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Journal of Sedimentary Research, 2006, v. 76, 1093–1105 Research Article DOI: 10.2110/jsr.2006.092

LATE QUATERNARY STRATIGRAPHY AND DEVELOPMENT OF TIDAL SAND RIDGES IN THE EASTERN YELLOW SEA SOO-CHUL PARK,1 BANG-HEE LEE,1 HYUK-SOO HAN,1 DONG-GEUN YOO,2

AND

CHI-WON LEE2

1

Department of Oceanography, Chungnam National University, Daejeon 305-764, Korea 2 Petroleum and Marine Research Division, Korea Institute of Geosciences and Mineral Resources, Daejeon 305-350, Korea e-mail: [email protected]

ABSTRACT: The eastern Yellow Sea is characterized by a number of tidal sand ridges which occur as a series of linear sediment bodies in the shelf (shelf sand ridges) and as a group of individual sand bodies in the nearshore (nearshore sand ridges). The shelf sand ridges are present in water depths of 50–90 m and show large, elongate shapes with a length up to 200 km. In contrast, the nearshore sand ridges are much smaller in size (up to 34 km length) and occur in water depths shallower than about 30 m. Detailed interpretation of seismic and lithologic data, using radiocarbon dating to constrain the ages of the ridges, has shown that the regional sea-level changes played a major role in the existence and development of different morphologic features of these sand ridges. The shelf sand ridges developed mainly during the postglacial transgression (ca. 14,000–9,500 yr B.P.), possibly during episodes of stillstand or very slow rise of sea level. The shape of the shelf sand ridges more or less results from an erosional process dominantly acting during the postglacial transgression. The substratum of the shelf sand ridges consists of the regressive or lowstand deltaic mud deposits, probably formed prior to the last glacial maximum (. ca. 17,000 yr B.P.). In contrast, the nearshore sand ridges have undergone tidal action during the recent highstand of sea level (ca. , 7,000 yr B.P.) and show a typical modern geomorphology of erosional sand ridges. The substratum of the nearshore sand ridges consists of remnants of the last interglacial tidal deposits. Large dunes indicate a strong hydrodynamic influence on the entire ridge surface at present.

INTRODUCTION

The eastern Yellow Sea is a well known macrotidal environment with tidal ranges of up to 9 m. Strong tidal action caused by high tidal ranges in this area significantly increases the potential for sediment reworking, affecting erosional–depositional processes on the seabed (Song et al. 1983; Adams et al. 1990; Wells and Park 1992). A large part of the sea floor in the eastern Yellow Sea is covered by sandy deposits which formed during the postglacial transgression (Lee et al. 1988). These sands have been reworked by modern tidal processes, and consequently sand ridges on the sea floor are prominent sedimentary features in the eastern Yellow Sea (Choi 2001; Park 2001). Most of the sand ridges in the eastern Yellow Sea occur either as a series of linear sediment bodies in the shelf (shelf sand ridges) (KIGAM 1996b; Jin and Chough 2002) or as a group of individual sand bodies in the nearshore (nearshore sand ridges) (Klein et al. 1982; Park and Lee 1994; KIGAM 1996b, 2000). Previous work suggests that the sand ridges around the Korean Peninsula are either active or moribund, depending on whether or not they respond actively to the present hydraulic regime (Park and Lee 1994; Liu et al. 1998; Park et al. 2003). Most of the nearshore sand ridges in the eastern Yellow Sea, which are present in water depths shallower than about 30 m, are interpreted as active tidal sand ridges on the basis of the observation of large bedforms (e.g., dunes) on the ridge surface, which are asymmetric with respect to the long axis of regional tidal currents (Klein et al. 1982; Bahng et al. 1994; Park and Lee 1994). These ridges modify their shape continuously in response to the prevalent tidal currents (Choi 2001; Park 2001) and show a typical modern tidal geomorphology which can be

Copyright E 2006, SEPM (Society for Sedimentary Geology)

comparable with those active tidal sand ridges in the eastern part of the Bohai Sea (Liu et al. 1998). In contrast, the sand ridges in the mid-shelf (50– 90 water depth) of the southern sea of Korea lack any superimposed significant bedforms, suggesting that the ridges are inactive or moribund (Park and Lee 1994; Park et al. 2003). These sand ridges are interpreted to have formed during the postglacial transgression when tidal currents were strong enough to generate sand ridges and remained moribund or inactive thereafter (Park et al. 2003). The same type of sand ridges is also reported from the East China Sea (Yang and Sun 1988; Liu et al. 1998). The shelf sand ridges in the eastern Yellow Sea are present in water depths deeper than 40–50 m, forming topographic highs on the bathymetric map. In general these sand ridges have large bedforms on the surface of them and extend in a direction parallel to the dominant tidal currents (KIGAM 1992, 1993, 1994, 1995, 1996a). They are known to have a tidal origin, but it is not yet clear whether or not they respond actively to the present tidal currents. The shelf sand ridges in the eastern Yellow Sea display remarkable similarities in size and scale with moribund sand ridges in the East China Sea shelf (Yang and Sun 1988), suggesting that local sea-level rise has played an important role in the formation of sand ridges. Recent studies also suggested that the shelf sand ridges in the eastern Yellow Sea are relict features formed prior to the recent sea-level rise (KIGAM 1996b, 2000), but details of their stratigraphy and formation have remained poorly understood. A more recent study based on one drill core from a shelf ridge crest (Jin and Chough 2002) suggests that the shelf sand ridges in the eastern Yellow Sea are erosional in origin as defined by Berne´ et al. (1998).

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In this study, we analyze a number of sediment cores and highresolution seismic (air-gun and sparker) profiles from the eastern Yellow Sea to understand the distribution pattern and stratigraphy of shelf as well as nearshore sand ridges. Interpreted seismic, lithologic, and sea-level data allow us to examine the role of regional sea-level changes in development of both types of sand ridges separately, using radiocarbon dates to constrain the ages of the ridges. We also describe the geometry and distribution pattern of these sand ridges and further discuss the development history during the late Quaternary. GEOLOGIC AND OCEANOGRAPHIC SETTING

The eastern Yellow Sea is a tectonically stable, postglacially submerged, shallow (, 90 m water depth) shelf with thick accumulations of land-derived clastic sediments (KIGAM 1996b). It is fringed by the Korean west coast, characterized by numerous rocky islands and lowlands of river mouths suggestive of a ria-type, submerged coast. The sea floor of the shelf is undulatory with topographic relief a few tens of meters high, forming ridges and swales (Fig. 1). These topographic features run in a NE–SW direction, nearly parallel to the direction of present tidal currents. The sea floor of the eastern Yellow Sea deepens progressively southwestward to form the northern extension of the East China Sea. Sediments in the eastern Yellow Sea are composed primarily of well sorted, fine to medium sands (Fig. 2), which have been intensely reworked during the Holocene rise of sea level (KORDI 1982; Lee et al. 1988). These sands are locally mixed with gravels or muds. The sea floor west of the sand field is covered by the central Yellow Sea mud deposits, which originated from the Huang He (Yellow River) (Alexander et al. 1991). The southeastern part of the eastern Yellow Sea is also characterized by a linear mud belt along the southwestern coast of the Korean Peninsula (Fig. 2), which is as thick as 50 m, covering an area of about 8,000 km2 (KIGAM 1996b; Jin and Chough 1998; Park et al. 2000). This mud belt is reported to have formed during the course of the postglacial transgression (Jin and Chough 1998; Park et al. 2000), but its origin is still a matter of controversy (Park et al. 2000; Lee and Chu 2001). The eastern Yellow Sea is strongly affected by semidiurnal tides and tidal currents (Choi and Fang 1993; Fang 1994). The tidal ranges are between 4 m and 9 m, with the maximum amplitudes found in the Kyonggi Bay, western central coast of Korea, where the tidal current velocity is over 100 cm/s. The tidal currents are quite variable in speed and direction because of complex bottom topography, islands, and an irregular coastline. It exceeds 200 cm/s in many narrows between islands and coastal embayments along the west and southwest coast of Korea. The general directions of tidal currents on the shelf are N–NE during flood and S–SW during ebb. The eastern Yellow Sea is also affected by the monsoon; the southerly and southwesterly winds are dominant in the summer and the northerly and northwesterly winds prevail in the winter. The northerly winds are persistent with an average speed of 8–9 m/s whereas the southerly winds are weaker and less persistent. The general circulation in the eastern Yellow Sea is dominated by the northward flow of the Yellow Sea Warm Current, a branch of the Tsushima Warm Current, characterized by warm water with high salinity (Guan 1994; Pang and Oh 2000; Lie et al. 2001). In contrast, the southward flow occurs as the Korean Coastal Current along the west coast of Korea. The Korean Coastal Current plays an important role in the southward transport of the fine-grained sediments derived from rivers, especially during the winter (Wells 1988). Recently, a regional sea-level curve was constructed in detail from the Yellow Sea (Liu et al. 2004). This sea-level curve shows that the postLGM transgression in the Yellow Sea is step-like: long periods of slow transgression punctuated by several, short rapid flooding events. Rapid and stepwise sea-level rise occurred during the melt-water pulses (Liu and Milliman 2004; Liu et al. 2004). Liu et al. (2004) have demonstrated that

FIG. 1.— Detailed bathymetric map of the eastern Yellow Sea. Contours are in meters.

this postglacial sea-level history played a very important role in formation of the Holocene mud deposits in the western Yellow Sea. MATERIALS AND METHODS

The data used in this study consist of: (1) about 9,000 line km of highresolution seismic reflection profiles (Fig. 3), (2) eleven shallow piston cores (1–2 m penetration depth), (3) five vibracores (2–4 m penetration depth), and (4) one drill core (13.5 m penetration depth). The seismic reflection data include sparker and air-gun seismic profiles collected by the Korea Institute of Geoscience and Mineral Resources (KIGAM). The shallow piston cores were collected from the shelf sand ridges (KIGAM 1992, 1993, 1994, 1995, 1996a), whereas the vibracores and drill cores were obtained from the nearshore sand ridges (KIGAM 1989, 2000). We analyzed the seismic data to define acoustic characteristics, bounding surfaces, thickness, and distribution of the sand ridges. Time-to-depth conversion was done using a sediment sound velocity of 1600 m/s (KIGAM 2000). The cores provide lithology, sedimentary structures, and radiocarbon ages. The radiocarbon ages of the shelf piston cores

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LATE QUATERNARY SAND RIDGES, THE EASTERN YELLOW SEA

FIG. 2.— Distribution map of sediment types in the study area. The data are compiled from the KIGAM’s reports (1992, 1993, 1994, 1995, 1996a).

were determined on foraminifera and wood fragments using accelerator mass spectrometry (AMS) at the Institute of Geological and Nuclear Sciences of New Zealand (KIGAM 1996a). The radiocarbon dates of the nearshore vibracores were made on molluscan shells at the Gakushuin University of Japan (KIGAM 2000). DISTRIBUTION AND MORPHOLOGY OF SAND RIDGES

The sand ridges in the eastern Yellow Sea can be grouped into the shelf sand ridges (SSR) and the nearshore sand ridges (NSR); the former occur in the shelf area at 50–90 m water depths, whereas the latter are located in the nearshore area at water depths shallower than about 30 m (Fig. 4). A total of twenty-three shelf sand ridges are identified on the seismic profiles from the shelf. They are aligned in a NE–SW or N–S direction, nearly parallel to the present tidal currents. They show principally linear, elongated shapes with dimensions of 30–200 km length, 3–13 km width, and 13–25 m height (Fig. 4). Some of the sand ridges (SSR9, SSR14, SSR15, SSR15) are slightly curved, showing a convex form toward the sea. Most of the shelf sand ridges, except for the ridges at the seaward

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FIG. 3.— Seismic (sparker and air-gun) track lines of the study area.

margin of the ridge field, are covered by large bedforms (dunes) with wave lengths of 200–500 m. The shelf sand ridges have generally a symmetric transverse profile, but at some locations, especially at the eastern margin of the ridge field, they show an asymmetric transverse profile with their steepest slopes generally facing landward. Most of the nearshore sand ridges are concentrated in the Kyonggi Bay, the central western coast of Korea. A total of ten sand ridges are distributed in the southern Kyonggi Bay in water depths less than 30 m, most of which are tied to rocky outcrops and islands (Fig. 4). They are much smaller in size and scale than the shelf sand ridges. The nearshore sand ridges have dimensions of 6–34 km length, 1–9 km width, and 7– 23 m height. They show lobate or linear shapes with their long axis parallel to the tidal currents. The nearshore sand ridges are oriented mainly in a NE–SW direction except for one sand ridge (NSR10) directed to NW–SE inside the Asan Bay. These nearshore sand ridges show an asymmetric tranverse profile with large bedforms (dunes) on them. Large dunes have in general a wave length of 100–200 m and a height of 3–5 m, with smaller dunes superimposed on them. The ridge topography disappears seawards to form a flat seabed covered by sandy sediments.

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FIG. 4.—Distribution map of the shelf sand ridges (SSR) and nearshore sand ridges (NSR) in the eastern Yellow Sea. The shelf sand ridges are numbered from SSR1 to SSR21 and the nearshore sand ridges from NSR1 to NS10. Heavy lines denote positions of seismic profiles shown in Figures 5, 6, 8, 9, and 10. The asterisk on the NSR10 indicates the position of one drill core (G7) described in Figure 10. The southeastern part of the study area is covered by the southeastern Yellow Sea mud deposits (SEYSM).

ACOUSTIC CHARACTER, LITHOLOGY, AND 14C AGES

Shelf Sand Ridges The shelf sand ridges rest on a gently sloping, continuous reflector that can be traced over a wide area of the ridge field (Figs. 5, 6). This reflector is irregular and characterized by a high acoustic-impedance contrast on the seismic profiles. The shelf sand ridges above this reflector can be divided into three seismic units (S1, S2, and S3 in descending order) based on the erosional boundaries and acoustic characters (Figs. 5, 6). The upper unit (S1) is a thin surface layer of the ridges less than 5 m in thickness. This unit shows acoustically transparent or chaotic reflection patterns. The middle unit (S2) has a thickness up to 20 m and exhibits acoustically oblique or prograding clinoforms. These two units form the main body of the shelf sand ridges. The lower unit (S3) is the ridge substratum, separated from the above two units by a strong, erosional mid-reflector. This unit displays variations in thickness (5–20 m) and is

characterized by parallel to subparallel reflection patterns which are laterally discontinuous. Analyses of shallow piston cores, collected from various parts of the shelf ridges, show that sediments on the surface and shallow subsurface of the shelf sand ridges can be classified into sand facies, sand–mud mixed facies, and mud facies (Fig. 7). The sand facies consists of massive sand with scattered shell fragments. It comprises principally the top surface layer (S1) of the ridges and does not show any structures in the cores. Sediments of the main body (S2) of the ridges cannot be easily identified in the cores because of short core length. However, the muddy sand layers at the top of core 93P29 and at the base of core 94P20 probably represent this unit (Fig. 7). The ridge substratum (S3) underlying the ridges is identified as the mud facies, consisting of mud or sandy mud in the cores (94P25, 93P29, 92P15, 94P33) from the ridge trough. A thin layer of massive sand or muddy sand overlying the mud facies in these cores corresponds to the S1 or S2 unit.

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FIG. 5.—High-resolution A) sparker and B) air-gun profiles from the shelf sand ridges (for profile locations, see Fig. 4). The ridges principally can be divided into three seismic units (S1, S2, and S3 in descending order) according to the erosional boundaries and acoustic characters, although the surface unit (S1) is sometimes too thin to be resolved by our seismic system. The surface (S1) and middle (S2) units form the main sand body of the ridges, and the lower (S3) unit represents the ridge substratum.

A total of seven 14C ages were obtained from the ridge trough cores comprising the substratum (S3) of the ridges (Table 1, Fig. 7). The AMS dates, measured from foraminiferal tests in the core 94P25, show an age of 17,350 yr B.P. in the upper part and 18,410 yr B.P. in the lower part. The AMS date made on the wood fragments in core 92P15 is 26,800 yr B.P. The AMS dates measured from foraminiferal tests in the core 94P33 are between about 27,000 yr B.P. and 45,000 yr B.P., generally decreasing upward in the core section. Consequently, the S3 unit consisting of mud facies has an age older than about 17,000 yr B.P., probably corresponding to the late Pleistocene regression or lowstand period. No age data are available for the main body of ridges, but one drill core from the SSR16 shows an age of 7,470 yr B.P. at the 1.2 m depth of the top sand layer, which corresponds to the S1 or S2 unit (KIGAM 1996b).

Nearshore Sand Ridges The nearshore sand ridges rest on the acoustic basement characterized by a strong reflector on the seismic profiles (Figs. 8, 9). This reflector is irregular and continuous, and can be traced over a wide area of the nearshore area, even beyond the limit of the ridges. The sand ridges above the acoustic basement can be divided into three seismic units (N1, N2, and N3 in descending order) based on mid-reflectors and acoustic characters (Figs. 8, 9). The bounding surfaces between these units are irregular and discontinuous on the seismic profiles. The upper unit (N1) has a thickness of 5–25 m. This unit is acoustically transparent or chaotic. Some multiples due to reverberation in shallow water depths mask the internal structure of this unit. The middle unit (N2) is less than 10 m in thickness and shows acoustically hummocky or chaotic reflection

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FIG. 6.— High-resolution air-gun profiles from the shelf sand ridges (for more explanation, see Fig. 5).

patterns. In some places, channel-fill structures with some prograding clinoforms are observed (Fig. 9). These two units form the main body of the ridges. The lower unit (N3) is the ridge substratum less than 15 m in thickness. This unit generally shows parallel to subparallel internal structures which are laterally discontinuous. The vibracores from a nearshore sand ridge (NSR8) provide the lithology and 14C ages of the sediments (Table 1, Fig. 9). The upper unit (N1) of the ridge is composed mainly of homogeneous sands with some gravels at the bottom. The sand is dominantly siliciclastic, consisting of quartz (35–62%), feldspar (31–63%), and rock fragments (1–13%). Three AMS 14C dates measured from the shell fragments show an age of 3,100 yr B.P., 4,900 yr B.P., and 5,160 yr B.P., indicating that the upper unit of nearshore sand ridges has a Holocene origin (Table 2). One vibracore (V-9B) demonstrates that the middle unit consists mainly of mud. One drilled core (G7) recovered from a nearshore sand ridge (NSR10) in the southeastern Kyonggi Bay (KIGAM 2000) shows that the

sediments consist predominantly of sands and muds (Fig. 10). Sand dominance in the upper core section (0–5.8 m) is positively correlated with seismic records of the upper ridge unit (N1) that are characterized by transparent or chaotic reflection configuration. The greenish mud or sandy mud layer at the middle core section (5.8–9.8 m) and the semiconsolidated, brownish mud layer at the lower core section (9.8– 13.5 m) coincide well with the middle (N2) and the lower (N3) units, respectively (Fig. 10). No age data were available from the drilled core because of the lack of dateable materials (KIGAM 2000). STRATIGRAPHIC INTERPRETATION

Table 2 describes the acoustic character, lithology, and 14C ages of the sand-ridge units and their stratigraphic interpretation. According to our 14 C data from the ridge field in the eastern Yellow Sea, the substratum (unit S3) of the shelf sand ridges, consisting mainly of mud, may represent the late Pleistocene regressive or lowstand deposits formed prior to the

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FIG. 7.—Lithology of piston cores from various parts of the shelf sand ridges. The sand or muddy sand layer from the ridge crest is correlated with S1 or S2 units, and the sandy mud or mud layer from the ridge trough corresponds to S3 unit. The insert map shows the positions of piston cores. Radiocarbon ages and lithology are compiled from the KIGAM’s reports (1992, 1993, 1994, 1996a). S, sand; mS, muddy sand; sM, sandy mud; M, mud.

LGM. This deposit is interpreted as a tide-influenced river deposit, on the basis of the lithology and microfossils (KIGAM 1996b). Consequently, the overlying erosional surface between unit S2 and S3 can be inferred as the maximum regressive surface during the LGM (Posamentier et al. 1988; Catuneanu et al. 1998), which has been well defined by highresolution seismic profiles and sedimentological studies in the eastern Yellow Sea basin (Lee and Yoon 1997; Jin and Chough 1998; Park et al. 2000). During the following postglacial transgression, much of the shelf sediments were eroded and redeposited due to intense tidal processes (Park et al. 2000; Uehara and Saito 2003). The main body (unit S2) of the shelf ridges is interpreted to have formed during this period. The oblique or prograding reflection patterns within this unit are strongly indicative of tidally influenced ridges. In contrast, the thin surface layer (unit S1) is interpreted as a reworked sand sheet after postglacial transgression. This unit probably began to form since rising sea level nearly approached the present level. Large bedforms on the ridge surface also indicate that the ridges have actively responded to modern tidal currents, although some

ridges without large bedforms are indicative of little influence of tidal currents on them at present. The brownish, semiconsolidated mud layer (unit N3) below the nearshore sand ridges is well correlated with the late Pleistocene mud deposits reported in many coastal areas of the eastern Yellow Sea (Park et al. 1998; Choi and Park 2000; Lim et al. 2002; 2003). This late Pleistocene mud deposit is interpreted as a tidal-flat deposit formed during the last interglacial period, on the basis of many tide-influenced signatures and sediment ages (Lim and Park 2003). This deposit was subaerially exposed and oxidized during the following glacial period (Lim and Park 2003). However, the overlying mud layer (unit N2) is very similar to the present intertidal deposits and is inferred to have formed in the late stage of postglacial transgression when sea level was about 10–20 m lower than at present. The erosional boundary between unit N2 and unit N3 is also well recognized in the sediment cores and is regarded as the sequence boundary (Lim et al. 2002; Lim et al. 2003). Our 14C data from the nearshore sand ridges show that the upper unit (N1) of sand ridges has developed mainly during the last 5,000 years, when sea level approached

TABLE 1.— Radiocarbon ages of cores collected from the shelf sand ridges and nearshore sand ridges (KIGAM 1996a, 2000). Sand ridges Shelf sand ridge (SSR)

Nearshore sand ridge (NSR)

Core No. 92P15 94P25 94P25 94P33 94P33 94P33 94P33 V–8B V–10B V–11A

Water depth (m) Depth in core (cm) 71 89 89 83 83 83 83 15 13 27

114 40 125 15 50 80 115 93 278 340

Materials Wood fragment Foraminifera Foraminifera Foraminifera Foraminifera Foraminifera Foraminifera Molluscan shell Molluscan shell Molluscan shell

AMS 14C age (yr B.P.) 26,800 6 370 17,350 6 140 18,410 6 200 27,370 6 470 28,170 6 350 35,220 6 530 45,400 6 1,600 3,100 6 250 5,160 6 140 4,900 6 100

Lab. No. R18539/1 R21015/2 R21015/1 R18781/1 R18781/2 R18781/3 R18781/4 Gak–14364 Gak–14363 Gak–14362

R denotes the laboratory number of the Institute of Geological and Nuclear Sciences of New Zealand, and Gak is that of the Gakushuin University of Japan.

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FIG. 8.—A, B) High-resolution sparker profiles from the nearshore sand ridges (NSR9 and NSR7) (for profile locations, see Fig. 4). The nearshore sand ridges are also divided into three seismic units (N1, N2, and N3 in descending order) according to the erosional boundaries and acoustic characters. The upper (N1) and middle (N2) units form the main sand body, and the lower (N3) unit represents the ridge substratum. C)The sonograph from NSR7 shows that the ridge surface is covered by large dunes (spacing, 100–200 m).

nearly the present level. Large and small dunes on the ridge surface suggest that the nearshore sand ridges actively respond to the present tidal currents. LATE QUATERNARY DEVELOPMENT

The late Quaternary was a period of considerable eustatic sea-level fluctuations, and sedimentation on the continental shelf was controlled

largely by changes in sea level. The development of the sedimentary sequence on the Yellow Sea and East China Sea shelf was also influenced by the Quaternary fluctuations of sea level coupled with high sediment supply from the surrounding landmass (Milliman et al. 1989; Alexander et al. 1991; Berne´ et al. 2002; Liu et al. 2002; Liu et al. 2004). Our stratigraphic interpretation of the eastern Yellow Sea sand ridges indicates that the development of each stratigraphic unit of the ridges is also closely related to the sea-level changes during the late Quaternary

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FIG. 9.—Lithology of vibracores from NSR8 (for profile location, see Fig. 4). The sand layer with some gravel at the bottom is correlated with the N1 unit, and the mud layer of V-9B corresponds to the N2 unit. The asterisks indicate the radiocarbon ages (yr B.P.). G, gravel; S, sand; M, mud.

TABLE 2.— Acoustic character, lithology, and 14C ages of the sand ridges and their stratigraphic interpretation. Sand ridges

Seismic units

Shelf sand ridge (SSR)

S1

, 5

S2

10–20

Nearshore sand ridge (NSR)

Thickness (m)

S3

5–20

N1

5–25

N2

, 10

N3

, 15

Acoustic character Transparent or chaotic Oblique or prograding Parallel Transparent or chaotic Hummocky or chaotic Parallel to subparallel

Lithology Massive sand

Age (yr B.P.) , 8,000

Sand or muddy sand

Stratigraphic interpretation Recently reworked sand sheets Transgressive sand–ridge deposits

Mud

. 17,000

Late Pleistocene regressive deposits

Homogeneous sand

, 7,000

Highstand sand–ridge deposits

Mud or sandy mud

Transgressive tidal mud deposits

Semiconsolidated mud

Late Pleistocene highstand mud deposits

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FIG. 10.—Lithology of one drill core (G7) from the nearshore sand ridge (NSR10), compiled from the KIGAM’s report (2000), is correlated with the sparker seismic section (for core and profile locations, see Fig. 4). The upper sand layer, the middle sandy mud layer, and the lower brownish mud layer are well correlated with N1, N2, and N3 units of the nearshore sand ridges, respectively. G, gravel; S, sand; sM, sandy mud; M, mud.

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FIG. 11.—Development of the shelf sand ridge units (S1, S2, S3) and nearshore sand ridge units (N1) associated with the sea-level changes during the last 50,000 yr. The newly studied regional sea-level curve by Liu et al. (2004) is used to explain the existence and development of the sand ridges in the eastern Yellow Sea during the postglacial transgression.

(Fig. 11). Especially, the main body (unit S2) of the shelf sand ridges developed largely during the course of the postglacial transgression period, whereas that (unit N1) of the nearshore sand ridges formed during the recent highstand of sea level. During the last glacial maximum, much of the shelf deposits were subaerially exposed, so the pre-Holocene surface between unit S2 and S3 is erosional and highly undulating, with depth variations of about 20 m between the highest and lowest parts. The underlying regressive or lowstand mud deposits (unit S3) show continuous parallel bedding, possibly indicating estuarine or deltaic deposits. In contrast, the main body (unit S2) of the sand ridges displays well organized foreset bedding (oblique and prograding reflection patterns), indicating that most of the sand ridges grew in constant, stable conditions, possibly during stillstands or very slow rise of sea level in the postglacial transgression period. According to the recent sea-level curve of the Yellow Sea and East China Sea (Liu et al. 2004), a stillstand of sea level with a possible short fall is recorded at about 265 m depth between about 14 and 12 ka, and another slowdown of sea-level rise from 245 m to 236 m between about 11.5 ka and 9.5 ka (Fig. 11). Tidal models in the Yellow and East China Sea (Oh and Lee 1998; Uehara and Saito 2003) also suggest that tidal currents at that time were strong enough to generate linear sand ridges in the eastern Yellow Sea. It is interpreted that the shelf sand ridges in the eastern Yellow Sea developed mainly during this period of stillstand or slowdown of sea-level rise. However, the large bedforms and chaotic or transparent reflection patterns of the topmost sand layer (S1) suggest that the shelf sand ridges have been continuously reworked by modern tidal currents. The shelf sand ridges in the eastern Yellow Sea display remarkable similarities to those on the East China Sea shelf (Yang and Sun 1988) and in the southern continental shelf of Korea (Park et al. 2003), which formed in transgressive settings and remained inactive or moribund on the shelf. In contrast, Jin and Chough (2002) suggested that the shelf sand ridges are erosional in origin, as revealed by internal strata truncated on either side of the ridges. Erosional sand ridges were initially described from the Celtic Sea by Berne´ et al. (1998), where ridges have been sculpted from former lowstand deposits because of extremely energetic combined tidal and wave currents. The erosional nature of the subhorizontal boundaries of the seismic units suggests that the shape of the shelf sand ridges in the eastern Yellow Sea more or less results from an erosional process dominantly acting during the postglacial transgression.

According to the Holocene sea-level curve in the East China Sea and Yellow Sea (Liu et al. 2004), sea level rose rapidly from 236 m to 216 m at 9.5–9.2 ka, after which slowdown occurred between 9.0 ka and 8.0 ka from 216 m to 210 m. Sea level approached the present level at about 7 ka and stabilized for a sufficiently long period of time to allow the formation of nearshore sand ridges. However, the development of the nearshore sand ridges was presumably not quite as fast as the shelf sand ridges, as attested by their small scale in size and distribution. Furthermore, some erosional surfaces in unit N1 are indicative of changing conditions during the development of the ridges, probably due to strong tidal currents and/or storm-induced wave actions. The nearshore sand ridges rest on the erosional surface of the ridge substratum (N3). These ridges show a negative angle of climb and internal strata truncated by erosion on the seismic profiles (Fig. 8), and are regarded as erosional sand ridges, as defined by Berne´ et al. (1998). This erosional nature is seen clearly on the lee sides of the ridges in Fig. 8B, where the subhorizontal boundaries of seismic units are truncated by the migration of the ridges. Berne´ et al. (1998) suggested that the result of this process is that the ridges incorporate some older deposits during migration. It would thus appear that all the nearshore sand ridges have undergone the same erosional shaping of their form during the recent highstand of sea level. Large dunes also indicate a strong hydrodynamic influence on the entire ridge surface at present. CONCLUSIONS

The shelf sand ridges are divided into three stratigraphic units (S1, S2, and S3 in descending order) based on the erosional boundaries and acoustic characters. The surface (S1) and middle (S2) units constitute the main sand body of the ridges, with a thickness up to 25 m. The middle unit (S2) is interpreted to have formed in the course of postglacial transgression, possibly during the episodes of stillstand or significant slowdown of sea-level rise. However, the ridge surface has been continuously reworked by modern tidal currents to produce a thin (, 5 m) surface sand layer (S1). The lower unit (S3) forms the substratum of the shelf sand ridges and displays variations in thickness (5–20 m). This unit indicates that the regressive or lowstand mud deposits formed in the deltaic environment prior to the last glacial maximum (. ca. 17,000 yr B.P.). The shelf sand ridges more or less result from an erosional process dominantly acting during the postglacial transgression.

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The nearshore sand ridges also consist of three stratigraphic units (N1, N2, and N3 in descending order) based on the mid-reflectors and acoustic characters. The upper unit (N1) is the main sand body of the ridges with a thickness of 5–25 m, which developed during the recent highstand of sea level (, 7,000 yr B.P.). The middle unit (N2) is less than 10 m in thickness and represents tidal mud deposits. The lower unit (N3) is the ridge substratum, which consists of semiconsolidated mud with a thickness less than 15 m. This substratum is correlated with the last interglacial mud deposits, which occur over a wide area along the western coast of Korea. It would appear that all of the nearshore sand ridges have undergone the same erosional shaping of their form during the recent highstand of sea level. ACKNOWLEDGMENTS

This work was supported by Korea Research Foundation Grant (KRF2002-015-CS0068). We thank the Korea Institute of Geoscience and Mineral Resources for providing seismic data and core materials. Drs. P. Liu and P. Wiberg, and an anonymous reviewer, are very much appreciated for their review of the manuscript and for their helpful comments. REFERENCES

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Received 5 August 2005; accepted 7 March 2006.