Xiuzigou Fm

The type section of the Xiuzigou Formation is located at Xiuzigou, about 9.5 km southwest of the Shengwan Town, Xichuan County, southwestern Henan Province (111°18’E, 32°53’N). It was remeasured by Henan Regional Geological Team in 1983. The Xiuzigou Formation was named by Southern Henan Regional Geological Survey Team of Beijing College of Geology (1961). The name is derived from Xiuzigou Village (秀子沟 in Chinese characters, which are mistakenly written as 袖子沟) in Shengwan Township, Xichuan County, Nanyang City, southwestern Henan Province. The formation spans Cambrian-Ordovician boundary.

Synonym: （袖子沟组）; As reported by Du et al. (2015), the Institute of Geology of Henan Bureau raised in rank the Xiuzigou Formation (袖子沟组) to Xiuzigou Group (袖子沟群, with correct Chinese characters for the unit but incorrect for the geographic name) in a 1962 unpublished report, Yan Guoshun et al. also in a 1988 unpublished report, erected the Xiuzigou Group (秀子沟群, with correct Chinese character for the geographic name), and Liu et al. (1991) erected Xiuzigou Formation (秀子沟组 lower in rank for the group of Yan et al.) for the strata in the same area and almost the same interval of the original Xijiadian Formation. All those names are synonyms of Xiuzigou Formation (袖子沟组), either in Chinese characters or in Hanyu Pinyin (the same as in English).

The Xiuzigou Formation is a carbonate sequence with top part unexposed. In type section, the formation can be roughly divided into three parts. Lower part consists of grey, dark grey, medium- to thick-bedded limestone, dolomitic limestone, argillaceous limestone, and sandy limestone, occasionally intercalated with fine-grained dolomite; Middle part consists of greyish white, micritic limestone, argillaceous striped limestone, and micritic dolomite. The bedding surface convers argillaceous film in red color; Upper part consists of grey, thick-bedded limestone with chert nodules or bands, laminated limestone with argillaceous nodules, and fine-grained dolomite. In the type section, the Xiuzigou Formation is 1132.8 m thick.

In the type section, the base of the Xiuzigou Formation yields trilobites Abharella sp., Eokaolishania sp., Shengia elongate, S. sichuanica and unidentified damesellids; the middle part yields unidentified mansuyiid trilobite; and the exposed top part yields conodonts “Acodus”oneotensis, Acontiodus staufferi, Acontiodus aff. iowensis, Chosonodina herfurthi, Drepanodus sp., Drepanodus subarcuatus, Glyptoconus? concavus, Scolopodus triplicatus, Scolopodus restrictus, Scolopodus xiutigouensis, S. cf. S. filosus, S. cf. S. triplicatus, and Teridontus sp.