Cakuohe Fm

The type section is located at Cakuohe, 10 km north of bohai in Luqu County, Gansu Province; the reference section is at Dangduogou, 24 km northwest of Diebu County, Gansu Province. It was named by Xi’an Institute of Geology and Mineral Resources and Gansu No. 1 Team of Regional Geological Survey in 1973, and quoted formally by Qin Feng and Gan Yiyan in 1976.

Limestone. The lithological character of the formation is marked by interbedded gray and dark-gray thin-bedded fine-grained crystalline limestone, clotted limestone, algal limestone, black calcareous shale, silty calcareous shale and yellowish-gray clayey siltstone, intercalated with more than ten beds of 0.2- to 10.8-m-thick calcirudite. The thickness is 547.4 m in total.

The limestone contains conodonts, trilobites and tentaculites, and the calcirudite contains coral and brachiopods characterized by abundant Cyrtospirifer. The lower part yields conodonts Palmatolepis proversa, P. gigas; the middle part yields P. poolei. Westward in the Cakuohe area of Luqu County, the formation contains abundant brachiopods and corals

The ripples, hummocky cross-bedding and the angular pebbles occurring in this formation indicate a sedimentary environment of shallow-sea shelf facies. Tempestite deposits have been interpreted in the lower and middle parts. Westward in the Cakuohe area of Luqu County, the formation contains abundant brachiopods and corals, is considered as carbonate platform facies, and has a thickness decreasing down to 149 to 260 m.

Because of thick cover and many folds and faults, the sequence and the relation of the formation to the overlying and underlying beds are not clear in the type section, so that the Dangduogou section was chosen as a reference section.