Environmental magnetic sequences of the terrestrial Late Cretaceous Qingshankou Formation of CCSD-SK-I south borehole in Songliao Basin, Northeast China and their paleoclimate implications

China Cretaceous Continental Scientific Drilling-Songke I(CCSD-SK-I) south borehole was drilled through the Late Cretaceous lacustrine sediment in the central depression of Songliao Basin, Northeast China, in order to compare the marine and terrestrial Cretaceous records and understand the Cretaceous climate change. The top of Quantou Formation (K2q), Qingshankou Formation (K2qn), Yaojia Formation (K2y) and the lower part of Nenjiang (K2n) Formation were recovered. The K2qn is 496.8 m thick, from 1286 m to 1782.8 m in the well log. The lower part of K2qn (K2qn1) mainly consists black shale and mudstone. The upper part of K2qn (K2qn2+3) is composed mainly of thick, gray to dark-gray mudstone, but changes to thinly laminated, red silty sandstone and mudstone towards the top. Magnetic susceptibility (χ) and susceptibility of anhysteretic remanent magnetization (χARM) were achieved at 5 cm intervals for K2qn1 and at 10 cm intervals for K2qn2+3. The χ values are in the range of ~5×10-8-3×10-7 m3kg-1 and the χARM values are between ~7×10-8 and 2×10-6 m3kg-1. Detailed mineral magnetic measurements were conducted for K2qn. Temperature-dependence of magnetic susceptibilities (χ/T curves) were measured for selected samples from K2qn. For K2qn1, saturation isothermal remanent magnetization (SIRM), S-ratio (IRM-100mT/SIRM), medial destroyed field of ARM (MDFARM) were measured and thermal demagnetization of ARM and SIRM and Lowrie experiment were finished for typical samples. The results indicate that the major remanence-carrier is magnetite. Spectral and continuous wavelet analyses on the full time series were carried out. Results of the χ sequence reveal four distinct cycles of 40.5-21.8 m, 13.1-8.9 m, 4.1-3.1 m and 2.3-1.7 m. The ratio of these major periods is 20:5:2:1, which matches well with the Milankovitch cyclicities of 405 ka:100 ka:40 ka:20 ka (long eccentricity, short eccentricity, obliquity, precession). The ratio of the major periods of 18.6-7.7 m, 3.8-3.0 m and 2.2-1.8 m from the χARM sequence is 5:2:1. It is also similar to the Milankovitch cyclicities of 100 ka: 40 ka: 20ka. So the sedimentary cyclicity discovered by the magnetic sequences in CCSD-SK-I south core was formed by orbital forcing (Weedon, 2003). The major periods of 40.5-21.8 m, 13.1-8.9 m, 4.1-3.1 m and 2.3-1.7 m were formed probably by long (405 ka) and short (100ka) eccentricity, obliquity (40 ka) and precession (20 ka), respectively. This result is consistent with what discovered from the logging records (Wu et al., 2009). Therefore, χ and χARM have provided the good proxies for high resolution paleoclimate changes research of Cretaceous in Songliao basin. References Weedon G, 2003. Time-series analysis and cyclostratigraphy. Gambridge University Press, Gambridge, pp. 1-259. Wu H C, Zhang S H, Jiang G Q, et al., 2009. The floating astronomical time scale for the terrestrial Late Cretaceous Qingshankou Formation from the Songliao Basin of Northeast China and its stratigraphic and paleoclimate implications. Earth and Planetary Science Letters, 278: 308-323.