Control effects of the synsedimentary faults on the basin-marginal fans in the central part of the deep-water area of early Oligocene Qiongdongnan Basin, South China Sea

The synsedimentary faults and basin-marginal fans located in the central part of the deep-water area of the early Oligocene Qiongdongnan Basin have been investigated using seismic profiles, boreholes, and well-log data. Through the formations of the characterized paleogeomorphology, such as transverse anticlines, fault ditches, and step-fault belts, the synsedimentary faults are known to have controlled the development position, distribution direction, and extension scales of the basin-marginal fans. For example, at the pitching ends of two adjacent faults, transverse anticlines developed, which controlled the development positions and distributions of the fans. During the early Oligocene, the faults controlled the subsidence center, and fault ditches were formed at the roots of the faults. In the surrounding salient or low salient areas, which were exposed as provenance areas during early Oligocene, the fault ditches acted as the source channels and determined the flow paths of the clastics, where incised valley fills were obviously developed. The fault ditches which developed in the sedimentary basins were able to capture the drainage systems and influenced the distributions of the fans. The large boundary faults and the secondary faults generated two fault terraces and formed step-fault belts. The first fault terrace caused the clastics to be unloaded. As a result, fans were formed at the entrance to the basin. Then, the second fault terrace caused the fans to move forward, with the fans developing in a larger extension scale. The results obtained in this study will potentially be beneficial in the future prospecting activities for reservoirs and coal-measure source rocks in the basins located in the deep-water areas of the South China Sea.