Chigou Fm

The Chigou Fm is the second lowest formation of the Zhashui Gr. The type section is located at Chigou in Niangniangmiao Township of Shanyang County, and reference section is at Xiaotai to Tongjiapo villages in Xiaotai Township of Zhashui County, Shaanxi Province. It was named by Shanxi Team of Regional Geological Survey in 1968 during mapping Xiaohekou Geological Map (the scale is 1:50,000) and formally quoted by Du Dinghan et al. in 1986.

Metamorphosed Greywacke. The Xiaotai section is used here as the stratotype instead of the Chigou section which is incomplete at top. In the Xiaotai section, the Formation is divided into two sub-formations. The lower part of the Lower sub-formation is composed of metamorphosed fine-grained feldspathic quartzose greywacke, intercalated occasionally with streaky sandy slate. The middle part is characterized by metamorphosed calcareous quartz siltstone and sandy slate interbeds, intercalated with metamorphosed fine-grained feldspathic greywacke. The upper part of the Lower sub-formation consists of thin-bedded calcareous feldspathic greywacke intercalated with sandy slate. The Lower sub-formation is 1410 m in thickness.

The lower part of the Upper sub-formation is composed of sandy slate intercalated with metamorphosed fine-grained quartz greywacke, calcareous siltstone and thin-bedded microcrystalline limestone; and thin-bedded sandy slate intercalated with clayey silty limestone in its upper part. The thickness of the Upper subformation is 833 m.

The metamorphic degree increases in the east of Shanyang County with tuff in the upper part at Linguan of Danfengzhu and Wuguanhe River; the clastic rocks increase and carbonated rocks decrease in the west of Shanyang County.

In the Jinqianhe section (Zhou Zhengguo et al., 1992) it yields a few spore fossils: Retusotriletes, Leiotriletes, Punctatisporites

The sandstone beds in the lower part of this formation extend more than 200 km to the east and west, and are near 100 m in thickness. These sandstone beds in the lower part have wedge or tabular cross-bedding and low-angle tabular oblique bedding with multistage washing planes, therefore suggest a shallow-water, high-energy environment with a sand barrier to castellated barrier character.

The metamorphosed middle and upper parts are dominated by parallel bedding, ripple lamination, and lenticular bedding, which indicate a shelf sedimentary environment.