Eocene-Oligocene provenance evolutions of the Tiantai Slope Belt in Xihu Sag of the East China Sea Basin

The northern Tiantai Slope of the Xihu Sag is a hot spot for oil and gas exploration in recent years. However, the area is located in the intersection area between the Haijiao Uplift and the Yushandong Uplift, in which multi-source and various intensity influx existed in different periods, and their unclear provenance evolutionary patterns have restricted the characterization of sandbody and research into reservoir physical property. Therefore, we studied the evolution of provenance system in the study area using heavy mineral assemblage, detrital zircon U-Pb chronology, genesis-morphology, and seismic reflection characteristics. In addition, we combined the study of paleogeomorphology and fault system, clarified the evolutionary relationships of different provenance systems, and revealed significant differences in parent rocks between the Yushandong Low Uplift and the Haijiao Uplift. During the deposition of the Eocene Pinghu Formation, sediment supply from the Yushandong Low Uplift was continuously reduced, while that from the Haijiao Uplift gradually increased. By the time of the Oligocene Huagang Formation deposition, the area had evolved to be dominated by sediments from the Haijiao Uplift. Analysis indicated that the pre-existing NW-trending faults in the study area resulted in early NW-trending valleys. The early Pinghu Formation represents the syn-rift phase, during which the NE-NNE-trending faults developed under regional extension, creating a structural high in the west and a structural low in the east. Sediments were deposited proximally within fault transition zones along pre-existing valleys. During the middle-late stages of the Pinghu Formation when rifting-depressing transition occurred, an arcuate zone formed under the controlled of fault system, resulting in mixed-provenance characteristics. During the deposition of the Huagang Formation, the fault activity had weakened and the paleotopography had become flattened. Under the paleogeomorphic conditions, sediment supply from the Yushandong Uplift diminished, and a large-scale west-to-east drainage system from the Haijiao Uplift became dominant. This study provided methodology for discriminating multi-source sedimentary systems and a useful reference for similar research areas.