A Floating Astronomical Time Scale for the Early Late Cretaceous Continental Strata in the Songliao Basin, Northeastern China

A continuous terrestrial succession was recovered from the Songke-2 (SK-2) borehole in the Songliao Basin, Northeastern China. This borehole provides a unique material for further research on the continental paleoclimate during Cretaceous greenhouse period, following a series of achievements of the Songke-1 (SK-1) core. In this study, thorium (Th) logging data were chosen as a paleoclimate proxy to conduct a detailed cyclostratigraphic analysis. The Th series varies quasi-periodically; power spectra and evolutionary fast Fourier transformation (FFT) analysis reveal significant cycles in the Quantou (K-2q), Qingshankou (K-2qn), Yaojia (Ky and Nenjiang (K-2n) formations. The ratio of cycle wavelengths in these stratigraphic units is approximately 20:5:2:1, corresponding to long orbital eccentricity (405 kyr), short orbital eccentricity (100 kyr), obliquity (37 kyr), and precession cycles (22.5 kyr and 18.4 kyr). The durations of the K-2n, K-2y, K-2qn and K-2q are estimated as 6.97, 1.83, 5.30 and 4.52 Myr, respectively, based on the constructed similar to 18.62 Myr "floating" astronomical time scale (ATS). Comparison of the durations between the SK-1 s and SK-2 boreholes exhibits a slight difference of 0.06 Myr and 0.459 Myr for K-2qn and K-2y. Nevertheless, our ATS of K-2n supports the chronostratigraphic frame constructed by the CA-ID-TIMS data of the SK-1s borehole. This new "floating" ATS provides precise numerical ages for stratigraphic boundaries, biozones and geological events in the Songliao Basin, and can serve as a basis for correlation of strata and events between marine and terrestrial systems.