Baota Fm
Type Locality and Naming
The type section is at Aijiashan (Leijiashan) near Longmaxi 3 km east of Xintan in Zigui, Hubei. The Baota Fm is derived from the Baota Limestone proposed by Li Siguang (J.S. Lee) and Zhao Yazeng (Y.T. Chao) in 1924.
The Baota Limestone had previously been referred to several meters thick limestone yielding abundant Orthoceras chinense (actually Sinoceras chinense) in the upper part of the Aijiashan Series. It was thought to be between the Silurian Longma Shale (including the Wufeng Fm) and the Ordovician Triplesia Bed (Yangtzeella Bed), and to be Caradoc or Early Ashgill in age. In 1945, Wang Yu restricted the Aijiashan Series to the strata underlying the Baota Limestone, and replaced the “Baota Limestone" with the “Matishi Limestone" proposed by Ding Wenjiang (V.K. Ting) (1929). In 1954, Yang Jingzhi subdivided the Baota Limestone into the Linxiang Fm in the upper part and the Baota Limestone in the lower part. The latter is the definition of the present Baota Fm, which is renamed the Baota Limestone Fm by Zhang Wentang in 1954. After 1974, the term Baota Fm was widely accepted.
Synonym: (宝塔组); Pagoda Fm
Reference Section:
The reference section is at Huanghuachang 5 km northwest of Yichang City, Hubei (Zeng Qingluan et al., 1983).
Lithology and Thickness
The formation is composed of light purple medium-bedded micrite with polygonal cracks, intercalated with very thin yellowish green shale. In the type area, the Baota Fm is 12 m thick.
Relationships and Distribution
Lower contact
It rests conformably on the dark gray, yellowish green shale at the top of the Miaopo Fm. In north-central Guizhou (Ordov column 43) it overlies the Shizipu Fm.
Upper contact
It is conformably overlain by the gray, greenish gray medium-bedded nodular marl at the base of the Linxiang Fm. In most exposure areas, this formation is conformably overlain by the strata above; but in the margin of the paleocontinent such as central Guizhou, northeastern Sichuan and southern Shaanxi, it is disconformably overlain by the Wufeng Fm and the Silurian.
Regional extent
Generally, this stratigraphic unit is 10-20 m thick, and changes little in lithology and thickness. It is widespread in Hubei, with a thickness of 4-15 m; eastern Sichuan and southern Shaanxi, 9-15 m thick; southern Sichuan and northern Guizhou, 9-20 m thick; eastern Yunnan, 9 m thick; Hexian and Susong of Anhui, 7-15 m thick. In northern Guizhou and eastern Sichuan, the Baota Fm rests on the Shizipu Fm including Gymnograptus linnarssoni zone, and its base is slightly lower than that in type section.
GeoJSON
Fossils
The Sinoceras chinense acme zone of cephalopod, Paraphillipsinella globosa range zone of trilobite and Hamorodus cf. europaeus zone of conodont have been distinguished in the Three Gorges district. These biozones can be correlated with the graptolite D. clingani zone to P. linearis zone.
Age
Depositional setting
Additional Information