Since the Cenozoic time, the South China Sea(SCS)has formed one of the largest semi-enclosed marginal basins along the East Asian continental margin through the geological processes such as South China Sea plate rifting, seafloor spreading, and plate subduction. In the South China Sea Basin and its surrounding regions, a series of active geological structures have developed, for example, the Manila subduction zones, the Littoral(Binhai) fault zone, and the Continental Slope(Lupo) fault zone. The activity of these tectonic zones is highly prone to triggering extreme natural disasters such as earthquakes and tsunamis. Along the coastal zone of the northern part of the South China Sea(the South China continental margin), there are densely populated large cities with critical infrastructure, which are also regions severely affected by extreme natural disasters like earthquakes and tsunamis. Therefore, identifying the sedimentary records of large-scale paleotsunami events in the northern part of the South China Sea and analyzing their potential triggering mechanisms are of great significance for seismic-tsunami hazard assessment.
This study focuses on the sedimentary strata of boreholes E15 and E12 from the Pearl River Estuary along the South China coast. Based on AMS 14C dating and a comparison of magnetic susceptibility data between boreholes E15 and E12, a high-precision chronological sequence was established for the core of borehole E15, spanning approximately the last 1000 years to the present. The core E15 is 5.9m long, with a progressively younger AMS 14C age sequence in the upper part of the core section from 4.24~0m. However, AMS 14C ages of the sediments in the lower part of core E15, from 5.9 to 4.24m, are sometimes reversed. The reversal ages may be attributed to the reworking or recycling of the sediments in the lower part of core E15.
To reveal the depositional processes of borehole E15, we conducted detailed analyses of sedimentary grain size, sedimentary color, and geochemical elemental composition. The lower part of the core section(5.9~4.24m) for E15, consists of dark gray to grayish-black medium-to-coarse sand layers with poor sorting and contains abundant marine biodetritus. In contrast, the upper part of the core section(4.24~0m) for E15 is composed of dark gray to grayish-brown silty mud, fine sandy silt, and delicate sand layers. The upper core section exhibits finer grain size, lighter color, faint horizontal bedding, and higher terrestrial-derived elemental content, representing typical delta-shallow marine depositional environment. The lower part core section is characterized by a coarser grain size(medium to coarse) of sands, which lack clear sedimentary structures and exhibit higher offshore marine-derived elemental content, but relatively lower terrestrial-derived elemental content.
Based on the sedimentary features and geochemical composition, the sands from 5.9~4.24m within the borehole E15 are completely different from the overlying normal, typical shallow sea-delta sediments. Considering the reversal AMS 14C ages, coarser grain size, poor sorting, darker color, higher offshore marine-derived elemental content, and lower terrestrial-derived elemental content in the lower part core of the E15, we propose that the sand layers with abundant marine biodetritus in the lower part of boreholes E15 (5.9~4.24m) may be deposits from an extreme hydrological event occurring approximately 1000 years ago. In fact, tsunami deposits dating back about 1000 years have been widely documented along the northern and western coasts of the South China Sea, the inner islands of the South China Sea, and the northwestern Philippines. Therefore, we suggest that the event deposits in the Pearl River Estuary region, along the northern part of the South China Sea, at ~1000 years ago, may also be the result of a tsunami event.
Combining sedimentary evidence and numerical simulations, we hypothesize that a strong submarine earthquake may have occurred along the Manila subduction zone in the eastern South China Sea approximately 1000 years ago, triggering a large-scale tsunami. The medium and coarse-grained sand layers in the lower part (5.9~4.24m) of the E15 borehole within the Pearl River Estuary may be the consequence of this tsunami event.
