Mixed Carbonate/Siliciclastic Deposition in the Evolution of the Margin Stratigraphy of the Northern Margin of the South China Sea

High resolution seismic reflection data and chirp and side-scan sonar data were acquired on the northeastern margin of the South China Sea in an investigation of the impact of variation of sediment supply, physiography and base level on the architecture of mixed carbonate-siliciclastic depositional sequences. As recently as the early Pliocene, the margin evolved from a carbonate ramp to a carbonate rim system and by 1.0 Ma margin sedimentation became dominated by siliciclastic sedimentation. Changes in the East Asian monsoon system have a large impact on the stratigraphy of the northern margin of the South China Sea by controlling the flux of terrigenous debris to the basin. The evolution from carbonate, to mixed, and finally to siliciclastic dominated sedimentation on the margin appears to be associated with a tipping point correlating with the strengthening of the summer monsoon around 1 Ma. Strengthening of the summer monsoon leads to a climate transition from cool and arid to warm and humid conditions. We hypothesize that this increase in humidity may have caused an increase in terrigenous flux to the margin causing the carbonate systems to begin to shut-down and transition from a carbonate dominated system to a mixed system while on its way to becoming a siliciclastic dominated system.

The character of sea level change may have also played an important role in the transition from a carbonate dominated system to a siliciclastic dominated system. Up until 3.5 Ma, sea level change was on the order of few 10's of meters, leaving much of the margin submerged and experiencing sea level change at rates with which carbonates can keep-up. After this interval, the magnitude of sea level variation changed significantly and it is possible that this variation in sea level dynamics helped trigger the change in depositional system dominance on the margin.