Nagaoling Fm
Type Locality and Naming
The stratotype section is exposed on the southern slope of the Xiayiling Hill in Liujing Town, Hengxian County, east of Nanning City, the capital of Guangxi Zhuang Autonomous Region. It is easily accessible by minibus along the paved road from Liujing to Xiayiling Hill, (Wang and Yu, 1962, Wang et al., 1964; Hou,1978; Kuang et al., 1984). It was named by Wang Yu in 1956.
Synonyms: Nagaoling Shale, Nahkaoling Fm, Nahkaoling Shale
[Figure 1. The geological map (left) and the landform (right, view from the top of the hill southward) of Liujing showing the location of Nagaoling Formation (red line A and arrow).]
[Figure Simplified geological map of Liujing area showing the location of type sections of some lithostratigraphic units (red lines) A-Nagaoling Fm, B-Yukiang Fm, C-Moding Fm, D-Najiao Fm, E-Mintang Fm and Gubi Fm.]
Lithology and Thickness
Claystone. It is dominated by a set of grayish-green shale deposits, dividing into two members. The lower Gaoling Member, 59 m thick, is characterized by green, yellowish green silty mudstone, muddy siltstone, intercalated with thin shelly limestone layers or lens, which contains large amount of brachiopods. The upper Mahuangling Member, 88 m thick, is composed mainly of grayish green to yellowish gray thin- to medium-bedded muddy siltstone and silty mudstone, partially interbedded with phosphorite concretions and limestone nodules.
[Figure Columnar section of Nagaoling Formation with photos showing the rock outcrops. Length of hammer 28 cm.]
Relationships and Distribution
Lower contact
The base of the Nagaoling Fm is separated by its grayish-green shale from the underlying purplish-red silty mudstone at the top of the Lianhuashan Fm at a conformable contact. At Wanjiang in Jingxi of western Guangxi, it overlaps directly on the Cambrian in an unconformable contact.
Upper contact
It is overlain by the Xiayiling Mbr of the Yukiang Fm (Yujiang Fm) in disconformable contact.
Regional extent
This formation is widely distributed in southern Guangxi with homogeneous lithology. In Wuming and Shanglin area, it is mainly composed of shale and siltstone, with rare limestone interbeds. In the Longmenshan area of Sichuan the equivalent Bailiuping Fm has similar brachiopods and lithology.
GeoJSON
Fossils
Brachiopods are mostly occurred in the Gaoling Member that are characterized by small sized brachiopod Orientospirifer fauna, moderately or abundantly accumulating along the bedding plane. The main species are Orientospirifer nahkaolingensis, O. wangi, Aseptalium guangxiensis, Kwangsirhynchus liyjingensis, Corrinopugnax sp., Chonetes sp., etc. (Wang et al.,1964; Wang et al., 1974; Hou et Xian, 1975 ).
Rugose corals are “Chalcidophyllum” nahkaolingensis and Eoglossophyllum minor (Wang et el., 1974 ). Chitinozoa Ancyrochitina ancyrea, A. tumida, A. spinosa, A. devonia, Sphaerochitina spherocephala, Desmochitina urna has been reported by Gao in1978. Three spore assemblages have been established. They are in ascending order as the Leiotriletes-–Punctatisporites Ass. Z., the Retusotriletis—Acanthotriletis— Stenozontriletis Ass. Z. and Veryhachium—Micrhystridium—Cymatisphaera Ass. Z.
Tentaculites Tentaculites lucasi, T. straeleni and Volynites sp. have been found (Ruan et al., 1979).
Some conodonts Ozarkodina sp., Hindeodella priscilla, H. equidentata, Trichonodella excavata, Spathognathodus linearis postelinatus have been described by Wang C. et Wang Z. in 1978 from the bioclastic limestone, about 30 m above the base of the Nahkaoling Formation. Later in 1989, Wang reported the occurrence of Eognathodus sulcatus (HL-2d) from this level, and assigned it to the Lower Eognathodus sulcatus Zone. Lu et al. (2016) revised it as a new species Eognathodus. huangi..
[Figure – Brachiopod Orientospirifer nakaolingensis (Hou), X 2.5, ventral views.]
Age
Depositional setting
According to its sedimentary character and fossils, the lower and middle part of this formation belongs to littoral subtidal mud-flat environment, while the upper part gradually changes into intertidal shore facies environment.
Additional Information