Soo-Chul Park

High-Resolution Seismic Study as a Tool for Sequence Stratigraphic Evidence of High-Frequency Sea-Level Changes: Latest Pleistocene-Holocene Example from the Korea Strait

ABSTRACT Sequence stratigraphic analysis of high-resolution seismic reflection profiles and sediment data reveals that the latest Pleistocene-Holocene deposits in the Korea Strait shelf off the southeastern Korean Peninsula form a high-frequency sequence consisting of a set of lowstand, transgressive, and highstand systems tracts that corresponds to a fifth-order (20 kyr) sea-level cycle. Eight depositional systems, each with different seismic facies, constitute the systems tracts. The lowstand systems tract, consisting of sandy mud, forms a deltaic wedge that pinches out updip near or at the relict shelf edge. The transgressive systems tract, consisting mainly of sands, includes six depositional systems: (1) inner-shelf transgressive layer, (2) transgressive estuarine-deltaic complex, (3) transgressive sand ridge, (4) mid-shelf transgressive layer, (5) incised-channel fill, and (6) beach-shoreface complex. Although the transgressive systems tract is widely distributed, it is much thinner than the lowstand and highstand systems tracts. The highstand systems tract is composed of a prodelta-shelf complex consisting almost exclusively of Holocene muds. The distribution and geometry of the systems tracts in the latest Pleistocene-Holocene sequence in the Korea Strait shelf is different from that of Vails model in that: (1) the lowstand systems tract consists only of the lowstand deltaic wedge, which forms an elongated sediment body along the paleoshelf edge, and (2) the highstand systems tract is completely confined to the inner shelf and forms a nearshore belt parallel to the coastline. High-frequency (20 kyr), high-amplitude (magnitude of about 130 m) sea-level change, together with strong currents flowing northeastward along the southeastern coast of the Korean Peninsula and varying sediment supply, have resulted in this rather unusual stratigraphic architecture.

Transgressive sand ridges on the mid-shelf of the southern sea of Korea (Korea Strait): formation and development in high-energy environments

Abstract The mid-shelf of the southern sea of Korea (Korea Strait) is covered by a series of shore-parallel sand ridges up to 63 km long, 3–9 km wide, and 22 m thick. The ridges are a moribund type but show well-organized, dipping strata, indicating appreciable progradation. The submerged paleo-channel system (the paleo-Seomjin River) associated with the sand ridge field suggests that the ridges primarily originated from river-mouth sandy shoals, which subsequently developed into shore-parallel, linear sand bodies in high-energy environments. The combined effect of coastal (tidal and longshore) and offshore ocean currents was possibly responsible for shore-parallel sediment transport and growth of the ridges. 14C dates, core lithology, and sea-level data suggest that the sand ridges mainly formed and developed during the middle period of the postglacial transgression. They are postulated to have developed sequentially from offshore to onshore in response to sea-level rise during that time. Very slow sea-level rise and high sediment supply through the paleo-channel system probably provided favorable conditions for forming the sand ridges. The ridges became inactive during the late transgression period as a result of a substantial reduction of wave and current strength and remained on the mid-shelf.

Depositional patterns of sand ridges in tide-dominated shallow water environments: Yellow Sea Coast and South Sea of Korea

Abstract The continental shelf of the Yellow Sea and the Korean South Sea is a tide-dominated shelf. A number of sand ridges occur in this area which form bathymetric highs. Surveyed were one large sand ridge (Jangan Satoe) along the western (Yellow Sea) coast and a group of mid-shelf sand ridges in the South Sea, to delineate their depositional pattern and probable origin in tide-dominated shallow water environments. Analyses of sediment samples, side-scan sonographs, and seismic profiles reveal that these ridges show regionally different characteristics in morphology, bedforms, and sediment. The sand ridge in the Yellow Sea shows a composite form of various bedforms including sand waves and megaripples. This ridge is interpreted to be in an accretional active stage, maintained by strong tidal currents. In contrast, the sand ridges on the mid-shelf of the South Sea show rounded, single forms without bedforms on them. They are interpreted to represent moribund-type sand ridges in the shelf environment, which were formed near the shoreline during the Holocene sea-level transgression. These ridges developed off the paleo-Seomjin River during a period of a stillstand or slow sea-level rise approximately between 10,000 and 7000 yrs B.P.

Evolution and chronology of late pleistocene shelf-perched lowstand wedges in the Korea strait

Abstract Detailed interpretation of high-resolution seismic profiles from the Korea Strait has shown the existence of a complex of three stacked sedimentary sequences on the outer shelf. The individual sequences, consisting of prograding wedge, are interpreted, from internal structure and sediment data, as a ‘shelf-perched lowstand wedge system’, related to alternating episodes of successive regression and transgression. Although the entire sequences have not been fully dated, it is believed that the sequences were deposited during late Pleistocene sea-level lowstands. Repeated falls of sea level (isotopic stages 8, 6 and 2) resulted in the formation of lowstand sedimentary sequences. During subsequent episodes of sea-level rise (isotopic stages 8/7, 6/5, and 2/1), the sequences were reworked and draped by thin transgressive sands.

Shallow seismic stratigraphy and distribution pattern of late Quaternary sediments in a macrotidal bay: Gunhung Bay, west coast of Korea

Abstract The shallow seismic stratigraphy of late Quaternary sediments in Gunhung Bay, a macrotidal coastal embayment of western Korea, was investigated by means of analyzing high-resolution seismic profiles and sediment samples. Late Quaternary sediments up to 20 m thick overlie the irregular surface of the acoustic basement. The sediments consist mainly of sands, muddy sands and sandy muds and show a zonal facies distribution. The distribution pattern appears to be largely controlled by the tidal currents in the bay. The sedimentary column above the acoustic basement can be divided into two sequences by a relatively strong mid-reflector which is interpreted as the erosional pre-Holocene surface formed during low sea level. Many V-shaped erosional patterns in this reflector indicate the existence of old tidal channels. The sediments underlying the mid-reflector are interpreted to be semi-consolidated pre-Holocene tidal deposits, presumably deposited during the last interglacial period, while the sediments overlying this reflector represent the late Holocene tidal desposits. Deposition of the late Holocene sediments began probably at about 5000 yrs B.P., when sea level approached its present level.

Seismic Stratigraphy and Depositional History of Holocene Transgressive Deposits in the Southeastern Continental Shelf, Korea

Analysis of high-resolution seismic profiles from the southeastern continental shelf of Korea reveals that the Holocene transgressive deposits consist of five sedimentary units characterized by retrograding or backstepping depositional arrangements. Unit I, forming a linear sediment body along the shelf margin, is an ancient beach/shoreface deposit formed during the early stage of transgression. During the transgression, the paleo-channels were backfilled with fluvial or coastal-plain sediments, forming Unit II as an incised-channel fill deposit. The near-surface sediment was reworked and eroded by shoreface erosion, forming a thin lag of sands (Unit III) on the midshelf. During the middle stage of the transgression, the shoreline may have stabilized at around 70 - 80 m below the present sea level for some period of time to allow the formation of sand ridge systems (Unit IV). Unit V in the inner shelf was deposited in an estuarine environment during the middle to late stage of transgression. Such transgressive stratigraphic architecture is controlled by a function of lateral changes in the balance among rates of relative sea-level rise, sediment input and marine processes at any given time.

Acoustic Characteristics of Gas-related Structures in the Upper Sedimentary Layer of the Ulleung Basin, East Sea

The upper sedimentary layer of the Ulleung Basin in the East Sea shows stacked mass-flow deposits such as slide/slump deposits in the upper slope, debris-flow deposits in the middle and lower slope, and turbidites in the basin plain. Shallow gases or gas hydrates are also reported in many area of the Ulleung Basin, which are very important in terms of marine resources, environmental changes, and geohazard. This paper aims at studying acoustic characteristics and distribution pattern of gas-related structures such as acoustic column, enhanced reflector, dome structure, pockmark, and gas seepage in the upper sedimentary layer, by analysing high-resolution chirp profiles. Acoustic column shows a transparent pillar shape in the sedimentary layer and mainly occurs in the basin plain. Enhanced reflector is characterized by an increased amplitude and laterally extended to several tens up kilometers. Dome structure is characterized by an upward convex feature at the seabed, and mainly occurs in the lower slope. The pockmark shows a small crater-like feature and usually occurs in the middle and lower slope. Gas seepage is commonly found in the middle slope of the southern Ulleung Basin. These gas-related structures seem to be mainly caused by gas migration and escape in the sedimentary layer. The distribution pattern of the gas-related structures indicates that formation of these structures in the Ulleung Basin is controlled not only by sedimentary facies in upper sedimentary layer but also by gas-solubility changes depending on water depth. Especially, it is interpreted that the chaotic and discontinuous sedimentary structures of debris-flow deposits cause the facilitation of gas migration, whereas the continuous sedimentary layers of turbidites restrict the vertical migration of gases.