Bocaitian Fm
Type Locality and Naming
The type section of Bocaitian Formation is the Tianpeng-Bocaitian section. This long section is also the type section for underlying, in descending order, the underlying Tanjiaba Fm, the Longha Fm, and the lower Tianpeng Fm. It lies 9.5 km northwest of the Tianpeng Town, or about 40 km southwest of the seat of Funing County. The section spans the border between Funning and Malipo counties with the Tianpeng belonging to Funning County on the east and Bocaitian belonging to Malipo County on the west. The Bocaitian Formation is exposed near the Bocaitian lot on the west side of the section, lying 6.2 km northwest of Donggan (105°13’E, 23°23’ N). The Bocaitian Formation was published by Liu Yuanzheng et al. (1965) from Geological Bureau of Guangxi Zhuang Autonomous Region. The name is derived from Bocaitian, a farming lot near Donggan Town, Malipo County, Yunnan Province, and was originally appeared in a 1964 manuscript by the No. 4 Brigade of Yunnan Regional Geological Survey Team. This formation spans Cambrian-Ordovician boundary.
Synonym: （博菜田组）
Lithology and Thickness
The Bocaitian Formation is mainly a fine clastic sequence in lower and middle parts and carbonate sequence in upper part. Lower and Middle parts of the Bocaitian Fm consist mainly of light grey, greyish green, greyish purple sandy mudstone, pelitic siltstone with a few of interbeds of dark grey limestone and argillaceous limestone; Upper part consists of greyish white, light grey thick-bedded limestone, grey dolomite and dolomitic limestone with interbeds of slate. In this section, the Bocaitian Formation is over 2505 m thick (Luo et al., 2009).
Relationships and Distribution
Lower contact
The- Formation conformably rests on the underlying Tangjiaba Fm and extends up to Ordovician
Upper contact
It is overlain conformably by the Xiamududi Fm of Early Ordovician age.
Regional extent
The Bocaitian Formation is exposed in the southeastern part of the Yangtze Area of South China Region, very close to the Jiangnan Slope Area. It is distributed in the counties in southeastern corner Yunnan Province, including Finning, Guangnan, Maguan, Malipo, Wenshan and Xichou counties belonging to the Wenshan Zhaun-Miao Autonomous Prefecture and Menzi City and Pingbian County both belonging to the Honghe Hani-Yi Autonomous Prefecture. It is also distributed in most western Guangxi Zhuang Autonomous Region, including Longlin and Jingxi counties. The thickness of the formation is relatively stable in the Pingbian-Monzhi area in the west, varying from 500 to 700 m, but is significantly increased in the Malipo and Funing area in the east, having maximum thickness of 2505 m at Bocaitian, from where it becomes thinner eastward and westward with a normal thickness of about 350 m.
GeoJSON
Fossils
The lower part of this formation yields trilobites Prosuakia, Saukia, Lophosaukia, Shergoldia, and Lonchopygella, and brachiopods Billingsella and Huenella; the middle part yields trilobites Changia and Saukia, and brachiopods Billingsella and Westonia; the upper part yields trilobites Mictosaukia, Koldinioidia, Shergoldia, Leiostegium, and Dictyites, and brachiopods Lingulella, Palaeostrophia, and Westonia (Luo et al., 2009).
Age
Depositional setting
Additional Information