CYCLOSTRATIGRAPHY AND ORBITAL TUNING OF THE TERRESTRIAL UPPER TURONIAN–LOWER DANIAN IN SONGLIAO BASIN, NORTHEASTERN CHINA

The Cretaceous represents one of the most remarkable peri-ods in Earth history with a “greenhouse climate”. Two overlapping drillcores (SK-1n (north core) and SK-1s (south core))，covering Late Turonian to Early Paleocene strata, were recovered with a total length of 2485.89 m in central part of the Songliao Basin in northeastern China, which is one of the largest and long-lived Cretaceous continental basins in the world. These cores provide a unique opportunity for studying the continental climate/environmental changes in the Cretaceous greenhouse world.

Establishing a high-resolution chronostratigraphic framework for these drillcores is the essential first step for studying the terrestrial paleoclimate signals and their correlation with marine records. Here we use natural gamma-ray (GR) log, thorium (Th) log, and magnetic susceptibility (MS) data as paleoenvironmental and paleoclimatic proxies to conduct a detailed cyclostratigraphic study on the SK-1 core. Power spectra, evolutionary fast Fourier transformation and wavelet analysis all reveal significant decameter-to meter-scale sedimentary cycles in SK-I core. The ratios of cycle wavelengths in these stratigraphic units are ∼20:5:2:1, and are interpreted as Milankovitch cycles of 405 kyr and 100 kyr eccentricity, 38.4 kyr obliquity and 20 kyr precession cycles, respectively. A duration of 27 myr astronomical time scale (ATS) is established by tuning filtered 405 kyr eccentricity cycles to a target curve of the astronomical solution La2010 based on four SIMS U–Pb zircon ages and magnetostratigraphic time framework of the SK-1 borehole. This ATS provides precise numerical ages for stratigraphic boundaries, biozones, geological and geophysical events, and serves as a basis for correlation of strata and events between marine and terrestrial systems. A ∼3.8myr-long hiatus between the Nenjiang (K2n) and Sifangtai (K2s) formations occurs from 76.1 to 79.9 million years ago. The ages and durations of magnetochrons C33r to C30n are precisely estimated and provide new constraints on the Late Cretaceous Geomagnetic Polarity Time Scale (GPTS) and South Atlantic sea-floor spreading rates.