Guniutan Fm
Type Locality and Naming
The type section is near Guniutan south of Fenxiang Town 16 km northwest of Yichang City, Hubei. The reference section is at Huanghuachang 5 km northwest of Yichang City, Hubei (Zeng Qingluan et al., 1983). The Guniutan Fm is derived from the Guniutan Limestone proposed by Zhang Wentang in 1957. Zhang (1962) renamed the Guniutan Limestone the Guniutan Limestone Fm, which was in turn renamed the Guniutan Fm (Zhang, 1964).
Synonym: (牯牛潭组）
Lithology and Thickness
The formation is dominated by a sequence of light purple, yellowish green or gray thin- to medium-bedded hydromica micritic limestone with common nodular texture. It has a recorded thickness of 2-74 m.
Relationships and Distribution
Lower contact
Upper contact
It is disconformably overlain by the mudstone at the base of the Miaopo Fm
Regional extent
It is exposed at Ganxi (24 m thick) in Yanhe and Heshui (74 m thick) in Yinjiang, northern Guizhou, southeastern Sichuan (21-36.8 m thick), central Sichuan and southwestern Sichuan (53.7 m thick), southern Shaanxi (2-8.7 m thick), most Hubei (12-30 m thick), Shimen (35.6 m thick) of northwestern Hunan and Chuxian (5.9 m thick) of Anhui. The Guniutan Fm is intercalated with minor shale around Shennongjia, Hubei, with manganiferous limestone in Yicheng Guanshan, and consists of oolitic limestone and limestone in northern Guizhou to southern Sichuan. The lithology of the Guniutan in other area is similar to that of the type area.
GeoJSON
Fossils
At Three Gorges area, the Guniutan Fm yields cephalopods of Dideroceras wahlenbergi zone, Ancistroceras zone and conodonts of Amorphognathus variabilis zone, Pygodus serrus zone, and is roughly equivalent to the Didymograptus artus zone to Didymograptus murchisoni zone.
Age
Depositional setting
Zeng (1991) suggested that the water depth of sedimentary basin in which the Guniutan Fm was deposited is similar to the depth indicated by the ecological assemblages BA4 and BA5 of benthic brachiopod. An outer continental shelf environment corresponding to the ecospace of BA5, however, was suggested by Zhou et al. (1993) based on the cephalopod assemblage characterized by Dideroceras-Protocycloceras.
Additional Information