Depositional architecture and evolution of basin-floor fan systems since the Late Miocene in the Northwest Sub-Basin, South China Sea

The sediment budget of the Northwest Sub-basin, South China Sea since the Late Miocene (11.6 Ma, average thickness > 1000 m) accounts for more than two-thirds of the total infill since the initial ocean spreading of the sub-basin (32 Ma). The sediment sources and architectural pattern of these deposits, however, are poorly known. Using high-resolution 2D reflection seismic data with age constraint from IODP boreholes, we have documented two interdigitating basin-floor fan systems that developed since the Late Miocene. These were fed by two of the largest deep-water canyon systems worldwide, from the west (the Central Canyon/Xisha Trough) and the northeast (the Pearl River Canyon), as well as from smaller headless canyons and gullies across the surrounding slopes. Based on careful analysis of seismic facies, their geometry and occurrence, we identify the principal deepwater architectural elements, including the multi-scale channels, channel-levee complexes, lobes, sheets and drapes, mass-transport deposits, volcanic intrusions, turbidity-current sediment-wave fields, and a contourite drift/terrace. Tentative reconstructions show that the development of these Late Miocene-Quaternary basin-floor fan systems was dominated by changes of sediment supply. The Xisha fan reached its largest extent during the Late Miocene, while the Pearl River fan was most active during the Late Miocene to Quaternary. During the Late Miocene, both the conduits of the Central Canyon and the Pearl River Canyon were active with abundant sediment supply, generating the two incipient fan systems. Sediment supply from the west via the Central Canyon persisted throughout the Late Miocene, being coarser-grained than that of the Pearl River fan. With the demise of the Central Canyon during the Pliocene and consequent sharp decrease in sediment supply, the Xisha fan size reduced significantly. By contrast, supply of mud-rich sediments from the Pearl River and northern slope increased through the Pliocene and into the Quaternary, leading to the modern sedimentary pattern of interdigitating basin-floor fans. Insights into the evolution of sediment supply and fan development through time derived in this study contribute to a better understanding of how source to sink systems feed marginal oceanic basins such as the South China Sea.

Automated summary

The study analyzes the sediment budget and fan system development in the Northwest Sub-basin of the South China Sea since the Late Miocene, revealing the impact of sediment supply changes on the formation of two interdigitating basin-floor fan systems. The Xisha fan and Pearl River fan experienced different sediment supply patterns during different periods, providing insights into the sediment source-to-sink processes in marginal oceanic basins like the South China Sea.