Astronomically Forced Variations in Well Logs of Lower Cretaceous Terrestrial Formations from the Songliao Basin, China

The International Continental Scientific Drilling Project (SK-2) in the Songliao Basin has obtained abundant well logging data from the Lower Cretaceous Shahezi Formation, providing a valuable archive for understanding terrestrial paleoenvironment and paleoclimate changes during the Early Cretaceous. The Shahezi Formation in the SK-2 borehole is dominated by fan-delta and lacustrine deposits, consisting primarily of mudstone, fine sandstone, coarse sandstone, and conglomerate. The correlation coefficient (COCO) and time scale optimization (TimeOpt) are applied to identify astronomical cycles from conventional, natural gamma-ray spectroscopy and elemental capture spectroscopy logs. The results show that resistivity, thorium content, natural gamma-ray, clay content and quartz content logs record astronomical cycle signals. Based on astronomical tuning, the duration of the 3342–5621 m interval of the Shahezi Formation in the SK-2 borehole is estimated to be about 5.1 Myr, which agrees well with the absolute age constraints. This study helps to understand the response of terrestrial environment to climate change in the Early Cretaceous.