贵州南部地区中三叠统青岩阶底界附近牙形石生物地层学研究

Research on Conodont Biostratigraphy near the Bottom Boundary of the Middle Triassic Qingyan Stage in the Southern Guizhou Province

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下—中三叠统地层均较发育 ,为奥仑尼克阶—安尼阶界线层型的研究提供了有利条件。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 :1N eospathdus cristagalli间隔带 ,2 N eospathodus pakistanensis间隔带 ,3N eospathodus waageni间隔带 ,4 N eospathodus homeri— N . triangularis组合带 ,5 Chiosella timorensis间隔带 ,6 N eogongdolellaregale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 N eospathodus— Chiosella—N eogongdolella的牙形石演化系列。在这个演化序列中 ,在 N eospathodus homeri— N . triangularis带上部出现N eospathodus homeri和 Chiosella timorensis之间的过渡类型 N eospathodus 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面之下 1.5 m处 Chiosellatimorensis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面之下 0 .5 m处 Chiosella timorensis带首现的位置。依据上述情况 ,本文?

The demarcation of the Lower—Middle Triassic boundary is a disputed problem in stratigraphic research in the world. Lower—Middle Triassic strata of different types, from platform facies to basin facies, are well developed in Southwest China. This is helpful for the research on the Global Stratotype Section and Point (GSSP). According to the research on conodonts in the Ganheqiao section in Wangmo County and the Qingyan section in Guiyang City, Guizhou Province, six conodont zones, which can be correlated with those in other regions, have been recognized. They are in ascending order as follows: ① Neospathdus cristagalli interval-zone, ② Neospathodus pakistanensis interval-zone, ③ Neospathodus waageni interval-zone, ④ Neospathodus homeri- N. triangularis assemblage-zone, ⑤ Chiosella timorensis interval-zone, ⑥ Neogongdolella regale range-zone. An evolutionary series of conodont Neospathodus—Chiosella—Neogongdolella occurred in the Ganheqiao section and Qingyan section in the Early—Middle Triassic. In this evolutionary series, a transitional Neospathodus qingyanensis appeared between Neospathodus homeri and Chiosella timorensis in the upper part of Neospathodus homeri- N. triangularis zone. This shows an excellent Early—Middle Triassic evolutionary series of conodont across the Lower—Middle Triassic boundary. The Lower—Middle Triassic boundary is located 1.5m below the top of the Ziyun Formation, where the Chiosella timorensis zone first appears on the Qingyan section; while the Lower—Middle Triassic boundary is situated 0.5 m below the top of the Ziyun Formation, where the Chiosella timorensis zone first appears on the Ganheqiao section. Based on the above-mentioned discussion, the present authors suggest that the Qingyan section can be regarded as a candidate of the GSSP of the Lower—Middle Triassic boundary and the Ganheqiao section as a reference of the GSSP of the Lower—Middle Triassic boundary for further necessary investigation.