The Nenjiang Formation responses to astronomically forced climate changes during the Upper Cretaceous in the Gulong Area, Songliao Basin, NE China

Abstract. The Songliao Basin preserves a complete Cretaceous continental sedimentary succession. Previous researchers have conducted extensive cyclostratigraphic studies on the Upper Cretaceous Nenjiang Formation, but paleoclimatic research has primarily focused on the lower part of the formation. The relationship between climatic changes and astronomical parameters in the middle and upper sections of the Nenjiang Formation has not yet been fully investigated. This study conducted cyclostratigraphic analysis of the second to third members of Nenjiang Formation Late Cretaceous (K₂n^2-3) in the Songliao Basin using high-resolution gamma-ray logging (GR) data, while also performing paleoclimatic analysis in conjunction with palynological data. The fossil spores and pollen from the K₂n³ indicate a geological age of the Late Cretaceous Campanian. Based on the ecological data of the parent plants of the spores and pollen, the vegetation types, climatic zones, and humidity types of the lower part of the K₂n³ were analyzed, confirming its semi-warm and semi-humid paleoclimatic characteristics. Time-series analysis reveals persistent 405-kyr (long eccentricity), 95-kyr (short eccentricity), 38.3-kyr (obliquity), and 22.8-kyr (precession) sedimentary cycles, further validated by an optimal sedimentation rate model. We established a 3.24-Myr floating astronomical time scale for K₂n^2-3 by tuning the GR series to orbital targets. This timescale is anchored by a volcanic ash age (83.269 ± 0.044 Ma) at the K₂n² base, generating an absolute age framework spanning 83.33–80.00 Ma. Vertical transitions in palynofloral assemblages correspond to the 38.3-kyr obliquity cycle, indicating significant obliquity forcing on regional paleoclimate dynamics.