Soo C. Park

Depositional history of Quaternary sediments on the continental shelf off the southeastern coast of Korea (Korea Strait)

Abstract The depositional history of Quaternary sediments on the continental shelf off the southeastern coast of Korea (Korea Strait) was investigated by means of analysing high-resolution seismic profiles and sediment samples. Quaternary sediments up to 50 m thick overlie the irregular surface of the acoustic basement in the inner shelf. The sedimentary column is comprised of two main sequences separated by a mid-reflector which is interpreted as the pre-Holocene surface. The Holocene sequence, reaching up to 30 m in thickness with a depocenter near the present river mouth, is characterized by deposition in a prodelta environment, while the Upper Pleistocene sequence represents deposition in deltaic or shallow water environments. During the late glacial period, most of the shelf was subaerially eroded and deposition took place only in the trough region. The present location of marine-built terraces indicates that the lowest position of sea level was about 130 m below its present level. The subsequent postglacial transgression reworked Pleistocene sediments to form a thin veneer of transgressive facies. Sea level rose rapidly in the early stages of the postglacial transgression and approached the present level at approximately 7000-5000 yrs B.P. Recent sedimentation (the Holocene sequence) probably began at that time.

Physical and acoustic properties of shelf sediments, the South Sea of Korea

Abstract Laboratory determinations of compressional wave velocity, porosity, density and sediment texture have been conducted on samples collected from the inner shelf Holocene mud sediment in the South Sea and southern part of the East Sea of Korea. Thirty-five piston core samples were used for the measurement. Mean grain size decreases seaward reflecting the distance from source. Velocity decreases with increasing water depth. Porosity and clay content increase gradually with deepening bathymetry on the inner shelf. The study area can be divided into three different areas (Namhae, Pusan and Ulsan) by sediment source, texture and physical properties. Velocity increases from west to east (Namhae→Pusan→Ulsan). Porosity has a pattern that is inverse to that of velocity. Sediment texture (mean grain size and clay content) is a very important factor to determine physical and acoustic properties of sediment. The geoacoustic properties of the three study areas generally coincide well with that of Hamiltons (1970) North Pacific continental terrace sediment. Ulsan area is a little different from the Hamiltons model, which has a relatively higher velocity at the same textural range. The difference is believed to be related to the lowest clay content in Ulsan area.

Late Quaternary lowstand wedges on the shelf margin and trough region of the Korea Strait

Abstract High resolution sparker profiles collected from the Korea Strait reveal a distinct pattern of stacked progradational sedimentary wedges on the shelf margin and trough region. These sedimentary wedges comprise a succession of three distinct units (A lower, B middle, C upper unit), bounded by erosional unconformities, and their internal structures show well-stratified, complex sigmoid-oblique types. The isopach map shows that the wedges thicken seaward and their landward limit is in about 100–120 m water depth. According to the internal structure, distribution pattern and radiocarbon dates, these sedimentary wedges are interpreted to have been formed during the glacioeustatic sea-level lowstands. Multiple falls of sea level during the late Quaternary, coupled with high sediment discharge from the paleo-Nakdong River system, resulted in the formation of thick lowstand wedges. Three intervals of wedge building are recorded; the youngest one (unit C) is interpreted to mark the last glacial period, while the middle one (unit B) represents the lowstand period before the last interstadial.

EVOLUTION OF LATE QUATERNARY DEPOSITS ON THE INNER SHELF OF THE SOUTH SEA OF KOREA

Abstract High-resolution (3.5 kHz) seismic profiles and sediment samples were analysed to delineate the shallow seismic stratigraphy of late Quaternary deposits in the central part of the Korean South Sea. The stratigraphy above the acoustic basement is represented by three distinct sedimentary sequences separated by erosional boundaries. The upper sequence (A) has an acoustically semi-transparent signature usually without distinct internal reflectors and overlies the flat-lying mid-reflector (R). The sediments in this sequence consist of bioturbated terrigenous mud derived mainly from the Seomijin River. They have developed during the last 5000 yrs and have an accumulate rate of 10–40 cm/100 yrs. reflector R represents the postglacial transgressive layer formed during the last stage of the Holocene sea-level rise, approximately between 7000 yrs B.P. and 5000 yrs B.P. The middle sedimentary sequence (B) below (R) shows parallel to subparallel internal reflectors and prograding clinoforms that have formed largely during the last glacial period. The top of this sequence displays various erosional channels and transparent fills, produced by subaerial and fluvial erosion of the pre-existing shelf. The submerged channels indicate the existence of a paleochannel system, presumambly the extension of the paleo-Seomijin River during the sea-level lowstand. The lower sequence (C) has an irregular, erosional upper surface and an acoustically transparent subbottom. It is interpreted as the erosional remnants of earlier Pleistocene sediments formed in shallow water environments.

Evidence for gassy sediments on the inner shelf of SE Korea from geoacoustic properties

The inner shelf of SE Korea is characterized by an up to 40 m thick blanket of soft sediments often characterized by acoustic turbidity (AT). This AT is caused by a layer of sub-surface gas, which prohibits the identification of geological structures below that gas layer. Sound speeds were measured directly in these sediments using the Acoustic Lance (AL) in both mid- and late-September 1999. In situsou nd speeds obtained in mid-September varied between 1400 and 1550 m/s, and thus did not confirm the presence of gas within the top 3.5 m of the seafloor. However, signal waveforms suggested that a gassy layer might have been just below the depth penetrated by the Lance. In late-September, on the other hand, two sites showed an abrupt decrease in signal amplitudes and in sound speed (less than 800 m/s) at depths as shallow as 2 m below seafloor, indicating the presence of free gas bubbles. Piston-cored sediments were retrieved at the same sites in February 1999. X-radiographs of some of the cores revealed numerous microcracks caused by the expansion of gas bubbles during core retrieval. In contrast to in situ acoustic data, ultrasonic sound speeds acquired in the laboratory in May 1999 on those cores did not differentiate convincingly between gas-bearing and gas-free sediments. Our measurements on the SE Korean shelf with the AL provide new data on the in situ acoustic behavior of gassy sediments and the sediments that overlie them in zones of AT. r 2003 Elsevier Science Ltd. All rights reserved.

High-resolution seismic study of modern fine-grained deposits: Inner shelf off the southeastern coast of Korea

The geometry and internal structures of modern sediments on the inner shelf off the southeastern coast of Korea were investigated by means of analysing high-frequency (3.5 kHz) seismic records. The records reveal a wedge-shaped sediment body, tapering off toward the sea. On the basis of reflection patterns, the sediments can be classified into two units; “foreset” (prodelta) unit and “bottomset” unit, consisting of sandy muds and clays, respectively. The lateral transition from foreset to bottomset deposits suggests a prograding delta system of the Nakdong River since the late Holocene.

Distribution and Acoustic Characteristics of Shallow Gas in the Korea Strait Shelf Mud off SE Korea

Shallow gas in the Korea Strait shelf mud (KSSM) off SE Korea, revealed by high-resolution subbottom profiles, is associated with acoustic blanking, acoustic turbidity, seepages with plumes in the water column, and seafloor depressions. The acoustic blanking, characterized by strong, consistent top reflection and wipeout below, is most dominant. The seaward edge of the acoustic blanking zone generally coincides with the 100-m water-depth contour, suggesting that the water depth (the pressure) may control the distribution of shallow gas. The acoustic turbidity, characterized by diffuse top reflection, is a dark smear, partially blanking the data below. The seepages with plumes, characterized by vertical smearing and disturbed seafloor, are seen only along the shallowest, landward edge of the acoustic blanking zone. This may suggest that the decreased gas solubility at shallow water depths, caused by the lowered pressure, increases the volume of free gas in the sediments, facilitating the gas escape. The seafloor depressions, interpreted as pockmarks, are accompanied by cone-shaped acoustic masking, which is probably the reflection from a narrow vent of gas. The gas-related acoustic anomalies appear to occur mostly in the upper, recent mud of the KSSM. Neither permeable beds nor faults, which can act as vertical migration pathways for deep thermogenic gas, are evident in the recent mud. We interpret that the bacterial degradation of organic matter in situ is the main source for the gas in the KSSM. The upwelling off SE Korea may be an important source for the increased organic matter in the area.

Near-surface seismic facies at the Korea Strait shelf margin and trough region

High-resolution seismic profiles across the shelf margin and trough region of the Korea Strait reveal five shallow, near-surface facies units. These are relict coastal deposits, relict delta deposits, slumps and slides, and trough lag deposits. Most deposits represent a lowstand systems tract, formed during the last lowstand of sea level. Relict coastal deposits represent a linear sediment body along the present shelf margin at water depths of 120–150 m, whereas relict delta deposits occur on the gentle, southwestern slope of the trough at water depths of about 150–200 m. Slumps and slides are dominant at the base of slope in the central trough region. Sediments on the central trough floor were partly eroded and redistributed by strong currents, resulting in lag deposits.

Deposition of Coarse-Grained Sediments in the Korea Strait during Late Pleistocene Low Sea Level

The wide area of the Korea Strait shelf is covered by a thin veneer of gravelly sands, sands, or muddy sands. Most gravels are sub- to well-rounded and their content reaches up to 25%. These sediments were derived from the Korean Peninsula through the paleochannel of the Nakdong River, which extended onto the mid-shelf during late Pleistocene low sea level. The gravel distribution pattern suggests reworking and transport of the sediments by strong currents during times of low sea level.

Acoustic characters and distribution pattern of modern fine-grained deposits in a tide-dominated coastal bay: Jinhae Bay, Southeast Korea

Jinhae Bay, a semi-enclosed, tide-dominated coastal embayment on the southeastern coast of Korea, receives large amounts of sediment derived from the Nakdong River. The irregular surface of the acoustic basement is overlain by a modern sedimentary sequence up to 25 m thick, characterized by an acoustically semitransparent subbottom. Sediments, consisting mainly of terrigenous and bioturbated mud, accumulate at a rate of 2–5 mm/yr. About 21% of the suspended sediments discharged from the Nakdong River, that is approximately 1.0 × 106 tons per year, accumulate in Jinhae Bay. Modern sedimentation began probably at about 5000 yr BP, when sea level approached its present level.