Dazhuka Fm
Type Locality and Naming
Yarlung Zangbo suture zone (formation spans 1500 km E-W, only 2 km N-S). The type section is located at the Dongga Coal Mine (N29°21′, E88°48′), Qiabulin, west of Xigazê City, Xizang (Tibet) Autonomous Region (BGMRXTAR, 1997). BGMRXTAR (1993) formally published this formation. Named by the Geological Party of Tibet Bureau of Industry in 1964.
Synonyms: The Dazhuka Fm is derived from the lower part of the Qiabulin Fm (BGMRXTAR, 1993). Actually, various conglomeratic “molasse” units crop out along the Yarlung Zangbo suture zone (YTSZ) and were commonly called the “Gangrinboche conglomerates” (Aitchison et al., 2002). The Dazhuka Fm, the “Gangrinboche conglomerates”, and the Luobusha Gr in Zêtang-Gyaca maybe are synonyms. This formation includs the original Luobusha Gr, Yemagou Fm, Rixugou Fm and Gangrenboqi Fm.
Lithology and Thickness
Mainly variegated (purplish red, grayish purple and gray) clastic rocks such as conglomerate, sandstone and shale, with tuffs and carbonate rocks. The total thickness is 458–624 m. It is divided into three parts: Lower part is formed by purplish-red tuff containing purplish-red shales. Middle part consists of grey and purplish-red shales containing sandstones (tuffaceous siltstones). Upper part is characterized by grey conglomerates containing purplish-red and greyish-green shales and sandstones. It contains abundant fossil plants and sporopollens. Its thickness is more than 880 m.
Relationships and Distribution
Lower contact
In the type section, the Dazhuka Fm is in reverse faulted contacts with the overlain Cretaceous Qubeiya Fm (sandy shales) and unconformably contacts with the underlying Oligocene Qiuwu Fm. (BGMRXTAR, 1997) or early Himalayan granites
Upper contact
Not known due to faulting. However, next younger regional unit on the China Lexicon strat chart for this column is the Wuyu Gr.
Regional extent
The formation is widely distributed to the Yarlung Zangbo suture zone (YTSZ) ranging from Ayilariju, Ngamring, Xigazê, Zêtang, to Gyaca. It extends west-east in length about 1,500 km but south-north in width less than 2 km. (BGMRXTAR, 1993) suggested that the Dazhuka Fm can be correlated to the upper tuff of the Menshi Fm or the Yemagou Fm in Menshi area.
GeoJSON
Fossils
Sporopollens: Alnipollenites, Betulaceoipollenites, Caryapollenites, Celtispollenites, Engelhardtioidites, Juglanspollenites, Momipites, Moraceoipollenites, Triporopollenites, Ulmipollenites, Ulmoideipites, Quercoidites, Cupuliferoipollenites, Cyrillaceaepollenites. (Li et al., 2009). The formation yields the plants Quercus sp. and Salix sp., the charophytes Amblyochara miranda and Gyrogona shannanensis, sporopollen, the gastropods Planorbis cf. rotundata and bivalves.
Age
Depositional setting
It is interpreted as a series of continental molasse depositional environment.
Additional Information