Sang-Joon Han

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.

Sediments and Sedimentary Sequences on a Modern Macrotidal Flat, Inchon, Korea

ABSTRACT Enormous, unbarred tidal flats fringe the west coast of Korea. Near Inchon, where spring tides range between 8 and 9 m, the intertidal expanse is more than 4 km wide. This low-energy regime results in three broadly intergradationai modern subfacies: l) an intensely bioturbated inner flat of slightly sandy mud; 2) a wavy-bedded middle flat of clayey sandy silt; and 3) a ripple-laminated outer flat of bioturbated sandy silt to silty sand. The mid-flat region is less distinctive sedimentologically than the mixed-flat subfacies of North Sea tidal flats, and flaser and lenticular bedding are rare. Well-developed intertidal drainage networks and landward salt marshes are absent. Vibracores reveal two additional sequences underneath the modern sequence, their contacts defined by scour horizons and shell concentrations. The basal sequence is characterized by irregularly oxidized, intensely-to-totally bioturbated argillaceous sediment with scattered wavy beds and abundant in-situ plant roots. These deposits suggest a transition from a shallow subtidal or low intertidal environment to a salt marsh developed in a protected intracoastal setting. The overlying intermediate sequence also consists principally of bioturbated fine sediment with scattered wavy beds, but plant roots are absent. Stratigraphic distributions of mollusk shells and other features ally this sequence with landward parts of the modern sequence.

The distribution of clay minerals in recent sediments of the Korea Strait

Abstract The bottom sediments of the Korea Strait contain, on average, 50% illite, 21% kaolinite, 17% intergrade clay, 8% chlorite, and 4% smectite. The geographical distribution of the clay minerals shows an increase in illite, a decrease in kaolinite and a slight decrease in chlorite seawards. The distributional patterns of smectite and intergrade clay are irregular. The clay fractions of the sediments near the land are supplied by river-borne sediments and are mixed with sediments transported landwards from offshore. The distribution of clay minerals in the Korea Strait appears to have been influenced by the supply or/and redistribution of fine-grained sediments by the Tsushima Warm Current from the East China Sea. Kaolinite seems to be transported mainly by river runoff; and illite, chlorite and smectite might have been derived partly from the East China Sea by the Tsushima Warm Current. Intergrade clay seems to have a complex origin.

A Geochemical Boundary in the East Sea (Sea of Japan): Implications for the Paleoclimatic Record

Sediment from six piston cores from the East Sea (Sea of Japan) was analyzed for evidence of paleoceanographic changes and paleoclimatic variation. A distinct geochemical boundary is evident in major element concentrations and organic carbon content of most cores near the 10-ka horizon. This distinctive basal Holocene change is interpreted to be largely the result of changing sediment sources, an interpretation supported by ratios. Organic carbon and carbonate contents also differ significantly between the Holocene and glacial intervals. The C/N ratio of organic matter is greater than 10 during the glacial period, but is less than 10 for the Holocene, suggesting that the influx of terrigenous organic matter was more volumetrically important than marine organic matter during glacial times. The chemical index of weathering (CIW) is higher for the Holocene than the glacial interval, and changes markedly at the basal Holocene geochemical boundary. Silt fractions are higher in the glacial interval, suggesting a strong effect of climate on silt particle transportation: terrigenous aluminosilicates and continental organic carbon transport were higher during glacial times than during the Holocene. Differences in sediment composition between the Holocene and glacial period are interpreted to have been climatically induced.

Clay mineralogy of bottom sediment in the Jinhae Bay, Korea

The bottom sediments of Jinhae Bay area contain, on average, 45 % illite, 23 % kaolinite, 17 % intergrade clay, 10 % chlorite, and 5 % smectite. The geographical distribution of the clay minerals shows, in general, an it crease in illite, a decrease in kaolinite, and a slight decrease in chlorite seawards. The distributional trends of smectite and intergrade clay are irregular. The clay fractions of the sediments of Jinhae Bay are supplied from stream-borne sediments and are mixed with sediments transported landwards from the East China Sea. Local erosion and redeposition of the clay fractions of the bottom sediments is caused by rapid tidal currents.