Abstract:

As a rare and distinctive geomorphological feature in arid regions around the world, a shell ridge in the Qaidam Basin preserves many bivalve fossils of different sizes and articulated shells, which provides important record for the paleoclimate and paleoenvironmental evolution of the middle-late Pleistocene NE Tibetan Plateau. The Nuomuhong Shell Ridge is newly discovered as fluvial sediments, which is different from early studies of lacustrine deposits. Through systematic analyses, we established eight ostracod assemblages for the Shell Ridge. Though scarce ostracod fossils were found in Assemblage 1 and they were hardly identified, fossils in other assemblages were relatively abundant. We named Assemblage 2 to 8 as follows: Ilyocypris-Candona-Candoniella, Ilyocypris-Candona-Leucocythere-Candoniella-Eucypris inflate, Leucocythere-Ilyocypris, Ilyocypris-Leucocythere-Candoniella-Candona, Ilyocypris-Candoniella-Leucocythere, Ilyocypris bradyi-Ilyocypris echinata and Ilyocypris bradyi-Candoniella. We clarified water evolution of the Shell Ridge based on ostracod fossil characteristics and their paleoecology information. We identified the Nuomuhong Shell Ridge fluvial deposits that went through frequent river diversions within the Assemblage 1 to 5 occurrence, during which the shell ridge sedimentation fluctuated between oxbow lake and shallow river environments. The shallow river environment was favorable for the living of ostracods, whereas the deeper water of oxbow lake was detrimental to their survival. The shallow river condition continued through the time of Assemblage 6 to 8, during which ostracods thrived in warmer climate till the river enclosed and salinity increased, which hampered ostracod survival. Subsequently, salt crust was likely formed after persistent droughts and the deposition of Nuomuhong Shell Ridge ended.

Key words: Qaidam Basin, Nuomuhong Shell Ridge, ostracod, fluvial deposit