Late Triassic–Jurassic bivalve biochronology, palaeogeography and palaeoenvironment of the Junggar Basin, NW China

Abstract

Abstract The middle Mesozoic of the southern Junggar Basin is a source of abundant Late Triassic–Jurassic non-marine and Early Jurassic marine–littoral bivalves. The bivalve chronology provides a framework for dating the strata and documents Early Jurassic transgressions and the end-Triassic mass extinction in the Junggar Basin. The first occurrences (FOs) and last occurrences (LOs) of Utschamiella cf. tungussica and Utschamiella cf. obrutschevi lie in the basal upper Rhaetian. The FOs of Ferganoconcha sibirica , Ferganoconcha subcentralis , Unio manasensis , Unio mirabilis and Waagenoperna are at, and the FOs of Margaritifera isfarensis , Tutuella rotunda and Tutuella chachlovi adjoin, the base of the middle Sinemurian. The LOs of Yananoconcha hengshanensis and Waagenoperna are at the Lower–Middle Jurassic boundary. The LOs of Psilunio , Cuneopsis and F . subcentralis are near the Middle–Upper Jurassic boundary. Non-marine bivalves disappeared in the late Rhaetian, due to a sudden Norian–Rhaetian temperature drop. New forms did not return until the Sinemurian, when the climate warmed. The transgressions created low-relief terrestrial environments, in which organisms including bivalves thrived, leading to the formation of large quantities of coal, oil and gas. The Junggar Basin shifted from Arctic to subtropical latitudes in the Northern Hemisphere between the Early and Middle Jurassic.