Laohudong Fm
Type Locality and Naming
The type section is located at Laohudong of Dalian Mt., Chunhua Town, Jiangning, Nanjing, Jiangsu. It was named by Xia Bangdong in 1959.
Synonym:
Dapu Fm (in Hunan and Guangxi)
Lithology and Thickness
Dolomite. Carnation and light-gray massive powdered and fine-crystalline dolomite, containing chert nodules in the middle part, with quartz sands occasionally seen in dolomite. Consists of syngenetic and epigenetic dolomites. The former is of micritic to fine-crystalline textures, without organic fossils or their relict textures; the latter shows apparent selective replacement, composed of microlithic to fine-crystalline dolomites, with a hypidiomorphic mosaic texture. The lower part is dominated by epigenetic dolomite, while syngenetic dolomite is more developed in the upper part. Stable in lithology, with thickness generally ~10 m.
Relationships and Distribution
Lower contact
Conformable contact: The bottom is demarcated by the gray-yellow clayey limestone or clayey limestone of the underlying Hezhou Fm. It may also overlie the sandy shale at the top of the Gaolishan and Wutong formations, between which there is generally a thin layer of quartz sandstone or quartz fine-grained conglomerate, showing an obvious disconformable contact. At Songzi, Changyang and Zigui of Xiadong, Hubei (thickness 2−20 m), the formation is mostly in disconformable contact with the underlying stratum.
Upper contact
Conformable contact with the overlying Huanglong Fm
Regional extent
Distributed extensively in the lower Yangtze River valley, and can be divided into two areas based on the fauna contained in the dolomite layer. The Southeast area include Yixing of Jiangsu, Changxing and Guangde of Zhejiang, Xuancheng, Jingxian County and Guichi of Anhui (thickness 6−14 m), yielding fusulinids and conodonts of the early Late Carboniferous; and the Northwest area include Hexian County, Hanshan and Caoxian County of Anhui (thickness 3−12 m), yielding corals and fusulinids of the late Early Carboniferous. And, in Jiangning lying between and in the vicinity of Nanjing, faunas of both the late Early Carboniferous and the early Late Carboniferous are found (thickness 8−12 m).
GeoJSON
Fossils
In the east suburb of Nanjing and the Caohu area of Anhui, there is the Fusulinid Eostaffella zone, and Corals are characterized by the abundance of Lithostrotion. The Lower part yields Conodont Gnathodus bilineatus of the late Early Carboniferous, and the Upper part contains the Idiognathodus sinuatus-Idiognathodus delicatus zone. In the Jiangsu-Zhejiang-Anhui border and Guichi area there occur Fusulinids Pseudostaffella antiqua and Profusulinella cf. brevissima.
Age
Depositional setting
It is interpreted as a dolomitic tidal deposit of shallow seawater.
Additional Information
In Hunan and Guangxi, it is called the Dapu Fm, overlying the Silurian System and the Devonian-Lower Carboniferous disconformably or unconformably.