Depositional architecture and evolution of Quaternary submarine canyon-fan system in the Baiyun Sag of the Pearl River Mouth Basin, northern South China Sea

Abstract

Submarine canyons in the shelf margin usually serve as essential conduits for delivering sediments to slopes and basins and coevally develop turbidite reservoirs within deep-water fan systems. To understand the control of the evolution of the canyon-fan system, we focused on Quaternary deep-water depositional systems imaged in 3D seismic reflection data covering the lower reaches of the canyons and the base of the slope in the Baiyun Sag, on the northern margin of the South China Sea. By analyzing seismic facies tied to seismic attribute maps and time-thickness variations, three main seismic units were interpreted and described with markedly different geomorphologies and facies assemblages. Variations in the depositional architecture of the fan system recorded changes in sediment conditions during the Quaternary. The evolution of the Quaternary canyon-fan system can be summarized in three stages. Stage 1 is characterized by fan complexes with backstepping and stratigraphic onlapping against the erosional base, indicating a reduction in sediment supply due to rapid transgression during the Early Pleistocene. During Stage 2, the sediment supply decreased to its lowest rate, producing elongate-shaped turbidite complexes and a high proportion of hemipelagic deposits. In contrast, stage 3 was expressed by canyon-fan systems with downlapping stratigraphy resulting from the enhancement of sediment input during shelf margin/slope progradational intervals. The Quaternary canyon-fan system evolution was controlled not only by the rapid transgression but also by the efficiency of the sediment transfer mechanisms. Although higher rate of sediment supply occurred during stage 1, the dominant canyon-direct fed systems at this stage had highly efficient sediment transfer mechanisms, generating thicker and larger canyon-fan systems compared to stage 3. During stage 3, the progradation-direct-fed system became the primary sediment transport pattern causing sediment to mostly accumulate on the slope and reduce sediment delivery to the basin floor. Our results indicate that the distribution of gas hydrate accumulation is strongly associated with the depositional pattern of turbidites within the Lower Pleistocene succession.