The Huoshiling Formation (HF) is the first basin filling sequence of the Songliao Basin during the fault depression stage. It is also one of the primary target formations of the International Continental Scientific Drilling Program SK2. Considerable debate persists regarding whether the SK2 Well penetrated the HF after the completion of drilling in 2018. The depth of the HF in SK2 ranges from 5613.41m to 5959.84m, with a thickness of 346.43m, and it is marked by a layer of tuff, which serves as the boundary with the overlying Shahezi Formation (SF) . The HF in SK2 is characterized by a high content of volcanic materials and mainly consists of pyroclastic rocks, sedimentary pyroclastic rocks, and volcaniclastic sedimentary rocks. The GR and RRD curves of the HF in SK2 show significant fluctuations and unstable characteristics; seismic data exhibit good continuity of cophasal axes, strong amplitudes, and high frequencies; and the sedimentation-filling rate is significantly lower than that of the SF. The HF is now defined as the first volcanic. sedimentary filling sequence that is unconformity on the basement, and overlain by the SF, or locally by Yingcheng (YF) /Denglouku/Quantou formations, exhibiting either conformable or unconformable contacts. It represents an early stage of intense volcanic activity in the fault depression stage. The evolution of fault depressions in the northern and western parts of the Songliao Basin is controlled by the northern Mongol-Okhotsk tectonic regime, while the evolution of depressions in the central and southeastern parts is controlled by the eastern Circum-Pacific tectonic regime. Differential tectonic regimes, the distance effect of plate subduction, and the migration of magma centers are essential reasons for the spatiotemporal variations in the initiation time and scale, as well as the volcanic-sedimentary filling and scale in fault depressions in the Songliao Basin. The basin fillings of the Songliao Basin during the fault depression stage overall experienced processes of volcanic-sedimentary construction, including the HF and YF, which were sandwiched between a volcanic quiescence period represented by the SF and terrestrial clastic sedimentation. However, there are significant differences in the initiation time, basin filling duration, and filling sequence of the fault depressions. The initiation time of the fault depressions and the onset of sedimentation of the SF in the Songliao Basin exhibit the characteristic of being earliest in the north, followed by the south, and latest in the central part. The volcanic-sedimentary construction of the HF is widely distributed within the basin. While the volcanic. sedimentary construction of the YF is mainly concentrated in the central part of the basin. The stratigraphic unit of the fault depression stage in the Songliao Basin may exhibit variability in isotopic ages. Since the end of the YF (ca. 103 similar to 104Ma), the fault depression stage of the basin ended, and the entire basin began to subside uniformly, establishing a consistent temporal framework for each stratigraphic unit thereafter