Bianmachong Fm
Type Locality and Naming
The type section is located at Bianmachong, lying 1.3 km southeast of Nangao Town or about 23 km northeast of Danzhai County (107°53’E, 27°23’N). In the section, the formation is 28 m thick. The Bianmachong Formation was first published by Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (1974). The name is derived from inhabitant site Biaomachong in the Nangao Township, Danzhai County, Qiandongnan (Southeast Guizhou) Miao-Dong Autonomous Prefecture, Guizhou Province. The name originally appeared in a 1970 manuscript by Zhang Zhenghua, Zhou Zhiyi and others, from the Institute of Geology, Guizhou Headquarter of Petroleum Exploration and the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences.
Synonym: （变马冲组）
Lithology and Thickness
The Bianmachong Formation in the type section is only 28 m thick, consisting of dark grey laminated carbonaceous shale and mudstone and quartzose siltstone with interbeds of sandstone, having a 3.7 m thick greenish grey carbonaceous and calcareous mudstone and politic siltstone in the basal part. The thickness of the formation varies significantly from 28 m to about 530 m with a tendency of deceasing eastward. It is thickest at Xiaosai, Yuqing County, where the formation is 531 m thick.
Relationships and Distribution
Lower contact
The Bianmachong Fm is conformably rests on underlying Jiumenchong Fm or Niutitang Fm. The lower boundary of the Bianmachong Fm is marked by lithological change from either the limestone of Jiumenchong Fm or the black shale of Niutitang Fm to the greyish green carbonaceous and calcareous shale at the base of the Bianmachong Fm
Upper contact
Regional extent
The formation is exposed in the Jiangnan Slope Area in southeastern Guizhou Province, which is the area east to the Wuchuan—Weng’an—Guiding line and west to Tainzhu—Jianhe—Sandu line, covering northern part of both the Qiannan (South Guizhou) Buyi-Miao Autonomous Prefecture and the Qiannan Miao-Dong Autonomous Prefecture. In the western part, the formation is dominated with silty shale whereas in the eastern part with carbonaceous shale and carbonaceous siltstone.
GeoJSON
Fossils
In the type section, the lower part of the formation contains trilobite Protolenella conica, P. lata, and Chengkouia pustolosa, and brachiopod Aclothele sp., and the younger strata contain also Chengkouia pustolosa, and brachiopod Aclothele sp. In other locality the top part of the formation yields trilobite Szechuanolenus sp.
Age
Depositional setting
Additional Information