A Transition From Tidal-Dominated Deltaic to Estuarine Environments, Eocene to Oligocene, East China Sea

Abstract

The East China Sea Basin is a shelf basin located at the active continental margin of western Pacific. As the significant source rocks and reservoirs, the Pinghu and Huagang Formations show distinctive characters of tidal dominated delta and estuary deposition respectively. This study aims to understand the sedimentary mechanisms of the tidal dominated deposition, and how the tectonic movements and sea level change induced the transition from deltaic to estuarine environments. Seismic profiles (2D) and volumes (3D) were employed for interpreting tectonic patterns, stratigraphic framework, and large-scale (third-order) sequence stratigraphic cycles. The cores and well logs were used integrately for identifying sedimentary facies and subdividing the forth-order cycles. The Pinghu Formation spanning in the Eocene is characterized by stacked coarsening upward successions with thickness of 20 m to 40 m on well logs. The stacked, coarsening upward intervals contain the sedimentary features including thick structureless sandstone, mud drapes, fluid muds, and highly bioturbated mudstone. These sequences represent deltaic progradations on the shelf with strong tidal influences. The Huagang Formation in the Oligocene mainly consists of a series of thick, block sandbodies with thickness ranging from 15 m to 60 m on well logs. The heterolithic sandy intervals are characterized by erosive bases with mud clasts and conglomerates, bi-directional cross-beds/ripples, mud drapes, and flaser/rhythmitic beds, which indicate the tidal channel/bar deposits in an estuarine environment. On the regional stratigraphic profile, an unconformity was developed between the Pinghu and Huagang Formations, corresponding to the tectonic movement of Yuquan uplifting. This movement triggered the Diaoyu Island uplifting and relative sea level fall in the East China Sea Basin. Finally, this study reaches the conclusion that the transition from tidal-dominated deltaic to estuarine environments in the East China Sea Basin were mainly influenced by tectonic movements, subsidence, relative sea level changes, and sediment influx. During the Late Eocene, regressive cycles of tidal dominated deltas were formed in the period of relative sea level fall causing by tectonic uplifting. From the early stage of Oligocene, transgressions were developed by increasing subsidence, relative sea level rise, and less sediment supply in the East China Sea Basin, forming the tidal-dominated estuary deposition.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015