Seung Soo Chun

Sedimentology of Modern, Inclined Heterolithic Stratification (IHS) in the Macrotidal Han River Delta, Korea

ABSTRACT An occurrence of inclined heterolithic stratification (IHS) is described from a tidal point bar in a 40-m-deep distributary of the macrotidal (tidal range 3.6-7.8 m), Han River delta, Korea. The channel bank demonstrates a convex-upward profile with intermittent presence of wave-formed scarps and terraces near the low-water level. The vertical succession of IHS is approximately 25 m thick and dips into the channel with angles reaching 14°. The IHS overlies 15 m of trough cross-bedded sand deposited in the channel thalweg and lower point bar. Even though the channel as a whole is ebb dominated, the preserved cross bedding is predominantly flood directed because the mutually evasive nature of the ebb and flood currents causes the point-bar surface to be flood dominated. This pattern may be a common feature of tidal point bars. The IHS itself consists of interstratified fine sand, sandy silt, and silt with an upward-fining textural trend. Tidal rhythmites are well developed in the middle and upper intertidal zone, and may also be present in the subtidal zone, but are poorly developed near the low-water level because of wave action. Seasonal discharge variations of the Han River are not obvious in the deposits, because the large size, distal location, and energetic tidal environment of the studied channel reduces the impact of river-stage fluctuations. Despite the moderate salinity levels, bioturbation is rare, except in the upper intertidal zone, because of the rapid sedimentation and energetic conditions.

LANDWARD MIGRATION OF ISOLATED SHELLY SAND RIDGE (CHENIER) ON THE MACROTIDAL FLAT OF GOMSO BAY, WEST COAST OF KOREA: CONTROLS OF STORMS AND TYPHOON

ABSTRACT A shelly sand ridge, 860 m long, 30-60 m wide, and up to 1.6 m high, parallels the shoreline on a high tidal mudflat in Gomso Bay, which opens to the eastern Yellow Sea. The ridge is composed entirely of fine to medium sand and shell material, with some subangular gravel, and overlies tidal mud discordantly. The lithostratigraphy of the bay deposits and 14C dates of shells from vibracores suggest that the shelly sand ridge, together with mid to lower sand flats, has accumulated on the tidal mud since 1800 yr B.P. This geologically recent development suggests that ridge evolution has been independent of regional sea-level change. River influence also seems to be negligible because of the apparently meager riverine sediment input into the bay. Judging from a series of aerial photographs spanning the past two decades (1967-1989), broad sand shoals on the mid to lower flat have moved landward and have become the shelly sand ridge. Vertical sections from a trench cut across the ridge also attest to its landward migration, with gently landward-dipping interbeds of sand and shell. Repeated measurements of the morphological change of the ridge over a two-year period (1990-1992) indicate that net transport was landward, on the order of 8 m/yr, except for during the typhoon season. The migration rate in the stormy winter monsoon season was generally two or three times greater than that of the remaining season. However, in the summer of 1992, a typhoon entering the Yellow Sea vigorously shifted the ridge landward, causing a displace ent of up to 11 m in a few days. This suggests that strong waves associated with storms or typhoons have predominantly driven the shelly sand ridge and offshore sands onto the tidal flat.

Depositional processes of late Quaternary sediments in the Yellow Sea: a review

This review focuses on the depositional processes of late Quaternary sediments in the eastern Yellow Sea, an epicontinental sea with a flat and broad seafloor (less than 100 m in water depth) and extensive tidal flats along the southeastern coast. The Yellow Sea was subaerially exposed during the last glacial period when sea level was about 120 m below the present level. During erosional retreat of shorefaces and river mouths, sedimentation was largely controlled by high-amplitude rise in sea level, forming transgressive sheets (echo type 1–3a) and sediment ridges (echo types 2-1 and 2–2) with extensive development of ravinement surfaces. The distribution of surface sediments reflects an interplay of sediment input from the surrounding landmass and the hydrodynamic regime in response to sea-level rise. Muddy sediments in the central part represent the Huanghe-River source and form a highstand sheet (echo type 1–3b). Large birdfoot-like sand bodies off the Jiangsu coast also represent highstand deposits when sea level reached the present position at about 6 ka. In the southeastern part of the Yellow Sea, sediments are dominated by sand ridges (echo types 2-1, 2–2 and 2–3), largely shaped by tidal currents. The southwestern corner of the Korean Peninsula is dominated by a thick deposit of mud, the Heuksan mud belt. The muds largely originate from the Geum River, whose distribution is controlled by strong southward coastal currents. In the coastal regions of the southeastern Yellow Sea, sedimentation is controlled by a combined effect of waves and tides with distinctive season-alities in sedimentary facies owing to the monsoonal climate: tidedominated mud deposition in summer and wave-dominated sand deposition/erosion in winter: Winter storms play a role in sedimentation on intertidal flats. Quantitative monitoring of sediment transport suggests that the textural variation results from the overwhelming role of winter waves superimposed on tidal currents in pulling sands and resuspending muds. Due to low sedimentation rate, the tidal flats formed retrogradational, coarsening-upward pattern during the Holocene sea-level rise.

A seasonal model of surface sedimentation on the Baeksu open-coast intertidal flat, southwestern coast of Korea

Detailed sedimentological studies such as surface sediment distribution, facies changes and sedimentation rate have been carrried out over two years (1997–1998) in the Baeksu open-coast intertidal flat, SW Korea. In the winter and spring seasons, sand facies is relatively dominant on the flat resulting from the strong and frequent influence of wind storms. Average sedimentation rates are ca. 4 and 3 mm/month in winter and spring, respectively. High temperature and weak wave energy would facilitate mud deposition over the sand-facies deposits during the summer with an average sedimentation rate of ca. 10 mm/month. In fall, mud sediments deposited in summertime, are subjected to erosion by strong waves. Sedimentation rate shows net erosion of ca. −10 mm/month in average. Such a seasonal cyclic pattern in deposition/erosion of surface sediments is most likely to reflect the importance of monsoonal controls in open-coast intertidal flats in temperate regions. The annual sedimentation rate of <0.5 mm/yr suggests that the intertidal flat would be in steady or quasi-equilibrium state.

Distribution Pattern, Geochemical Composition, and Provenance of the Huksan Mud Belt Sediments in the Southeastern Yellow Sea

2 Korea Institute of Geosciences & Mineral Resources, Daejeon 305-350, Korea Abstract: In order to determine the provenance of the Huksan Mud Belt sediments in the southeastern Yellow Sea, the major and rare earth elements of the same sediments were analyzed. The surface sediments were sampled from top of piston-cores and box-cores taken at 51 sites within the Huksan Mud Belt. With the mean grain size of 5-6 φ , the sediments of the study area are mud-dominated. The spatial distribution patterns show that silt content is high in the northern Mud Belt, whereas clay content increases as it moves toward the southern Mud Belt. Interestingly, the geochemical compositions both of major and rare earth elements have resulted in differences of sediment provenance. Among the major elements, plots of Fe/Al vs. Mg/Al ratios, Al 2O3 vs. MgO ratios, and Al2O3 vs. K2O reveal that the Huksan Mud Belt sediments are dominated by the Korean river-derived sediments. However, the characteristics of rare earth elements infer sediments originating from the Chinese rivers. This discrepancy between the above provenances is attributed to the different contributory factors in the content of chemical elements. Considering strong correlation between major elements with grain sizes, the contents of the major elements are thought to be influenced by the grain size. However, there is a weak correlation between rare earth elements and grain sizes. The behaviour of rare earth elements may be controlled by heavy minerals, rather than grain sizes. Further study requires to solve the discrepancy arose from the difference in applied chemical tracers.

Armored mudstone boulders in submarine debris-flow deposits, the Hunghae Formation, Pohang Basin: An evidence for the large-scale slumping of adjacent area of a submarine channel or scar wall

In the Hunghae Formation of the Pohang Basin, some heavily armored, calcareous mudstone balls (boulders) occur in a chaotic conglomerate bed which was deposited from debris flow on a steep slope. Sparsely armored or unarmored mudstone balls are also found in the same bed. Also some isolated armored mudstone boulders occur in a homogeneous mudstone bed of the formation which is interpreted as hemipelagite. The armored mudstone balls were most likely formed by retrogressive failures of submarine channel or scar wall and subsequent rolling over sand and pebble pavement along the submarine channel floor prior to incorporation into the debris flow. Their occurrence also suggests that a large-scale slumping should be happened on the upper slope apron or lower prodelta including the submarine channel and be evolved into debris flows, resulting in the corporation of the boulders into them. Some armored mud boulders rolled along and passed away the channelized gravel pavement, and deposited on hemipelagic mud in lower slope, not having been reworked.

Factors controlling mud accumulation in the Heuksan mud belt off southwestern Korea

The Heuksan mud belt (hereafter HMB) is 20~50 km wide, ~200 km long, and ~50 m thick, having accumulated in the course of the Holocene transgression on the tide-dominated epicontinental shelf southwest of Korea. The internal architecture of the HMB is characterized by offshore prograding clinoforms. Of particular interest are the depositional processes responsible for this anomalously thick mud accumulation within a relatively short period of time. Tidal currents are important in the dispersal of mud in the HMB, although these alone cannot explain such an enormous mud deposit. In order to understand the formative processes of the HMB, a detailed sedimentary facies analysis, including high-resolution grain-size measurements, has been conducted on more than 30 short cores and three long drill cores recovered from the mud belt. Five major mud facies were identified. Of these, mud sequences showing a thickening–thinning trend of alternating silt and clay laminae suggestive of a tidal origin occur dominantly at inner to mid shelf locations. By contrast, internally structureless muds with sharp bases and no bioturbation, which are interpreted of representing fluid-mud deposits, are widespread at mid to outer shelf locations. Wave-generated mud ripples and storm beds on the inner shelf suggest that storm waves in winter resuspend previously deposited mud to form near-bed fluid-mud suspensions with resulting gravity-driven mud transport across the low-gradient outer shelf. This previously not recognized process is probably a major factor controlling depositional processes on the giant mud belt, enabling rapid accumulation and offshore progradation even during transgression, i.e., at times of sea-level rise.

Age and morphodynamics of a sandy beach fronted by a macrotidal mud flat along the west coast of Korea: a lateral headland bypass model for beach-dune formation

The Dasari beach-dune system fronted by an intertidal mud flat is a typical example of numerous small beaches found both in embayments and along the open macrotidal west coast of Korea. The beach is frequently exposed to energetic wave action at high tide in winter. Although this coastal dune-sandy beach-intertidal mud flat system has previously been described, its origin and morphodynamic behavior has to date not been firmly established. To clarify these issues, elevation profiles and surficial sediment samples were collected seasonally along five monitoring transects across the tidal flat. In addition, box-cores as well as vibro- and drill-cores were acquired along the middle transect. Optically stimulated luminescence (OSL) and 14C– AMS (accelerator mass spectrometry) dating methods were applied to determine the age of the tidal flat, the beach and the dune deposits. The results show that Dasari beach is topographically composed of two distinct morphological and sedimentological sectors, comprising a high-tide sandy beach that merges seaward into an extensive low-tide tidal flat composed of mud. The transition between the two sectors is marked by a sharp break in slope and change in internal sedimentary structures. At the boundary, the subtle shoreward fining trend in mean grain size on the intertidal flat switches to a pronounced shoreward coarsening trend. Near the transition, mixing between the beach sand and the mud is observed. Another striking feature is a seasonal rotation of the beach system centered on the middle sector, with the northern sector eroding in winter and accreting in summer, and the southern sector accreting in winter and eroding in summer. The spatial grain-size pattern reveals that the beach is fed from the neighboring beach in the north by lateral headland bypassing, rather than onshore transport across the tidal flat, the intermittent lateral supply of sand explaining the seasonal rotation of the beach. Stratigraphically, the beach-dune deposits are underlain by muddy tidal flat deposits, which results in a clear upward coarsening grain-size trend and thus intimates transgressive deposition associated with sea-level rise over the past 7–8 ka. However, a time gap of 4–5 thousand years between the tidal flat and the beach-dune deposits indicates that there is no genetic link between the two depositional systems. As the modern beach-dune system is remote from the former landward limit of the tidal flat, a continuous retreat model must be rejected. Instead, a lateral headland bypassing model, in which sand supplied alongshore progressively encroached the tidal flat, is more plausible. In the light of this interpretation, many of the barrier beach-lagoon models proposed for the macrotidal west coast of Korea need to be reconsidered.

The 2009-based detailed distribution pattern and area of Phragmites communis-dominant and Suaeda japonica-dominant communities on the Suncheon-bay and Beolgyo estuarine wetlands

Halophyte distribution pattern and area in the Suncheon-bay and Beolgyo estuary coastal wetlands were analyzed using KOMPSAT-2 landsat images were taken in 2008 and 2009, and field investigations were fulfilled for confirming the precise boundaries of individual halophyte areas. The salt-marsh vegetation in those areas can be classified mainly into two dominant communities: Suaeda japonica-dominant and Phragmites communis-dominant communities. In order to identify sedimentary characteristics, tidal-flat surface leveling and sedimentary facies analysis had been conducted. The sedimentary facies of marsh area are mostly silty clayey and clay facies with a little seasonal change and its slope is very gentle (0.0007~0.002 in gradient). Phragmites communis and Suaeda japonica communities were distributed in the mud-flat zone between 0.7 m and 1.8 m higher than MSL (mean sea level): zone of 1.1~1.8 m in the former and zone of 0.7~1.3 m in the latter. In the Suncheon-bay estuarine wetland, on the basis of 2009 distribution, Phragmites communis-dominant and Suaeda japonica-dominant communities are about 0.79 km and 0.22 km in distribution area, respectively. On the other hand, Bulgyo estuarine marsh shows that the distribution areas of Phragmites communis-dominant and Suaeda japonica-dominant communities are about 0.31 km and 0.031 순천만과 벌교 하구 연안습지의 2009년 기준 갈대 및 칠면초 우세 군집 분포양상과 면적 제시 Journal of Wetlands Research, Vol. 17, No. 1, 2015 27 km in distribution area, respectively. Individual 105 and 60 dominant community areas and their distribution patterns can be well defined and indicated in the Suncheon-bay and Bulgyo estuarine marshes, respectively. The distribution pattern and area of hylophyte communities analyzed in this study based on 2008/2009 satellite images would be valuable as a base of future monitoring of salt-marsh related studies in the study area which is the most important salt-marsh research site in Korea.

Micro-characteristics of sustained, fine-grained lacustrine turbidites in the Cretaceous Hwangsan Tuff, SW Korea

A distinction between the classic Bouma turbidite and a hyperpycnite is needed because hyperpycnal flows provide a direct link between the terrestrial source and the depositional sink. These deposits are a potential record of river floods and hence contain tectonic and climatic information. Unlike the sandy turbidite or hyperpycnite, the muddy counterpart is generally poorly documented. Based on detailed sedimentological analysis of a deltaic mudstone unit within the Cretaceous Hwangsan Tuff, SW Korea, this paper addresses the characteristics and depositional processes of the lacustrine muddy turbidite or hyperpycnite. The Hwangsan Tuff is composed of reworked volcaniclastics up to 300 m thick, deposited in a lake margin, on a delta front to the basin plain. The thin-bedded mudstones constitute a diverse type of turbidites. Most mudstone beds generally show normal grading and are wavy in form, but internally demonstrate the existence of a depositional break such as an erosional surface and the repetition of rippled units. Thin-bedded mudstone with a pronounced normal grading is interpreted to have been deposited by small, dilute turbidity currents. However, a composite bed of lower massive siltstone overlain by a structureless claystone suggests deposition from slow-moving, high-density flows with high clay content. The hyperpycnal flow deposits in muds are characterized by a thickness change in the horizontal laminae, internal erosion scour, and the recurrence of rippled units, all of which are features indicating deposition from flow fluctuations in long-lived hyperpycnal flows. Abundant plant debris and a lack of bioturbation also support this hypothesis. The hyperpycnal flows form the mouth bars to delta front bodies of the lake margin.