云南西北部三叠纪地层的划分及对比

THE SUBDIVISION AND CORRELATION OF THE TRIASSIC STRATA IN NORTHWEST YUNNAN

云南西北部三叠纪地层分布较广,横跨3个地层分区,从西向东分别是澜沧地层分区、兰坪地层分区和金沙江地层分区,主要讨论后两个地层分区的三叠纪地层。兰坪地层分区三叠纪地层零星分布,呈推覆块体或飞来峰形式展现,有晚三叠世歪古村组、三合洞组、挖鲁八组和麦初箐组,不整合于上二叠统羊八寨组之上、中侏罗世漾江组之下;产双壳类、菊石、腹足类、牙形石及植物化石,时代属卡尼期至瑞替期。金沙江地层分区的三叠纪地层有中三叠世上兰组、攀天阁组,晚三叠世崔衣比组和石钟山组;产丰富的双壳类、腕足动物、菊石、有孔虫、牙形石、叶肢介等化石。攀天阁组、崔衣比组为双峰式火山岩系,产有孔虫、牙形石等化石,火山岩中获得的同位素年龄为224Ma、236Ma、238Ma、248±4.4Ma,时代包括早、中、晚三叠世。上兰组以往被认为不整合在早二叠世拉竹河组之上,经后来研究证实,原划的不整合为上兰组内部的断层,不存在拉竹河组。上述两个地层区的三叠纪地层虽然有许多相似,但仍有较大差异,为了便于对比,建议各自使用独立的地层名称。

Triassic strata are widely distributed in northwestern Yunnan, which can be divided into, from west to east, the Lancang, Lanping, and Jinsha River stratigraphic regions. In the Lanping stratigraphic region, Triassic strata consist of the Upper Triassic Waigucun, Sanhedong, Waluba, and Maichuqing groups, which are sporadically distributed as nappes or klippes unconformably overlying the upper Permian Yangbazhai Group and underlying the Middle Jurassic Yangiiang Formation. The Triassic strata in the Lanping stratigraphic region contain bivalves, ammonoids, gastropods, conodonts and plant fossils, and they are Carnian to Rhaetian in age. In the Jinsha River stratigraphic region, Triassic strata include the Middle Triassic Shanglan and Pangtiange groups, and the Upper Triassic Cuiyibi and Shizhongshan groups. They contain rich bivalves, braehiopods, ammonoids, foraminifera, conodont fossils, and concho-stracans. The Pangtiange and Cuiyibi groups contain bimodal volcanic rocks and yield radiometric ages of 224 Ma, 236 Ma, 238 Ma, 248±4.4 Ma, suggesting a Triassic age. It was previously thought that the Shanglan Group unconformably overlies the lower Permian Lazhuhe Group. Subsequent studies showed that the unconformity was a fault within the Shanlan group and the Lazhuhe Group does not exist. Al- though there are many similarities between the Lanping and Jinsha River stratigraphic regions, the differ- ences justify the usage of independent stratigraphic systems in these two regions.