Pingliang Fm

The type section is near Village Yindongguan, 8 km southwest of Pingliang City, Gansu. The Pingliang Fm is derived from the Pingliang Shale proposed by Yuan Fuli in 1923. In 1972, Gansu Party of Regional Geological Survey suggested that the Pingliang Shale is actually composed of shale and limestone, and rests conformably on the Sandaogou Fm and is disconformably overlain by the Permian, and renamed it the Pingliang Fm. However, considering the incomplete type section of the Pingliang Fm and the fact that the Pingliang Fm contains more limestone and small conodonts such as Microcoelodus and Erismodus than the “Longmendong Fm”, An et al. (1990) suggested that the Pingliang Fm was deposited in shallower environment than the Longmendong Fm did, and these two formations should be maintained. In Longxian, the strata equivalent to the Pingliang Fm was previously named the Longmendong Fm (Chen et al., 1984), and is in conformable contact with the underlying strata, with a completely exposed sequence intercalated tuffaceous sandstone. Considering the similar lithology of the Longmendong Fm and the Pingliang Fm, Fu et al. (1993) suggested that these two formations are equivalent.

Synonym: (平凉组)

Reference Section:

The reference section is at Longmendong in Xinji, 26 km northwest of Longxian, Shaanxi (Fu, 1977). It is 225.6 m thick.

The formation is dominated by black, dark gray, yellowish green calcareous, muddy and silty shale intercalated with many thin limestone beds and lenses, with calcirudite in the Lower-middle part, and green siltstone and sandstone in the middle-Upper part.

In Longxian, the Pingliang Fm includes five graptolite zones, i.e. Husterograptus teretiusculus zone, Nemagraptus gracilis zone, Climacograptus peltifer zone, Pseudoclimacograptus scharenbergi minor zone and Climacograptus geniculatus zone, in ascending order, and three conodont zones, i.e. Pygodus serra zone, Pygodus anserinus zone and Yaoxianognathus neimengguensis zone, and is equivalent to the Husterograptus teretiusculus zone to Dicranograptus clingani zone. The Pingliang Shale is incomplete, and yields abundant graptolite, such as Husterograptus teretiusculus, Nemagraptus gracilis. On the basis of the study on the Longmendong section in Longxian, Shaanxi, Fu (1977) considered the Pingliang Fm to be equivalent to the Husterograptus teretiusculus zone to Dicranograptus clingani zone consisting of shale, calcirudite and sandstone. Li et al. (1983) reported and described the conodonts. In 1984, Chen Junyuan et al. named the strata equivalent to the Pingliang Fm in Longmendong, Longxian, Shaanxi the Longmendong Fm, which was followed by Shaanxi Bureau of Geology and Mineral Resources (1989) and was widely applied to the strata in Longxian-Qishan.