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Cukur, Deniz,Um, In-Kwon,Bahk, Jang-Jung,Chun, Jong-Hwa,Horozal, Senay,Kim, So-Ra,Kong, Gee-Soo,Kim, Kyong-O.,Kim, Seong-Pil Elsevier 2018 Marine and petroleum geology Vol.98 No.-
<P><B>Abstract</B></P> <P>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°. 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 SU5 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.</P> <P>Seismic reflection data from the margin reveal the coexistence of four structural styles including the basement-involved normal faults, reverse faults, reactivated normal faults, and anticline folds. The basement-bounding normal faults are associated with the initial extension of the continental margin while the others are postulated as resulting from contractional tectonism of the eastward movement of the Amur Plate that began in the early Pliocene. The prominent seafloor erosion over the folded/inverted blocks further suggests that inversion continues today.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A tectonostratigraphic framework for the northeastern margin of Korea is developed. </LI> <LI> Six seismic units have been defined in analysis of the seismic reflection profiles. </LI> <LI> Primary deposits consist chiefly of hemipelagites interbedded with turbidites. </LI> <LI> Basement structure and seismic stratigraphy suggest a rift origin for the margin. </LI> <LI> Regional compression in the Pliocene reactivated the earlier extensional faults. </LI> </UL> </P>
Cukur, Deniz,Kong, Gee-Soo,Chun, Jong-Hwa,Kang, Moo-Hee,Um, In-Kwon,Kwon, Taekhyun,Johnson, Samuel Y.,Kim, Kyong-O Elsevier 2019 Marine geology Vol.415 No.-
<P><B>Abstract</B></P> <P>We identify and describe five giant seafloor depressions from the southeastern continental shelf of the Korean Peninsula using multibeam bathymetry, sub-bottom profiler, and multi-channel seismic reflection data, supplemented by piston cores. Multibeam bathymetry data from the shelf show four crescent-shaped depressions (SD1 to SD4) and one near-circular depression (SD5) within a group of NW-SE trending depressions, the largest covering an area of about 7 km<SUP>2</SUP> on the seafloor. The depressions reach up to ~4.5 km in width and ~2 km in length and have asymmetric cross-sections. Some have depths as large as 40 m below the surrounding seafloor with walls as steep as 45°. The depressions are confined to water depths between 130 and 170 m and bounded on the north by a large submarine channel that was plausibly formed by fluvial or tidal processes during the Last Glacial Maximum (LGM) sea-level lowstand. Multi-channel seismic and sub-bottom profiler data reveal truncated depression walls and the presence of sediment drift deposits within the depressions, indicating that both erosion and deposition are active processes. Flaser and lenticular bedding in the cored drift deposits along with variable grain size (ranging between ~2.6 phi and ~4.3 phi) are diagnostic features of the bottom currents influenced by tidal forces. Depressions SD1 to SD4 lack evidence of fluid or gas escape. In contrast, many features of depression SD5 are characteristic of gas escapes and blowouts, including acoustic anomalies, a 20-m-high carbonate mound or carbonate-encrusted mound, and mud dikes and mud patches in cores. Based on the SD5 example, we think it is likely that the other crescent-shaped seafloor depressions formed originally as pockmarks by gas/fluid venting, and have since become inactive. The pockmarks represent zones of weakened sediment that were eroded, expanded, and merged by bottom currents to form larger seafloor depressions. Modern currents are strong enough to transport shelf sediments, and these currents were probably much stronger at lower sea levels when the Korea Strait was a more restricted passage between the East China Sea and East Sea.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Four crescent-shaped seafloor depressions (SD1 to SD4) from the SE shelf of Korea </LI> <LI> These depressions have 25 to 40 m of depths below the adjacent shelf </LI> <LI> Evidence of drift deposits inside the depressions and erosion at the depression walls </LI> <LI> SD1 to SD4 probably began as pockmarks and then were modified by submarine currents </LI> <LI> We also document a smaller circular depression (SD5) that we consider as pockmark </LI> </UL> </P>
Lee, Gwang-Soo,Cukur, Deniz,Yoo, Dong-Geun,Bae, Sung Ho,Kong, Gee-Soo Elsevier 2017 QUATERNARY INTERNATIONAL Vol.459 No.-
<P><B>Abstract</B></P> <P>High-resolution seismic data (Chirp and Sparker profiles) were used to investigate the sequence stratigraphy and evolution of the continental shelf of the South Sea, Korea, since the Last Glacial Maximum (LGM). Approximately 1950 km of chirp and sparker profiles were acquired. Along with seismic profiling, 30 piston core samples were collected and two previous long drill cores (SSDP-103 and 104) were tied to the seismic data. Our data show nine types of seismic facies on the basis of seafloor morphology and sub-bottom acoustic characters. Based on the analysis of seismic profiles, shelf deposits of the South Sea accumulated after the onset of the LGM can be divided into five sedimentary units; S1 to S5, from top to bottom. Correlation between sediment cores and seismic data suggests that Unit S5 is an incised channel fill formed by fluvial or coastal sediments during the early transgressive stage, accompanied by backstepping of the shoreline. Unit S4 is a transgressive sand layer reflecting the deposition of coarse sediments due to the strong currents and tides, following the rapid retreat of the coastline. Unit S3 represents paleo channel- and basin-fill deposits formed in estuary or delta environments near the inner continental shelf. Unit S2 forms an inner shelf transgressive sand layer including sand ridges. Unit S1 is interpreted as the most recent mud formed during the sea level highstand stage when the sea-level rise ended. Our results show that the sedimentary evolution of the continental shelf of the South Sea, since the LGM, has been closely related to the sea-level changes. The distribution of paleo-channels is associated with the paleo-Seomjin River, local sediment supply, and continental shelf erosion.</P>
Roger Urgeles,박장준,이상훈,Senay Horozal,Deniz Cukur,김성필,김길영,정승원,엄인권 한국지질과학협의회 2019 Geosciences Journal Vol.23 No.3
In this study we use a scenario-based approach to highlight potential tsunami hazard from actual Late Pleistocene submarine landslides in the Ulleung Basin: two submarine landslides on the western slope of the Ulleung Basin, north and south of the Hupo Bank (2.53 and 1.12 km3 respectively) and a landslide (15.1 km3) on the continental slope south of the Ulleung Basin. The simulations attempt to highlight the consequences, should one of these events occur at Present. Results of the simulations indicate potential local hazard zones with very local waves < 2 m high in the Korean coast of the East Sea that could result from midsized landslides, not exceeding 3 km3 on the western slope of the basin. Time available for early warning since onset of these events is between 15–30 minutes. On the other hand, the continental margin south of the Ulleung Basin is an area where landslide tsunamis with significant hazard potential could originate. New landslides of similar size to those of the Late Pleistocene could produce tsunami waves > 3 m in the stretch of coastline from Ulsan in the south to Uljin in the north. The timing available for early warning from landslide tsunamis originating in this area is 15–30 minutes along the affected section of the shoreline. We also suggest a Probabilistic Tsunami Hazard Assessment (PTHA) for comprehensive assessment of the Korean coast of the East Sea. PTHA accounts for uncertainties in location, release mechanisms, evolution, and return periods of submarine landslides as well as epistemic uncertainty. However, to constrain these uncertainties detailed information on source areas, recurrence period and dynamics of submarine landslides is necessary and calls for additional data collection and further studies.