SEISMIC SEQUENCE FRAMEWORK AND DEPOSITIONAL EVOLUTION OF THE SEDIMENT DRIFT AT ODP SITE 1144, DONGSHA SLOPE, NORTHERN SOUTH CHINA SEA

Seismic stratigraphic framework and depositional evolution of the sediment drift at ODP Site 1144, Leg 184 Dongsha Slope, Northern South China Sea, are studied using high-resolution 2-D seismic data, combined with the drilling data and geophysical well logs. Seven seismic discontinuities with reflection terminations (onlap, downlap, toplap and erosional truncation), named SB1 to SB7 upwards, are identified within the drift. As a result, seven seismic sequences S1 to S7, from bottom to top are defined. The bottommost sequence is characterized with chaotic, weak amplitude reflections and interpreted as pre-drift slumping or debris deposits. The other sequences (S2-S7),which consist mainly of reflections with sub-parallel to wavy, continuous, medium to strong amplitude, are interpreted as the main components of the drift. Well-to-seismic correlation with the synthetic seismogram of Site 1144 suggests that the sequence boundaries of SB2 to SB7 were approximately developed at 1.09, 0.88, 0.65, 0.30, 0.13 and 0.02 Ma, respectively. Time thickness maps of the sequences suggest that depositional centers shifted frequently, mostly in a direction parallel to the slope strike. The drift extends in NE-SW along the slope. According to the thickness distribution, the drift is estimated at least 1 100 km² in area, with a maximum thickness of about 520 m.