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Yangtianba Formation
Click to display on map of the Ancient World at:
Yangtianba Fm base reconstruction

Yangtianba Fm

Period: 
Neoproterozoic

Age Interval: 
late Qingbaikouan (late Tonian), Qb (39)


Province: 
Gansu

Type Locality and Naming

Southeast corner of Gansu. The Yangtianba Formation was named by the West Qinling Geological Team of the Comprehensive Geological Party of the Gansu Bureau of Geology (1964), which was published by the Gansu Compiling Group for the Regional Stratigraphical Scale (1980), the Regional Stratigraphic Scale of North China. The type section is located in Yangtianba, Wenxian County, Gansu Province. Lowest formation in the Bikou Gr.

Synonym: （秧田坝组）


Lithology and Thickness

Southeast corner of Gansu. The Yangtianba Fm consists of a series of low-grade metamorphosed terrigenous clastic flysch units. The particle size of clastic rock is upward coarsening. Lower part, with a basal conglomerate, consists of light green-light gray-green metamorphosed lithic sandstone, metamorphosed lithic arkose, meta-greywacke and meta-siltstone, sandy pebbly phyllite, silty phyllite and sericite phyllite, forming various thin inter-beddings with meta-conglomerate and meta-basalt and chlorite schist lenses. Upper part is medium to gray to dark gray fine-grained meta-lithic sandstone, interbedded with silty slate, sericite phyllite and lenses or irregular masses of conglomerates. Bouma sequences are well developed. The thickness of this formation changes considerably from 0 to 5000 m thick.


Lithology Pattern: 
Sandstone


Relationships and Distribution

Lower contact

The formation is disconformable or unconformable with underlying Baiyang Fm of the middle Bikou Gr (see Bikou Gr for details)

Upper contact

The top is unconformably overlain by the Guanjaigou Fm tillite of Cryogenian (or the Guangou Fm of early Cambrian)

Regional extent

Southeast corner of Gansu. The formation is distributed south of Shangde, Wenxian County to Douba, Kangxian County and in the north of Hengdan, Wenxian County to Sanheba, Kangxian County, and Luotang, Yaodu, Wudu County.


GeoJSON

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Fossils

Microplants are found in slate of the upper part, such as: Trachysphaeridium dengyingense, T. simplex, Leiofusa nauiculara, Lophominuscula sp., Bavlinella faveolata, Paleamorpha punctulata etc.


Age 

See age discussion for Bikou Gr, and below in Depositional Setting. Late Qingbaikouan (late Tonian) in schematic stratigraphic column.

Age Span: 

    Beginning stage: 
Tonian

    Fraction up in beginning stage: 
0.55

    Beginning date (Ma): 
846.00

    Ending stage: 
Tonian

    Fraction up in the ending stage: 
0.8

    Ending date (Ma):  
776.00

Depositional setting

There are currently different opinions on the facies and age of this formation: (1) It is interpreted as the turbidite-flysch formation as a very thick suite of glaciomarine sediments corresponding to the lower tillite of Yangtze region, with the age of Nanhuan System, belonging to the cover strata above the Bikou Gr (sensu stricto) (Qin Keling et al., 1990); (2) It is interpreted as a younger suite of terrigenous clastic rocks dominated by immature terrigenous clasts, with the same age as the Bikou Gr of the Qingbaikouan Period (Li Yaomin et al., 1991).


Depositional pattern:  


Additional Information


Compiler:  

Extracts from The Neoproterozoic Erathem (chapter in Stratigraphic Lexicon of China, draft of 2022) by Gao Linzhi, Ding Xiaozhong, Zhang Chuanheng, Zhang Heng.