Ostracod characteristics of the eastern Hetao Basin and its sedimentary environmental significance during the Middle and Late Quaternary

By the comprehensive analysis of lithology, lithofacies and ostracod biological assemblages of the DTLC core in the eastern Hetao Basin, the ostracod biostratigraphy and its sedimentary environmental significance during the middle and late Quaternary was emphatically discussed. The conclusions are as follows:In the early stage of Middle Pleistocene (Q₂¹), it reflected a fluvial-lacustrine alternative sedimentary environment mainly dominated by alluviation with no ostracod fossils. In the late stage of Middle Pleistocene (Q₂²), it reflected a closed brackish and salt water lake environment under cold and dry climatic condition with a brackish-salt water lacustrine ostracod enrichment characterized by Leucocythere plethora-Eucypris inflate-Limnocyther dubiosa. During the early period of Late Pleistocene (Q₃¹), it represented a closed brackish water and deep lake environment with a relative lacustrine ostracod enrichment characterized by Leucocythere plethora-Limnocythere dubios. During the late period of Late Pleistocene (Q₃²), it mainly reflected an alluvial-proluvial environment with a small number of freshwater ostracods characterized by Ilyocypris manasensis-Candoniella albicans-Cypria luminosa. It was a river-floodplain environment during the Holocene (Q₄). According to the comprehensive analysis of the regional sedimentary environment of the Hetao Basin, it is concluded that the Hetao Basin was a flows-in lake basin where the Yellow River was an inner river from the late stage of Middle Pleistocene (Q₂²) to the early period of Late Pleistocene (Q₃¹). Since the late period of Late Pleistocene (Q₃²), the Yellow River gradually turned into an outflow river and the Hetao Basin became an outflow lake basin.