Establishment of Floating Astronomical Time Scale for the Terrestrial Late Cretaceous Qingshankou Formation in the Songliao Basin of Northeast China

Abstract

The Upper Cretaceous Qingshankou Formation (K 2 qn) in the Songliao Basin (SLB) of Northeast China, which consists of lacustrine mudstone and shale, is one of the most important rock sources in the Daqing oil field. However, the geochronology framework of the K 2 qn has been debated for a long time. In this study, detailed evolutionary wavelet analysis and sliding windows of spectral analysis were conducted on the natural gamma-ray logging and density logging data of the K 2 qn from the south well of the SLCORE–1 (Mao206), and the results reveal that the Mao206 section preserves stable Milankovitch cycles. The major cycles of ∼39, 13.5–9, 5–3.8, and 2.5–1.7 m are considered to be caused, respectively, by long and short eccentricity, obliquity, and precession. The accumulation rate of the K 2 qn varied slightly, and the mean accumulation rate was estimated as 9.63 cm/ka. The accumulation time of the K 2 qn of the Mao206 can then be estimated as 5.16 Ma. A “floating” astronomical time scale was established by filtering the long and short eccentricity cycles of ∼39 and 13.5–9 m, respectively. The K 2 qn was found to have 12.7 long and 47.7 short eccentricity cycles. The duration of the lacustrine anoxic event 1 (LAE1) in the K 2 qn is estimated as 250 ka, which is close to the duration of ocean anoxic event 2 (OAE2) at the Cenomanian–Turonian boundary. The establishment of floating astronomical time scale of the K 2 qn provides geochronological constraints on the duration of different kinds of geological events.