Redefinition of the Lower Permian Qudi Formation in Yadan area, Rongma Town, northern Tibet, and its tectonic significance

The continuous separation of accretionary complex in central Qiangtang of Tibet is significant for defining the framework of strata from Paleozoic to Triassic and studying the evolution of Tethys. The authors measured a section of the accretionary complex in Yadan area, Rongma Town, Nyima County, Tibet, and discovered a sequence of submarine fan clasolites formed by deep-water gravity flow on the continental slope in Early Permian. The north of the strata is covered by Cenozoic red Kangtuo Formation, and the south is in faulted contact with basalt blocks. It is intercalated with sandstone lenses from the outer part to the middle fan. The intercalated layer of bioclastic limestone formed by collapse deposition contains a fusuline fauna and consists of ten species in four genera such as Parafusulina, Pseudofusulina, Schwagerina, Chalaroschwagerina, suggesting Artinskian period. The 1:250000 geological map of Rigan Peicuo defined the strata as Tunlong Gongba Formation which was mainly formed by diamictite continental shelf deposits. Based on the lithostratigraphy and the time of the fossil assemblage, The authors redefined the submarine fan strata as Qudi Formation. In combination with previous widely reported limestone formation containing fusuline and the continental rift background of basic dikes in central Qiangtrang from Late Carboniferous to Early Permian, the authors hold that the early Permian sedimentary formation represented the initial evolution of the Meso-Tethys Ocean and there existed a small ocean basin after the Kungurian Stage.