Xiannudong Fm
Type Locality and Naming
The type section of the Xiannüdong Formation is the Shatan-Qiaoting section, measured between Shatan and Qiaoting villages, which is close to Xiannüdong. The section lies about 0.4 km north of Ganchang Town or about 15 km northeast of the seat of Nanjiang County, Sichuan Province (116°52’20” E, 32°30’10” N). The Xiannüdong Formation was first published by Lu (1963). The name is derived from Xiannüdong in Ganchang Township, Nanjiang County, Bazhong City, northeastern Sichuan Province. It was originally appeared in a 1960 manuscript by Ye Shaohua. The unit was named Xiannüdong Member belonging to Guojiaba Fm by Lu (1963) and Liu et al. (1964) and raised the member in rank as Xiannüdong Formation by Li et al. (1975).
Synonym: （仙女洞组）; Xiannüdong Fm
Lithology and Thickness
The Xiannudong Formation is largely a carbonate sequence bearing rich archaeocyathids. The formation can be divided into three parts. Lower part, 48.2 m thick, consists of grey to dark grey, massive oolitic limestone with oblique bedding. Middle part, 56.4 m thick, consists of light to dark grey, fine-grained calcareous sandstone with calcareous nodules, alternated with dark grey, thick-bedded limestone, and with sandy shale at the base. Upper part, 20.2 m thick, consists of pink, purplish red, grey and greyish green, thick-bedded limestone and siliceous limestone bearing rich algae. In the type locality, Xiannüdong Formation is 124.8 m thick.
Relationships and Distribution
Lower contact
The Xiannudong Fm rests conformably on either the underlying Chiungchussu Fm or Niutitang Fm. The lower boundary of Xiannudong Fm is marked by the appearance of oolitic limestone at its base.
Upper contact
It is in conformable contact with the overlying Tsanglangpu Fm. The upper boundary by appearance of sandy shale at the base of the Tsanglangpu Fm.
Regional extent
The Xiannüdong Formation is exposed in the Yangtze Area of South China Region, distributed in northeastern Sichuan (Nanjiang, Wangcan, and Wanyuan counties), southwestern Shaanxi Province (Mianxian, Nanzheng, Ningqiang, and Xixiang counties), and northern Chongqing Municipality. The thickness of the formation varies from 21 to 168 m.
GeoJSON
Fossils
The Xiannüdong Formation is rich in archaeocyathids (sporadic individuals or accumulated to form reef), which mainly belong to Septoidea with merely minor elements to Taetioidea, including Ajacicyathus, Archaeofungia, Coscinocyathus, Dictyocyathus, Rotundocyathus and Taylorcyathus. The formation yields also trilobites Yiliangella, Yunnanaspis and Malungia.
Age
Depositional setting
The formation is a deposit under an oxygen-enriched, warm continental shelf environment.
Additional Information
Zhang (2009, p. 1221) attributed the nomenclature of the unit to Pan and Peng (1939). As checked by the present author, the paper by Pan and Peng (1939) had never named this unit, although they had mentioned, in the Chinese text, they had been “searching for fossils above the Lingulella horizon of the Sanhuichang Limestone over around Banhe and Xiannüdong villages” (Pan and Peng, 1939, p. 51). The Xiannüdong is believed to be mentioned nothing more than a working locality in that paper.