Abstract: The Bonaparte Basin is rich in oil and gas resources.There occur more than one thousand meters of Cenozoic shallow marine carbonate deposits.At the bottom of the Miocene,there is a mixed sequence of siliciclastic and carbonate sediments up to 12~52 m thick.The terrigenous siliciclastics in the mixed sequence,however,have not been carefully studied so far as a potential reservoir by researchers.Based on the logging and 3D seismic data,the authors found that the alternation of terrigenous siliciclastic delta facies and shelf-derived carbonate platform facies in space is commonly seen in the northern Bonaparte Basin.The deposition of the mixed sediments depends upon regional tectonics and relative sea level changes.In the late Oligocene,regional tectonic uplifting caused erosion of the source area.A great amount of terrigenous siliciclastics were brought into the basin which was dominated by the deposition of carbonate platforms.In the early stage of the Miocene,when the region was tectonically stabilized again,short-term relative sea level changes played significant roles in the formation of the mixed sediments through affecting the re-distribution of terrigenous and the production of carbonate sediments.