Changes in inner- to outer-shelf delta architecture, Oligocene to Quaternary Pearl River shelf-margin prism, northern South China Sea

Abstract Inner shelf to shelf-margin delta and slope-fan deposits in the Cenozoic Pearl River Mouth Basin in the northern continental margin of the South China Sea comprise the most important petroleum reservoirs in the basin and their evolution reflects a changing interplay of tectonics, sea level change and sediment supply. Integrated analysis of 3D seismic, well logs and core data allows interpretation of depositional architecture and evolution of the delta systems as they responded to the major controls. Inner shelf-delta systems, sited on the landward reaches of the continental margin, are characterized by thickly stacked distributary channel, thin (20-40 m) but relatively coarse delta-front and thin, fine-grained, distal delta to prodelta deposits. In contrast, outer shelf to shelf-margin deltaic systems formed 200 m (merging outwards to 1000 m) high clinoforms with steep slopes imaged on 3D seismic profiles as S-shape or tangential progradational reflections. Slumps and debris flow as well as sandy turbidite slope-fan deposits are common on the slopes of the clinoforms. Basinward and landward, cross-shelf transits of the delta systems are evident, as they formed some 18–20 stacked, high-frequency transgressive-regressive deltaic cycles (0.8–3 Myr) and five megacycles (3 to 10 Myr). Development of the deltaic cycles is interpreted as the result of high-amplitude sea level falls and rises related to glacio-eustasy as well as to changing sediment supply. The large-scale transgressive-regressive evolution of the deltaic clastic wedge is likely to have been controlled mainly by long-term changes in tectonic subsidence rate and sediment supply. The shelf-edge deltaic regressions are likely to have developed during sea level fall, but also during periods of high sediment input enhanced by uplifted tectonic relief and catchment enlargement or at times by monsoonal intensification events.