Abstract: Based on analysis of micropaleontology on the cuttings and core samples from Jianshi Well 1 in Jianshishan area of middle Qaidam basin, a few ostracoda fossils dominated by Eucypris obtus and some others like Eucypris stenoformi, Cyprideis and Cyprinotus (Heterocypris) have been found in the Upper and Lower Youshashan formations. The era distribution of these fossils indicates that the strata with ostracoda in this well belongs to Miocene, abundant Cyprideis-bearing strata should be the top boundary of Lower Youshashan formation. The ecological distribution of the ostracoda shows that the sedimentary environment of Upper Youshashan formation and Lower Youshashan formation belongs to seldom-real brine lake, and the former is more salty than the latter. From Miocene to Pliocene, the lake was transported eastward and underwent the evolution from shallow to deep, then to shallow with brine being gradually salty in this area.

Key words: Qaidam basin, paleogeographic environment, ostracoda, Miocene, stratigraphic division