Zhengmuguan Fm

Helan Mtns., Inner Mongolia-Ningxia border. The Zhengmuguan Fm was named by Zheng Zhaochang (1975), which was published in Ningxia Compiling Group for Regional Stratigraphical Scale (1980), Regional Stratigraphic Scale of North China. The type section is located in Alxa Zuoqi, Nei Mongol Zizhiqu (Inner Mongolia Autonomous Region).

Synonym: （正目观组） , The current Zhengmuguan Formation includes the "Zhenmuguan Group" and "Jingdiquan Formation" established by the Ningxia No. 2 Party of Regional Geological Survey. Perhaps Tu'erkeng Fm (Turekeng Fm) for dolomite-bearing strata overlying tillite in this region?

The Zhengmuguan Fm consists of two members. Lower member is yellow-brown, light-red and gray tillite whose pebbles are mainly gray siliceous banded dolomite and dark gray dolomite with minor quartz sandstone and slate. The pebbles are poorly sorted and the typical diameter is 10-20 cm, with the largest over l m, with different forms. Cement is mainly calcareous-magnesian or clayey-calcic, with lesser cements of muddy-sandy and ferruginous characteristics. The tillite has stable horizons but variable thickness in locations in the middle of Helan Mountains. It is 144 m thick in Zhengmuguan and 79 in thick in Binggou, Huangqikou. The lower member thins to 9 m thick when going southward to Zihuagou, is 14 m thick in Dagangou, 10 m thick in Jingdiquan, and 10-70 m thick near Suyukou. Upper member (former Jingdiquan member) consists of a series of slate, including gray, gray-green and gray-black silty slate in the middle part of Helan Mts; and it is distributed the same as the tillite of the lower member. There is continuous deposition between the lower and upper members where there is a sequence of conglomerate-pebbly - calcareous slate - sandy slate - clayey slate. The thickness is quite variable, usually 31 to 252 m.

Microplants found in the slate of upper member: Symplasso- sphaeridium sp., Gloeocapsomorpha sp., Polyedrosphaeridium sp., Synsphaeridium sp., Laminarites sp., Taeniatum dozssum, Asperatopsophosphaera sp., Pseudozonosphaera verrucosa, Trachysphaeri- dium hyalinum, Leiopsophosphaera minor; and some fossils are found in the cement: Trachysphaeridium cultum, Leiopsophosphaera sp.and L. min