Niuerchuan Fm
Type Locality and Naming
The Niuerchuan Fm is the lowest formation of the Zhashui Gr. The type section is located between Jiangjia Courtyard and Wulongzhai of Wuwanggou Village of Niu’erchuan Town, Shanyang County. The reference section is at Xiaohongling of Xiaotai Town to Huotai of Xingping Town in Zhashui County, Shaanxi. It was named by Shaanxi Team of Regional Geological Survey during measured Xiaohekou Geological Map (1:50,000) in 1968, and Du Dinghan et al. quoted formally in 1986.
Lithology and Thickness
Siltstone. Lower part is composed of gray to light brownish-gray metamorphosed quartz siltstone and feldspathic quartz siltstone, intercalated with calcareous quartz sandstone and feldspathic quartz greywacke. Middle part is silty slate and calcareous silty slate, intercalated with metamorphosed fine-grained feldspathic quartz greywacke and quartz siltstone, and occasionally intercalated with silty crystalline limestone. Upper part is silty slate, intercalated with metamorphosed quartz siltstone and a few metamorphic fine-grained feldspathic quartz sandstones. The formation is more than 3519.3 m thick in total.
Relationships and Distribution
Lower contact
Its base is not exposed.
Upper contact
The top of the formation is a conformable contact to the metamorphosed fine-grained quartz sandstone of Chigou Fm.
Regional extent
This formation is distributed in Zhouzhi, Shanyang, Shangnan area of south side of the Zhashui Group.
GeoJSON
Fossils
No mega fossils have yet been found in this formation. A few fossil spores: Leiosphaeridium, Punctatisporites, Trematosphaeridium in Dongzigoukou of Chefanggou; and Aneurospora greggsii, Veruciretusispora magnifica var. magnifica, Samarisporites inaequus etc. have been reported in middle and upper parts of Niuerchuan Fm in Jinqianhe section by the Xi’an Institute of Geology and Mineral Resources (1976-1977) and Zhou Zhengguo et al. (1992).
Age
Depositional setting
In east of Shanyang, cross-bedding, ripples, drying crack and Skolithos facies trace fossils occur, which indicates an intertidal deposit environment; while in the west of Shanyang, horizontal-bedding markers are characteristic, which would indicate a weak hydrodynamic neritic shelf deposit environment.
Additional Information
Because its base is not exposed, the Niuerchuan Fm in Wuwanggou is only equivalent to the middle and upper parts of the Xiaotai to Huotai section. Its outcropping thickness is more than 1773 m. To the east, the metamorphic grade increases, and in Luoyugou area of Shanyang there are more intercalations of clayey dolomite, dolomite and marble interbeds; and to west, the content of clastic rock increases and carbonate rock decreases.