Lithostratigraphic and paleontological characteristics of the Carboniferous Yunzhug Formation in the Yunzhug area, northern Tibet

The Yunzhug area is located in the Paleozoic Xainza basin, where Early Ordovician-Middle Permian successive marine sedimentary sequences are developed. The rock association of the Carboniferous Yunzhug Formation consists of fine quartzose sandstone, shale and siltstone with several bioclastic limestonebeds, which contain abundant fossils, mainly including brachiopods, bivalves, conodonts, corals,bryozoans,ammonites and trilobites. The conodont Gnathodus girtyi-Gnathodus texanus assemblage is a late Early Carboniferous assemblage; and the brachiopod Productus-Rhipidomella tibetena assemblage is also a late Early Carboniferous assemblage, the brachiopod Choristites- Spirigerella assemblage occurs in the lower part of the early Late Carboniferous and the brachiopod Trigonatreta cf. paucicostulata- Elivella baschkirica assemblage occurs in the upper part of the early Late Carboniferous. Uncinunellina is a typical cold-water brachiopod and Cyathaxonia is a typical cold-water coral, which indicates that the global Carboniferous glaciation had influenced the whole area then. It is confirmed that the age of the Yunzhug Formation is late Early Carboniferous to early Late Carboniferous, and that the boundary between the Early and Late Carboniferous is placed between bed 11 and bed 12.