Abstract: The present paper focuses on the boundary and the sedimentary strata between the middle submember (E₂ h₂²) and upper submember (E₂ h₂¹) of the second member of the Hetaoyuan Formation in the Xidagang outcrop areas, Nanyang depression. The authors contend on the basis of thin section examination, granulometric analysis, X-ray diffraction patterns and geochemical signatures that the boundary should be a breakup unconformity rather than the formerly "palaeoexposure surface". This breakup unconformity resulted from the highly extension of the Quwa fault, a NE-trending fault, and represents the transition from the southern faults to the eastern faults in the Nanyang depression.