Yanyuan Fm

The naming section is located at Caigoutou (E101°30′18″, N27°25′57″), Ganhaizi Township, Yanyuan County, Liangshan Yi Autonomous Prefecture, Sichuan. Named by Chang Longqing in 1937.The section is measured by the First Regional Survey Team of Sichuan Bureau of Geology in 1971.

Primarily semi-mature, gray, lignite-bearing muddy shale. The lithology and thickness vary from different places. In the Yanyuan basin, this formation is mainly gray claystone, with up to several dozen lignite beds and sand and gravel lenses; on the basin margins sandy conglomerate increases and coal beds decrease. The thickness is generally 300-500 m and is up to 640 m in the Maiyu-Heshao area in the basin center. In the Zhongpuxiong basin, it is grayish white or dark gray mudstone and siltstone with several beds of lignite, being 40 m thick. In the Butu basin, it is marked by yellow sandstone with grayish purple, sand-bearing claystone lenses in the lower part and yellowish brown conglomerate with greenish gray or grayish black shale and thin lignite beds in the upper part, being 148 m thick.

The fossil mammals total five genera and species, namely, Sinomastodon yanyuanensis, Stegodon cf. elephantoides, Chilotherium sp., Axis sp. and Cervavitus sp. The sporopollen is mostly Polypodium and Unatextisporites.