Seismic stratigraphy and structural characteristics of the northeastern continental margin of Korea in the East Sea (Sea of Japan)

摘要

The northeastern continental margin of Korea in the East Sea (Japan Sea) is a narrow sedimentary apron 20-30 km wide, 50-1300 m below sea level, with an average gradient of less than 2 degrees. The present study focuses on seismic stratigraphy of the sedimentary section and the underlying basement structure of the margin based primarily on multi-channel seismic reflection profiles. Major accumulation of sediment is centred in a graben depocenter in the northern part of study area, where more than 2 km of sediments have been deposited. This graben widens towards north and is flanked by two major basement bounding faults. The sedimentary succession of the margin can be divided into six seismic stratigraphic units based on reflection character, onlapping, and erosional truncation. The lowermost seismic unit (SU1) is characterized by parallel-to subparallel-reflections with varying amplitudes and is interpreted to be representing shallow-to non-marine sediments that were deposited during the active extensional phase of margin development. The presence of erosional channels in the uppermost section of SU1 further suggests a lowstand period after the deposition of SU1. Unconformably overlying the SU1 is a few hundred metres of SU2 that is characterized by continuous, high-amplitude reflections. Continuous seismic reflections within SU2 are indicative of sea level rise immediately following the lowstand period. The overlying seismic unit sets include SU3, SU4, and SUS with well-stratified seismic reflections. The consistent reflection pattern and high continuity of the reflections within these units are indicative of relatively stable depositional environments within the margin. Hemipelagic sediments and turbidites are probably the main components of these units. The youngest seismic unit SU6 comprises well-stratified seismic reflections in the upper slope and chaotic-to transparent-seismic reflections in the lower slope. The sedimentary units characterized by well-stratified seismic reflections are typical for hemipelagic slope sediments while chaotic seismic reflections are diagnostic features for mass-transport sediments (MTDs; i.e., slides/slumps, debris-flow deposits). The preferential occurrence of MTDs adjacent to the major faults suggests that they may be due to earthquakes associated with tectonic activity.