Xihengshan Fm
Type Locality and Naming
Subei Basin. The Xihengshan Formation was erected by the Sixth Petroleum Prospecting Brigade in 1960. The reference section is the section measured from Shangzhuang to Lijiawan south of Jiangning county of Jiangsu.
Synonym: Xihengshan Gr
Lithology and Thickness
It is represented by a set of clastic rocks, which is roughly divided into two members. Lower member is built up by light yellow, gray white and purple conglomerate, gravel-bearing coarse-grained quartz sandstone, moderately coarse-grained feldspar-quartz sandstone, siltstone, calcareous and clayey siltstone, silty mudstone and mudstone, locally with clayey and carbonaceous siltstone, coal streaks and sandy mudstone lenses. Upper member is built up by variegated moderately coarse-grained quartz sandstone, calcareous siltstone and silty mudstone, constituting rhythms that are coarse in the lower part and fine in the upper part. Each rhythmic bed is seen to contain pyroclastic rocks and quartz trachyandesite lenses and calcareous thin shale beds in the upper part. The base of the formation is made up of conglomerate and sandy conglomerate, locally with pyroclastic and quartz trachyandesite partings. The Xihengshan Formation is 607.44 m thick.
Relationships and Distribution
Lower contact
The lower limit marked by 2 m conglomerate is distinguished from calcareous siltstone of the underlying Beixiangshan Fm, showing an unconformable contact with the latter.
Upper contact
The top is contacted conformably with the overlying volcanics of the Longwangshan Fm.
Regional extent
The formation is distributed along the line linking Nanjing and Jiangdu and along the line linking Gaochun and Yixing in the western part of south Jiangsu. The upper part of the formation is not developed in the west Hengshan area, and the top of the formation is not observed there. It is more than 600 m thick. Northward to the Hanfu mountain it is up to 1044 m thick. From Tianshengqiao to Xiasiqiao of the Lishui County, it is common. The upper limit marked by calcareous siltstone and fine sandstone lies disconformably under the overlying Longwangshan Formation marked by limy sandy conglomerate. The formation is 473.40 m thick therein. In the Hanfushan area, sandstone and conglomerate are developed with two layers of purple gray andesite and coal streaks. In Wushan of the Lishui County there occur relatively thick limestone and bioclastic limestone. In Cangshan of the Jurong County, limestone and limestone with sandstone are observed. In the Yixing area, breccia is observed. In the Ningzhen area, microclastic rocks are dominant, with lenticular gravel-bearing limestone and calcareous conglomerate.
GeoJSON
Fossils
The upper member contains ostracods Cypridea sp.; estherias Dictyestheria lishuiensis, Orthestheria cf. intermedia; floras Gleichenites cf. nipponensis, Brachyphyllum obesum, and Sphenopteris cf. nitidula.
Age
Depositional setting
The lower member belongs to fluvial facies while the upper member belongs to fluvial-lacustrine facies, locally volcanic eruption facies
Additional Information