1:250 000 Oma Sheet in Xizang

The stratagraphic systems in the Oma Sheet are redivided into thirty-three formal and twenty informal mappable units. The Late Triassic molasse formations are identified for the first time in the South Qiangtang landmass, and the Tinggongla Formation is established. The Jienu Group on the southern flank of the Bangong-Nujiang suture zone is redivided into the Duoren and Risum Formations. It is demonstrated that the Qangba Formation should overlie the Jingzhushan Formation, and the flysch-silicalite formations in the Muggar Kangri Group Complex are overlain with an angular unconformity by terrigenous clastic rocks of the Shamuluo Formation, implying that the Bangong-Nujiang suture zone was closed during the Late Jurassic to the Early Cretaceous. There exists a large-scale transgressive onlap boundary between the Langshan Formation and its underlying strata. The plagiogranites with an Ar-Ar age of 128 Ma are recognized for the first time in the Goicang ophiolitic mélange zone. The Ar-Ar age of 141 Ma for the ophiolite zone in the Lhabu Lake area shows that the tectonic emplacement of the ophiolites took place during the Jurassic.