Chemical weathering characteristics of the Late Cretaceous Nenjiang Formation from the Songliao Basin (Northeastern China) reveal prominent Milankovitch band variations

Abstract

The Songliao Basin (SLB) in Northeast China has a succession of terrestrial sedimentary deposits that span much of the Cretaceous. The sedimentary records recovered from SLB allow us to investigate the relationship between astronomically forced climate and clastic sediment accumulation in a terrestrial setting from the mid to high latitudes. Here, we compiled high-resolution X-Ray Fluorescence (XRF) datasets (K/Ti, Rb/Sr, Ca/Ti), and gamma ray (GR) logging data from Member 2 of the Nenjiang Formation in the SK-1n borehole. Power spectral analysis of the GR data revealed that sedimentary cycles had wavelengths of 27.4 m, 6.56 m, 2.60 m, and 1.27 m. These wavelengths are interpreted as the orbital cycles of long-eccentricity, short-eccentricity, obliquity, and precession, respectively. We established an astronomical time scale by assuming that the 27.4 m cycles represent ~405 kyr eccentricity. We also applied correlation coefficient (COCO) analysis to identify the astronomical forcing periods. The chemical weathering history was classified by the relative changes of Rb/Sr and K/Ti ratios. The lower Rb/Sr and higher K/Ti values correspond to weak weathering, interpreted as arid periods, and vice versa. Ca/Ti ratios correlate with the humid periods and we attribute this correspondence to a salinity control on Ca/Ti. Furthermore, the unconventional 173-kyr obliquity cycle associated with the secular frequency interference of orbital inclinations between Earth and Saturn was recognized and manifested by humid/arid cliamtic oscillations, suggesting that high obliquity contributed to strengthening the hydrological cycles in SLB and bringing more precipitation and runoff towards the basin. The unique paleogeographic location and the existing paleoclimate simulations of SLB further demonstrated the existence of paleomonsoon variations in East Asia during the Late Cretaceous.