Duoni Fm

North Lhasa Block – easternmost. The Duoni Formation was established by Si Xingjian and Zhou Zhiyan in 1962. The section for the designation is Balilang-Zituo-Luolong-Lhorong in eastern Xiziang (96°00; 30°00′). The reference section is at Lhorong of Xizang. In 1955 Li Pu et al called black shale with sandstone interbedded with sandy shale and black coal-bearing sandy shale as the Duoni coal measures and assigned them to Jurassic -Cretaceous. In 1962 Si Xingjian and Zhou Zhiyan renamed the Duoni coal measures as the Duoni Formation and assigned it to Early Cretaceous.

Lower part of the Duoni Formation is represented by calcareous sandstone and shale interbedded with siltstone, with coal seams. Middle part is gray and gray green siltstone and black shale with sandstone and coal seams. Upper part is gray sandstone and siltstone with purple red-variegated clastic rocks and marl lenses. The formation ranges from 1200 to 4313 m or more in thickness.

The formation bears floras Zamiophyllum buchianum, Weichselia reticulata, Cladophlebis sp., Klukia sp., Otozamites sp. in Lhorong; Klukia browniana, Glossozamites sp., Onychiopsis sp., O. elongata, Nissonia sp., Sphenopteris sp. in Duoni and Ruoba; bivalves Velsella cf. imbricata, Chlamys (Radulopecten) cf. ineaquicostata, Astarte sp., Cucullaea sp., Trigonia sp., Inoceranus sp., Pseudotrapezium sp., ammonites and Crinoids in the Lhorong area.