Siyangshan Fm
Type Locality and Naming
The type section of Siyangshan Formation is the Xiyangshan section, lying 1.3 km south of the seat of Changshan County, Zhejiang Province (118.517122°E, 28.896249°N). The section was measured by Lu et al. (1955). In the type section, the formation is 63.9 m thick. The Siyangshan Formation was named by Lu et al. (1955). The name is derived from Siyangshan (spelled Xiyangshan in Hanyu Pinyin) Hill, Changshan Town, Changshan County, Quzhou Prefectural City, western Zhejiang Province. Originally it was called Siyangshan Shale by Lu et al. (1955), and was renamed Siyangshan Formation by Lu et al. (1963).
Synonym: （西阳山组）; Siyanshan Formation (spelling)
Lithology and Thickness
The Siyangshan Formation is mostly a carbonate sequence, consisting of grayish black, thin-bedded, laminated limestone and striped argillaceous limestone, alternated with marlaceous or calcareous shale. The marl and calcareous shale become usually yellowish brown, brownish yellow or grayish purple, loose slaty or foliated shale after weathering. In lithology, the lower part of the formation is mostly stable but the upper part varies notably with a tendency of increasing argillaceous limestone northwestward.
Relationships and Distribution
Lower contact
The Siyangshan Fm is in conformable contact with underlying Huayenssu Fm. As noted by Yu (1996) that the lower boundary of the Siyangshan Fm is marked by lithological change from the striped limestone of underlying Huayenssu Fm to the laminated limestone of the Siyangshan Fm. Actually, both the lower and the upper boundaries of the Siyangshan Formation are very hard to be defined in field as the lithology is transitional and gradually changed between the Siyangshan Fm and the underlying or overlying formations. As noted by Lu and Lin (1989), in practice, the lower and the upper boundaries of the formation are defined mostly on the basis of the appearance of trilobites. The Lower boundary is defined by the appearance of trilobite Lotagnostus, Hedinaspis or Onchonotina. These methods apparently made the formation a form of the lithostratigraphic unit with chronostratigraphic or biostratigraphic meaning.
Upper contact
The Siyangshan Fm is in conformable contact with overlying Yinchupu Formation that is latest Cambrian through Early Ordovician in age. The upper boundary is marked by lithological change from the laminated limestone of the Siyangshan Fm to the calcareous mudstone at the base of overlying Yinchupu Fm. Actually, both the lower and the upper boundaries of the Siyangshan Formation are very hard to be defined in field as the lithology is transitional and gradually changed between the Siyangshan Fm and the underlying or overlying formations. As noted by Lu and Lin (1989), in practice, the lower and the upper boundaries of the formation are defined mostly on the basis of the appearance of trilobites. The upper boundary by the appearance of Hysterolenus. These methods apparently made the formation a form of the lithostratigraphic unit with chronostratigraphic or biostratigraphic meaning.
Regional extent
The Siyangshan Formation is exposed in the Jiangnan Slope Area of South China Region, including western and northwestern Zhejiang Province (Jiangshan, Changshan, Tonglu, Fuyang, Kaihua, Changhua, Anji); northern Jiangxi (Wuning) and southeastern Anhui Province (Dongzhi, Jixi, Ningguo, Shitai, Xiuning, Yixian). The thickness increases northwestward, being 40 to 383 m.
GeoJSON
Fossils
The Siyangshan Formation is well fossiliferous with trilobites, especially with agnostoid trilobites. In the type section the formation yields trilobites Archaeuloma？angustilimbata, Charchaqia norini, Chekiangaspis chekiangensis, Diceratopyge yuqianensis, Duibianaspis typicalis, Hedinaspis shengi, H. regalis, Jegorovaia expansa, Lotagnostus asiaticus, L. hedini, L. punctatas, Micragnotus orientalis, Neoagnostus sp., Niobella cf. yangjiawanensis, Olenus sinensis, Onchonotina vigilans, Pseudaphelaspis elongatus, Plicatolina changshanensis, P. kindlei, P. xiyangshanensis, Promacropyge carinata, Pseudagnostus communis, P. rugosus, P. sp., Pseudoyuepingia brevis, P. elongata, P. intermedia, P. zhejiangensis, Rhaptagnostus changshanensis, Sinoproceratopyge constrictus, Wujiajiania expansa, W. tachenensis, and Yuepingioides ocellata (Lu and Lin, 1984, 1989). The formation yields also diverse nautiloids Acaroceras spp., Anhuiceras spp., Archendoceras brevicollumo, Bambusoceras spp., Dongshanoceras jiangshanense, Ectenolites spp., Eocameroceras cambria, Huaiheceres hanjiaense, Parapalaeoceras spp., Walcottoceras spp., Yanheceras spp., and Zhuibianoceras spp. (Lu et al., 1984; Lu and Lin, 1984, 1989).
Age
Depositional setting
Additional Information