Rock magnetic records of the Qingshankou Formation of SK-1 south borehole in Songliao Basin, Northeast China, and their paleoclimate implications

The south borehole (SK-1s) of the China Cretaceous Continental Scientific Drilling project (Songke 1) penetrated the Late Cretaceous lacustrine sediments in the central depression of the Songliao Basin, Northeast China, in order to reveal the terrestrial records of the Cretaceous climate changes. High-resolution rock magnetic records were obtained on the Qingshankou Formation (K2qn) of SK-1s. The results indicate that detrital magnetite is the main remanence-carrier of the K2qn Formation; while both magnetite and paramagnetic minerals, including clay minerals and pyrite, dominantly contribute to magnetic susceptibility. Although reductive diagenesis may have more or less effect on the magnetic signals of the K2qn Formation, they recorded ancient lake-level changes well, resulting both from the tectonic development of the basin and from the Milankovitch astronomical paleoclimatic changes. The first-order vertical trends of the rock magnetic records were controlled by the shallowing-upwards of the lake-level, caused by the tectonic uplift of the basin. The short-term variations of the rock magnetic records were controlled by the lake-level periodical drift, caused by the fluctuations of the rainfall of the catchment area of the basin, which were controlled by astronomical orbital forcing. In addition, the magnetic records also distinguished two most anoxic periods during the lacustrine anoxic event 1 (LAE1): the sections of 1758–1761 m and 1769–1771.5 m, which may suggest the two most active periods of surface productivity of the lake.