Late Quaternary seismic stratigraphy in response to postglacial sea-level rise at the mid-eastern Yellow Sea

Quaternary International 392 (2016) 125e136

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Late Quaternary seismic stratigraphy in response to postglacial sea-level rise at the mid-eastern Yellow Sea Dong-Geun Yoo a, b, *, Tae-Soo Chang a, Gwang-Soo Lee a, Gil-Young Kim a, Seong-Pil Kim a, Soo-Chul Park c a b c

Petroleum and Marine Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejon 305-350, Republic of Korea Petroleum Resources Technology, University of Science and Technology (UST), Daejon 305-350, Republic of Korea Department of Oceanography, Chungnam National University, Daejon 305-764, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 8 August 2015

Late Quaternary seismic stratigraphy and depositional history at the mid-eastern Yellow Sea were investigated using high-resolution seismic profiles and core sediments. The results show that the shelf sequence consists of five sedimentary units formed since the LGM: incised-channel fill (SU1), estuarine deposit (SU2), thin sand veneer (SU3), tidal sand ridge (SU4), and central deltaic mud (SU5). The lowermost unit (SU1) above the sequence boundary is interpreted as channel fill deposits mainly formed during the LGM, which belongs to the lowstand systems tract. Three units (SU2, SU3, and SU4), regarded as transgressive systems tract, can be grouped into paralic and marine components separated by a ravinement surface. SU2 lying below the ravinement surface represents a paralic unit that consists of estuarine sediments left behind from shoreface erosion. The top surface of SU2 is truncated by an erosional surface and is overlain by two marine units (SU3 and SU4), which were produced by shoreface erosion that shifted landward during the transgression. SU3, mainly distributed over a wide area of the central part, is very thin, whereas SU4 on the eastern part off the Korean Peninsula forms serial sand ridges, partly modified by modern tidal currents. The uppermost unit (SU5) above the maximum flooding surface, regarded as the highstand systems tract, formed the thin deltaic mud patch derived from the Huanghe River developed after the highstand sea level approximately 7 ka BP. © 2015 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Seismic stratigraphy Late Quaternary Postglacial sea-level rise Mid-eastern Yellow Sea

1. Introduction The Yellow Sea is a post-glacially submerged epicontinental sea with a flat and broad seafloor less than about 100 m in water depth (Fig. 1; Alexander et al., 1991). During the Holocene transgression, the Yellow Sea experienced environmental changes associated with progressive landward migration of the shoreline (Lee and Yoon, 1997; Liu et al., 2002, 2004). The rate of transgression in the Yellow Sea could have been very rapid because of the low gradient of the seafloor (Milliman et al., 1989). Furthermore, the large tidal amplitude and strong tidal currents seem to have produced a complex and dynamic hydraulic regime for sediment erosion and deposition (Alexander et al., 1991; Lee and Yoon, 1997; Park et al., 2000). Thus, during the postglacial transgression, various

* Corresponding author. Petroleum and Marine Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejon 305-350, Republic of Korea. E-mail address: [email protected] (D.-G. Yoo). http://dx.doi.org/10.1016/j.quaint.2015.07.045 1040-6182/© 2015 Elsevier Ltd and INQUA. All rights reserved.

sedimentary units, including subaqueous delta, transgressive sand sheet, and tidal sand ridges, were formed and left over a wide area of the shelf showing various seismic facies and lithologic associations (Alexander et al., 1991; Lee and Yoon, 1997; Jung et al., 1998; Park et al., 2000; Jin and Chough, 2002; Liu et al., 2002, 2007, 2004; Shinn et al., 2007; Yang and Liu, 2007). Such deposits are well recorded on the seafloor and bear witness to complex interplay between depositional and erosional processes, associated with sealevel changes and sediment supply. Because of these features, the Yellow Sea shelf is an important site for better understanding of depositional and erosional processes during the late Quaternary. To reconstruct the depositional history of these deposits, sequencestratigraphic concepts (e.g. Vail, 1987; Posamentier et al., 1988; Hunt and Tucker, 1992; Catuneanu, 2006) have been used to study modern continental shelves. After the successful test for the late Quaternary sequence of high sediment accumulation on the Gulf of Mexico (Boyd et al., 1989), the application of the concept to the Quaternary sequence has been common sense, using high-

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Fig. 1. Physiography of the Yellow Sea. The insert box is the study area shown in Fig. 2. Arrows show a general trend of current systems in the Yellow Sea and the East China Sea (modified from Milliman et al., 1989; Liu et al., 2007). Contours in meters. TWC; Taiwan Warm Current, YSWC; Yellow Sea Warm Current, BCC; Bohai Coastal Current, JCC; Jiangsu Coastal Current, KCC; Korean Coastal Current.

resolution seismic profiles and sediment analyses (e.g., HernandezMolina et al., 1994; Okyar et al., 1994; Saito, 1994; Trincardi et al., 1994; Morton and Suter, 1996; Tortora, 1996; Tesson et al., 2000; Yoo and Park, 2000; Karisiddaiah et al., 2002; Yoo et al., 2002; Labaune et al., 2005; Lobo et al., 2005; Rabineau et al., 2005; Zecchin et al., 2008). The present study focuses on the mideastern part of the Yellow Sea, where abundant seismic evidence is linked to the existence of transgressive depositional and erosional processes combined with the high-energy condition of tide and waves. In this paper, we describe the acoustic characteristics and depositional pattern of late Quaternary sediments using high-resolution seismic records and sediment data. We then discuss the stratigraphy and depositional history of the Yellow Sea in a low-gradient, tide-dominated, and high-energy shelf setting during the late Quaternary. 2. Regional setting The Yellow Sea is separated from the Gulf of Bohai by the Shandong Peninsula to the north and is in contact with the East China Sea to the south (Fig. 1). It is a shallow, low-gradient epicontinental sea, and it is less than 100 m deep, with an average water depth of about 55 m. The isobaths in the western Yellow Sea are parallel to the coastline, whereas the eastern part is characterized by ridge-and-swale morphology and numerous islands. These topographic features run in a NEeSW direction, nearly parallel to the direction of present tidal currents (Jung et al.,

1998; Park et al., 2006). The seafloor gradually deepens toward the south reaching more than 100 m in water depth in the vicinity of Jeju Island (Fig. 1). The shelf sediments in the Yellow Sea consist mainly of four types: mud, sand, sandy mud, and muddy sand (Lee and Chough, 1989). Mud occurs in the central Yellow Sea, the old Huanghe delta along the Jiangsu coast, and the southeastern Yellow Sea to the northwest of Jeju Island. Sand is predominantly present in a wide area of the northeastern Yellow Sea as a large-scale sand ridge field. The transitional facies, i.e., sandy mud and muddy sand, are also seen between mud and sand and they seem to originate from a heterogenous mixture of modern and relict sediments. The general circulation is characterized by a counterclockwise gyre with the northward inflow of the Yellow Sea Warm Current along the Korean coast and the southward flow of the Yellow Sea Cold Current and Jiangsu Coastal Current along the Chinese coast (Beardsley et al., 1985). The Yellow Sea is largely affected by tidal currents. Semi-diurnal tides in this area range from 1.5 to 8 m with the maximum amplitude found in Kyonggi Bay on the western coast of Korea. The general directions of tidal currents on the shelf are NeNE during flood and SeSW during ebb. The tidal currents (50e100 cm/s) play an important role in depositing and redistributing the near-surface sediment in the Yellow Sea (Song et al., 1983; Alexander et al., 1991). The major source of terrigenous sediment to the Yellow Sea is the Huanghe River. The Huanghe River, the second longest river in China, discharges about 1.1 * 109 tons/yr of suspended sediments

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into the Gulf of Bohai (Schubel et al., 1984). The seasonal variation of sediment discharge is even more dramatic than that of runoff. Sediment discharge is greatest in August, averaging 31.2% of the annual total. Coarse-grained sediments settle near the river mouth, whereas fine-grained sediments are transported southward to the Yellow Sea. A number of small-scale rivers in the Korean Peninsula discharge small amounts of clastic sediments into the eastern Yellow Sea (Lee and Chough, 1989). The Han River is the largest river entering the Yellow Sea from South Korea. Its average annual volume discharge is 25 km3. The Han River has a characteristic seasonal variation of flow, with its maximum flow in July and August when summer monsoons and occasional typhoons carry heavy rainfall. The concentration of suspended sediment ranges from about 3 to 50 mg/l in the upper reaches of the estuary. The variation is caused primarily by changes in river discharge and tidal currents. The Keum River is 401 km long and has a drainage area of 10,000 km2. The average freshwater discharge is about 200 m3/s, and the annual river volume discharge is 6.4 km3. The annual suspended sediment discharge of the Keum River is about 1.3  106 t, and approximately 79% occurs in the flood season (Schubel et al., 1984).

to 80 m between the nearshore area and the deep outer shelf (Fig. 3A). The isobaths are almost parallel to the coastlines which are indented. In the nearshore area, the seafloor is generally flat, and it becomes steeper toward the inner shelves. Farther offshore, the seafloor broadens and becomes flat gently, although there are ridge-and-swales characterized by a NEeSW trending topography. The ridges are tens of meters high, tens of kilometers long, and a few kilometers apart (Park et al., 2006). Thus, the seafloor forms a bank- or belt-like feature, a bathymetry being parallel to the coastline. As shown in Fig. 3B, sand dominates over the inner-shelf seafloor, but mud content tends to increase offshore. Seafloor sediments along the ridges and swales comprise purely sand, but muddy sand intercalated locally. In the vicinity of numerous bedrock islands and nearshore zone, the seafloor is dominated by muddy sand, and spotted gravels and mud patches. However, farther offshore where the finest sediments occur, muddy deposits form a wedge on the almost flat seafloor in water depth of ~80 m. The muddy sediments in the outermost part are a portion of the Huanghe River-derived muds that are distributed along the central axis of the Yellow Sea (Lee and Chough, 1989; Milliman et al., 1989).

3. Data

5. Interpretation of seismic data

The data used in this study consist of high-resolution (3.5 kHz and sparker) seismic reflection profiles and sediment cores acquired by the Korea Institute of Geosciences and Mineral Resources (KIGAM). High-resolution seismic investigations were carried out along the profiles shown Fig. 2, deploying a 3.5 kHz sub-bottom profiler and 1e2 K J multi-electrode sparker systems (model EG&G 231A triggered capacitor bank, 232A power supply, 402-7 sparkarray) and a single-channel Benthos MESH 50/24P hydrostreamer. Analog recording of the data was performed on EPC recorders at a 0.5 s sweep rate. Positioning was done using the shipboard global positioning system (GPS) and Loran GPS system. Ship speed was maintained at about 5 knots. Sediment cores were collected using a piston corer at 11 locations selected along the seismic lines (Fig. 2). X-radiography was carried out to examine the internal structure, texture, and lithological characteristics of samples. X-radiographs were made from

Five seismic units were identified in the study area by the interpretation of high-resolution seismic profiles and correlation with sediment data (Figs. 4e6). Five seismic units with various seismic facies and geometries are referred to as a SU1, SU2, SU3, SU4, and SU5 from oldest to youngest, respectively. The acoustic characteristics of these units are summarized in Table 1, and their distributions are shown in Fig. 7. SU1 is represented by channel-like features with an erosional base (Figs. 4 and 5A). The channel fill is over 10 m thick and about 1e1.5 km wide. On seismic records, it is characterized by semi-transparent or weakly stratified reflections with divergent or prograded fill patterns (Figs. 4 and 5A). The channels are located at the central part of the study area and are generally oriented in a NeS, NEeSW, and NNW-SSE directions (Fig. 7A). They are merged into one channel southward. SU1 is completely buried by SU2 or SU3.

Table 1 Classification and characteristics of sedimentary units in the study area. Systems tracts

Seismic units

Occurrence

External form

Acoustic characteristics

Lithology

Interpretation

Reference

HST

SU5

Western part

Wedge or sheet

Semi-transparent; prograding

Mud

Distal mud patch

TST

SU4

Eastern part

Bank

Seaward inclined; chaotic

Tidal sand ridge

SU3

Wide area

Sheet or drape

Semi-transparent or hummocky

Sand or muddy sand Sand

SU2

Central part

Ponded shape

SU1

Central or western part

Channel fill

Semi-transparent with weakly stratified Divergent, prograded or complex fill

Sandy mud or Muddy sand Muddy sand or Gravelly sand

Milliman et al., 1989 Liu et al., 2004 Park et al., 2006 Jung et al., 1998 Jin and Chough, 2002 Shinn et al., 2007 Lee and Yoon, 1997 Shinn et al., 2007 KIGAM, 1996 Jin and Chough, 2002

LST

Transgressive sand veneer Estuarine deposit Incised-channel fill

HST: Highstand systems tract; TST: Transgressive systems tract; LST: Lowstand systems tract.

sediment slabs (7  30  1 cm). Grain size analysis was done by the standard dry sieving for the sand fraction (>63 mm) and by the pipette method for the mud fraction (<63 mm). 4. Bathymetry and shelf sediments The seafloor topography of the study area, in general, deepens progressively west-to southwestward. Water depth ranges from 20

SU2 overlays either SU1 or older sedimentary strata and acoustically shows hummocky or semi-transparent reflections alternating with faintly stratified reflections (Fig. 4). In some area, it is characterized by small-scale progradation or coastal onlap patterns. Its thickness ranges from less than 5 m to more than 10 m. This unit is well preserved around the seaward extent of paleo-rivers, including the Yellow and Han rivers. SU2 is covered by either SU3 or SU5.

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Fig. 2. Tracklines of seismic profiles and the locations of sediment cores (filled circles) illustrated in Fig. 8. Heavy lines indicate the selected profiles shown in Figs. 4e6.

SU3 prevails on the flat seafloor with a very gentle gradient (Figs. 4 and 7A). It is relatively thin (less than a few meters thick) with a sheet-like external form. In most case, SU3 is exposed on the seafloor, but to the west it is covered by SU5 (Figs. 4 and 5). SU3 is

acoustically semi-transparent or weakly stratified. In the central part, where SU3 is exposed on the seafloor, the surface of SU3 is fashioned by wavy bedforms (Fig. 5B). These bedforms are less than 5 m high, 150e300 m wide and either asymmetrical or symmetrical

Fig. 3. (A) Detailed bathymetry and (B) distribution of surface sediments. Contours in meters.

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Fig. 4. High-resolution seismic profile collected from the western part of the study area, showing four (SU1, SU2, SU3, and SU5) seismic units (for location, see Fig. 2).

in cross section. SU3, consisting of the sand sheet, may merge with sand ridges (SU4) with mound shapes in the nearshore region (Fig. 7). SU4 occurs in the eastern Yellow Sea (Fig. 7A) where the water depths range from 40 to 80 m. It consists of a series of sand ridges, which cover an area of more than 11,000 km2. The main ridges are about 30e80 km long and some can exceed 100 km oriented in a NEeSW direction (KIGAM, 1993). In general, the ridges are about 5e10 km wide. SU4 is usually 5e20 m thick, but it reaches up to 30 m at the crest (Fig. 6). The internal structure of SU4 is acoustically characterized by well-stratified to inclined reflectors with a low gradient. Some hummocky or chaotic reflection patterns also occur (Fig. 6). Its lower boundary consists of a flat erosional unconformity, and internal reflectors taper seaward downlapping onto the erosional surface. In cross section, SU4 shows a moundshaped external form which has either an asymmetric or symmetric profile with a round-shaped crest. The upper surface is often covered by regularly spaced dunes superposed on the ridge (Fig. 6). SU4 is mostly exposed at the seafloor. SU5, the uppermost unit in the study area, is only observed in the western half of the study area (Fig. 7A). It is characterized by semi-transparent reflections with weakly stratified reflectors (Fig. 4). In the westward portion near the Shangdong Peninsula, however, the internal reflectors show subparallel to slightly southeastward prograding reflections (Liu et al., 2002; Yang and Liu, 2007). It has a sheet-shaped or wedge-shaped external form that progressively thins toward the south to southeast. The detailed isopach map shows thick accumulations of SU5 near the northwestern tip (Fig. 7B). In most area, SU5 is usually less than 5 m thick through the central Yellow Sea, but thickness of more than 40 m is found near the Shangdong Peninsula (Fig. 7B; Liu et al., 2004; Yang and Liu, 2007). 6. Lithology In the course of examination and evaluation of selected piston cores (Fig. 8), four sedimentary units were identified based on the

lithology and sedimentary structures: a basal sandy mud or muddy sand (SU2), sands (SU3 and SU4), and a top clayey mud (SU5). Each unit is characterized by a number of environmentally diagnostic sedimentary facies, including estuarine deposits, shelf sand sheet, tidal sand ridges, and deltaic mud, in ascending order. Four sedimentary units are tied closely to the seismic units above. However, the lowermost seismic unit (SU1), interpreted as channel fills on seismic profiles, is missing in the piston core data due mainly to their deep location, so that this unit is excluded in this lithofacies description and its analysis. The basal sandy mud or muddy sand, correlating with SU2, occurs dominantly in the central part of the area, and is sharply overlain by the upper sandy units (SU3 and SU4) (Fig. 8). The olive gray colored sedimentary unit is highly bioturbated, but do not contain shells and peat remains. Muddy sand with high degree of bioturbation and low water content is often considered to be of paralic deposits formed during early transgression in estuarine environment (Lee and Yoon, 1997; Shinn et al., 2007). The sandy units occur mainly in the central and eastern part of the study area respectively, and directly overlie the lower muddy sand or sandy mud unit (Fig. 8). The massive sand unit is almost 1e2 m thick with variable sizes of shell fragments. Cores from central part consistently show a surficial sand sheet (SU3), by contrast, it forms sand ridges (SU 4) in the eastern nearshore area. Based on three cores (P-04, P-05, and P-06), SU3 consists of massive, medium to fine sands; the sand content reaches up to 80%, but the mud content is less than 10% (Fig. 8; KIGAM, 1992, 1993). Within this unit, no physical structures are discernible, and shell fragments and some gravels are present locally. Gravels with shell fragments are generally concentrated at the base of this unit, as reported by KIGAM (1992) and Shinn et al. (2007). A fining upward succession from gravelly sand to fine sand with erosional surface at the bottom was also observed. However, sediments from the sand ridges (SU4) show relatively intense bioturbation with dominantly horizontal burrows. Shell layers are also present in some places, and are thought to be of probable storm origin. The sand ridges in the study area are considered to

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Fig. 5. High-resolution seismic profile collected from the central part of the study area, showing two (SU1 and SU3) seismic units (for location, see Fig. 2).

be active at present, with no mud blankets on either crests or troughs. Confined to the westernmost part of the area, the clayey mud unit (SU5) is characterized by the entire mud with an alternation of silt and clay laminae and intense bioturbation with vertical and horizontal burrows in the deepest shelves. Shell debris and peat remains are not observed in the unit. The uppermost mud unit directly overlies the sand sheet unit in some cores (Fig. 8), reflecting part of a basinwide muddy prodelta wedge that has been derived from the Huanghe via suspended littoral drift along the coast of the Gulf of Bohai (Alexander et al., 1991; Lee and Yoon, 1997; Liu et al., 2002). 7. Discussion 7.1. Deposition of late Quaternary deposits Together with sediment input, the rise of postglacial sea level has played an important role in the development of sedimentary units in the Yellow Sea (Liu et al., 2002, 2004; Park et al., 2006; Shinn et al., 2007). During the lowstand of sea level at the LGM (Last Glacial Maximum), the sea level was about 120e130 m lower than at present and the paleo-shoreline was positioned in the far

south or southeast of Jeju Island (Fig. 9; Liu et al., 2004; Yang et al., 2014). Then, the Huanghe and Han Rivers might have extended to the southern tip of the Yellow Sea near Jeju Island. Being nowhere deeper than about 90 m (Fig. 3A), the present study area was entirely exposed, resulting in subaerial erosion associated with fluvial incision (Lee and Yoon, 1997; Xu et al., 1997; Liu et al., 2004; Park et al., 2006; Shinn et al., 2007; Yang et al., 2014). The terrigenous sediments derived from paleo-rivers preferentially filled the incised-valleys, leading to formation of SU1 (Fig. 10A). In borehole data, rounded, partly oxidized sandy gravel beds without marine faunal fossils indicate that SU1 formed during the lowstand of sea level, as fluvial channel lags (KIGAM, 1996). However, the transition into non-oxidized upper pebbly sand layers suggests that the fluvial deposits (SU1) were partly redistributed into a transgressive lag in the course of sea-level rise, similar to the results for the Korea Strait shelf (Yoo et al., 2014). Consequently, SU1 was formed during the lowstand in sea level, and it appears to be partly backfilled with fluvial or coastal sediments during the transgression. The subsequent sea level rise, which began at approximately 19 ka BP, flooded the exposed shelf (Fig. 9). The major depocenter migrated from the southern tip of the Yellow Sea near Jeju Island to the present-day shoreline north- and northeastward across the shelf (Park et al., 2006; Shinn et al., 2007). By approximately 14 ka

Fig. 6. High-resolution seismic profile collected from the eastern part of the study area, showing seismic unit (SU4) (for location, see Fig. 2).

Fig. 7. (A) Distribution of four seismic units, except for SU2. Three units (SU3, SU4, and SU5) are exposed at the seafloor, whereas SU1 and SU2 are completely covered by three units. SU1 is partly modified from Xu et al. (1997) and KIGAM (1993). (B) Isopach map (contour in meters) of SU5 is modified by Liu et al. (2002), and Yang and Liu (2007). SU5 is only restricted to the western part of the study area.

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Fig. 8. Core lithology of sedimentary units (for core locations, see Fig. 2).

Fig. 9. Sea-level curve, constructed from radiocarbon dates (Liu et al., 2004). Note that the postglacial transgression in the Yellow Sea was step-wise: long period of slow transgression punctuated by several short and rapid flooding events.

BP the shoreline approached the southern part of the study area around 80e90 m water depth (Fig. 9). As sea level rose further, an estuarine environment probably developed around the central part of the study area seaward extent of the paleo-Huanghe and Han Rivers (KIGAM, 2001; Shinn et al., 2007). The paleo-river may have supplied abundant sediments to the study area. These sediments seem to have been trapped in this estuary creating SU2 (Fig. 10B and C). SU2 is, thus, classified as the estuarine deposit associated with the extent of paleo-rivers. An estuary is a likely place for riverderived sediment to be trapped until a shoreline is displaced further landward (Park et al., 2006). This resulted in backstepping or retrograding depositional arrangements as reported by Lee and Yoon (1997), and Shinn et al. (2007). These features have also been studied in other areas, such as the Adriatic Sea shelf (Trincardi et al., 1994), the Tyrrhean shelf (Tortora, 1996), the Korea Strait shelf (Yoo and Park, 2000; Yoo et al., 2014), and the East China Sea (Yoo et al., 2002; Lee et al., 2013). As the shoreline migrated landward, the surface sediments on the shelf were presumably reworked and redistributed to form a

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Fig. 10. Schematic diagram illustrating the development of five seismic units in the mid-eastern Yellow Sea in response to different stages of relative sea-level change shown in Fig. 9.

thin lag of sands (SU3) (Figs. 5B and 10C). The study area, especially the central part, forms a relatively flat and wide platform with a low gradient (Alexander et al., 1991). As the shoreline migrated more rapidly landward, the rate of increase in accommodation space probably exceeded the rate of sediment supply. As a result, most of

the transgressvie sand layer on the central part of the study area is relatively thin (less than a few meters) and remains as relict facies uncovered by recent sediments (Fig. 7A; Lee and Yoon, 1997). This finding has been supported by some previous studies (Qin et al., 1996; Jin and Chough, 2002; Park et al., 2006; Shinn et al., 2007).

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Fig. 11. Late Quaternary shelf sequence model for the mid-eastern Yellow Sea. Note that Type-I located only on the western half consists of four sedimentary units forming a set of lowstand, transgressive, and highstand systems tracts, whereas Type-III at the eastern part only includes SU5 directly overlying the acoustic basement. SB; sequence boundary, RS; ravinement surface, MFS: maximum flooding surface, LST; lowstand systems tract, TST; transgressive systems tract, HST; highstand systems tract.

They have shown that SU3 represents relict facies that experienced subaerial and/or subaqueous weathering for a long time during the Holocene transgression. As the transgression continued, tidal currents could have become the most important dynamic factor, especially in the eastern part of the Yellow Sea (Song et al., 1983; Uehara and Saito, 2003). Today, the study area has a macro-tidal regime with tidal currents exceeding 2 knots in coastal areas off the Korean Peninsula (Korea Hydrographic Office, 1990). Sternberg et al. (1985) also suggested that tidal currents are the major cause of sedimentation in the Yellow Sea and the East China Sea. Presumably, tidal currents during the Holocene transgression were much stronger than today, playing an important role in the shaping and formation of SU4, characterized by a series of sand ridges (Fig. 10C; Jung et al., 1998; Park et al., 2006). Recently, Uehara and Saito (2003) indicated that the high bottom-stress value migrated from Jeju Island toward the west coast of Korea. On the basis of tidal ellipses of M2 component, tidal currents are strong off the mid-west coast of Korea where SU4 mainly occurs (Lee and Jung, 1999). As a result, SU4 is regarded as tidal sand ridges mainly developed during the period of stillstand or slow rise of sea level. However, large-scale bedforms with chaotic reflection patterns of the topmost part of the ridges suggest that the sand ridges (SU4) have been continuously modified and subjected to re-deposition by modern tidal currents (Fig. 6; KIGAM, 1993; Jung et al., 1998; Park et al., 2006). The sand ridges in the study area are also considered to be active at present, with no mud blankets on either crests or troughs.

In the Yellow Sea, the sea level reached its present position approximately 7 ka BP (Fig. 9; Liu et al., 2004). Since that time, the sediment originating in the Huanghe could have been transported through the Bohai Sea to the Yellow Sea. The coarse fractions of sediments settled near the river mouth and formed a delta. The remaining fine-grained sediments moved further south into the Yellow Sea along the coast of the Shandong Peninsula and were deposited in the central Yellow Sea forming SU5, as a distal mud (Fig. 10D; Liu et al., 2004). Such a dispersal pattern is proved by the presence of a high-turbid water plume near the tip of the Shandong Peninsula extending to the south (Milliman et al., 1989) and by the southward decrease of sediment accumulation rates from the Shandong Peninsula (Alexander et al., 1991; Liu et al., 2007). Recent mud (SU5) prograded over transgressive sands (SU3), forming a thin deltaic mud patch. 7.2. High-resolution sequence stratigraphy The shelf deposits in the mid-eastern Yellow Sea consist of five depositional systems: incised-channel fill (SU1), estuarine deposit (SU2), sand veneer (SU3), tidal sand ridge (SU4), and deltaic mud patch (SU5). The lowermost depositional system (SU1) above the sequence boundary is regarded as channel fill deposits mainly formed during the last glacial period, which belongs to the lowstand systems tract (Fig. 11). The transgressive deposits, including three depositional systems, comprise paralic and marine components separated by a ravinement surface. The lower depositional

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system (SU2) lying below the ravinement surface represents a paralic component that consists of estuarine sandy mud or muddy sand preserved from shelf erosion, as proposed by Shinn et al. (2007). The top surface of the paralic unit is truncated by an erosional surface. Such erosional truncation of the paralic unit marks a dramatic change in the depositional environments, from tidal/estuarine conditions to open marine conditions (Demarest and Kraft, 1987; Nummedal and Swift, 1987; Thorne and Swift, 1991; Saito, 1994; Trincardi et al., 1994). This is confirmed by the distinctive changes in the lithofacies from sandy mud to sand with shell debris (KIGAM, 1996). This ravinement surface is overlain by two depositional systems (SU3 and SU4), which correspond to a marine component that consists of a sand veneer and a tidal sand ridge system produced through shelf erosion during the Holocene transgression (Fig. 11). Such a transgressive systems tract, including paralic and marine deposits, has been reported in the Adriatic Sea shelf (Trincardi et al., 1994), the continental shelf off northeastern Japan (Saito, 1994), the Tyrrhenian continental shelf (Tortora, 1996), the Yellow Sea (Lee and Yoon, 1997; Shinn et al., 2007), the East China Sea (Yoo et al., 2002; Lee et al., 2013), and the Korea Strait shelf (Yoo et al., 2014). The uppermost depositional system (SU5) above the maximum flooding surface, which is composed of the distal mud patch derived from the Yellow River, represents a highstand systems tract developed after the last 7 ka BP when sea level was close to the present level. On the basis of geometries and depositional patterns, the shelf sequence in this area can be divided into three types (Fig. 11). Type-I occurs only on the western part of the study area and contains four depositional systems, which constitutes a set of lowstand, transgressive, and highstand systems tracts. The transgressive deposits also consist of paralic (SU2) and marine units (SU3) separated by a ravinement (Fig. 11). In contrast, type-III occurs dominantly in the eastern half of the study area, and consists only of one depositional system (SU4) which forms a series of sand ridge systems directly overlying the sequence boundary. Type-II, distributed on the central part between type-I and type-III, consists of one lowstand (SU1) and two transgressive deposits (SU2 and SU3). 8. Conclusions In the mid-eastern Yellow Sea, the late Quaternary deposits comprise five sedimentary units formed since the LGM. Each unit was deposited during distinctive portions of the sea-level curve associated with hydrodynamic conditions. SU1 above the sequence boundary is interpreted as incised-channel fills mainly deposited during the LGM. SU2 is correlated with the estuarine sediment of sandy mud or muddy sand formed during the transgression. SU3 consists of a thin veneer of transgressive sands with shell fragments, which originate from the reworking of shelf sediments during transgression. SU4, regarded as a serial sand ridge, was formed since the postglacial transgression and seems to have been partly modified by strong tidal currents at the present condition. SU5 is correlated with the central deltaic mud, originating from the Huanghe River, deposited after the highstand sea level. SU5 prograded over transgressive sands and formed a thin deltaic mud patch. Acknowledgements This work was financially supported by the Korea Institute of Geoscience and Mineral Resources (KIGAM). We thank the officers and crew of the R/V Tamhae II of the KIGAM for helping with seismic survey and sampling operations. Dr. S.H. Lee kindly read the earlier version of the manuscript.

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References Alexander, C.R., DeMaster, D.J., Nittrouer, C.A., 1991. Sediment accumulation in a modern epicontinental-shelf setting: the Yellow Sea. Marine Geology 98, 51e72. Beardsley, R.C., Limeburner, R., Cannon, G.A., 1985. Discharge of the Changjiang (Yangtze River) into the East China Sea. Continental Shelf Research 4, 57e76. Boyd, R., Suter, J., Penland, S., 1989. Relation of sequence stratigraphy to modern sedimentary environments. Geology 17, 926e929. Catuneanu, O., 2006. Principles of Sequence Stratigraphy. Elsevier, Amsterdam, p. 375. Demarest, J.M., Kraft, J.C., 1987. Stratigraphic record of Quaternary sea levels: implications for more ancient strata. In: Nummedal, D., Pilkey, O.H., Howard, J.D. (Eds.), Sea-level Fluctuation and Coastal Evolution, vol. 41. Society of Economic Paleontologists and Mineralogists, Special Publication, pp. 223e239. Hernandez-Molina, F.J., Somoza, L., Rey, J., Pomar, L., 1994. Late PleistoceneHolocene sediments on the Spanish continental shelves: model for very high resolution sequence stratigraphy. Marine Geology 120, 129e174. Hunt, D., Tucker, M.E., 1992. Stranded parasequence and the forced regressive wedge systems tract: deposition during base-level fall. Sedimentary Geology 81, 1e9. Jin, J.H., Chough, S.K., 2002. Erosional shelf ridges in the mid-eastern Yellow Sea. Geo-Marine Letters 21, 219e225. Jung, W.Y., Suk, B.C., Min, G.H., Lee, Y.K., 1998. Sedimentary structure and origin of a mud-cored pseudo-tidal sand ridge, eastern Yellow Sea, Korea. Marne Geology 151, 73e88. Karisiddaiah, S.M., Veerayya, M., Vora, K.H., 2002. Seismic and sequence stratigraphy of the central western continental margin of India: late-Quaternary evolution. Marine Geology 192, 335e353. KIGAM (Kigam Institute of Geology, Mining and Materials), 2001. Marine Geological and Geophysical Study for Compiling Marine Geological Maps in the Korean Seas and Marine Mineralogical Study, p. 85. KIGAM Research Report KR-01(C)04, Daejeon, Korea. KIGAM (Kigam Institute of Geology, Mining and Materials), 1992. Marine Geological Study of the Continental Shelf off Taecheon, West Coast, Korea, p. 151. KIGAM Research Report KR-92e3B, Daejeon, Korea. KIGAM (Kigam Institute of Geology, Mining and Materials), 1993. Marine Geological Study of the Continental Shelf, off Kunsan, Korea. KR-93e5A, p. 261. KIGAM Research Report KR-93-5A, Daejeon, Korea. KIGAM (Kigam Institute of Geology, Mining and Materials), 1996. Yellow Sea Drilling Program for Studies Quaternary Geology (Analyses of YSDP-102, YSDP-103, YSDP104, YSDP-105 Cores), KR-96(T)-18, p. 595. KIGAM final Report, Daejeon, Korea. Korea Hydrographic Office, 1990. Tidal Current Charts (Kyongnyolbic Yolto Approaches). Ministry of Transportation, republic of Korea. Publication No. 1433. Labaune, C., Jouet, G., Berne, S., Gensous, B., Tesson, M., Delpeint, A., 2005. Seismic stratigraphy of the Deglacial deposits of the Rhone prodelta and of the adjacent shelf. Marine Geology 222e223, 299e311. Lee, G.S., Kim, D.C., Yoo, D.G., Yi, H.I., 2013. Sedimentary environment and sequence stratigraphy of Late Quaternary deposits in the East China Sea. Marine Georesources and Geotechnology 31, 17e39. Lee, J.C., Jung, K.T., 1999. Application of eddy viscosity closure models for the M2 tide and tidal currents in the Yellow Sea and the East China Sea. Continental Shelf Research 19, 445e475. Lee, H.J., Chough, S.K., 1989. Sediment distribution, dispersal and budget in the Yellow Sea. Marine Geology 87, 195e205. Lee, H.J., Yoon, S.H., 1997. Development of stratigraphy and sediment distribution in the northeastern Yellow Sea during Holocene sea-level rise. Journal of Sedimentary Research 67 (2), 341e349. Liu, J., Saito, Y., Wang, H., Yang, Z., Nakashima, R., 2007. Sedimentary evolution of the Holocene subaqueous clinoform off the Shandong Peninsula in the Yellow Sea. Marine Geology 236, 165e187. Liu, J.P., Milliman, J.D., Gao, S., 2002. The Shandong mud wedge and post-glacial sediment accumulation in the Yellow Sea. Geo-Marine Letters 21, 212e218. Liu, J.P., Milliman, J.D., Gao, S., Cheng, P., 2004. Holocene development of the Yellow River's subaqueous delta, North Yellow Sea. Marine Geology 209, 45e67. Lobo, F.J., Fernandez-Salas, L.M., Hernandez-Molina, F.J., Gonzalez, R., Dias, J.M.A., Diaz del Rio, V., Somoza, L., 2005. Holocene highstand deposits in the Gulf of Cadiz, SW Iberian Peninsula: a high-resolution record of hierarchical environmental changes. Marine Geology 219, 109e131. Milliman, J.D., Qin, Y.S., Park, Y.A., 1989. Sediments and sedimentary processes in the Yellow and East China Seas. In: Taira, A., Mesuda, F. (Eds.), Sedimentary Facies in the Active Plate Margin. TERRAPUB, Tokyo, pp. 233e249. Morton, R.A., Suter, J.R., 1996. Sequence stratigraphy and composition of late Quaternary shelf-margin deltas, Northern Gulf of Mexico. American Association of Petroleum Geologists 80, 505e530. Nummedal, D., Swift, D.J.P., 1987. Transgressive stratigraphy at sequence-bounding unconformities: some principles derived from Holocene and Cretaceous examples. In: Nummedal, D., Pilkey, O.H., Howard, J.D. (Eds.), Sea-level Fluctuation and Coastal Evolution, vol. 41. Society of Economic Paleontologists and Mineralogists, pp. 241e260. Special Publication. Okyar, M., Ediger, V., Ergin, M., 1994. Seismic stratigraphy of the southeastern Black Sea shelf from high-resolution seismic records. Marine Geology 121, 213e230. Park, S.C., Lee, B.H., Han, H.S., Yoo, D.G., Lee, C.W., 2006. Late Quaternary stratigraphy and development of tidal sand ridges in the eastern Yellow Sea. Journal of Sedimentary Research 76, 1093e1105.

136

D.-G. Yoo et al. / Quaternary International 392 (2016) 125e136

Park, S.C., Lee, H.H., Han, H.S., Lee, G.H., Kim, D.C., Yoo, D.G., 2000. Evolution of late Quaternary mud deposits and recent sediment budget in the southeastern Yellow Sea. Marine Geology 170, 271e288. Posamentier, H.W., Jervey, M.T., Vail, P.R., 1988. Eustatic controls on clastic deposition I - conceptual framework. In: Wilgus, C.,K., Hastings, B.S., Kendall, C.G., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea-level Changes: an Integrated Approach, vol. 42. Society of Economic Paleontologists and Mineralogists, pp. 109e124. Special Publication. Qin, Y.H., Zhao, Y., Chen, L., Zhao, S., 1996. Geology of the East China Sea. Science Press, Beijin, pp. 15e44p. Rabineau, M., Berne, S., Aslanian, D., Olivet, J., Joseph, P., Guillocheau, F., Bourillet, J., Ledrezen, E., Granijeon, D., 2005. Sedimentary sequences in the Gulf of Lion: a record of 100,000 years climatic cycles. Marine and Petroleum Geology 22, 775e804. Saito, Y., 1994. Shelf sequence and characteristic bounding surfaces in a wavedominated setting: Latest Pleistocene-Holocene examples from Northeast Japan. Marine Geology 120, 105e127. Schubel, J.R., Shen, H.T., Park, M., 1984. A comparison of some characteristic sedimentation processes of estuaries entering the Yellow Sea. In: Park, Y.A., Pilkey, O.H., Kim, S.W. (Eds.), Proceedings of Korea-U.S. Seminar and Workshop on Marine Geology and Physical Processes of the Yellow Sea, Seoul, pp. 286e308. Shinn, Y.J., Chough, S.K., Kim, J.W., Woo, J., 2007. Development of depositional systems in the southeastern Yellow Sea during the postglacial transgression. Marine Geology 239, 59e82. Song, Y.O., Yoo, D.H., Dyer, K.R., 1983. Sediment distribution, circulation and provenance in a macrotidal bay: Garolim Bay, Korea. Marine Geology 52, 121e140. Sternberg, R.W., Larsen, L.H., Miao, Y.T., 1985. Tidally driven sediment transport on the East China Sea continental shelf. Continental Shelf Research 4, 105e120. Tesson, M., Posamentier, H.W., Gensous, B., 2000. Stratigraphic organization of late Pleistocene deposits of the western part of the Golfe du Lion shelf (Languedoc Shelf), western Mediterranean Sea, using high-resolution seismic and core data. American Association of Petroleum Geologists 84, 119e150. Thorne, J.A., Swift, D.J.P., 1991. Sedimentation on continental margins, VI: a regime model for depositional sequences, their component systems, and bounding surfaces. In: Swift, D.J.P., Oertel, G.F., Tillman, R.W., Thorne, J.A. (Eds.), Shelf Sand and Sandstone Bodies, vol. 14. Society of Economic Paleontologists and Mineralogists, pp. 189e255. Special Publication.

Tortora, P., 1996. Depositional and erosional coastal processes during the last postglacial sea-level rise: an example from the central Tyrrhean continental shelf (Italy). Journal of Sedimentary Research 66, 391e405. Trincardi, F., Correggiari, A., Roveri, M., 1994. Late Quaternary transgressive erosion and deposition in modern epicontinental shelf: the Adriatic Semienclosed Basin. Geo-Marine Letters 14, 41e51. Uehara, K., Saito, Y., 2003. Late Quaternary evolution of the Yellow/East China Sea tidal regime and its impacts on sediments dispersal and seafloor morphology. Sedimentary Geology 162, 25e38. Vail, P.R., 1987. Seismic stratigraphy interpretation using sequence stratigraphy, Part 1: seismic stratigraphy interpretation procedure. In: Bally, A.W. (Ed.), Atlas of Seismic Stratigraphy, vol. 27. American Association of Petroleum Geologists, Studies in Geology, pp. 1e10. Xu, D., Liu, X., Zhang, X., Li, T., Chen, B., 1997. China Offshore Geology. Geological Publishing House, Beijing, p. 310. Yang, S.Y., Wang, Z., Dou, Y., Shi, X., 2014. A review of sedimentation since the Last Glacial Maximum on the continental shelf of eastern China. In: Chiocci, F.L., Chivas, A.R. (Eds.), Continental Shelves of the World: Their Evolution during the Last Glacio-Eustatic CycleGeological Society of London, pp. 293e303. Memoir 41. Yang, Z.S., Liu, J.P., 2007. A unique Yellow River-derived dispersal subaqueous delta in the Yellow Sea. Marine Geology 240, 169e176. Yoo, D.G., Kim, S.P., Lee, C.W., Chang, T.S., Kang, N.K., Lee, G.S., 2014. Late Quaternary transgressive deposits in a low-gradient environmental setting: Korea Strait shelf, SE Korea. Quaternary International 344, 143e155. Yoo, D.G., Lee, C.W., Kim, S.P., Jin, J.H., Kim, J.K., Han, H.C., 2002. Late Quaternary transgressive and highstand systems tracts in the northern East China Sea midshelf. Marine Geology 187, 313e328. Yoo, D.G., Park, S.C., 2000. High-resolution seismic study as a tool for sequence stratigraphic evidence of high-frequency sea-level changes: Latest PleistoceneHolocene example from the Kores Strait. Journal of Sedimentary Research 70, 210e223. Zecchin, M., Baradello, L., Brancolini, G., Donda, F., Rizzetto, F., Tosi, L., 2008. Sequence stratigraphy based on high-resolution seismic profiles in the late Pleistocene and Holocene deposits of the Venice area. Marine Geology 253, 185e198.