Dongchuan Fm
Type Locality and Naming
Named in 1948 by Meng Xianmin and others. The named section is located in Chexun, Huize, Yunnan. It is called "Dongchuan System" when it is named, referring to the purple-red sandy mudstone in the lower part of the Triassic in the Dongchuan-Huize area of Yunnan. In 1962, Zhao Kejin and others renamed it as Dongchuan Formation. In 1992, Wang Zunzhou et al. emphasized that the Feixianguan Fm should not be used to replace the Dongchuan Fm and classified the Dongchuan Fm from the Early Triassic to the Early Middle Triassic.
Synonym:
Lithology and Thickness
It is a purple-red sandstone, mudstone, and shale interbedded with different thicknesses, with typical cross bedding and well-developed wave marks. Sandstone has a high content of debris components, mostly basalt debris; the size of the debris changes from bottom to top, from west to east, obviously from coarse to fine; the upper part of this group is occasionally mixed with marl, and the lower part is often mixed with fine conglomerate, sandstone. The thickness is 112 to 766 m.
Relationships and Distribution
Lower contact
In the reference section, the bottom, the basalt conglomerate is in integrated contact with the underlying Upper Permian Xuanwei Fm (partially in pseudo-integrated contact), or with the underlying Upper Permian basalt (Emeishan Basalt Fm) in pseudo-integrated contact. However, regionally in east Yunnan, the underlying unit is the Kayitou Fm of late-Changhsingian to earliest Induan.
Upper contact
Overlain by the Leikoupo Fm of Anisian.
Regional extent
It is distributed on the eastern edge of the "Kangdian Oldland", roughly west of 104°E, including Dongchuan, Huize, Qiaojia in northeastern Yunnan, and Zhaojia in Liangshan, Sichuan. Jue, Meigu, Leibo, Mabian, Leshan and Emei. This group is about 198 m thick in Longmen Cave of Emei, 265 m thick in Leshan Tongjiezi, 112.2 m thick in Yingjinghuatan, 135 to 497 m thick in Meigu and Zhaojue, and 513 m thick in Zhaotong, Huize and Yongshan. ~766 m.
GeoJSON
Fossils
The fossils of this group are rare, and the upper part of this group contains bivalves in the Meigu and Zhaojue areas of Sichuan Costatoria goldfussi mansuyi, C. cf. radiata, C. cf. laevigata. The age of this group varies from place to place
Age
Depositional setting
It is a fluvial deposit. In 1977, the Triassic Formation of the Southwest Team of the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences and Chen Chuzhen et al. (1979) pointed out that the Dongchuan Fm is a fluvial deposit from the Feixianguan Fm westward to the edge of the Kangdian ancient land.
Additional Information