Characteristics of major unconformities and their control over sedimentation in rifting stage: A case from the East China Sea

Based on 2D and 3D seismic data and well drilling data from a Sag in the East China Sea, through quantitative analysis of active sag-controlling faults, the characteristics of the major unconformities and their control over sedimentation in rifting stage are discussed in this paper. The study shows that during the rifting stage, fault activities changed from violent to calm, and the topographic differences changed from large to small, that resulted in three major unconformities: second-order unconformities between rifting episodes T₄⁰ and T₃⁴, second-order unconformity between rifting episode and rift-depression episode T₃². Upon the facts, the rifting stage of the sag may be subdivided into 3 rifting episodes and 1 rift-depression episode. The rifting episode I of Palaeocene is the initiation time of tectonic movement, controlled by the Santan fault and the Baidi fault with limited distribution and near-source depositional systems. The rifting episodeⅡ of Baoshi Formation and the rifting episode Ⅲ of Lower Pinghu Formation are the period of intense tectonic movement. During the period, the strata were strongly dislocated by faults, the region was covered by deep water. Tidal delta and tidal flat deposits dominated the gentle slope belt in the west, and fan delta developed in the steep slope belt in the east of the basin. Restricted by rifting depression, the fan delta is characterized by high sedimentation rate, large thickness, limited depositional area and dramatic facies change. The rift-depression episode in the middle and upper stages of the Pinghu Formation represents a shrinking period, the region began to convert to a depression. Water exchange between the basin and the seas in the south was intensified, water became shallower, large-scale asymmetric deltas and tidal sand ridges were well developed. The rift-depression episode and rift episode are favorable for the formation of lithological reservoirs, and the excellent targets for deep oil and gas exploration.