Xuanwei Fm
Type Locality and Naming
The type section is located in the Dasuopo Coal Mine of Xuanwei County, Yunnan Province (104°15′, 26°03′). It was named by Xie Jiarong in 1941.
Synonym: Hsuanwei Fm
Lithology and Thickness
Clastics and Coal. The formation is divisible into the four members: First member is composed of yellow-green, light-yellow and light-grey siltstones, intercalated with claystone, silty siltstone and carbonaceous siltstone, yielding plant fossils (phytolites) and containing 8 coal layers, with a thickness of 53 m. Second member is composed dark-grey siltstone and siltstone, intercalated with fine-grained sandstone, being characterized by the development of large-scale cross-beddings, yielding a great amount of plant fossils (phytolites) and containing 17 coal layers, with a thickness of 95 m. Third member is composed of dark-grey claystone, siltstone and siltstone, containing 5 coal layers, with a thickness of 25 m. Fourth member is composed of yellow-grey and yellow-green siltstone and silty siltstone, intercalated with fine-grained sandstone and claystone, containing only coal seams or thin coal layers, with a thickness of 67 m, and with such a sequence of conglomerate→sandstone→siltstone→claystone→carbonaceous shale or coal seams in ascending order being seen and with its basal part being commonly possessed of scouring surface. Total thickness is 241.7 m.
Relationships and Distribution
Lower contact
Disconformable or conformable contact with the underlying Emeishan.Basalt Fm
Upper contact
Conformable contact with the Kayitou Fm Shale of the overlying Feixianguan Fm or with the Liangfengpo Fm
Regional extent
Distributed in the Yanjin-Zhaotong, Ludian-Huizhe, Xuanwei-Fuyuan and Shizhong-Mile areas of Yunnan Province, as well as in the area to the west of an alignment from the Magu of Hezhang County to the Tucheng of Panxian County in Guizhou Province in a S-N trend. It has a small change in lithology in the south-north direction, but a remarkable change in the east-west direction, with a thickness of 72-384.5 m. In the Tangdi area of Huizhe County near the Kam-Yunnan Oldland in the western part of the province the lower part of the formation is composed of basaltic conglomerate, and its upper part is composed of yellow-green sandy shale, with no coal seams being found, with a thickness of 44.3 m; in the Meigu area of Sichuan Province it is only 10-20 m thick, and it is known as the Shawan Fm in the Leshan area, with a thickness of 55 m. Easterly to the Dasuopo area of Xuanwei County the formation is composed of grey and grey-green sandy shales, with its middle and upper parts containing 3 layers of mineable coal, with a thickness of 384.5 m In the Nuomu area of Fuyuan County and Qingshuigou area of Qujing County in the southeastern part of the province there are found a small amount of marine-facies intercalations consisting of siltstone, fine-grained sandstone, clay band and shale, containing siderite ores and 30 coal layers, with each layer of which being 0.2-5.37 m thick, as well as over 10 mineable coal layers, with a thickness of 260-321.46 m. In the western part of Guizhou Province the lower part of the formation is composed mainly of clay, while its upper part is composed largely of siltstone, with a thickness of 100-150 m; southeasterly it is transiting into the Longtan Fm, representing coal-bearing deposits. The coal beds are distributed in the Yanjin-Zhenxiong, Xuanwei-Fuyuan and Mile areas of East Yunnan, where the amount of mineable coal horizons in the middle and upper parts of the formation is of over 10 layers.
GeoJSON
Fossils
In the Xuanwei (Hsuanwei) area the First, Second and Third members yield Plant fossils (phytolites) represented by the Gigantopteris nicotianaefolia-Lobatannularia multifolia assemblage, while the Fourth member yields plant fossils (phytolites) represented by the Gigantonoclea guizhouensis-Ullmannia cf. bronnii assemblage zone.
Age
Depositional setting
It is interpreted as river-alluvial-facies and coastal swamp-facies deposits.
Additional Information