THE DANSGAARD ~ OESCHGER CYCLES-A DYNAMIC MECHANISM OF MARINE INCURSIONS DURING LATE CRETACEOUS NENJIANG(K2N1) FORMATION IN CHINA SONGLIAO BASIN

The Songliao Basin in China is the largest continental petroliferous basin in the World and the Late Cretaceous Nenjiang (K₂n¹) Formation is a set of representative high-quality source rock with high TOC content. Lots of predecessors have explored the origin mechanism of Nenjiang source rocks, the paleontological and organic geochemical evidences as well as biomarkers all indicates that marine incursions during K₂n¹ exert a significant impact on the formation of Nenjiang source rocks. However, there are few studies on the dynamics of marine incursions.

To explore the dynamic mechanism of marine incursion during K₂n¹, we take high-resolution Chlorine series (average 1.33ka time paces and 812ka long) in the Nenjiang Formation (K₂n¹) from SK1(s) borehole as studying materials. In consideration of the nonlinear and non-static characteristics of geological data, the Chlorine series is been decomposed into ten intrinsic components (IMF) using the empirical mode decomposition method, and then the spectrum analysis approached through REDFIT program is pursued for the first five components to obtain the main frequencies in the series.

Spectral results, above 90% confidence level, of intrinsic module functions can recognize remarkable millennial-scale to ten-millennial-scale periodicities. The evident millennial periodicities of 3.0~8.5ka are in pace with Dansgaard~Oeschger (D~O) cycles, especially the 3.0ka cycle, double of 1.47ka. The ten-millennial-scale signals of 17.5ka, 20.4ka and 43.3ka, 49.2ka are coincided with orbital precession and orbital obliquity and 12.3ka maybe half of precession reflecting the solar insolation forcing. Combined with the variance contributions of IMFs, the orbital-forced ten-millennial-scale signals show low variances (5.3%), thus the marine incursions are mainly controlled by high-frequent D~O cycle. Until now, there are several prevailing interpretations to the trigger mechanism of this D~O cycle, including orbital forcing, tidal effects and solar forcing. In consideration of the lack of polar ices during Cretaceous Greenhouse and low variance contribution of orbital signals in our time series, we think the millennial D~O cycle maybe initiated by tidal effects and solar forcing. At last, the further studies should be conducted to ascertain the detailed dynamic mechanism.