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Bikou Gr

Bikou Gr

Period: 
Neoproterozoic

Age Interval: 
Qingbaikouan (Tonian), Qb （39, 40, 41）


Province: 
Gansu, Shaanxi, Sichuan

Type Locality and Naming

Southeast corner of Gansu. The Bikou Gr was named by Ye Lianjun (L.T.Yeh) and Guan Shicong (Kwan S.C.) in 1944. The typical section is located in Bikou of Wenxian County, Gansu Province.

Synonym: （碧口群） , Beikou Gr


Lithology and Thickness

The redefined Bikou Gr includes, in ascending order, the Yangba Fm, the Baiyang Fm and the Yangtianba Fm.

The Yangba Fm (and the correlative Guihuaqiaogou Fm in north Sichuan) is subdivided into two parts. Lower part consists of silty phyllite, sericite-phyllite and meta-siltstone intercalated with chloritold meta-sandstone. Upper part is composed of greenschist, metamorphic acidic tuff intercalated with meta-andesite, basalt, phyllite and meta-sandstone, bearing intercalations of ferruginous quartzite, white dolomite and purple jasper bands. The Yangba Fm (Guihuaqiaogou Fm) is over 8000 in thick.

The Baiyang Fm is composed of light green to gray metamorphic sandstone, sandy pebbly phyllite, sericite-phyllite intercalated with meta-conglomerate, meta-basalt and greenschist, with a thickness more than 7151 m.

The Yangtianba Fm (and the correlative Yinping Fm in north Sichuan) is composed of light green to gray metamorphic siltstone, silty slate, medium-to fine-grained lithic sandstone intercalated with lenticular meta-conglomerate, with Bouma sequence developed. A thickness of 2177 m.

In Sichuan Province, the Bikou Group is subdivided, from bottom to top, into Dashaba Fm, Guihuaqiaogou Fm and Yinping Fm. For details on the Yinping Fm, please see the Neoproterozoic System of Stratigraphic Lexicon of China.


Lithology Pattern: 
Sandstone


Relationships and Distribution

Lower contact

Base is not seen.

Upper contact

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

Regional extent

Based on the investigation data in recent years, the Bikou Gr is dominantly distributed in the border of Gansu, Shaanxi and Sichuan Provinces, including Wenxian, Bikou (Gansu Province), between Mianxian and Yangpingguan in Lüeyang (Shaanxi Province), Pingwu-Qingchuan district (Sichuan Province).


GeoJSON

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Fossils

The basal conglomerate of the Yangtianba Fm was found to yield microplants, which is considered to belong to the Qingbaikouan age (or to the Nanhuan era, by some researchers),


Age 

Until now, the age of the Bikou Gr is still in dispute. The isotopic ages determined from different areas include 1044± 22 Ma (metamorphic age, by K-Ar dilution method) and 1565±16 Ma (by Ar-Ar method), the zircon U-Pb age of 1367 Ma and a metamorphic age of 970±12.4 Ma (by Rb-Sr method) are acquired from the Guihuaqiaogou Fm in Pingwu-Qingchuan district in north Sichuan Province. Recently, a LA-ICP-MS zircon U-Pb date of 932-723 Ma was measured by Su Lianguo et al., 2019. All of the above-mentioned age data support that the sequences are below the Yangtianba Formation are Qingbaikouan (Tonian). The Yangtianba Fm is considered by most to belong to the Nanhuan System, representing the lower tillite of the glacio-fluvial to gravity flow deposit (Zhao Xiangsheng et al., 1990) or belong to the Qingbaikouan System by some researchers. [Schematic stratigraphic column shows Bikou Gr, including Yangtianba Fm, as spanning entire Qingbaikouan, with lowermost Nanhuan absent – used here for graphic column also.]

Age Span: 

    Beginning stage: 
Tonian

    Fraction up in beginning stage: 
0.0

    Beginning date (Ma): 
1,000.00

    Ending stage: 
Tonian

    Fraction up in the ending stage: 
0.8

    Ending date (Ma):  
776.00

Depositional setting

Bouma sequence developed.


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.