Xijiadian Fm
Type Locality and Naming
The type section of the Xijiadian Formation is the West Hill Ridge section. It is located on the hill ridge west of Fengjia’ao Village, about 7 km north of the Xijiadian Town, Danjiangkou, Hubei Province (111°12’58”E, 32°50’52”N). It is the upper portion of the same section shared by the underlying Fengjia’ao Fm and Yanwugou Fm. It was measured by Zhu and Tao (1988) from Hubei School of Geology (now Huibei University of Geological Staff). The Xijiadian Formation was named by Zhu and Tao (1988). The name is derived from Xijiadian Town, Danjiangkou City (county level), Shiyan City, northwestern Hubei Province.
Synonym: （习家店组）
Lithology and Thickness
The Xijiadian Formation is a carbonate sequence. The type section embraces five lithological beds, from bottom up: 1, greyish yellow, purplish red and greyish brown, thin-bedded marlstone, intercalated with red shale and argillaceous limestone lenses (18 m thick); 2, greyish yellow, bluish grey and greyish purple, thin-bedded marlstone, intercalated with thin-bedded to platy limestone (14 m); 3, dark grey, thin- to medium bedded limestone (7 m); 4, grey, thin-bedded limestone (18 m); 5, grey, thin-bedded limestone intercalated with yellowish grey, argillaceous striped limestone (27 m). In the type section, the Xijiadian Formation is 84 m thick.
Relationships and Distribution
Lower contact
The Xijiadian Fm is in conformable contact with underlying Fengjia’ao Fm. The lower boundary of the formation is marked by lithological change from medium-bedded limestone of the Fengjia’ao Fm to its thin-bedded marlstone.
Upper contact
The Xijiadian Fm is in conformable contact with overlying Xiuzigou Fm. The upper boundary is marked by lithological change from thin-bedded limestone to medium- to thick-bedded, striped limestone of the Xiuzigou Fm.
Regional extent
The Xijiadian Formation is exposed in the Jiangbei Slope Area of South China Region, distributed in northwestern and northern Hubei Province (Danjiangkou and Laohekou cities) and southwestern Henan Province (Xichuan and Dengzhou counties). The thickness of the formation increases eastward, varying from 84 to 321 m.
GeoJSON
Fossils
Bed 1 of the formation in the type section (=Bed 9 of Zhu and Tao, 1988) yields trilobites Corynexochus sp.; and the Bed 4 (=Bed 12 of Zhu and Tao, 1988) yields trilobites Goniagnostus aff. nathorsti, Luia sp., Peronopsis rakuroensis, and Solenoparia calcon. In other sections, the formation yields rich additional trilobites Anomocarella zushimiaoensis, Diplagnostus panicauda, Fuzhouia oratolimba, Huzhuia typica, Hypagnostus vulgarus, Linguagnostus kjerulfi, Mapania sp. Palaeadotus sp., Parablackwelderia qinlingensis, Pianaspis attenuata, P. sinensis, Ptychagnostus aculeatus, and Yakutiana ovalis etc. Yang et al. (1991) recognized four trilobite zones for the formation, in ascending order, they are: 1, Ptychagnostus gibbus-Doryagnsotus incertus Zone; Aspidagnostus laevis Zone; 3, Linguagnostus kjerulfi-Diplagnostus planicauda Zone, and 4. Xichuania-Eoshengia dongqinlingensis Zone.
Age
Depositional setting
Additional Information