Nanjinguan Fm
Type Locality and Naming
The type section in at Nanjinguan about 5 km southeast of Yichang City, Hubei. The reference section is at Huanghuachang 5 km northwest of Yichang City (Zeng et al., 1983).
Synonym: (南津关组）
Lithology and Thickness
The Nanjinguan Fm is divided into three members. Lower member is dominated by thick-bedded limestone intercalated with bioclastic limestone and local oolitic limestone, with calcareous mudstone or very thin shale at the base. Middle member is represented by light gray thick-bedded dolomite. Upper member is characterized by gray medium- or thick-bedded ooid-bearing calcirudite, calcarenite and bioclastic limestone, with siliceous and dolomitic bands. In the type area, the Nanjinguan Fm is 154 m thick.
Relationships and Distribution
Lower contact
The Nanjinguan Fm rests conformably on the dolomitic limestone of the Sanyoudong Gr (or regionally, also the dolomite of the Loushankuan Fm) of late Cambrian.
Upper contact
It is conformably overlain by the yellowish green shale at the base of the Fenxiang Fm. In 1983, Zeng Qingluan found a 45 m thick calcareous limestone between the previously defined Nanjinguan Fm and the Fenxiang Fm in Huanghuachang, Yichang, and drew the upper boundary of the Nanjinguan Fm at the base of the bioclastic limestone without banded chert. On the lithological features, the present book raises the upper boundary of the Nanjinguan Fm 45 m at the base of the yellowish green shale, i.e., the base of the Fenxiang Fm
Regional extent
The Nanjinguan Fm is widespread in the middle and upper Yangtze regions. The Shang' ouchong Fm (Zhu Zhaoling et al., 1979) consisting of limestone in the lower part and dolomitic limestone in the upper part in Chuxian, Anhui is lithologically similar to the Nanjinguan Fm. The strata of the Nanjinguan Fm had previously been referred to the lower part of the Yichang Limestone, between the underlying Sanyoudong Limestone and overlying Cretaceous red beds, and had previously been thought to be equivalent to Yichang formation of Yang Jingzhi et al. (1954) and Yichang Bed of Zhang Wentang et al. (1957).
GeoJSON
Fossils
In eastern Gorge area, the Nanjinguan Fm includes conodont Cordylodus angulatus zone, Acanthodus costatus zone and the lower and middle of Glyptoconus quadraplicatus-Paltodus deltifer zone; graptolite Dictyonema yichangensis zone; trilobite Asaphellus inflatus-Dactylocephalus dactyloides zone and Asaphopsis immanis zone, in ascending order (Wang et al., 1992). Additionally, chitinozoan Lagenochitina cf. destombesi, brachiopods and cephalopods have been found from the Nanjinguan Fm.