Base level cycles division and switch mechanism of oligocene epoch Yacheng formation in Qiongdongnan basin

Abstract：Based on the data of borehole cores, geophysical loggings, seismic reflections, the characteristics and switch mechanism of base level cycles of Oligocene Epoch Yacheng forma- tion in the Qiongdongnan basin were studied. The lithologies of core samples was used to iden- tify short-period base-level cycles. The logging curves, calibrated with the core data, were used to recognize the mid-period cycles. A template using logging curves to identify the hierar- chy of middle and short-period cycles wasestablished, which can recognize the base level cyclein non-coring section. The termination styles of reflections in seismic profile, including onlap, downlap and truncation, which reflect the incompatibility of strata, can identify the long and super long-period cycles. By restoring the thickness of Yacheng formation by a strata-stripping method, the tectonic subsidence rate of each sag of 312.5--750 m/Ma was achieved. The results show that the Yacheng fromation can be subdivided into 1 super long period, 3 long period and at least 12 mid-peroiod cycle sequences. The short-period cycle types are complete, but their number varies largely in different locations. The tectonic subsidence rate was much higher than the global sea-level change rate of 0.2--40 m/Ma, which show that the development of base-level cycles and switch mechanism in this area were controlled more obviously by tectonic subsidence than by global sea-level change.