Xidashan Fm
Type Locality and Naming
The type section of Xidashan Formation is the Mohershan section, situated on the north slope of the Mohershan Mountain in the border area between the Yuli and Heshuo counties, Bayingolin Prefecture, Xinjiang Uygur Autonomous Region (88°21’05”E, 41°24’20”N). It is also the type section of the overlying Mohershan Formation. The section lies 35 km southeast of Qinggir Village or about 40 km northeast of Xingdi Village in Yuli County, and was measured by Zhang et al. (1959). The Xidashan Formation was first published by Zhang et al. (1959). It is the same type section shared by the underlying Xishanbulak Formation and the overlying Mohershan Formation. The name is derived from Xidashan Mountain, about 40 km west of Qinggir Village in Yuli County, Bayingolin Mongolian Autonomous Prefecture, Xinjiang Uygur Autonomous Region. It was originally appeared in a 1958 manuscript by the No.13 Geological Team of Ministry of Geology of China. Originally it was called Xidashan series and subsequently renamed Xidashan Formation by Hu et al. (1965a).
Synonym: （西大山组）
Lithology and Thickness
The Xidashan Formation is a sequence with carbonate and clastic rocks in alternation. In the type section, the formation is divided into two lithologic beds, from the bottom up: 1, light grey, thin-bedded limestone, alternated with carbonaceous shale (30 m thick); 2, light grey, thin-bedded limestone, intercalated minor carbonaceous shale (35 m).
Relationships and Distribution
Lower contact
The Xidashan Fm is in conformable contact with underlying Xishanbulak Fm. The lower boundary of the formation is defined by the disappearance of the black chert of the Xishanbulak Fm.
Upper contact
It is conformably overlain by the Mohershan Fm. The upper boundary is clearly defined by lithological change from the thick-bedded sandy limestone at the top of the formation to the thin-bedded limestone of the Mohershan Fm.
Regional extent
The Xidashan Formation is exposed in the east of Southern Tianshan Area of Tarim Region, distributed in the north slope of the Kuruktag Mountain (Qinggir, Xingdi, Dapingtai, and Baleguenbulak villages and the area of Bosten Lake). Usually, the Xiadashan Fm and underlying Xishanbulak Fm are outcropped at the foot of the Kuruktag Mountain, the overlying Mohershan Fm at about half way of the mountain and the youngest Torsuqtagh Fm at the mountaintop. The thickness of the formation ranges from 42 to 507 m.
GeoJSON
Fossils
The Xidashan Formation embraced three trilobite zones, from bottom up: 1, Ushbaspis-Chengkouia Zone containing Chengkouia constricta, C. xinjiangensis, Metaredlichia xibeiensis, Parazhenbaspis mohershanensis, P. kuruktagensis and Ushbaspis rectangularis, and archaeocyathids Aldanocyathus, Coscinocyathus and Rotundocyathus; 2, Tianshanocephalus Zone containing Tianshancephalus tianshanensis, Zhenbaspis xidashanensis and Kootenia sp.; 3, Arthricocephalus-Changaspis Zone containing Arthricocephalus sp., Changaspis sp., and Redlichia sp.
Age
Depositional setting
Additional Information