Gaoqiao Fm

Type section of Gaoqiao Formation is the Guanyinba-Qianhekou section, measures along the side of the highway between the Guanyinba and Qianhekou villages in Gaoqiao Township, Ziyang, Ankang, Shaanxi Province (108°25’00” E, 32°27’36” N). The section was measured by Xu Baozheng and colleagues in 1970’s, and was remeasured by the Shaanxi Regional Survey Team in 1988. The Gaoqiao Town lies about 13 km southwest of the seat of Ziyang County. The Gaoqiao Formation was named by the Compiling Group for Shaanxi Regional Stratigraphic Scale (1983). The name is derived from Gaoqiao Town, close to the estuary of Quanhe River, Ziyang County, Ankang City, southern Shaanxi Province. Originally the Gaoqiao Formation was appeared in a 1977 manuscript by Xu Baozhen and colleagues from Shaanxi Regional Geological Suryey Team. This formation spans the Cambrian-Ordovician boundary. The name actually was preoccupied at first by the Gaoqiao Formation named by the Jiangsu Regional Geological Survey Team of Heavy Industry Bureau (1970) for an Ediacaran formation in Jiangsu Province, but the Jiangsu’s Gaoqioa Formation is now abandoned because it is a junior synonym of the Sujiawan Fm.

Synonym: （高桥组）; Gaoqiaozhen Fm

The Gaoqiao Formation is an epimetamorphic clastic sequence, and can be roughly divided into two parts. Lower part consists of mainly grey or dark grey slate with a few of interbeds of calcareous slate or striped argillaceous limestone. Upper part consists of mainly green slate, greenish grey striped calcareous slate with a few of interbeds of micritic limestone. In the type section, the formation is 1014.1 m thick.

The formation yields graptolites Tetragraptus quadribrachiatus, Tetragraptus sp. from the upper part and trilobites Geragnostus sp., Hysterolenus sp., and Niobella sp. from the lower part.