Sandu Fm

The type section of the Sandu Formation is the Sandu section lying on the riverside of the Duliujiang River. It was measured between Zhalagou and Sanhe Town, about 2 km east of Sanhe Town, the seat of Sandu Shui Autonomous County. Except for the thickness and fossils of the formation, no lithology was described by Qian (1961) when he first publishes the formation. A better section measured at the Paizhao Village in Sandu County was selected as representative section of the formation (117°54’E, 26°06’N) by Guizhou Bureau of Geology and Mineral Resources (1987). The Paizhao section lies about 6 km north of the Sanhe Town, and was measured by Cai Ying and Yin Gongzheng and others from Guizhou Regional Geological Survey Team in 1983. The Sandu Formation was first published by Qian (1961). The name is derived from Sandu Township, Sandu Shui Autonomous County, Qiannan Buyi-Miao Autonomous Prefecture, Guizhou Province. The name was originally appeared in a 1959 manuscript by Yang Jingzhi, Qian Yiyuan and colleagues from Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. The formation was said to be first named as Sandu Shale by Lu (1954) by a number of authors (e.g., Guizhou Bureau of Geology and Mineral Recourses, 1987; Dong, 1997; Zhu in Compiling Committee for Chinese Stratigraphic Lexicon et al., 1999, Guizhou Institute of Geological Survey, 2017). It is untrue as Lu had never named the Sandu Shale. In his 1954 paper, he just mentioned his fossils (6 taxa) were “collected from the greyish yellow, carbonaceous shale, about one meter thick, at the highway side, 2 km north of Sandu (Sanhe) County Town.” Those fossils were called by Lu (1954) as Sandu trilobite fauna rather than a lithological unit. Actually, the name was first published by Qian (1961).

Synonym: （三都组）

The Sandu Formation is essentially a carbonate sequence with interbeds of clastic rocks, In the Paizhao section, the Sandu Formation is roughly divided into 4 parts, from bottom up: Basal part consists of grey, thin to medium-bedded limestone, micritic limestone, and marlstone, with interbeds of medium-bedded brecciated limestone and minor of yellow carbonaceous shale, and with a layer of thick to massive brecciated limestone at base (94.91 m thick); Lower part consists of grey, thin-bedded micritic limestone, and greyish green, thin-bedded shale, with minor interbeds of yellowish green shale and with a layer of grey, massive sandy brecciated limestone, 1.5 m thick, at the base (68.83 m); Middle part consists of greenish grey, thin-bedded marlstone, with a layer of grey, thick-bedded brecciated limestone, 1 m thick, at the base (153.45 m); Upper part consists of greenish grey, bluish grey, thin-bedded marlstone and bluish grey, greenish grey carbonaceous mudstone (117.44 m). In this section, the formation is 434.63 m thick.

In the Paizhao section, the basal part of the Sandu Formation yields trilobites Charchaqia cf. sanduensis, Hedinaspis regalis, Lotagnostus sp., Jegorovaia cf. expansa and Pseudagnostus sp.; the lower part yields trilobites Acrocephalina cf. aruta, Guizhoucephalina sp., Euloma (Proteuloma) sp., Jegorovaia sp., Pseudagnostus sp., and Sanduspis sp.; the middle part yields trilobites Euloma (Proteuloma) sp., and Jegorovaia sp.; and the upper part yields trilobites Charchaqia sanduensis, Macropyge (Promacropyge) sp., Palaeoharpes sp., and Sanduspis sp.