Establishment of the Duoren and Risum formations in the Risum area, western Tibet, China and its geological significance.

A suite of flysch-flyschoid clastic deposits is widespread in the area of Bangong Co-Gerze-Ge′gyai. Generally, its lithology is monotonous, contained fossils are rare and the level of research is low. Owing to lacking of age evidence, especially for its upper part, there has been a heated debate about its age and upper boundary of deposits and it is difficulty to get a consensus on the stratigraphic division of the suite of deposits. During the regional geological survey of 1∶250000 Rutog Sheet, an intensive study was conducted of this suite of deposits and plenty of fossils were obtained from its middle and upper parts. Its age is assigned to Late Jurassic. According to the features of sedimentary formations, rock associations, paleogeography and fossil aspects, it is advisable to separate it from the original Mugagar Kangri stratigraphic area and attribute it to the Baingoin-Baxoi stratigraphic area and set up the Duoren Formation and Risum Formation.