Xishuangyingshan Fm
Type Locality and Naming
Beishan region. The type section of the Xishuangyingshan Formation is Xishuangyingshan section, located in the south of Shuangyingshan Hills in Mazongshan Township, Subei Mongolian Autonomous County (96°25’E, 41°28’N). The section was measured by the No. 2 Brigade of Gansu Regional Geological Survey Team in 1968. In the type section the formation is over 45 m thick with upper boundary is not exposed. The Xishuangyingshan Formation was first published by Gansu Second Regional Geological Survey Team (1968). The name is derived from Xishuangyingshan Hills in Mazongshan Township, Subei Mongolian Autonomous County, Jiuquan City, western Gansu Province. The name was originally appeared in a 1964 manuscript by Xiu Zelei and Zhao Xiangsheng, who called the formation Xishuangyingshan Group. It was lowered in rank as Xishuangyingshan Formation by Yu (1994).
Synonym: （西双鹰山组）
Lithology and Thickness
The Xishuangyingshan Formation is basically a sequence of chemically-formed rocks. It consists mainly of siliceous rock, thin-bedded stinkstone and biogenic limestone. Lower part of the formation comprises black phosphoric siliceous rock, intercalated with thin-bedded limestone. Upper part consists of grey limestone, intercalated with phosphoric siliceous rock.
Relationships and Distribution
Lower contact
As revealed by other sections around the type locality, the Xishuangyingshan Fm rests conformably on underlying Shuangyingshan Fm. However, in the areas west of Xishuangyingshan, the formation rests disconformably on the Xicangjing Fm (Xichangjing Fm) of Ediacaran System because both the underlying Shuangyingshan Fm and even older Pochengshan Fm are pinched out there. The lower boundary of the Xishuangyingshan Fm is defined by lithological change from the coarse-crystallized biogenic limestone of the underlying Shuangyingshan Fm to the black siliceous rock at its base.
Upper contact
The formation is disconformably overlain by the Luoyachushan Fm of Ordovician System. The upper boundary is marked by a disconformable plane.
Regional extent
Beishan region. The Xishuangyingshan Formation is exposed in the Middle Tianshan-Beishan Region with fairly wide distribution on the border area between Xinjiang Uygur Autonomous Region (Luoyacushan in Xingxingxia Valley) and Gansu Province (Shajing, Zerumu, Xishuangyingshan, Shuangyingshan, Dahuoluoshan and Wutongjing in Mazongshan area). Eastward, it extends to Ejin Banner (Yueyashan and Zhusilenhaierhan), Alxa Left Banner (Olijisumu) and Alxa Right Banner (Haobiru) of Inner Mongolia Autonomous Region.
GeoJSON
Fossils
The lower part of the Xishuangyingshan Formation yield trilobites Dinesus spinellosa, Galahetes opimus, Pagetia sp. sp. and Xystridura sp.; brachiopods Lingulella sp. and Acrothele sp. and monoplacophora; the middle part yields trilobites Centropleura sp., Corynexochus pulcher, Hypagnostus sp., Lejopyge sp. and Ptychagnostus sp.; and the upper part yields trilobites Glyptagnostus reticulatus, Hedinaspis regalis, Pseudagnostus communis and Proceratopyge rectispinatus.
Age
Depositional setting
The formation represents rather deep marine setting.
Additional Information