Xiaodong Fm
Type Locality and Naming
Naming section is located in Nadao Village, Xiaodong Town, Qinzhou City, Guangxi Province (E108°38′00″; N22°13′00″). Proposed by Guangxi Bureau of Geology & Mineral Prospecting & Exploitation in 1974, and was published in 1982 by Bai Shuanglian et al. Due to the incomplete exposure of the formation and the fault contact with the overlain Shijia Fm, there’s been a lack of a complete typical profile.
Synonyms: Xiadong Fm; and Xiaodong Gr
[Figure Simplified geological map of Xiaodong area showing the distribution of Qinzhou Fm and Xiaodong Fm. The map on the lower right shows the geographical position of Xiaodong Town]
Lithology and Thickness
Sandy claystone. The formation has its maximum known thickness about 500 m, including three units from base to top: (1) dark gray thin- to medium-bedded metamorphic mudstone, silty mudstone intercalated with calcareous sandstone, sandy limestone and carbonaceous iron-manganese claystone lenses, 117 m; (2) dark gray thin- to medium-bedded metamorphic mudstone, siliceous and silty mudstone intercalated with calcareous sandstone, sandy limestone, locally with siliceous rocks, containing dacryoconarids, ostracods, brachiopods and trilobites, 130 m; (3) light gray thin- to medium-bedded mudstone and siltstone with thin fine-grained sandstone and conglomeratic sandstone, 60-120 m..
Relationships and Distribution
Lower contact
It rests conformably on the Qinzhou Gr without distinct lithologic differences, but can be partly distinguished from the latter by the abundant tentaculite fossils, flysch rhythmites and manganese deposits.
Upper contact
Faulted or angular contact with Shijia Fm of earliest Carboniferous
Regional extent
Scattered outcrops of are distributed on a narrow belt extending NE-SW direction from Jishiping to Daoliu villages in the southeast of Xiaodong-Taiping Towns of Qinzhou County. In addition, the formation also appears on the south of Nali Town of Qinzhou County and around Fangcheng and Lingshan Counties in Guangxi.
GeoJSON
Fossils
It contains Tentaculites Nowakia cancellata, N. richteri, N. richteri, N. holynensis, N. otomari; Ostracods Hollinella sp., Healdianella sp., Bairdia sp., Beyrichia sp., Parabolbina sp. Primitia sp.; Trilobites: Dechenella sp., Proetus sp.; and Brachiopods Plectodonta sp., Costanoplia sp., Leptostrophia sp. in the lower part of the formation and N. otomari, Striatostyliolina sp. in the upper part.
Age
Depositional setting
Deep-water basin is suggested.
Additional Information
Lithology of this formation is similar to underlying Qinzhou Fm. In general, however, this formation is characterized by carbonaceous-iron-manganese claystone lenses, and more flysch rhythm. Lithologically, it is also similar to those of Tanding Fm and Luofu Fm in the NW Guangxi, therefore, it has been called Luofu Fm or Tangxiang Fm by Mu et al. (1988)..