Sanqiao Fm

The type section is located ∼3 km west of Sanqiao in Guiyang City, Guizhou. It was named as Sanqiao Limestone and considered as Permian by Ding Wenjiang and Ge Lipu in 1936. In 1937, Yin Zanxun believed that the Sanqiao Limestone was located under the Retike Coal Measures and its age should be the Late Triassic. According to the brachiopod found in 50 m thick sandy shale, Xu Deyou in 1940 suggested that the Sanqiao Limestone was divided into two parts of a Upper Sanqiao Limestone of Norian and Lower Sanqiao Limestone of Carnian. According to the original meaning of Sanqiao Limestone in 1962, Yin Hongfu attributed its age to the Ladinian of the Middle Triassic, which should specifically refer to the thick layer of dolomite underlying the above-mentioned fossil-bearing strata. In 1963, Wang Yu et al. renamed the Sanqiao Formation and redefined the Sanqiao Formation, which specifically refers to the fossil-bearing sandy shale intercalated with limestone. This definition was generally accepted by later researchers.

Synonym: Sanchiao Fm

The Formation, 78 m thick, is mainly composed of clastic rocks and carbonate. It can be divided into four lithologic segments in ascending order. First member (lower sandstone) is the calcareous mica sandstone interbedded with sandstone, with a small amount of calcareous mudstone and shale, and intercalated with limestone lens in the upper part. Second member (lower limestone) consists of gray and fleshy red medium to thick-bedded limestone and bioclastic limestone, and the upper part is marlstone. Third member (upper sandstone) is yellowish gray, yellowish green medium to thick quartz sandstone interbedded with argillaceous sandstone, sandy mudstone and shale. Fourth member (upper limestone member) consists of a upper unit of gray oolitic and bioclastic limestone, and an upper unit of grayish-black thin-bedded micrite with chert nodular bands. The sandstone of this formation is abundant with parallel bedding, diagonal bedding and ripple marks. Thickness varies from 0 to 93 m.

This formation yields abundant bivalves with Costatoria kweichowensis-Cassianella beyrichii bivalve assemblage in the lower part and Burmesia sp. in the upper part. Other fossils include brachiopods, coral, plant fragments and dinosaur bones.

It is interpreted as a shallow water environment.