Analysis of Base Level Cycles in Coarse Lower Huagang Formation of Oligocene on Eastern Slope of Xihu Sag

To the terrestrial coarse clastic strata in the lower Huagang Formation of Oligocene on Eastern slope of Xihu Sag, East China Sea, the sequence stratigraphic framework can be established by comparing the base level cycles. Long-term cycles can be defined by tectonic evolution stages. In the classification of medium-term cycle by system tract analysis and the stacking pattern of short-term cycles, a new feature reveals, i.e. a semi-cycle of accommodation space shallowing-upward can be identified under background of TST. The observation of core samples indicates that the micro-facies alteration of coarse clastic rocks is not necessarily related with the change of water depth, unsuitably used to define the short-term and ultra short-term cycle.