Xiaodingxi Fm
Type Locality and Naming
The type section is located in the Xiaodingxi, Mianhuadi, Yun County, Yunnan Province. It was named by the first area survey team of Yunnan Geological Bureau Guangxi Geological Survey in 1977.
Synonym: The former Xiaodingxi Fm represents a set of strata dominated by volcanic and pyroclastic rocks of the Upper Triassic Carnian–Norian on both banks of the Lancang River in Yunnan. In 1985, the regional survey team of Yunnan Bureau of Geology and Mineral Resources delineated the lithologic section with conglomerate and volcanic breccia in the upper part of the formation, and established another Manghuihe Fm. In 1990, Yunnan Bureau of Geology and Mineral Resources followed this definition and determined the age of the Manghuihe Fm as Norian - Rhaetian of Late Triassic. In 2000, Yang Zunyi et al. maintained the meaning of the original Xiaodingxi Fm; which is used in this lexicon.
Lithology and Thickness
The formation is mainly composed of grey-green, dark green and purplish red basalt, andesite, volcanic breccia and rhyolite. Sandstone and shale are enriched in the lower part; Upper part is packed with conglomerate. The thickness is 2453 m.
Relationships and Distribution
Lower contact
There is a pseudo-conformable contact between this formation and the underlying Manghuai Fm of Middle Triassic.
Upper contact
An unconformable contact between the group and the overlying Huakaizuo Fm of Middle Jurassic.
Regional extent
The formation is mainly distributed on both sides of the Lancang River.
GeoJSON
Fossils
Bivalves: Trigonodus bittneri, T. rablensis, Yunnanophorus gracilis, Pergamidia nakuensis, etc.; Ammonoids: Acrochordiceras; Conchostracans: Euestheria xiangyunensis, E. mupangensis; Plants: Equisetites sarrani, E. longidus, E. arenaceus, etc.
Age
Depositional setting
The Xiaodingxi Formation is a set of volcanic and pyroclastic strata of the Upper Triassic Carnian to Norian.
Additional Information