Mingshan Fm

The naming section is located in the Dongcaogou-Yuguangpo area, Chengdong Township, Mingshan County, Sichuan. Named the Mingshan Group by the Second Regional Geological Party, Sichuan Bureau of Geology, in 1975 and renamed the Mingshan Formation in 1982.

The formation is divided into two members. The Lower member, called the Jinjiguan Member, is subdivided into three beds (beds 1–3), 123 m thick, and consists of brownish red and dark brown, thick-bedded, muddy siltstone with fine sandstone, and locally there appears brownish red mudstone with dark gray marlstone. Its base is dominated by sandy conglomerate with muddy siltstone, which is called the “Zhuyaozi sandstone” in the vicinity of Qionglai and Jiaguan and the “Jinjiguan sandstone” in Mingshan and Ya’an. The basal conglomerate forms steep slopes and has a distinct boundary with mudstone at the top of the underlying Guankou Formation. It thickens progressively westward: it is 450 m thick in the Lushan syncline and 680 m thick at Laochang, Tianquan. The Upper member, called the Yuguangpo Member, i.e. beds 4–9, is 370 m thick. Its lower part is represented by alternating beds of brownish red siltstone and mudstone with 3–4 beds of grayish black mudstone-shale at the base, which separate the member from the Jinjiguan Member. The middle and upper parts are brownish red and purplish red siltstone and mudstone with dark brown brecciated mudstone and grayish green, lime breccia (a gypsum and glauberite bed is represented in the well). The upper part is brownish red mudstone with minor muddy siltstone and grayish black mudstone-shale. The Yuguangpo Member has been almost all eroded east of Hongya; only ~150 m of it is left in the Meishan-Jiajiang syncline; and it is completely preserved between Ya’an and Hongya, being 400–500 m thick. With Lushan and Mingshan as the lake basin center, the formation has a tendency to become coarse and thick.

Fossil ostracods are contained in the Jinjiguan Member. Fossils include charophytes, Estheria, and the ostracod assemblage represented by Sinocypris-Eucypris in the Yuguangpo Member.

From the lower member (Jinjiguan Member) to the upper member (Yuguanpo Member), the formation shows the upward-fining sedimentary cycle, belonging to the lacustrine-saline lacustrine deposits.