Dalong Fm
Type Locality and Naming
The type section is located in the vicinity of the Dalong Coal Mine, Heshan City, Guangxi, with the section at Matan by the Hongshui River of Heshan City having been studied in the most details, and thus having been taken as the standard section for the Dalong Fm. It was named by Zhang Wenyou and Cheng Jiatian in 1938.
Synonym: Talung Fm
Lithology and Thickness
Chert and Volcanic tuffs. Chert and siliceous limestone. Being rich in intermediate-acidic tuff represents a local salient feature for the Dalong Fm in the type section, but its lower member contains emerald green tuffaceous layers as well. From here easterly to the Liuzhou, Laibin and Zhangmu areas, closing to the region of volcanic effusion, the Dalong Fm is intercalated with several layers of tuffs with a thickness of over 10 m, and with the top part of the formation being replaced by a volcaniclastics-alluvial fan composed of tuffaceous sandstone and shale, with a thickness of 400 m. While in other areas the Dalong Fm tends to contain only thin-bedded distal carbonatized tuffs or sludging volcanic-ash layers.
Relationships and Distribution
Lower contact
Generally, a conformable contact with the underlying Wuchiaping Fm or Longtan Fm (see Regional extent discussion).
Upper contact
Generally, a conformable contact with the overlying Daye Fm of earliest Triassic.
Regional extent
Coeval with the carbonates of the Changxing Fm. In the northern and eastern parts of Sichuan Province the Dalong Fm that was deposited between the carbonate depressions (Li Zishun et al., 1989) is in a conformable contact with the underlying Wuchiaping Fm, and is intercalated with more thin-bedded siliceous limestones, with its base being corresponding to the upper part of the Wuchiapingian Stage. The Dalong Fm in the southern part of Hunan Province was formed due to a gradual transition from marine strata of the top part of the Longtan Fm.
GeoJSON
Fossils
The marine strata yield Conodonts as represented by Clarkina postbitteri occurring at the very base of the Wuchiapingian Stage, while the basal part of the Dalong Fm yields Ammonoid community as represented by Anderssonoceras, and for this reason its stratigraphic horizon is corresponding to the lower part of the Wuchiapingian Stage. The Dalong Fm yields the ammonoid and conodont zones of the Changhsingian Stage (Li Zishun, 1989), with its fossils predominantly being small-sized and thin-shelled ones, belonging to the brachiopod-fauna Cathaysia sulcatifera—Crurithyris pygmaea assemblage zone (Liao Zhuoting, 1979).
Age
Depositional setting
It is interpreted as a tuffs deposited into marine basins.
Additional Information
GeoJSON estimate by Can Cai and Xinyi Zhang (Chengdu Univ. Tech. students)