ASTRONOMICALLY FORCED DEPOSITION IN THE EARLY CRETACEOUS SONGLIAO SYNRIFT BASIN, CHINA AND ITS PALEOCLIMATIC IMPLICATIONS

Driving forces for Early Cretaceous climate and environmental change remain speculative and disputed, due in large part to time scale uncertainties and a lack of information on terrestrial environments. Fortunately, recent exploration of the Songliao Synrift Basin in northeast China now provides a new opportunity to improve the Early Cretaceous time scale and unlock the mysteries of Early Cretaceous terrestrial climate change. In this study, we conduct cyclostratigraphic analysis on natural gamma-ray logs from extended boreholes from the terrestrial Songliao Basin. The target is the Lower Cretaceous Shahezi Formation (K₁S), a 836-m-thick succession with black and dark grey mudstone, siltstone, fine grained sandstone, gravel-bearing sandstone and conglomerate, together with meter-scale black coal units distributed throughout the upper part of the formation. The mudstones have the highest gamma-ray values and the conglomerates the lowest gamma-ray values. Cyclic evolution of the lithologies indicates a paleolake that expanded and contracted repeatedly. Time series analysis of the gamma-ray logs from selected boreholes reveals power spectra that are consistent with Earth’s astronomical periodicities of precession, obliquity and orbital eccentricity, providing strong evidence for astronomically driven climate changes in this Early Cretaceous basin. Sedimentation rates significantly decreased from lower to upper K₁S as the basin evolved from synrift toward post-rift conditions. Based on the interpreted astronomical cycles and other available chronostratigraphy, we conclude that the age of K₁S is approximately early Valanginian to late Hauterivian. An abrupt decrease in gamma-ray values in the lower K₁S may reflect the well-known Weissert Event, which was a transient cooling event in the mid-Valanginian accompanied by polar ice as evidenced by marine glendonites at Svalbard Island, and a positive ¹³C excursion in marine carbonates and terrestrial organic matter.