Abstract: In the chronostratigraphy framework, utilizing the high resolution sequences base-level cycle to identify sequences boundaries, the Xiayoushashan formation of Nancan-2 well in Wunan area of Qaidam basin can be divided into two medium base-level cycles (Class-3 sequence). The changes in sedimentary environment and base-level cycles is interactional, so the sedimentary facies and paleoecology characters are significant evidences on the cycle division. The well sedimentary facies can be divided by references to sedimentary characters of fluvial and lacustrine sands and mudstone at Xiayoushashan formation of Xichagou outcrop, the paleoecology types include shallow water, deep water, fresh water and brackish water. Therefore, the isochronous stratigraphic framework of Wunan area is established, the relations between base-level cycles and paleontology responses are then analyzed.

Key words: Qaidam basin, Xiayoushashan formation, base-level cycle, sedimentary environment, paleoecology, sequence stratigraphic framework