Late Oligocene to early Miocene delta and linked slope fan systems: Depositional architecture and sediment dispersal, the Pearl River Mouth Basin

Abstract

AbstractThe Upper Oligocene to Lower Miocene (Zhuhai Formation) in the Pearl River Mouth Basin of the northern South China Sea contains extensive shelf‐edge delta and linked slope fan deposits, and records the sediment dispersal and constraint processes. According to the integrated analysis of well‐log, seismic and core data, depositional architecture and delivery mechanism of sandy deposits from the shelf margin to the lower slope are documented systematically. Four major depositional facies are identified: shelf‐edge incised channels and shelf‐edge deltas; amalgamated slump‐channel complexes; slope gullies; and slope fan deposits. Incised channels at shelf‐edge delta front (2.0 to 4.4 km wide) comprise V‐shaped single channels and W‐shaped or irregular‐shaped channels which eroded shelf‐edge sandy deltaic deposits and retransferred them to the lower slope. Further slumping along channel walls may be triggered by the continuously channel downcutting process; and then these incised channels amalgamated and collapsed downslope to generate large‐scale coalesced slump‐channel complexes (15 to 35 km wide), which are regarded as the main delivery system to transport sandy deltaic deposits to the deep‐water basin. There are a series of V‐shaped or W‐shaped slope gullies (tens to hundreds of metres wide) developed along the lower slope, which further delivered sediments to slope fan systems. Slope fan systems are composed predominantly of basal debris flow massive and sandy turbidite channel and frontal splay deposits comprising one of the most important oil and gas reservoirs in the study area. Channelization and large‐scale slumping along the shelf‐edge delta front are regarded as the most important dispersal mechanisms delivering sandy sediments across the shelf margin. Falling sea level was suggested to be one of the important factors causing the incision and large‐scale shelf margin slumps, whereas the unstable high (800 to 1200 m) and steep (5° to 8°) shelf‐edge delta and slope clinoforms provide a pre‐condition for large‐scale shelf‐edge collapse.