Doushishan Fm

The type section is located at Dangduo Gully, 24 km northwest of Dêwo County, Gansu Province. It was named by Xi’an Institute of Geology and Mineral Resources and No 1 Branch Team of Gansu No 1 Team of Regional Geological Survey in 1973. Qin Feng and Gan Yiyan formally quoted it in 1976. It is rather hard to find a complete section; at Luqu on the west, it shows higher muddy content. The most complete exposure is located at Dangduo Gully.

Limestone. The formation is composed of dark-gray to gray medium- to thick-bedded limestones of micrite, psammitic micrite and grainstone intercalated with gray dolomitic micrite and black banded thin-bedded nodular chert. The thickness is 659.7 m in total.

Conodonts could be found in many beds; and brachiopod and coral are mainly seen in the middle and upper parts. The middle part is characterized by the brachiopod Yunnanella-Yunnanellina assemblage zone, the coral Gorizdronia-Synaptophyllum assemblage zone, and the conodont Polygnathus perplexus; the upper part yields the brachiopod Tenticospirifer hsikuangshanensis-Cyrtosspirifer cf. pamiricus assemblage zone, and conodonts Apatognathus cuspidate, Polygnathus semicostatus.

In the lower part of this Formation is algal limestone with wave marks showing shallow-water deposit characteristics, therefore a neritic shelf to open platform facies is suggested.

The resistant thick-bedded carbonate rock in the formation usually forms high or steep cliffs in the landscape. The formation generally has a consistent lithology and thickness.