Heicigou Fm

Type section of Heicigou Formation is located at the Xiaheicigou Village in Anyuan Township, Tianzhu County, Wuwei City, Gansu Province. The section lies about 21 km southwest of the Anyuan Town, or about 52 km northwest of the seat of Tianzhu County (102°38’E, 37°12’N), and was measured in 1972 by the Gansu First Regional Geological Survey Team. In the type section, the exposed part of formation is 1336 m thick. The Heicigou Formation was first published by Xiang (1981b). The name is derived from Xiaheicigou (Lower Heicigou) Village, in Tianzhu Tibetan Autonomous County, Wuwei City, Gansu Province, and was originally appeared in a 1965 manuscript by the Gansu First Regional Geological Survey Team. Originally it was called Heicigou Group by Xiang (1981b) and was lowered in rank as Heicigou Formation by Yang (1997). The nomenclator of the Heicigou Formation was referred to Song Shuhe and Zhang Luyi, 1958, by Gansu Bureau of Geology and Mineral Resources (1989), Zhou, Cao, et al. (1996), and Compiling Committee for Chinese Stratigraphic Lexicon et al (1999), and to Song Shuhe (1959) by Zhang (2009) By checking those literatures it reveals Song and Zhang or Song had not nominated the unit, either as Heicigou Group or as Heicigou Formation but the “Baiyinchang Volcanic Formation”. The unit, as Heicigou Formation, was firstly appeared in a 1965 manuscript by the Gansu First Regional Geological Survey Team, but was published firstly by Xiang (1981b) as Heicigou Group that has been commonly followed by subsequent authors. Yu Boda in Yang (1997) returned unit to Heicigou Formation that is followed in this book.

Synonym: （黑茨沟组）

The Heicigou Formation is mainly a sequence of metamorphic volcanic rocks, consisting of dark green andesitic basalt, greyish green andesitic tuff, greyish green andesitic porphyrite and purplish red andesitic agglomerate. The upper part includes interbeds of ferriferous siliceous rock and with a layer of greyish white limestone, 17 m thick, at the top of the formation.

The limestone at the top of the formation yields trilobites Amphoton sp., Kootenia gansuensis, Peronopsis sp., and Ptychagnostus sp.