Suyukou Fm
Type Locality and Naming
The type section of the Suyukou Formation is the Wudaotang section. It is located at Mining Area of the Suyukou Phosphorite Mine, Helan, Yinchuan, Ningxia Hui Autonomous Region (105°54’E, 38°45’N). It is the lower portion of the same type section with the overlying Wudaotang Formation. The section lies about 7.5 km northwest of Suyukou, or 44 km northwest of seat of Helan County, and was measured by No. 2 Geological Team of Ningxia Geological Bureau in 1975. In the type section the formation is 30.28 m thick. The Suyukou Formation was firstly published by Zhou and Zheng (1976). The name is derived from Suyukou, Jinshan Village, Hongguang Township, Helan County, Yinchuan City, Ningxia Hui Autonomous Region. Originally the name was appeared in a 1958 manuscript by the former Suyukou Phosphorite Team of Geological Bureau of Gansu Province. Xiang (1981b), Compiling Committee for Chinese Stratigraphic Lexicon et al. (1999) and Yuan et al., (2016) followed Zhou and Zheng (1976) in regarding the Suyukou Formation as a valid unit, which is tentatively followed in this book. However, Shen in Gu (1996) assigned strata referred previously to Suyukou Formation as Xinji Formation. Indeed, there is a strong possibility that formation is synonymous with the Xinji Formation by the phosphoric lithology.
Synonym: (苏峪口组)
Lithology and Thickness
The Suyukou Formation is principally a clastic sequence, with interbeds of phosphoric and carbonate rocks at base. In the type section, the formation is divided into 4 lithologic beds, from bottom up: 1, yellowish brown phosphoric conglomerate (0.77 m thick); 2, grey, greyish yellow, medium-bedded silty calcareous dolomite (3.75 m); 3, greyish yellow phosphoric sandstone (0.77 m); and 4, greyish white, medium-bedded dolomitic feldspathic siltstone (24.99 m).
Relationships and Distribution
Lower contact
The Suyukou Fm is in disconformable contact with underlying Tuerkeng Fm (former upper part of Zhengmuguan Fm) of Ediacaran System. The lower boundary of the formation is marked by disconformable plane.
Upper contact
It is conformably overlain by Wudaotang Fm. the upper boundary is defined by lithologic change from the siltstone of Suyukou Formation to silty calcareous dolomite at the base of the Wudaotang Fm.
Regional extent
The Suyukou Formation is exposed in the Western Margin Area of North China Region, distributed in the middle of Helanshan Mountain (Suyukou and Wudaotang of Helan County) and it is sporadically distributed in Tongxin County and Guyuan City, south-central Ningxia, and Pingliang City of northeastern Gansu Province. In the middle section of Helanshan Mountain the formation is stable in lithology and thickness, usually 30‒39 m thick around Suyukou area (Helan County), thinning southward to 21‒23 m thick in west Yinchuan City, and further southward at Qinglongshan in Tongxin County the formation becomes merely 6.5 m thick. However, stretching even further southward to Dataizi in Pingliang City and in Huanxian County of Gansu Province, the formation returns to 32~56 m in thickness and consists of light purplish red quartzose sandstone intercalated with minor dolomitic limestone in lithology with phosphoric conglomerate at base.
GeoJSON
Fossils
In the type section, the formation yields Bergeroniellus.
Age
Depositional setting
Additional Information