Shuangqiaoshan Gr

Northwesternmost Jiangxi. The Shuangqiaoshan Gr was named by Wang, C. C. (Wang Zhuquan) in 1930. The type locality is in Shuangqiao Mt., north of Wuning County, Jiangxi Province. The Xiaruan Fm and Pishanqiao Fm are regarded as its synonyms.

Synonym: （双桥山群）

The Shuangqiaoshan Gr is subdivided into five formations, and the description for each formation from top to bottom is as follows.

Xiushui Fm, the uppermost formation, was named by Kang Yuyi in 1962. The type section is located at Dakeng, Yining, Xiushui County, Jiangxi Province. It is composed of interbedded grey-green, grey-black thin-bedded tuffaceous slate with silty slate, tuffaceous sandstone. It is 2500 m thick, sandwiched with pebbly sandstone. It is in unconformable contact with overlying Luokedong Fm or Dengshan Gr and conformable (?) contact with underlying Anlelin Fm, separated by a basal conglomerate. The formation is distributed in Xiushui River drainage area, on the north bank of Poyang Lake and northeastern Jiangxi Province. The Xinmin Fm, Shicheng Fm, Wushijian Fm and Zhujia Gr are thought to be synonyms of the formation.

Anlelin Fm was named by the Jiangxi Party of Regional Geological Survey in 1964. The type section is in Anlelin Village, Chengxi Township, Wuning County, Jiangxi Province. The Li'an Fm, Jiuling Gr and Sancha-gang Fm are thought to be synonyms of the Anlelin Fm. It is composed of interbedded grey to dark-grey calcareous sandstone, sericite chlorite slate with carbonaceous tuffaceous slate. The tuffaceous sandstone increases in upper part, occasionally sandwiched with spilite and quartz keratophyre. Its thickness varies from 1258 m to 3735 m. It is in conformable contact with underlying Jilin Fm. It is widespread in Xiushui, Wuning and Duchang counties. It is [was?] attributed to the Jixian Period.

Jilin Fm was named by Ma Changxin in 1984. The type section is in Jilin, Boyang County, Jiangxi. Synonym is Jilinzu Fm. It is composed of purple-red thin-bedded banded slate, silty slate, fine blasto-sandstone, sandwiched with grey-green slate, fine-grained sandstone, siltstone and minor spilite and quartz keratophyre. Its thickness varies from 570 m to 1541 m. It is in conformable contact with underlying Hengyong Fm. The formation is distributed in Xiushui River drainage area, Boyang, Wuning and Guayuan Counties. It is [was?] attributed to the Jixian Period.

Hengyong Fm was named by the Jiangxi Party of Regional Geological Survey in 1970. The type section is in Hengyong, Boyang County, Jiangxi. The formation is dominated by grey, dark-grey metamorphosed tuffaceous sandstone, silty slate and black banded slate, intercalated with siltstone and carbonaceous slate, occasionally with spilite and keratophyre. The thickness varies from 350 to 2000 m.

Zhanggongshan Fm, the lowermost formation, was named in 1962 by the Anhui-Zhejiang-Jiangxi Party of Stratigraphy. It was originally named Zhanggongshan Gr. The type section is in Zhanggong Mountain, Wuyuan County, Jiangxi Province. It occurs in Zhanggong Mountain, located at the juncture between Jiangxi and Anhui provinces. It is composed of quartz schist in lower part, and the upper part is phyllite intercalated with siliceous slate, spilite and keratophyre. The thickness varies from 350 to 3750 m.

[Figure: The column of Hengyong Fm, Jilin Fm (Jilinzu Fm) and Anlelin Fm of the Shuangqiaoshan Gr. a) The depositional facies in the Hengyong Fm.; b) Sampling locality of tuff at the Anlelin Fm.]

The Anlelin Fm contains microplants: Trachysphaeridium, Protoleio sphaeridium, Leiopsophosphaera, Zonosphaeridium, Dictyosphaera. The Jilin Fm has rich microplants: Leiominuscula, Trachyminuscula, Trematosphaeridium etc. Microplants in Hengyong Fm include Trachysphaeridium, Lophosphaeridium, Leiominuscula, Protoleiosphaeridium, Zonosphaeridium, Asperatopsophosphaera etc.

SHRIMP U-Pb age of 831±6 Ma in the Hengyong Formation and 829±6 Ma in Anleling Formation (Gao Linzhi et al., 2008)