Abstract:The large-scale shelf-margin delta and deep-water fan systems developed in the Zhuhai Formation of the south subsag of Baiyun Sag are revealed by 3D seismic and drilling data. So far, researchers have not yet studied in detail the evolution characteristics of the Zhuhai Formation and clarified the evolution law of the shelf break zone during the transition from shallow-water shelf depositional environment to deep-water continental slope in the Zhuhai Formation and its relationship with deep-water deposition. Thus, this paper makes a detailed analysis of the sedimentary system structure of the shelf break zone in each sedimentary period with the new 3D seismic data over 2000km2 in the study area and the fine interpretation results of more than 20 seismic profiles by referring to the latest research progress on the shelf break zone. Further, the paper identifies the evolution characteristics of each stage and summarizes the formation patterns and main controlling factors of the shelf break zone, with the following results obtained. ① The shelf break zone of the south subsag of Baiyun Sag is distributed in the NE-SW direction, and the progradation distance of the shelf break zone varied in different periods. From the fourth member to the first member of the Zhuhai Formation, the width of the shelf break zone decreased, which gradually advanced toward the SE. The average slope of the shelf break zone is 0.96°~4.76°, the seaward advance distance 21~28 km, and the progradation rate about 0.9~10.6 km/Ma. There are obvious differences in the characteristics of the shelf break zone between the north and the northwest. ② The evolution of the shelf break zone can be divided into three stages:the initial formation period (sedimentary period of the fourth member of the Zhuhai Formation), the development period (sedimentary period of the third member of the Zhuhai Formation) and the stable period (sedimentary period of the second and the first members of the Zhuhai Formation). During the development period, the shelf break zone advanced rapidly seaward, and its shape changed from bending to straight. In the stable period, the shape of the shelf break zone changed from jagged to smooth (sedimentary period of the second member of the Zhuhai Formation) and finally stabilized (sedimentary period of the first member of the Zhuhai Formation). ③ The evolution of the shelf break zone is influenced by the differential supply of sediments, sea level changes, paleo-uplifts, etc. Adequate provenance supply is the key to the formation of the shelf break zone. The differential supply of double provenance results in the difference in the shape of the shelf break zone in the north and northwest. The relative sea level rose first and then fell which modified the layout of the shelf break zone. The overall tectonic activity was stable during the depression period, and the paleo-uplift limited the range of the shelf break zone. The research is of important significance for the sedimentary evolution and oil and gas exploration in the deep-water area of the northern South China Sea.
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