Depositional evolution of sediment drifts inside intra-slope basins on the lower southeastern slope of the Dongsha Islands (South China Sea) and their paleoceanographic implications

Abstract

Confined drifts generally contain abundant paleoceanographic information. The lower southeastern slope of the Dongsha Islands, South China Sea, was partly disassembled into several intra-slope basins by igneous bodies formed during the post-spreading process. Based on a series of high-resolution, two-dimensional seismic profiles, this study presents seismic evidence of sediment drifts inside several intra-slope basins at present water depths between approximately 2100 and 2800 m. The main discontinuities in the study area since the middle Miocene were carefully correlated with nearby seismic lines reported by previous studies. Moreover, their accurate ages were determined according to the dating results of the China National Offshore Oil Corporation with microfossil combinations. From 15.97 to 7.16 Ma, sheeted drifts were formed near the obstacles, indicating fairly low-velocity bottom currents. Subsequently, low-relief mounded drifts and associated small-scale moats were formed along the obstacles (7.16–5.33 Ma), showing that bottom currents were relatively strong. From 5.33 Ma onwards, high-relief mounded drifts and associated large-scale moats prevailed, formed by intensified bottom currents. Additionally, high-relief mounded drifts associated with large-scale moats occurred near the northern obstacle, and low-relief mounded drifts and associated small-scale moats were only formed near the southern obstacle in the largest intra-slope basin. In contrast, in the narrowest intra-slope basin, high-relief mounded drifts were confined by large-scale moats at the foot of both obstacles. This stark difference is attributed to the lateral variation of bottom current intensities caused by the variable widths of the intra-slope basins, resulting in different confining effects on bottom currents. The changes recorded in the sediment drifts reveal that the sedimentary succession should be subdivided into three phases: the onset stage (15.97–7.16 Ma), the slow growth stage (7.16–5.33 Ma), and the rapid growth stage (since 5.33 Ma). The gradual intensification of bottom currents is closely related to the influx of the Northern Pacific Deep Water since ~ 7 Ma and the gradual formation of the semi-enclosed environment due to the eastward subduction of the South China Sea basin into the Philippine plate since the late Miocene.