Huaichun Wu, China University of Geosciences (Beijing) (China)
Shihong Zhang, China University of Geosciences (Beijing) (China)
Qinghua Huang, Exploration and Development Research Institute of Daqing Oil Field Corporation Ltd. (China)
The Songliao Basin(SLB) in Northeastern China is one of the largest Cretaceous continental basins in the world. Well-preserved Cretaceous terrestrial deposits in this basin provide a unique opportunity for building the terrestrial type stages of Cretaceous in China(Stratigrphy Committee of China, 2002; Sha, 2007). The Upper Cretaceous Qingshankou Formation(K2qn), mainly composed of black mudstone and shales, is the most important source strata in the Daqing oil field (Yang et al., 1985; Li et al., 1995). It is also regarded as one of the best strata for studying the response of Cretaceous major geological events and global climate change in continental succession. However, the absence of reliable radiometric age and difficulties for age determination and correlation between terrestrial and marine fossils hinder a profound understanding of the geological processes in the K2qn(Yang et al., 1985; Wang et al., 2002; Sha, 2007).
The K2qn of the south well of the SLCORE-1(M206), the first terrestrial Cretaceous scientific core in the world, mainly consists of deep-lacustrine gray and black mudstone and siltstone interbedded with shale and marls. They are particularly suitable for high-resolution cyclostratigraphic studies and establishing astronomical time scales with a 0.02 to 0.40 Ma resolution(Rio et al., 2003; Hinnov and Ogg, 2007). We conducted a detailed evolutionary wavelet analysis and sliding windows of spectral analysis on the natural gamma-ray logging and density logging of the K2qn of the M206 section. The ratios of major periods of ∼39m, 13.5-9m, 5-3.8m and 2.5-1.7m in the natural gamma-ray and density logging is about 20:5:2:1, and they are considered to be caused by long and short eccentricity, obliquity and precession, respectively.
Our results also reveal that the accumulation rate of the K2qn varies slightly with mean sedimentary rate of 9.63cm/ka, and the accumulation time can be estimated as 5.16Ma. Two Gaussian band-pass filters were designed to establish a floating astronomical time scale for the K2qn of the M206 section. By filtering the signals of the long and short eccentricity cycles of 39m and 13.5-9m, a floating astronomical time scale was established, which recorded 12.7 long and 47.7 short eccentricity cycles. The original natural gamma-ray logging preserves 265 precession cycles, which means that the sedimentary processes were influenced intensively by the Milankovitch climate changes. According to the astronomical time scale, the duration of the lacustrine anoxic event 1(LAE1) in the K2qn is estimated as 250ka, which is close to the duration of ocean anoxic event 2(OAE2) at the Cenomanian-Turonian boundary(Prokoph et al., 2001; Kuhnt et al., 2004). The establishment of the floating astronomical time scale of the K2qn allows for high-resolution stratigraphic correlations between continental and ocean deposits, and precisely estimation of the duration of distinct paleoenvironmental and paleoecological events in the K2qn.