Yaosuo Fm
Type Locality and Naming
The lectostratotype is located at Sifangpo - Wuliqiao of suburban of Dushan City (GPS 25°50′N, 107°32′E), by Liao et al in 1979. It was named by Ting V.K. (Ding Wenjiang) in 1929 and quoted formally by Grabau W. in 1932, without stratotype assigned.
Lithology and Thickness
Dolomitic limestone. The Formation is divided into two members.
Lower – dolomitic Sifangpo Member, consists of dolomitic limestone and dolomite with tubular or laminated fenestrae and cauliflower structure as well as algae lamination, 212.4 m thick.
Upper – limestone Wuliqiao Member, is composed of limestone and siliceous rocks with some dolomite beds in the top, 162.1 m thick.
[Figure Column section with outcrop (right) of Yaosuo Formation in the type locality of Dushan (after Wang Yue, 2004).]
[Figure Outcrop of dolomite with laminations in Yaosuo Fm at Wangyou County, Guizhou.]
Relationships and Distribution
Lower contact
The formation rests conformably on the Wangchengpo Fm, and its lower boundary is easy distinguished by the appearance of grayish-black medium-bedded dolomitic limestone.
Upper contact
It has a conformable contact with the overlying Zhewang Fm, and the upper boundary is placed at the top of a bed dolomitic limestone rich in ostraocods in the topmost Wuliqiao Mbr. Southward to Huishui-Changshun area, there is abrupt change of facies from dolostone to micrite of Daihua Fm of basin facies, where the dolostone of Yaosuo Fm extents up to the Devonian-Carboniferous boundary without Zhewang and Gelaohe Fms distinguished.
Regional extent
The Formation is widely distributed in southern and southeastern Guizhou. Similar dolostone is also present in the Qujing-Panxi area of east Yunnan and is replaced by other names of Zaige Fm or Zaijieshan Fm.
GeoJSON
Fossils
Abundant ostracods present in the dolostone including Baiartina equiangularia , Leperdtia spp., rare Amphipora and gastropods.
Age
Depositional setting
It is interpreted as tidal flat, lagoon and sabkha environments.
Additional Information
Zhang Jianwei et al. (2021, Frontiers of Earth Science) reported an anomaly of mercury (Hg) and total organic carbon (TOC) within a portion the Sifango Member of the lower Yaosuo Fm, which they interpreted as evidence of global volcanism coeval with the F–F mass extinction. No details on the placement of this horizon within the Sifango Member were given.