Dawangshan Fm
Type Locality and Naming
Subei Basin. The Dawangshan bed was named by Gu Xiongfei in 1955. The section for the designation is in Dawangshan of the Maanshan City of Anhui. The reference section was measured from the Zhouan Village to the Guli Town of the Jiangning County of Jiangsu.
The Dawangshan bed represents trachytic and trachyandesitic tuff and lava between the Jianshan bed and the Niangniangshan bed. It was renamed as a formation during the National Stratigraphic Conference in 1959.
Synonym: In 1960 the Anhui Regional Geological Survey Team discovered a set of pyroclastic sedimentary rocks with floras and sporopollen between the Dawangshan Formation and the Longwangshan Fm, which was then named the Yunheshan Fm. In 1976 the compilation group of the monographic study of the Ningwu Porphyrite Iron Deposit assigned the Yunheshan Fm to the lower part of the Dawangshan Formation. [The Yunheshan Fm was named by Zhou Renlin in 1960. The naming section is located in the area extending from the Yunhe Mts to the Tongshan Mts of Lishui County, Jiangsu Province. Its Synonym: （云合山组）]
Lithology and Thickness
The Dawangshan Formation is represented by a set of trachyandesitic and andesitic volcanic rocks and pyroclastic sedimentary rocks, which is divided into two parts. Lower part is built up by purple gray andesitic sediment tuff with sedimentary breccia tuff, with purple gray tuffaceous siltstone lenses. Upper part is purple gray, gray and gray yellow pyroxene andesite, andesite, trachyte, andesitic agglomerate breccia, volcanic breccia and andesitic sediment tuff with a small amount of dark purple silty mudstone. It is 1100.82 m thick.
Additional details on the former Yunheshan Fm [now the Lower part of the Dawangshan Fm; from an entry originally prepared for the Jurassic lexicon set]: Its lower part is composed of variegated andesitic tuffaceous conglomerates, intercalated with grey tuffaceous breccias, with a thickness of about 40 m; its middle part, of purple and violet-grey tuffaceous sediments, intercalated with tuffaceous breccias, siltstones and calcareous shales, with a thickness of about 61 m; and its upper part, of yellow and grey-violet silty shales, intercalated with marl lenses, with a thickness of 127 m. It has a disconformable contact with the upper part of the Dawangshan Fm.
Relationships and Distribution
Lower contact
The basal part is disconformably separated from andesitic agglomeratic volcanic breccias of the underlying Longwangshan Fm by andesitic sediment tuff. and sometimes the formation overlies unconformably the “Xiangshan Group”.
Upper contact
The top of the formation is disconformably separated from the purple red andesitic agglomerate volcanic breccia of the overlying Gecun Fm by gray biotite pyroxene trachyandesite. Regionally, the Niangniangshan Fm volcanics in this same Ning-Wu Basin are slightly younger.
Regional extent
The formation is mainly exposed in the Ningwu and Lishui areas. On the section at Yingzishan of the Jiangning County, the upper part of the formation is not developed but the lower part is relatively occur, consisting of sediment volcanic breccia, sediment tuff, tuffaceous siltstone and clayey siltstone with black shale and mudstone, measuring 64.79 m in thickness. In Ningwu the Dawangshan Formation is dominated by intermediate volcanic rocks and in Lishui by alkaline rocks.
The former Yunheshan Fm [now the Lower part of the Dawangshan Fm; from an entry originally prepared for the Jurassic lexicon set]: It is distributed mainly in the areas of the Yunhe Mts, Tongshan Mts and Donglu Mts of Lishui County, in the area extending from the Wanli region to the Ersi region of Liyang County, in the Zhouchong Village of Nanjing City, as well as in the areas of Niushou Mts, Longshan Mts, Zhongshan Buddhist convent, Zhatang and Qiangang of the Nanjing-Wuhu Basin. There are variations in both the lithology and thickness of the formation, commonly in a range from 9 m to 230 m. In some regions there are found hematitization, germanium-bearing hematite or jasper horizons, with some segments containing many layers of skarns and iron-mineralized skarns, which would serve favorable horizons for searching for iron ores.
GeoJSON
Fossils
It yields Estherias Yanjiestheria cf. sinensis, Y. aff. kyonganensis, Y. zhantangensis; and floras Pagiophyllum, Potozamites, Otozamites sp. in the lower part.
The former Yunheshan Fm [now the Lower part of the Dawangshan Fm; from an entry originally prepared for the Jurassic lexicon set]: yields “conchostracan” fossils as represented by Orthestheria yongkangensis, Orthestheriopsis tianshengqiaoensis, O. lishuiensis, Yanjiestheria cf. sinensis, Y. zhatangensis and Y. aff. kyongsangensis; Ostracod fossils as represented by Cypridea sp.; and plant fossils as represented by Cladophlebis cf. osimaensis, Otozamites sp., Pagiophyllum sp. and Pityophyllum sp., as well as the spore-pollen fossils predominated by the Classopollis assemblage.
Age
Depositional setting
It is volcanic eruptive and volcanic sedimentary facies. The lower part (former Yunheshan Fm) is suggested as fluvial-lacustrine facies deposits formed during the volcanic-eruption interval, but is still associated with eruption-facies deposits
Additional Information