Xianshuihu Fm

The type section is at area 30 km southeast of Wulanbulag in Erjin Qi (99°42′E, 42°12′N), Inner Mongolia. The Xianshuihu Fm is derived from the Xianshuihu Gr proposed by Xiu Zelei and Zhao Xiangsheng in 1964 and published by Gansu Compiling Group for Regional Stratigraphic Scale (1980). The Xianshuihu Gr was divided into lower clastic rock formation and upper “volcanic rock formation”. In 1980, No.2 Gansu Party of Regional Geological Survey included the lower clastic rock formation in the Xiaohulishan Fm (present Shajing Fm), and the upper “volcanic rock formation” in the Xianshuihu Gr, and considered them to be Middle Ordovician in age. In 1991, Inner Mongolia Bureau of Geology and Mineral Resources renamed the Xianshuihu Gr the Xianshuihu Fm, and considered the Xiaohulishan section in Erjin Qi as the stratotype. Coeval with the upper part of Huaniushan Gr.

Synonym: (咸水湖组）

Reference Section:

The reference section is in Xiaohuli Hill, Shanshan County, Xinjiang.

The formation is divided into three parts. Lower part is composed of thick-bedded pebbly sandstone, breccia-bearing dacite, amygdaloidal and vesicular andesite, andesitic crystal rock intercalated with andesitic basalt. Middle part is represented by siliceous and muddy slate, siliceous siltstone, limestone and crystalline limestone yielding graptolite, brachiopod and trilobite. Upper part is dominated by andesite intercalated with andesitic breccia, andesitic basalt and basalt. Generally, the upper part is composed dominantly of andesite and minor crystalline limestone intercalated with siltstone, chert and thin-bedded mudstone, conglomerate, and yields brachiopod and trilobite. In the type area, the Xianshuihu Fm is 5141 m thick.

This stratigraphic unit yields graptolites Dicellograptus sextens, D. exilis, D. divaricatus, D. inutus, Climacograptus and brachiopods Plectorthis, Glyptorthis, Orthis, Hesperorthis, Platystrophia.