Changxing Fm
Type Locality and Naming
The naming locality is situated at Changxing (Changhsing), Zhejiang Province. The reference section (and GSSP for the Changhsingian Stage) is located at Meishan, 20 km to the northwest of the Changxing County Town, Zhejiang Province. It was named by Grabau A.W. in 1931.
Synonym: Changhsing Fm
Lithology and Thickness
Limestone. The Baoqing Member of the lower part of the formation is composed of dark-colored medium-, and thick-bedded limestones, containing chert-bands and carbonatized tuffs, with a thickness of 14 m; its base is composed of dark-grey limestone, conglomerate or dolomitized limestone. The lower part of the Meishan Member in the upper part of the formation consists of grey-black bituminous lamellae-bearing medium-bedded microcrystalline limestone with horizontal stratification. The upper part of the Meishan Member is composed of grey medium-bedded microcrystalline limestone, containing cherty bands, with its top part containing an increasing amount of clayey component, being intercalated with many layers of clay bands, with a thickness of about 23 m.
Relationships and Distribution
Lower contact
Depending on region, conformable contact on the underlying Longtan Fm, Xuanwei Fm (S Sichuan, 66),
Upper contact
Conformable contact with the overlying Qinglong Fm or Feixianguan Fm (basal Triassic)
Regional extent
This formation is widely distributed in various places of the Yangtze River stratigraphic subregion, while in the Datian area of Fujian Province, in the Qujiang area of Guangdong Province, in the southern part of Hunan Province, and in the Han Zhong area of Shaanxi Province this formation is distributed in a scattered manner. In the eastern part of Sichuan Province and in the western part of Guizhou Province this formation represents restricted carbonate-platform facies deposits, with a thickness of 30-60 m, with the medium- and thick-bedded micritic limestone being predominant. In the transitional area between it and the Longtan Fm its top part is commonly possessed of a several-meter-thick layer of calcareous claystone, while in the transitional area between it and the Dalong Fm its top part is commonly composed of tuffaceous claystone and marl. In the Wuxi area of Jiangsu Province, the Huzhou area of Zhejiang Province, and the Chenxian area of Hunan Province, the Changxing Fm is predominated by light-grey massive limestones, belonging to shallow-sea tidal-flat facies, platformal-margin facies, biologic-beach or biologic-reef facies.
GeoJSON
Fossils
The base of the Baoqing Member yields Conodonts in the Clarkina orientalis zone of the Wuchiapingian Stage, with its top part yielding Ammonoids as represented by the Pseudogastrioceras, Tapashanites zone, while the Conodonts of this member are represented by the Clarkina subcarinata zone.
The top part of the formation yields Ammonoids known as the Rotodiscoceras zone, while the Ammonoids occurring in the rest of the formation should be assigned to the Pseudotirolites-Plearonodoceras zone. The Conodonts in the upper member are known as the Clarkina deflecta-C. changxingensis zone, and the Fusulinida is known as the Palaeofusulina sinensis zone.
Age
Depositional setting
It is interpreted as slope-facies carbonate in the type section, with its top boundary belonging to eupelagic-facies condensed deposits. But see Regional extent for other carbonate-facies types.
Additional Information
Magnetostratigraphy: In accordance with the studies conducted by Li Huamei (1989), the stratotype section of the Changxing (Changhsing) Fm comprises at least five normal-polarity zones.
Radio-isotopic dating: SHRIMP uranium lead age of the clay from the top part of the Changxing Fm is of (251.0±3.6) Ma (Claue Long J.C. et al., 1991), the 39Ar/40Ar age is of (249.9±1.5) Ma (Renne P.R. et al., 1995), the ID-TIMS age is of (251.4±0.4) Ma, and the age of the basal boundary is of (253.4±0.2) Ma (Bowring S.A. et al., 1998).