Abstract: The relationship between Xiaoquangou group of the Middle-Upper Triassic and underlying Cangfanggou group of the Lower Triassic in Junggar basin has been considered to be conformable and continuous. However, a great number of information indicate that the central and northern parts of this basin belong to a large unconformity in regional distribution, and the upper Cangfanggou group is absent to a large extent except for the southern area of the basin. The discovery of this unconformity has been delayed 5 years, while the universal confirmation of it has delayed more than 20 years due to restriction of traditional ideas. The key for thoroughly solving this big problem attributes to the drillings of Moshen-1 well and Shimo-1 well.

Key words: Junggar basin, Middle-Upper Triassic, Lower Triassic, contact relation, unconformity, Indosinian movement