Xiaxiang Fm
Type Locality and Naming
The name was proposed by the Anhui Party of Regional Geological Survey in 1965 (Bureau of Geology and Mineral Resources, Anhui Province, 1987; Du Senguan, 1989). The type locality is in Ningguo County. The type section is located near Xiaxiang, 2.5 km northeast of Hule Town, Ningguo County in southern Anhui Province. Coordinate: 118°45′48″E, 30°22′8″N. The Anji Fm established in Zhejiang Province shares similar lithology and sequence with the Xiaxiang Formation named in Anhui Province, in addition, their type sections are located closely. Therefore, we reject the Anji Fm and retain the Xiaxiang Fm by the priority law.
Synonym: （霞乡组）
Lithology and Thickness
Lower part is gray green, yellow green argillaceous sandstone, siltstone with silty shale; with gray green, yellow green thick-bedded sandstone and silty stone intercalated with black carbonaceous shale and silty shale at the bottom of the formation. Upper part consists of gray green, yellow green, blue gray and gray silty shale with siltstone, argillaceous siltstone. The formation is 1074-1934 m in total thickness.
Relationships and Distribution
Lower contact
It is in conformable contact with the underlying the Xingkailing Fm of the Upper Ordovician.
Upper contact
It is in conformable contact with the overlying Helixi Fm
Regional extent
The Xiaxiang Fm is well exposed in Ningguo, Guangde and Taiping counties in southern Anhui Province and Anji County in northwestern Zhejiang Province where the unit was named as Anji Formation previously.
GeoJSON
Fossils
The lower member yields graptolite from the Normalograptus persculptus zone to the Pristiograptus leei zone, and the graptolite-bearing horizons correspond to the time span from late Hirnantian age of latest Ordovician to Rhuddanian age of Llandovery epoch. The age of the upper member is still hard to be defined because of no fossil evidence but can be inferred to Aeronian age according to the age of the overlying sequences. A chitinozoan assemblage of low abundance and low diversity was also found near the base of the formation in Chunan, Zhejiang Province (Tang et al., 2012).
Age
Depositional setting
The formation belongs to an identical sedimentary facies’ region. The grain getting finer and the thickness getting thinner from south to northwest shows environment changes from the shallow shelf to deep shelf. In other words, the coarse-gained clasts are developed and the graptolite zones are less developed (only 1 to 2 graptolite zones at the base of the formation) near the ancient continent; the clasts become finer-grained and up to 5 graptolite zones can be observed at the sequence far from the continent.
Additional Information