Astrochronology of the terrestrial mid-Cretaceous Quantou and Denglouku formations in the SK-3 borehole from the Songliao Basin, Northeastern China

The continental scientific drilling borehole Songke 3 (SK-3) in the Songliao Basin, Northeastern China, recovers a continuous mid-Cretaceous stratigraphic record which offers a unique opportunity to study terrestrial climate and environmental change. In this study, high-resolution gamma-ray (GR) data were utilized as a paleoclimate proxy to conduct the cyclostratigraphic analysis of the Quantou Formation (K₂q) and Denglouku Formation (K₂d) within the SK-3 borehole. The cyclic variations in GR values, which reflect fluctuating clay influxes, document Cretaceous Milankovitch cycles including the 405-kyr long eccentricity, ∼100-kyr short eccentricity, ∼38-kyr obliquity, and ∼20-kyr precession cycles. Astronomical tuning of GR data series to the 405-kyr and 100-kyr eccentricity cycles yields an 8.89 ± 0.32 Myr duration for the K₂d² to K₂q interval at SK-3. Based on the age of 91.81 ± 0.22 Ma at the boundary of Qingshankou (K₂qn) and K₂q formations derived from Songke 1 south borehole (SK-1s), our newly constructed astronomical time scale (ATS) suggests that the Turonian/Cenomanian and Cenomanian/Albian boundaries are at the depth ∼1055.5 m within the mid-K₂q³ and ∼2350 m within the lower K₂d², respectively. Our study also indicates the complex processes of the sedimentary system in response to the Milankovitch cycles. The eccentricity cycles are more effectively preserved in the lacustrine-deltaic facies. In contrast, both obliquity and precession cycles exhibit better preservation in the meandering river and braided river facies.