Discovery of Late Triassic bivalves from Jurassic deep-water deposits in Riganpeicuo area, Tibet and their geological significance

Abstract The Jurassic sequences in the South Qiangtang Basin of Tibet are essential for understanding the paleogeography and tectonic evolution of this basin and the Bangong-Nujiang Meso-Tethys Ocean (BNMO). However, the partial absence of fossils hinders the study of the stratigraphic distribution and tectonic paleogeography of the basin. Late Triassic bivalves, including nine species in nine genera, were identified for the first time in olistostromes from the Sêwa Formation in the Riganpeicuo area. Based on detailed geological field surveys and sedimentary facies analysis, the lower-middle part of the Rigenco section is a deep-water turbidite fan deposit and the upper part is a shallow sea mixed shelf deposit, which differs distinctively from the carbonate platform facies of the Jiebuqu Formation. Therefore, the strata formerly assigned to the Jiebuqu Formation (Rigenco section) is formally reassigned to the Lower-Middle Jurassic Sêwa Formation and Middle Jurassic Shaqiaomu Formation. These results not only provide further understanding of the sedimentary infill history of the South Qiangtang Basin, but also further support that the BNMO was already open in the Late Triassic and formed a mature ocean in the Early-Middle Jurassic, with subsequent rapid subduction. Both the opening and subduction processes were completed in a relatively short time interval with a large number of attendant olistostromes.