Transition from Tide-influenced Delta to Tide-dominated Estuary in the East China Sea Shelf Basin: Implication for Changes from Source to Sink Controlling on Sedimentation

The East China Sea Shelf Basin (ECSSB) was a back-arc basin located in the active continental margin of the western Pacific Plate. This study aims to illustrate the facies distribution and architectures of delta and estuary... [ view full abstract ]

The East China Sea Shelf Basin (ECSSB) was a back-arc basin located in the active continental margin of the western Pacific Plate. This study aims to illustrate the facies distribution and architectures of delta and estuary systems in tidal conditions, and demonstrate how the changes from source to sink induced the transition from the deltaic to estuary environments in the back-arc basin. On the basis of sedimentary cores, wireline well-logs, and high resolution seismic data, sedimentary architectures and models of the depositional systems were reconstructed. In the southern Xihu Sag, the Pinghu Formation spanning in the Eocene is interpreted as deltaic progradations with facies assemblages of distributary channels and bars, interdistributary bays, and pro-delta mud on the shelf with tidal influence. The Huagang Formation in the Oligocene was interpreted as tide-dominated estuary deposits with facies of tidal channel-bar complexes and tidal flats. An unconformity was developed between the Pinghu and Huagang Formations, corresponding to the tectonic event of Yuquan movement, which triggered the turnaround from rifting to depression of the Xihu Sag in the back-arc basin of ECSSB. In the Late Eocene, rifting stage of the Xihu Sag undergone the high rates of subsidence (163 m/Ma) and sedimentation (17.12 cm/ka) with sea level fall led to the deltaic progradations with slight tidal influence during regressions. After the Yuquan Movement, in the Early Oligocene the Xihu Sag experienced in depression stage developing relatively low rates of subsidence (110 m/Ma) and sedimentation (6.88 cm/ka) with sea level rise, which led to the estuary deposition with significant tidal influence during transgressions. Therefore, the combination of tectonic context, sea level change, and sediment supply determine the nature of depositional systems, and the interaction between various sedimentary processes rework or shape their facies distribution, geomorphology, and architectures of sedimentation in the back-arc basin.