Search

The depositional environments of the Jeju Strait shelf since the last interglacial period are analyzed using sedimentary structures, grain size textures, sediment color, and optically-stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) ages of vibro-cores. The shelf deposits can be divided into 12 sedimentary facies grouped into four depositional units: 1) shelf sand, 2) estuarine sand/mud, 3) fluvial sand/mud, and 4) shelf sand/mud. Each unit is separated by a sharp and unconformable erosion boundary. Unit 1 shows massive to slightly bedded sandy shells with oyster fragments, representing a shelf environment during sea-level highstand. The OSL date of the base of the unit is 124.4 ± 10.0 ka. Unit 2 is characterized by thin periodic parallel laminated mud, suggesting tidal signatures in tidal flats. Sediments are typically moderate brownish, suggesting extreme weathering after deposition. The OSL dates of this unit range from 81.1 ± 5.3 to 74.2 ± 4.7. Unit 3 consists of lower cross-bedded sand and laminated mud with fining-upward successions. It is interpreted as the product of channel deposits in a fluvial environment during a sea-level falling. The OSL ages of Unit 3 range from 50.6 ± 2.9 to 26.1 ± 1.6 ka, belonging to the Weichselian glacial period. The uppermost unit (Unit 4) is composed of homogeneous mud and massive sand with molluscan shell fragments in the modern shelf environment during sea-level highstand. OSL and 14C-AMS ages range from 0.32 ± 0.02 to 11.2 ± 0.1 ka. Consequently, the late Pleistocene-Holocene sedimentary facies in this area consist of marine and non-marine environments, which mainly correspond to sea-level changes.

Thirty-two core samples were collected from the continental shelf area of the South Sea, Korea. They have been used to characterize physical properties in relation to sedimentary units (interpreted from seismic and core data) and to interpret physical property variation as a function of burial depth. The physical property data were readily classified into sedimentary units: Units S1, S2, S3, S4, and S5, in descending order. The variations in physical properties reflect well the relative characteristics of sedimentary units (equivalent to seismic units). Moreover, the relationships between physical properties are well grouped by each sedimentary unit. In addition, the distributions of physical properties match well with the grain size distribution and bathymetry of the study area. In particular, the lithology (e.g., grain size) plays an important role in the variation of physical properties. These results suggest that the physical properties of these sediments are mainly controlled by depositional processes in the vicinity of the paleo-Seomjin River, and that sea-level change specifically was a dominant sedimentary processes in the study area during the Quaternary. In particular, the physical properties of Unit S2 do not show transitional characteristics between Units S1 and S3, although it belongs to the transition zone in sequence order. This finding is attributed to the fact that compared with the sediments of Units S1 and S3, Unit S2 is composed of more coarse-grained sediments with abundant shell fragments.

Three drill core samples collected from the Heuksan Mud Belt off SW Korea have been used to characterize sediment properties in relation to seismic units and to interpret geotechnical properties as a function of burial depth. The physical property data were readily classified into seismic units: Unit I, Subunit II-a, Subunit II-b, Subunit III-a, Subunit III-b, and Pre-LGM (last glacial maximum), in descending order. Thus, the variations in physical properties reflect the relative characteristics of the seismic units. Moreover, the relationships between physical properties are well grouped by each seismic unit. These results suggest that the physical properties of these sediments are mainly controlled by depositional processes related to sea-level change as one of dominant sedimentary processes in the study area. Interestingly, the shear strength and velocity of Subunit II-b are characterized by cyclic variation. These characteristic patterns are likely related to periodic variation in sedimentary environments (e.g., wave- or tide-dominated environments) in the study area. Clay fabric investigations with depth using scanning electron microscopy are characterized by different clay particle arrangements (e.g., from edge-to-edge to face-to-face contact with depth) owing to compaction effect caused by overburden pressure with depth. The geotechnical properties of the Heuksan Mud Belt suggest that the shallow regions are apparently over consolidated probably due to the flocculated fabrics at shallow depths and that the regions have moderate compressibilities in spite of the high mud content in the sediments.

This study is the first attempt at describing and interpreting incised-channels on the continental shelf of the South Sea, Korea. The distribution, pattern, shape, and infilling of submerged channels incised across the continental shelf of the South Sea have been investigated based on approximately 1940 line-km of chirp and sparker data. The incised-channel system of the paleo-Seomjin River on the continental shelf is approximately 109 km long, 800–5000 m wide, and is more than 890 km2 in area. Meandering and straight types of channel are dominant on the inner shelf, with a change to straight and braided planforms on the mid to outer shelf. Sparker seismic data suggest the paleo-channels formed under fluvial systems when the shelf was exposed during the Last Glacial Maximum (LGM). Channel fills are divided into five types, based on erosional surfaces and internal seismic reflectors: (I) transparent to semi-transparent incised channel fill; (II) parallel to sub-parallel incised channel fill; (III) complex incised channel fill; (IV) divergent incised channel fill; and (V) chaotic incised channel fill. The acoustically transparent facies of the channel fill type (Type I) suggest the presence of basin muddy deposits in a low energy environment, which were dominant in the inner shelf. These muddy sediments were likely during a period of higher sea level during the Holocene transgression. Types II, III, and IV are characterized by onlap, downlap, and wave or tidal ravinement surfaces, respectively, indicating that they were deposited during a transgressive stage, which type V represent basal incision surface. The evolution of paleo Seomjin River has been strongly controlled by sea level change and sediment transport processes since the LGM.

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.

A total of 157 piston and box core samples were collected and analyzed to characterize physical properties and geoacoustic provinces of surficial sediments in the southwestern part of the Ulleung Basin in the East Sea. The results and distribution patterns of surficial sediment texture, physical and acoustic properties reflect well the sedimentary environment like distribution and composition of sediment in the study area. The inner shelf and basin slope in this study are dominated by fine-grained sediments and show high porosity and water content, with low bulk density and velocity as well. The trough of the Korea Strait and outer shelf, which is composed of coarse-grained sediments, shows low porosity and water content and high bulk density and velocity. Based on in-situ velocity and sediment properties, the study area is divided into five provinces: (1) Province IA is hemi-pelagic mud partially mixed with intermittent sandy sediments which are derived from the outer shelf due to slide/slump or mass flows (in-situ velocity: 1445 m/s, mean grain size: 8.4Φ, porosity: 84%, and bulk density: 1.24 g/cm3), (2) Province IB is covered with muddy sediments that were deposited during the Holocene (in-situ velocity: 1445 m/s, mean grain size: 7.9Φ, porosity: 78%, and bulk density: 1.35 g/cm3), (3) Province II is marked by mixed recent and relict sediments (in-situ velocity: 1477 m/s, mean grain size: 6.1Φ, porosity: 68%, and bulk density: 1.53 g/cm3), (4) Province III is dominated by coarse-grained relict sediments formed during the Pleistocene (in-situ velocity: 1564 m/s, mean grain size: 3.8Φ, porosity: 54%, and bulk density: 1.77 g/cm3), and (5) Province IV consists of coarser sediments than Province III (in-situ velocity: 1671 m/s, mean grain size: 3.0Φ, porosity: 46%, and bulk density: 1.91 g/cm3). The compressional wave velocity, mean grain size, and bulk density increase from Province IA to Province IV, whereas the porosity and water content decrease from Province IA to Province IV. Variability of the physical and acoustic properties tends to follow the general trend of the mean grain size. The classification of each province using the in-situ velocity corrected with the temperature and pressure provides a better reflection of the sediment properties and sedimentary environment.

The Korea Strait is a seaway connecting the East Sea with the East China Sea. The Plio-Quaternary stratigraphy and sedimentation on the Korea Strait shelf was studied using a dense network of high-resolution, single-channel seismic reflection profiles. The shelf sequences above the acoustic basement consist of seven depositional sequences separated by erosional unconformities corresponding to falls in sea level. Individual sequences are characterized by well-stratified and complex sigmoid-oblique prograding reflection configurations, reflecting a history of upbuilding and outbuilding in response to repetitive transgressive and regressive events driven by sea level changes. During the regression to lowstand of sea level, lowstand prograding wedges were formed seaward along the present shelf margin, whereas during the highstand of sea level, as in the present condition, highstand deposits were deposited in the inner shelf along the coast. However, as the subsequent regression began, the inner-shelf highstand deposits were severely eroded out and/or formed a condensed section. Volumetrically, the forced regressive and lowstand systems tracts are the most important components of the Korea Strait. Consequently, the Plio-Quaternary sequence in this area consists of a succession of stacked progradational lowstand wedges, mainly formed during the regression and lowstand of sea level.

Late Quaternary seismic stratigraphy and depositional history at the mid-eastern Yellow Sea were investigated using high-resolution seismic profiles and core sediments. The results show that the shelf sequence consists of five sedimentary units formed since the LGM: incised-channel fill (SU1), estuarine deposit (SU2), thin sand veneer (SU3), tidal sand ridge (SU4), and central deltaic mud (SU5). The lowermost unit (SU1) above the sequence boundary is interpreted as channel fill deposits mainly formed during the LGM, which belongs to the lowstand systems tract. Three units (SU2, SU3, and SU4), regarded as transgressive systems tract, can be grouped into paralic and marine components separated by a ravinement surface. SU2 lying below the ravinement surface represents a paralic unit that consists of estuarine sediments left behind from shoreface erosion. The top surface of SU2 is truncated by an erosional surface and is overlain by two marine units (SU3 and SU4), which were produced by shoreface erosion that shifted landward during the transgression. SU3, mainly distributed over a wide area of the central part, is very thin, whereas SU4 on the eastern part off the Korean Peninsula forms serial sand ridges, partly modified by modern tidal currents. The uppermost unit (SU5) above the maximum flooding surface, regarded as the highstand systems tract, formed the thin deltaic mud patch derived from the Huanghe River developed after the highstand sea level approximately 7 ka BP.

Core sediment sampling has been conducted to investigate the physical and acoustic properties of inner shelf sediments in the South Sea, Korea. Thirty-two piston core samples were analyzed for sediment texture (grain size as well as, fractions of sand, silt, and clay), physical properties (porosity, water content, bulk density, grain density, and shear strength), and acoustic properties (compressional wave velocity and attenuation). The sediments in the study area consist mostly of homogenous mud. The study area can be divided into two different areas (south of Namhae Island and southeast of Naro Island) by texture, and physical and acoustic properties. The mean grain size generally decreases seaward from the mouth of Yeosu Sound and eastward in the southeast area of Naro Island. The compressional wave velocity decreases southeastward from 1500 to 1480 m/s and then increases to 1520 m/s due to relict sediments deposited in mid-shelf of the South Sea. Porosity, water content, and bulk density show an inverse pattern to velocity. The geoacoustic properties of the study area are generally similar to those of continental terrace sediment studied by Hamilton in the North Pacific. The area south of Namhae Island area matches Hamilton's model better than the area southeast of Naro Island. This result is believed to be related to the higher silt and sand contents as well as lower clay content than southeast of Naro Island.

High-resolution (Chirp and Sparker system) seismic profiles were analyzed to investigate the sequence stratigraphy of late Quaternary deposits in the southeastern Yellow Sea. Approximately 1560 line-km data of chirp and sparker profiles were acquired, together with 11 piston cores. Two previous long drill cores (YSDP-102 and 103) were used for comparison with seismic data. High-resolution seismic profiles image the Holocene mud deposits and the complex sedimentary structure in this area. Sequence analysis of high-resolution seismic profiles reveals that the shelf deposits form a succession of high-frequency (five-order) sequences consisting of one depositional sequence developed during the late Quaternary. The depositional sequence includes five sedimentary units, each with different seismic facies and geometry: (1) incised channel fill and lowstand deltaic wedge (unit SY1), (2) sand ridges (unit SY2), (3) estuarine/deltaic mud (unit SY3), (4) redeposited mud (unit SY4), and (5) recent distal mud (unit SY5). Based on the interpretation of high-resolution seismic records and correlation with the YSDP-102, 103 long cores and piston cores, late Quaternary deposits in the southeastern Yellow Sea consists of a set of the lowstand (unit SY1), early transgressive (unit SY2), middle transgressive (unit SY3), late transgressive (unit SY4), and highstand systems tract (unit SY5) formed since the last-glacial period. The rather unusual stratigraphic architecture including three systems tracts is largely controlled by the postglacial sea-level changes and regionally circulation pattern associated with sediment erosion and redeposition. The results of this study present firstly clear seismic evidence that the southeastern Yellow Sea mud belt (SEYSM) can be divided into three stratigraphic units (units SY3, SY4, and SY5) bounded by distinct bounding surfaces.

In order to characterize geoacoustic provinces of the South Sea shelf of Korea, sediment texture, physical, and geoacoustic properties (p-wave velocity) were measured and calculated based on approximately 350 core samples collected from the study area. The empirical relationship between mean grain size and velocity suggested by Hamilton was used for calculation of p-wave velocity. The inner shelf sediments of the South Sea are characterized by Holocene muddy sediments derived from nearby rivers (i.e., mostly supplied from Nakdong and Seomjin rivers near the Korean shore), whereas the outer shelf areas are dominated by Quaternary sediments (e.g., relict sediments, due to a rising of sea level during the Quaternary) depending on seafloor bathymetry. In this study, the seafloor of the South Sea shelf is differentiated into 4 provinces based on sediment properties (esp. mean grain size and velocity). Province I consisted of muddy sediments affected directly by the Nakdong and Seomjin Rivers discharge. Province II was covered by the sediments transported from two rivers. Province III is marked by mixed Holocene fine materials and coarse Pleistocene materials. Province IV is dominated by relict sediments characterized by Pleistocene coarse sandy materials. Mean grain size, velocity, and wet bulk density increase from Province I (>7.4o, 1515 m/s, and 1.52 g/cm 3 , respectively) through Province II (5.4o, 1597 m/s, and 1.69 g/cm 3 ) to Province III (2.9o, 1766 m/s, and 2.0 g/cm 3 ). Porosity, on the other hand, exhibits an opposite trend, decreasing from Province I (71.3%) to Province II (60.3%) and decreasing significantly in Province III (44.6%). Province IV is characterized by the highest velocity (1979 m/s average) and the coarsest material (2.1o). Nevertheless the values of physical properties such as wet bulk density, porosity, and density lie between Province II and III.

Geochemical (opal, total organic carbon (TOC), and CaCO3) analyses of three sediment cores from the South Korea Plateau in the East Sea (Japan Sea) revealed that the late Quaternary (