Posongchong Fm
Type Locality and Naming
The type section is located nearby Posongchong of Xizhang, about 20 km northeast of Guangnan County (105o08’E, 24o08’ N), Yunnan. It was named by Liao Weihua et al in 1978.
Lithology and Thickness
Fine-grained sandstone. It is a set of grayish-brown medium- to thick-bedded fine-grained sandstone, siltstone and silty mudstone. It is 415 m thick.
Figure Outcrop showing the sandstone and siltstone of Posongchong Formation in the type locality.
Relationships and Distribution
Lower contact
It has an angular unconformable contact with underlying Bolaitian Fm dolomite of Cambrian. The basal contact is sharp with a weathered iron-manganese crust or basal conglomerate at the base of Posongchong Fm.
Upper contact
It is conformably overlain by the black carbonate mudstone of Pojiao Fm.
Regional extent
It is distributed in Guangnan, Wenshan, Mengzi, Maguan, Xichou, Qiubei, Funing, southeast of Yunnan. Revised strat columns (Hongfei Hou, Apr2021) extended this name to NW Guangxi (Nandan, revised column 63). It often overlaps on the Cambrian or Ordovician, and its thickness changes considerably.
GeoJSON
Fossils
Plants and spores are very common, including Zosterophyllum australianum, Protopteridium sp., Sucheocladus sp., and fish fossils are reported in Gumu of Wenshan Shi. Six fish fossils are found from the Posongchong Fm. in Wenshan, including Kwangnanaspis subtriangularis, Gantarostratoaspis gengi, Gumuaspis rostrata, Macrothyraspis longicornis, Wenxianaspis zhichangensis and Sanqiaspis rostrata. The last one has been reported from Chaotong of northeast Yunnan and the Pingyipu Group of Beichuan, Sichuan.
Age
Depositional setting
It is interpreted as a beach facies.
Additional Information