The Songliao Basin's tectonic evolution comprises three stages: the syn-rift stage, post-rift stage, and structural inversion stage. The Yingcheng Formation and the Denglouku Formation record the strata corresponding to the transitional period from the syn-rift stage to the post-rift stage. However, the stratigraphic boundary between these two formations and the sedimentation time of the Denglouku Formation remain controversial. This paper undertakes a zircon isotope chronology study on the tuff from the lower part of the Denglouku Formation, based on the continuous coring data from the International Continental Scientific Drilling Program (ICDP) boreholes SK2 and SK3 in the Songliao Basin. The LA-ICP-MS U–Pb age of the tuff at 2359.35 m depth in the lower part of the Denglouku Formation in borehole SK3 is determined to be 105.4 ± 0.6 Ma, constraining the initial sedimentation time of the Denglouku Formation, while an age of 103.4 ± 0.8 Ma from the tuff at 2970.65 m at the base of the Denglouku Formation in borehole SK2 marks the end of the first member's sedimentation. By integrating the existing chronostratigraphic data with basin-scale stratigraphic correlations from the Songliao Basin, this study re-evaluates the controversial sandy conglomerates within the fourth member of the Yingcheng Formation in the northern Songliao Basin, demonstrating their affinity to the first member of the Denglouku Formation.