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Dashuitang Formation

Dashuitang Fm

Period: 
Triassic

Age Interval: 
Late Middle Triassic (TJ48)


Province: 
Yunnan

Type Locality and Naming

Named in 1966 by the survey team of the First District of the Yunnan Geological Bureau. The named profile is located in Dashuitang Village, Mengbang, Zhenkang County, Yunnan. The Dashuitang Fm originally refers to a set of clastic rocks and carbonate rocks located between the Upper Permian basalt and the Upper Triassic Nansoba Fm (Nanshuba Fm in this lexicon usage), distributed in Luxi and Ruili, Yunnan.

Synonym:


Lithology and Thickness

It is mainly composed of gray limestone with bioclastic limestone, dark gray with flint concretions and striped argillaceous limestone with calcareous sand shale and dolomite, with purple argillaceous mudstone and conglomerate at the bottom. The thickness is 300 to 400 m.


Lithology Pattern: 
Siliceous limestone


Relationships and Distribution

Lower contact

It is disconformable contact with the underlying Upper Triassic Niuhetang Fm

Upper contact

The top is in conformity contact with the overlying Nansoba Fm (Nanshuba Fm).

Regional extent

This group is a shallow-water deposit, distributed in Baoshan, Zhenkang, and Fengqing, Yunnan, and scattered in Tengchong and Luxi. In the Baoshan- Zhenkang area, the formation is stable, but the thickness varies greatly. The thickness of the Nanding River is only tens of meters, and sometimes it becomes calcareous sand and shale. The formation north of Baoshan directly overlies the limestone of the Hewanjie Formation.


GeoJSON

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Fossils

The bivalve fossils include: Daonella sp., Neoschizodus sp., Eumorphotis sp. Halobia? sp. etc. The conodont zonation is divided into Epigondolella posteria zone and E. bidentata zone. There are also corals, foraminifera, gastropods, bryozoans and so on.


Age 

Its age was initially determined to be Middle Triassic. In 1985, Wang Zhihao and Dong Zhizhong discovered the Late Triassic Middle and Late Norian conodonts in this group. In 1990, the Yunnan Bureau of Geology and Mineral Resources set the age of this group as the Late Triassic Carnian. This lexicon is temporarily accepts this view; and assigns it to the Carnian (base constrained as Middle Carnian). Conodonts imply it extends into upper Norian (Sevatian substage).

Age Span: 

    Beginning stage: 
Carnian

    Fraction up in beginning stage: 
0.4

    Beginning date (Ma): 
233.12

    Ending stage: 
Norian

    Fraction up in the ending stage: 
0.85

    Ending date (Ma):  
208.97

Depositional setting

"Wu et al. (2020) recognized eight types of microfacies in the Dashuitang and Nanshuba formations including endoclastic breccia limestone. They concluded that the Dashuitang Formation was deposited on a deep slope, and the overlying Nanshuba Formation was a deep-water shelf." (From " Carbon-isotope excursions in the Norian Stage (Upper Triassic) of the Baoshan terrane, South China " submission by Xin Jin et al., 2022.)


Depositional pattern:  


Additional Information


Compiler:  

Zhang Shunxin, Ji Kaixuan and Tong Jinnan.