Shuanghe Fm

Lijiang region. Named based on the Shuanghe coal (N26°35′00"; E99°50′00″), Jianchuan County, Yunnan. It was called the Shuanghe Coal Formation by the No. 12 Geological Party, Yunnan Bureau of Geology, in 1963. It is measured firstly by the First Regional Geological Survey Team of the Yunnan Bureau of Geology in 1959 and then second by the Twelve Geological Survey Team of the Yunnan Bureau of Geology in 1963. The reference section is located in the vicinity of Jiuzhiyan-Shicaijiang Village in the Shuanghe area, 8 km northwest of the seat of Jianchuan County, Yunnan Province.

[Figure: Geological map of the Jianchuan Basin, northwest Yunnan Province (Wang et al., 2020).]

[Figure: Stratigraphic column diagram of Jianchuan Basin (sample location, number and results). 1, sandy conglomerate; 2, mudstone; 3, limestone; 4, tuff; 5, coal seam; 6, marlite; 7, intrusive rock; 8, Ar-Ar age; 9, U-Pb age (Wang et al., 2020).]

Mainly grayish yellow sandstone and shale, containing coal. It is divided into two parts: Lower part, calcareous fine sandstone and gray sandy mudstone with lignite; Upper part, several beds of gray marl with 2 to 3 beds of minable bituminous coal. The thickness is 198-450 m.

[Figure: Coal-bearing strata of Jianchuan Formation: a, the Jianwetang section of Madeng Basin; the tuff layer prospect (b) and close range (c) of the Shuanghe coal mine area (Wang et al., 2020).]