- rensis at 0.5 m with the first appearance. Based on the above, this paper suggests that the Qingyan section of Guiyang can be used as candidate stratigraphic section of the Lower-Middle Triassic boundary line, and the Wangmoganhe Bridge section can be studied as an auxiliary section of the candidate stratigraphic layer of the Lower-Middle Triassic boundary.