Abstract: Recent results from the regional geologica survey of the 1:50 000 Gaowu and Zaibian Sheets in southeastern Guizhou have disclosed that the Xiajiang Group consists of the rift wedge strata which are overlain upon the Mesoproterozoic Sibao Group. More precisely, the sedimentary basal conglomerates from the Jialu Formation at the bottom of the Xiajiang Group are overlain with a high angle (unconformably)upon the flysch turbidites, or upon the Motianling granites intruded into the Sibao Group. The Motianling granite sample (S1)collected beneath the onlap boundary gives a TIMS U-Pb zircon age of 825 ±2.4 Ma, implying that the deposition within the Neoproterozoic rift systems began at the time later than 825 ±2.4 Ma. The syndepositional basic volcanic rock sample (S2)collected from the bottom of the Jialu Formation above the boundary gives a TIMS U-Pb zircon age of 814 ±13 Ma, which approximately represents the initial age of the Neoproterozoic sedimentary onlap in the study area, and approaches that (820 Ma)in other regions in southern China. The results of research in this study strongly support the viewpoint that the initial age of the sedimentary onlap within the Nanhua rift systems should be dated at ca. 820 Ma.