The Late Palaeozoic stratigraphic sequences and basin evolution in the Bomi region, Xizang

The unconformity between the Lower Carboniferous Norco Formation and Middle-Upper Devonian Sumzom Formation in Bomi and its adjacent areas such as the Sumzom and Rawu zones is believed to be an important boundary that represents the conversion of the nature of the Late Palaeozoic basins on northern Gondwana in southeastern Xizang. The Sumzom Formation beneath the unconformity is interpreted as stable carbonate platform deposits, while large-scale volcanism and basin disintegration appear above the unconformity where the deepening-upward depositional sequences represented by the Carboniferous Norco and Laigu Formations rested down following the steadily subsidence of the sedimentary basins and relative rising of sea level. The maximum sea level rise during the Late Carboniferous was manifested by the occurrence of the ferruginous slates and turbidites in the upper part of the Laigu Formation. The successive latest Late Carboniferous to Permian deposits represented by the Luobadui Formation recorded a gradually shallowing-upward depositional sequence. The deepening- and shallowing-upward depositional cycle in the basin has disclosed the evolutionary path from a strone one to a weak one of volcanic activity. The angular unconformable contact of the red continental molasse deposits of the Middle Jurassic Mali Formation with the underlying strata shows that a major orogenic event once took place in the study area during the Late Triassic to the Early Jurassic.