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Transgressive and highstand systems tracts and post-glacial transgression, the East China Sea
ELSEVIER
Sedimentary Geology 122 (1998) 217–232
Transgressive and highstand systems tracts and post-glacial transgression, the East China Sea Yoshiki Saito a,Ł , Hajime Katayama a , Ken Ikehara a , Yoshihisa Kato b , Eiji Matsumoto c , Kazumasa Oguri c , Motoyoshi Oda d , Mariko Yumoto d a
Marine Geology Department, Geological Survey of Japan, Higashi 1-1-3, Tsukuba, Ibaraki 306-8567, Japan b Department of Marine Science, Tokai University, 3-20-1, Orido, Shimizu, Shizuoka 424-8610, Japan c Institute for Atmospheric–Hydrospheric Science, Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan d Department of Earth Sciences, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan Received 15 May 1996; accepted 16 September 1997
Abstract The Late Pleistocene–Holocene depositional sequence on the shelf in the East China Sea (ECS) is interpreted on the basis of the analyses of four sediment cores and high-resolution seismic reflection sub-bottom profiler records along a NE– SW across-shelf transect. Sedimentary strata deposited above a lowermost planar erosional surface that was formed during sea-level fall of the last glacial are divided into two units along a NE–SW across-shelf transect (Changjiang–Okinawa). The Lower Unit is characterized by seaward-dipping tangential clinoforms with a thickness of 30–40 m at mid-shelf depths and less than 30 m beneath the outer shelf. Prograding clinoforms are more obvious in mid-shelf environments. The Upper Unit is characterized by an upper transparent layer that is formed into ridge-and-swale topography. The boundary between the Upper and Lower Units is sharp and erosional. Surficial sediments taken by cores from the central ECS shelf are also divided into two facies: a sandy facies consisting of sand or sandy gravel with moderately abundant molluscan shell fragments, and a muddy facies comprising mud intercalated with thin sand layers. The sandy facies is widely distributed on the middle to outer shelf seafloor and has a measured thickness of 30 cm, up to several metres. Radiocarbon ages of molluscan shells in this sediment are less than approximately 10 ka BP except for ages from basal shell lag deposits, which are >10–40 ka BP. The muddy facies underlies the sandy facies and has radiocarbon ages of 2.7–2.9 ka BP (previous work) and 20–38 ka BP (this study) and TL ages of 27–50 ka BP (previous work). The boundary between these lithologic units is an erosional sharp contact. The sandy and muddy facies are correlated with the Upper and Lower Seismic Units, respectively. The Lower Unit is characterized by prograding clinoforms and is interpreted to represent deltaic or nearshore tidal ridge sediments of the paleo-Changjiang River that were deposited during the last glacial lowstand of sea-level as a lowstand systems tract. The underlying erosional surface is interpreted to be a sequence boundary formed as a regressive marine erosional surface during the fall of sea-level. The erosional boundary between the Lower and Upper Units is interpreted as a transgressive surface of erosion formed during the subsequent rise of sea-level. The Upper Unit has a modern sand ridge topography and is interpreted to represent offshore shelf sand ridges of the transgressive to highstand systems tract. 1998 Elsevier Science B.V. All rights reserved. Keywords: East China Sea; Changjiang; sand ridge; shelf sequence; last glacial; sequence stratigraphy
Ł Corresponding
author. Tel.: C81 298 54 3772; Fax: C81 298 54 3533; E-mail: [email protected]
c 1998 Elsevier Science B.V. All rights reserved. 0037-0738/98/$ – see front matter PII: S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 1 0 7 - 9
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1. Introduction Continental shelves are a globally important reservoir for terrigenous sediments. Shelves occupy only about 7.5% of the ocean’s surface (Postma, 1988), but most terrigenous materials are delivered through
rivers to these areas and must be transported through these areas to deep-sea basins. About 70% of riverine sediments in the world are supplied to shelf areas of South Asia, Southeast Asia, East Asia and Oceania (Milliman and Meade, 1983; Milliman and Syvitski, 1992). These sediments are transported to the oceans
Fig. 1. Location and bathymetric map of the Yellow Sea and the East China Sea. The boundary between the East China Sea and the Yellow Sea is the line between the Cheju Island and the river mouth of the Changjiang.
Y. Saito et al. / Sedimentary Geology 122 (1998) 217–232
through large rivers such as the Ganges–Brahmaputra River, Indus River and Red River, and form huge deep-sea fans such as the Bay of Bengal, the Arabian Sea and the South China Sea. However, two of the largest rivers in the world in terms of sediment discharge, the second-largest Yellow (Huanghe) River and fourth-largest Changjiang River, do not form such deep-sea fans in the Okinawa Trough in the East China Sea (Fig. 1). These rivers have built the 500 km wide East China Sea shelf since the Pliocene with riverine sediments preserved in shelf-to-slope areas as stacked sequences (Liu, 1989). The purpose of this paper is to present a sequence stratigraphic model for a wide siliciclastic continental shelf, which has a huge modern supply of sediments. The model is derived from analysis of a Late Pleistocene–Holocene characteristic sequence that has formed during known sea-level changes over the last 50 ka.
219
2. Regional setting The East China Sea (ECS) shelf is about 500 km wide from west to east and 1500 km from north to south including the Yellow Sea (Figs. 1 and 2). Sediments on the ECS shelf consist of sand and mud. Muddy sediment occurs in nearshore to inner shelf environments off the Changjiang River on the south coast of China (Fig. 3). About 70% of the Changjiang River sediments are deposited in the river mouth area and the remainder are transported mainly to the south (Milliman et al., 1985a). Another area of muddy sediment is located south of Cheju Island and derived from the Yellow River or Old Huanghe River region at Jiangsu (Milliman et al., 1985b; Saito and Yang, 1995). Sandy sediment with ripples and megaripples is distributed on the inner shelf between both mud depocentres (Butenko et al., 1985) (Fig. 3). Sandy sediment also occurs
Fig. 2. Map showing the seismic reflection tracklines and core locations in the East China Sea. DZQ-4 borehole is after Yang and Lin (1996)
31 500
81
17
19
40
45
17
18
34 183 93
22
26
1022 418
73
73
50
molluscan shell molluscan shell
benthic foraminifer benthic foraminifer foraminifer (bulk 125–250 µm) foraminifer (bulk 125–250 µm) planktonic foraminifer benthic foraminifer molluscan shell
benthic foraminifer benthic foraminifer molluscan shell molluscan shell molluscan shell
molluscan shell molluscan shell
molluscan shell
molluscan shell
molluscan shell
AMS
AMS
AMS AMS
AMS
AMS
AMS
Circe scripta (Schumacher) Saccella sp.
AMS AMS
AMS
AMS
AMS
AMS
AMS
AMS
7830 5870
5400
7260
3900
22810
20730
30180
23850
42690 40030 9190
31320
38290
11410 9510
7810
7550
2340
50 60
60
90
60
100
90
210
100
1400 970 60
240
570
60 60
60
60
60
14 C C age š method (yr BP)
14
fragments AMS fragments AMS Barbatia lima AMS (Reeve) or Hawaiarca uwaensis (Yokoyama) AMS
Limopsacae gen. et sp. indet Turridae gen. et sp. indet Circe scripta (Schumacher) Paphia sp. Chlamys nobilis (Reeve)
Species
8250
7980
2760
1.9 0.4
1.8
1.9
1.8
8270 6270
5840
7700
4340
0.2 23230
0.2 21150
1.2 30570
0.8 24250
0.8 43110 0.6 40450 2.0 9640
0.6 31720
0.2 38700
50 60
60
90
60
100
90
210
100
1400 970 60
240
570
60 60
60
60
60
Conven- š tional 14 C age (yr BP)
1.3 11800 1.8 9950
1.4
1.0
0.6
δ13 C
80564 80565
80563
106400
106401
93367
93366
106399
106398
80560 80561 80562
106397
106396
80558 80559
80569
80568
80567
Code No. Beta
Conventional radiocarbon ages and errors have been calculated by Beta Analytic Inc. and Geo-Science Laboratory, Japan. There are no bivalve mulluscan shells indicating life position for radiocarbon dating.
29–03.45N 125–55.06E 31–30.07N 125–59.90E
107 84
70 22
29–03.45N 125–55.06E
B 94-20 PN-6GC 3-3, 70 K 93-05 PN-8MC bottom of core
128
107
128–133
B 94-20 PN-5GC 3-3, 95–100
28–41.98N 126–26.53E
128
34
123–128
B 94-20 PN-5GC 3-3, 90–95
28–41.98N 126–26.53E
128
B 94-20 PN-6GC 3-3, 34
107–109
B 94-20 PN-5GC 3-3, 74–76
28–41.98N 126–26.53E
128 128 128
128
73–75
B 94-20 PN-5GC 3-3, 40–42
28–41.98N 126–26.53E 28–41.98N 126–26.53E 28–41.98N 126–26.53E
128
B 94-20 PN-5GC 3-3, 97.3–99.5 130.3–132.5 28–41.98N 126–26.53E
44 50–51 62
B 94-20 PN-5GC 3-3, 11 B 94-20 PN-5GC 3-3, 17–18 B 94-20 PN-5GC 3-3, 29
28–41.98N 126–26.53E
128
128
35–37
B 94-20 PN-5GC 3-3, 2–4
28–41.98N 126–26.53E
128 128
B 94-20 PN-5GC 3-3, 97.3–99.5 130.3–132.5 28–41.98N 126–26.53E
33–35
B 94-20 PN-5GC 3-3, 0–2
28–41.98N 126–26.53E 28–41.98N 126–26.53E
128
271
128
20 28
B 94-20 PN-5GC 3-2, 20 B 94-20 PN-5GC 3-2, 28
28–41.98N 126–26.53E
28–24.18N 126–53.88E
271
Water Sample Material depth wt. longitude (º) (m) (mg)
28–24.18N 126–53.88E
latitude (º)
Location
28–41.98N 126–26.53E
16–17
B 94-20 PN-5GC 3-2, 16–17
104
Subbottom depth (cm)
B 94-20 PN-4GC 3-2, 98
Section depth (cm)
21–27
Core
B 94-20 PN-4GC 3-2, 15–21
Cruise
Table 1 List of the radiocarbon dates determined by this study
220 Y. Saito et al. / Sedimentary Geology 122 (1998) 217–232
Y. Saito et al. / Sedimentary Geology 122 (1998) 217–232
221
ness is approximately 200 m in the Yellow Sea (Qin et al., 1989). At least nine marine units have been discriminated since 1.7 Ma in the southwestern Yellow Sea and the western East China Sea (Zheng, 1988; Jin, 1992; Yang, 1994).
3. Methods
Fig. 3. Map showing the distributions of sand ridges, and grainsize of sediments (after Yang and Sun, 1988; Saito and Yang, 1994). Closed circles show core locations with radiocarbon ages of older than 10 ka within 1 m from the seabottom (Saito and Yang, 1994).
in the middle to outer shelf areas, where accumulation rates are very low in comparison with muddy sediments (DeMaster et al., 1985; Alexander et al., 1991). These surficial sandy sediments are thought to be transgressive sediments deposited since the last glacial maximum at ¾20–18 ka BP (Suk, 1989; Bartek and Wellner, 1995). In a NW–SW transect from the inner shelf off the Changjiang River mouth to the outer shelf, ridge-and-swale topography is recognized. These sand ridges are tidal sand ridges that formed during the last post-glacial sea-level rise (Yang and Sun, 1988). In response to the rise in sealevel, the zone of active tidal sand ridges migrated shoreward and older sand ridges in the offshore region were progressively abandoned (Yang and Sun, 1988; Chen and Tang, 1996). The sand ridge zone is located in a valley-like depression on the shelf that was formed by the paleo-Changjiang River during the last lowstand of sea-level. Quaternary sediments in the Yellow Sea and the East China Sea consist of alternating marine and fluvial sediments formed in response to glacio-eustatic sea-level changes (Qin et al., 1987; Yang, 1994; Bartek and Wellner, 1995). Their maximum thick-
Field surveys were conducted during the BO 94-20 cruise from September 26 to October 12, 1994, with the R.V. Bosei-Maru of Tokai University. The total length of the seismic reflection survey was approximately 1700 km. Hull-mounted 3.5-kHz subbottom profiling (SBP) was carried out on all lines. A 200 J EG and G model 240 subtow-type Uniboom was only used along the PN line (Changjiang to Naha on Okinawa Island), which had a total length of approximately 300 km (Fig. 2). The PN line was located in the northern part of the sand ridge zone (Fig. 3). Ship speed was about 6 knots for the Uniboom survey and 6, 10, or 11 knots for the 3.5-kHz SBP survey. Subsurface depths and thicknesses are based on a sound velocity of 1500 m s 1 . Sediment cores were taken using a gravity corer with a length of 3 m and multi-corer with a length of 0.6 m (Table 1). Cores were cut vertically, logged (slabs of 25 ð 5 ð 1 cm), and X-radiographed. Radiocarbon dating was performed using Accelerator Mass Spectrometry (AMS) methods by Beta Analytic Inc. (Geo-Science Laboratory, Japan). All dates were calculated using the Libby half-life of 5568 years and a reference of AD 1950.
4. Results 4.1. Seismic reflection survey Seismic reflection profiles are shown in Figs. 4–7. A prominent flat-lying reflector (R) that truncates reflectors of underlying strata, is found below the seafloor in the study area, at 10–50 m. Strata above the erosional surface R are divided into two units. The Lower Unit is characterized by seaward-dipping, progradational tangential clinoforms. The unit is 30– 40 m thick in the middle shelf environment and less than 30 m in the outer shelf. Its depocentre is located
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in the 100–110 m isobath area. The Upper Unit is characterized by a smooth and flat-lying basal reflector and a ridge and swale top. The seismic reflection profiles indicate that the Upper Unit is composed of transparent or slight dark-tone structureless features. The lower boundary surface of the Upper Unit is an erosional surface that truncates clinoforms of the Lower Unit. This bounding surface is named reflector H. The thickness of the Upper Unit is less than 12 m at middle shelf depths. The thickness of the Upper Unit is almost 0 m in the outer shelf. It thins in a seaward direction. Surficial ridge topography in the middle shelf environment has a smooth uppermost surface and consists of the Upper Seismic Unit. However, the surface topography of ridges at outer shelf depths is irregular and the ridges comprise the Lower Seismic Unit. Internal structures of the Lower Unit are truncated by the seafloor. 4.2. Core analysis Sediment cores taken from the shelf and slope areas in the East China Sea are shown in Figs. 8 and 9. Core locations are shown in Figs. 2 and 4. Radiocarbon dates are indicated in Table 1. Sediment facies of cores are divided into two facies: sandy facies and muddy facies. PN-8GC, and PN-6GC samples from the shelf area and PN-4GC sample from the upper slope consist of sandy sediments, particularly PN-6GC sample which is composed of well-sorted fine sand. Five radiocarbon dates of shells from these sandy cores indicate ages of less than 11 ka BP. PN6GC and PN-8GC are located in the Upper Seismic Unit. Sample PN-5GC was taken from a sand ridge on the outer shelf, where the Lower Seismic Unit almost crops out. This core is divided into two parts by a sharp erosional contact between an upper shelly sand with a thickness of 29 cm and ages of 7.8–11.4 ka BP, and a lower part that consists of dark olivegrey marine silt, intercalated with thin sandy layers. Radiocarbon dates from this part range mainly from 20 to 38 ka BP, although ages of more than 40 ka BP were determined for molluscan shell samples which look old and weathered, and also ages of less than 10 ka BP for molluscan shell and foraminifer samples. The preparation for diatom analysis was done for the muddy part of PN-5GC; however, the muddy
sediments do not contain any diatoms. The muddy lower part in the core is correlated with the Lower Unit described from seismic reflection profiles.
5. Discussion and conclusion 5.1. Shelf sequence Results of seismic sequence and sediment core analysis are summarized in Table 2. The erosional surface R was probably formed during the fall of sea-level of the last glacial. Strata overlying reflector R are divided into two units. The Lower Unit is characterized by seaward-dipping progradational clinoforms in seismic reflection records and consists of muddy sediments with radiocarbon ages of 20–38 ka BP. The Upper Unit is characterized by sandy sediments with radiocarbon ages of less than 11 ka BP and is formed into a ridge and swale topography. Yang and Lin (1996) showed the chronostratigraphy of the DZQ-4 borehole taken from the location at 29º240 4500 N and 125º210 5100 E with a water depth of 88.7 m, and located near the PN line between PN8MC and PN-6GC (Figs. 2 and 7). They indicated that sediment facies changes at 28.1 m below the seafloor from underlying shallow marine facies with TL ages of 87–105 ka BP to overlying deltaic facies with TL ages of 50–27 ka BP. This boundary and the overlying deltaic sediment are correlated with reflector R and the Lower Unit, respectively. Radiocarbon ages from the Lower Unit of PN5GC indicate a wide range from about 4 ka BP to 42 ka BP. Moreover, these ages are stratigraphically reversed and=or random. Yim et al. (1990) showed that radiocarbon dates of pre-Holocene marine deposits in Hong Kong (22–46 ka BP) indicate younger ages than the uranium-series dates (130–142 ka BP) because of subaerial exposure during the lowstand of sea-level in the last glacial and that old radiocarbon dates are likely to be minimum age estimates. Moreover, ages of the lower muddy part of PN-5GC must be younger than the oldest ages of the Lower Unit in the DZQ-4 borehole because PN-5GC and DZQ-4 are located at the seaward and landward margins of the seaward-dipping progradational clinoforms. Therefore, estimated radiocarbon ages of the lower muddy part of PN-5GC are thought to be 30–40 ka
Y. Saito et al. / Sedimentary Geology 122 (1998) 217–232
Fig. 4. A 3.5-kHz sub-bottom profiler record in the middle to outer shelf of the central East China Sea (horizontally compressed). Location is shown in Fig. 2. PN-7GC (gravity core) was not recovered.
Fig. 6. A high-resolution seismic reflection record (Uniboom) in the middle to outer shelf of the central East China Sea. Location is shown in Fig. 2.
pp. 223–224
pp. 225–226
Y. Saito et al. / Sedimentary Geology 122 (1998) 217–232
Fig. 5. A 3.5-kHz sub-bottom profiler record of the continental slope to the Okinawa trough in the central East China Sea. Location is shown in Fig. 2.
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227
Fig. 7. Interpretation of seismic records in the East China Sea shelf. The upper one from a 3.5-kHz record, the lower one from a Uniboom record. Arrow shows the location of deep borehole DZQ-4.
BP, though there is still the question of why the lowest part of PN-5GC shows young ages of less than 10 ka BP. Similar lithologies and stratigraphic interpretations in the survey area are shown by the Editorial Board for Marine Atlas (1990). Their core logs also show that the upper sandy facies has radiocarbon ages of less than 8 ka and that the lower muddy facies has radiocarbon ages of 27–29 ka, except for older radiocarbon dates from shelly lag deposits at the base of the Upper Unit. The thickness of the upper sandy facies is 0.2 to 2.7 m, and that of the lower muddy facies is 0.7 to 3 m (Editorial Board for Marine Atlas, 1990). A composite of all radiocarbon dates and TL ages for the Lower Seismic Unit indicates a range of 4–50 ka BP, mostly 25–50 ka BP, coinciding with the epoch of the last glacial and of the lowstand
in sea-level before the lowest sea-level. Estimated sea-level during the 25–50 ka BP is 70–90 m below the present level (Shackleton, 1987; Chappell et al., 1996). The age of reflector R at the DZQ-4 site is between 87 and 50 ka BP. These cores do not indicate direct evidence of emergence at this time. However, there is no good core indicating continuous marine deposition through the last glacial maximum (18–20 ka BP). The formation of reflector R is considered not to be a fluvial incision surface, but instead is interpreted as a marine erosion surface, because reflector R is a flat-lying erosional surface without fluvial sediments. If it is assumed that the formation of the reflector is linked to the fall of sea-level during the oxygen isotope stage 4 with an age of about 65 ka BP, or the lowering of sea-level during oxygen iosotope stage 2 between 25 and 50 ka BP, then the nearshore deposition of the Lower Unit
Table 2 Summary of seismic units and sediment facies in the middle to outer shelf of the East China Sea
Seismic facies Thickness Surface topography Sediment ages
14 C
Upper Unit
Lower Unit
Transparent <10–12 m 0m Ridge and swale
Seaward-dipping clinoform 30–40 m <30 m Smooth, flat Ridge and swale Mud with sand 20–38 ka BP Late Pleistocene
Sand (gravel) <10 ka BP Holocene
Middle shelf (PN-8MC to PN-6-GC) Outer shelf (PN-6GC to shelf edge) Middle shelf (PN-8MC to PN-6-GC) Outer shelf (PN-6GC to shelf edge)
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Fig. 8. Core logs taken from the East China Sea. Core locations are shown in Fig. 2 and Table 1. Materials for radiocarbon ages are shown in Table 1.
is contemporaneous with offshore marine erosion on the shelf. Fig. 10 shows a schematic depositional model of the study area. In relation to the fall and lowstand of sea-level during the last glacial with ages
of 25–50 ka BP, the paleo-Changjiang delta prograded seaward and formed the Lower Unit (stage A). The ensuing fall in sea-level leading up to the last glacial maximum and related changes of the paleo-Changjiang delta are not clear. The subse-
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229
Fig. 9. X-radiographs (negative) of core samples. From left to right: PN-5GC 8–28 cm (sandy facies: shelly lag deposits), 33–43 cm (muddy facies: intercalated 1–2-cm-thick silt–sand layers, s) and 43–63 cm (muddy facies: silt–sand layer, s, bioturbated, some burrows in the lower part). Each slab is 5 cm ð 20 cm.
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Fig. 10. A schematic model for the East China Sea shelf sequence.
quent rise in sea-level induced erosion in nearshore environments, at which time the bounding surface, reflector H, was formed (stage B). Related to the post-glacial transgression, sandy sediments were deposited forming a sand ridge topography after 8–11
ka (stage C). At 9 ka BP sea-level was 25–40 m below the present sea-level and rose to 10–20 m at 8 ka BP (Zhao et al., 1979; Chen et al., 1985). Therefore the water depth at site PN-5GC during the early stage of sand ridge deposition was 88–103 m
Y. Saito et al. / Sedimentary Geology 122 (1998) 217–232
deep at 9 ka BP and 108–118 m deep at 8 ka BP. The shoreline was located about 400 km westward. Yang and Sun (1988) thought that these tidal sand ridges were formed at river mouths or nearshore areas of the paleo-Changjiang River. However, radiocarbon ages of subsurface sediments presented in this study suggest that the tidal sand ridges were formed in offshore areas on the middle to outer shelf. The sea-level reached the present level at about 6 ka BP and has been essentially stable for the last 6 ka (Zhao et al., 1979; Chen et al., 1985). Transgressive sandy sediments have been partly covered by modern muddy sediments as shown in Fig. 3 (south of Cheju Island). Radiocarbon ages of the base of the muddy sediments show less than 8 ka BP (Yang et al., 1995). Therefore these muddy sediments are interpreted as late transgressive systems tracts to highstand systems tract (stage D). The Lower Unit and Upper Unit are interpreted as lowstand systems tract and transgressive to highstand systems tract, respectively (Van Wagoner et al., 1988; Vail et al., 1991). Reflectors R and H are interpreted as bounding surfaces formed during phases of regressive and transgressive marine erosion, respectively. The sequence boundary of reflector R was formed during the lowstand of sea-level between 25 and 50 ka BP, but not during the lowest sea-level. Finally this study is based on only one seismic reflection line, offshore from the Changjiang River. Further work using a 3-D seismic survey and an intensive coring program will enable the determination of the precise route of the paleo-Changjiang River as well as elucidating the position of paleo-shorelines and the geometry of each unit related to the sea-level changes.
Acknowledgements We would like to thank A. Sato and K. Kawabe of Kokusai Kogyo Co. Ltd. for the operation of the seismic surveys, and N. Araki, S. Sato, M. Hanada and the crew of the R.V. Bosei-Maru for their support during the cruise. We thank Scott Nodder and an unknown reviewer for critical reviews of the manuscript and Bob Cater and Tim Naish for encouraging us to publish in this special volume, and for their helpful reviews. This study was done as part
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of the Marginal Sea Flux Experiment in the West Pacific (MASFLEX), supported by Special Coordination Funds for Promoting Science and Technology of the Science and Technology Agency of Japan.
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Postma, H., 1988. Physical and chemical oceanographic aspects of continental shelves. In: Postma, H., Zijlstra, J.J. (Eds.), Continental Shelves. Elsevier, Amsterdam, pp. 5–37. Qin, Y., Zhao, Y., Chen, L., Zhao, S. (Eds.), 1987. Geology of The East China Sea. Science Press, Beijing, 290 pp. Qin, Y., Zhao, Y., Chen, L., Zhao, S. (Eds.), 1989. Geology of The Huanghai Sea. China Ocean Press, Beijing, 289 pp. Saito, Y., Yang, Z., 1994. The Huanghe River: its water discharge, sediment discharge, and sediment budget. J. Sedimentol. Soc. Jpn. 40, 7–17. Saito, Y., Yang, Z., 1995. Historical change of the Huanghe (Yellow River) and its impact on the sediment budget of the East China Sea. In: Tsunogai, S., Iseki, K., Koike, I., Oba, T. (Eds.), Global Fluxes of Carbon and Its Related Substances in the Coastal Sea–Ocean–Atmosphere System. M and J International, Yokohama, pp. 7–12. Shackleton, N.J., 1987. Oxygen isotopes, ice volume and sea level. Quat. Sci. Rev. 6, 183–190. Suk, B.-C., 1989. Sedimentology and history of sea level changes in the East China Sea and adjacent seas. In: Taira, A., Masuda, F. (Eds.), Sedimentary Facies in the Active Plate Margin. Terra Sci. Publ., Tokyo, pp. 215–231. Vail, P.R., Audemard, F., Bowman, S.A., Eisner, P.N., PerezCruz, C., 1991. The stratigraphic signatures of tectonics, eustasy and sedimentology — an overview. In: Einsele, G., Ricken, W., Seilacher, A. (Eds.), Cycles and Events in Stratigraphy. Springer-Verlag, Berlin, pp. 617–659. Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M. Jr., Vail, P.R., Sarg, J.F., Loutit, T.S., Hardenbol, J., 1988. An overview of the fundamentals of sequence stratigraphy and key defi-
nitions. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), SeaLevel Changes: An Integrated Approach. Soc. Econ. Paleontol. Mineral., Spec. Publ. 42, 39–45. Yang, C.-s., Sun, J.-s., 1988. Tidal sand ridges on the east China Sea Shelf. In: de Boer, P.L., van Gelder, A., Nio, S.D. (Eds.), Tidal-Influenced Sedimentary Environments and Facies. Reidel, Dordrecht, pp. 23–38. Yang, Z., 1994. The sedimentary sequence and palaeogeographic changes of the South Yellow Sea since the Olduvai Subchron. Acta Geol. Sin. 7, 195–207. Yang, Z., Lin, H. (Eds.), 1996. Quaternary Stratigraphy in China and Its International Correlation. Geological Publishing House, Beijing, 206 pp. Yang, Z., Saito, Y., Guo, Z., Cui, Q., 1995. Distal mud area as a material sink in the East China Sea. In: Tsunogai, S., Iseki, K., Koike, I., Oba, T. (Eds.), Global Fluxes of Carbon and Its Related Substances in the Coastal Sea–Ocean–Atmosphere System. M and J International, Yokohama, pp. 1–6. Yim, W.W.-S., Ivanovich, M., Yu age, K.-F., 1990. Young age bias of radiocarbon dates in pre-Holocene marine deposits of Hong Kong and implications for Pleistocene stratigraphy. Geo-Mar. Lett. 10, 165–172. Zhao, X., Geng, X., Zhang, J., 1979. Sea level changes of the eastern China during the past 20,000 years. Acta Oceanol. Sin. 1, 269–281. Zheng, G., 1988. Quaternary stratigraphic division of the Hole QC2 in the southern Huanghai Sea. Mar. Geol. Quat. Geol. 8 (4), 1–9.
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Transgressive and highstand systems tracts and post-glacial transgression, the East China Sea Yoshiki Saito a,Ł , Hajime Katayama a , Ken Ikehara a , Yoshihisa Kato b , Eiji Matsumoto c , Kazumasa Oguri c , Motoyoshi Oda d , Mariko Yumoto d a
Marine Geology Department, Geological Survey of Japan, Higashi 1-1-3, Tsukuba, Ibaraki 306-8567, Japan b Department of Marine Science, Tokai University, 3-20-1, Orido, Shimizu, Shizuoka 424-8610, Japan c Institute for Atmospheric–Hydrospheric Science, Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan d Department of Earth Sciences, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan Received 15 May 1996; accepted 16 September 1997
Abstract The Late Pleistocene–Holocene depositional sequence on the shelf in the East China Sea (ECS) is interpreted on the basis of the analyses of four sediment cores and high-resolution seismic reflection sub-bottom profiler records along a NE– SW across-shelf transect. Sedimentary strata deposited above a lowermost planar erosional surface that was formed during sea-level fall of the last glacial are divided into two units along a NE–SW across-shelf transect (Changjiang–Okinawa). The Lower Unit is characterized by seaward-dipping tangential clinoforms with a thickness of 30–40 m at mid-shelf depths and less than 30 m beneath the outer shelf. Prograding clinoforms are more obvious in mid-shelf environments. The Upper Unit is characterized by an upper transparent layer that is formed into ridge-and-swale topography. The boundary between the Upper and Lower Units is sharp and erosional. Surficial sediments taken by cores from the central ECS shelf are also divided into two facies: a sandy facies consisting of sand or sandy gravel with moderately abundant molluscan shell fragments, and a muddy facies comprising mud intercalated with thin sand layers. The sandy facies is widely distributed on the middle to outer shelf seafloor and has a measured thickness of 30 cm, up to several metres. Radiocarbon ages of molluscan shells in this sediment are less than approximately 10 ka BP except for ages from basal shell lag deposits, which are >10–40 ka BP. The muddy facies underlies the sandy facies and has radiocarbon ages of 2.7–2.9 ka BP (previous work) and 20–38 ka BP (this study) and TL ages of 27–50 ka BP (previous work). The boundary between these lithologic units is an erosional sharp contact. The sandy and muddy facies are correlated with the Upper and Lower Seismic Units, respectively. The Lower Unit is characterized by prograding clinoforms and is interpreted to represent deltaic or nearshore tidal ridge sediments of the paleo-Changjiang River that were deposited during the last glacial lowstand of sea-level as a lowstand systems tract. The underlying erosional surface is interpreted to be a sequence boundary formed as a regressive marine erosional surface during the fall of sea-level. The erosional boundary between the Lower and Upper Units is interpreted as a transgressive surface of erosion formed during the subsequent rise of sea-level. The Upper Unit has a modern sand ridge topography and is interpreted to represent offshore shelf sand ridges of the transgressive to highstand systems tract. 1998 Elsevier Science B.V. All rights reserved. Keywords: East China Sea; Changjiang; sand ridge; shelf sequence; last glacial; sequence stratigraphy
Ł Corresponding
author. Tel.: C81 298 54 3772; Fax: C81 298 54 3533; E-mail: [email protected]
c 1998 Elsevier Science B.V. All rights reserved. 0037-0738/98/$ – see front matter PII: S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 1 0 7 - 9
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1. Introduction Continental shelves are a globally important reservoir for terrigenous sediments. Shelves occupy only about 7.5% of the ocean’s surface (Postma, 1988), but most terrigenous materials are delivered through
rivers to these areas and must be transported through these areas to deep-sea basins. About 70% of riverine sediments in the world are supplied to shelf areas of South Asia, Southeast Asia, East Asia and Oceania (Milliman and Meade, 1983; Milliman and Syvitski, 1992). These sediments are transported to the oceans
Fig. 1. Location and bathymetric map of the Yellow Sea and the East China Sea. The boundary between the East China Sea and the Yellow Sea is the line between the Cheju Island and the river mouth of the Changjiang.
Y. Saito et al. / Sedimentary Geology 122 (1998) 217–232
through large rivers such as the Ganges–Brahmaputra River, Indus River and Red River, and form huge deep-sea fans such as the Bay of Bengal, the Arabian Sea and the South China Sea. However, two of the largest rivers in the world in terms of sediment discharge, the second-largest Yellow (Huanghe) River and fourth-largest Changjiang River, do not form such deep-sea fans in the Okinawa Trough in the East China Sea (Fig. 1). These rivers have built the 500 km wide East China Sea shelf since the Pliocene with riverine sediments preserved in shelf-to-slope areas as stacked sequences (Liu, 1989). The purpose of this paper is to present a sequence stratigraphic model for a wide siliciclastic continental shelf, which has a huge modern supply of sediments. The model is derived from analysis of a Late Pleistocene–Holocene characteristic sequence that has formed during known sea-level changes over the last 50 ka.
219
2. Regional setting The East China Sea (ECS) shelf is about 500 km wide from west to east and 1500 km from north to south including the Yellow Sea (Figs. 1 and 2). Sediments on the ECS shelf consist of sand and mud. Muddy sediment occurs in nearshore to inner shelf environments off the Changjiang River on the south coast of China (Fig. 3). About 70% of the Changjiang River sediments are deposited in the river mouth area and the remainder are transported mainly to the south (Milliman et al., 1985a). Another area of muddy sediment is located south of Cheju Island and derived from the Yellow River or Old Huanghe River region at Jiangsu (Milliman et al., 1985b; Saito and Yang, 1995). Sandy sediment with ripples and megaripples is distributed on the inner shelf between both mud depocentres (Butenko et al., 1985) (Fig. 3). Sandy sediment also occurs
Fig. 2. Map showing the seismic reflection tracklines and core locations in the East China Sea. DZQ-4 borehole is after Yang and Lin (1996)
31 500
81
17
19
40
45
17
18
34 183 93
22
26
1022 418
73
73
50
molluscan shell molluscan shell
benthic foraminifer benthic foraminifer foraminifer (bulk 125–250 µm) foraminifer (bulk 125–250 µm) planktonic foraminifer benthic foraminifer molluscan shell
benthic foraminifer benthic foraminifer molluscan shell molluscan shell molluscan shell
molluscan shell molluscan shell
molluscan shell
molluscan shell
molluscan shell
AMS
AMS
AMS AMS
AMS
AMS
AMS
Circe scripta (Schumacher) Saccella sp.
AMS AMS
AMS
AMS
AMS
AMS
AMS
AMS
7830 5870
5400
7260
3900
22810
20730
30180
23850
42690 40030 9190
31320
38290
11410 9510
7810
7550
2340
50 60
60
90
60
100
90
210
100
1400 970 60
240
570
60 60
60
60
60
14 C C age š method (yr BP)
14
fragments AMS fragments AMS Barbatia lima AMS (Reeve) or Hawaiarca uwaensis (Yokoyama) AMS
Limopsacae gen. et sp. indet Turridae gen. et sp. indet Circe scripta (Schumacher) Paphia sp. Chlamys nobilis (Reeve)
Species
8250
7980
2760
1.9 0.4
1.8
1.9
1.8
8270 6270
5840
7700
4340
0.2 23230
0.2 21150
1.2 30570
0.8 24250
0.8 43110 0.6 40450 2.0 9640
0.6 31720
0.2 38700
50 60
60
90
60
100
90
210
100
1400 970 60
240
570
60 60
60
60
60
Conven- š tional 14 C age (yr BP)
1.3 11800 1.8 9950
1.4
1.0
0.6
δ13 C
80564 80565
80563
106400
106401
93367
93366
106399
106398
80560 80561 80562
106397
106396
80558 80559
80569
80568
80567
Code No. Beta
Conventional radiocarbon ages and errors have been calculated by Beta Analytic Inc. and Geo-Science Laboratory, Japan. There are no bivalve mulluscan shells indicating life position for radiocarbon dating.
29–03.45N 125–55.06E 31–30.07N 125–59.90E
107 84
70 22
29–03.45N 125–55.06E
B 94-20 PN-6GC 3-3, 70 K 93-05 PN-8MC bottom of core
128
107
128–133
B 94-20 PN-5GC 3-3, 95–100
28–41.98N 126–26.53E
128
34
123–128
B 94-20 PN-5GC 3-3, 90–95
28–41.98N 126–26.53E
128
B 94-20 PN-6GC 3-3, 34
107–109
B 94-20 PN-5GC 3-3, 74–76
28–41.98N 126–26.53E
128 128 128
128
73–75
B 94-20 PN-5GC 3-3, 40–42
28–41.98N 126–26.53E 28–41.98N 126–26.53E 28–41.98N 126–26.53E
128
B 94-20 PN-5GC 3-3, 97.3–99.5 130.3–132.5 28–41.98N 126–26.53E
44 50–51 62
B 94-20 PN-5GC 3-3, 11 B 94-20 PN-5GC 3-3, 17–18 B 94-20 PN-5GC 3-3, 29
28–41.98N 126–26.53E
128
128
35–37
B 94-20 PN-5GC 3-3, 2–4
28–41.98N 126–26.53E
128 128
B 94-20 PN-5GC 3-3, 97.3–99.5 130.3–132.5 28–41.98N 126–26.53E
33–35
B 94-20 PN-5GC 3-3, 0–2
28–41.98N 126–26.53E 28–41.98N 126–26.53E
128
271
128
20 28
B 94-20 PN-5GC 3-2, 20 B 94-20 PN-5GC 3-2, 28
28–41.98N 126–26.53E
28–24.18N 126–53.88E
271
Water Sample Material depth wt. longitude (º) (m) (mg)
28–24.18N 126–53.88E
latitude (º)
Location
28–41.98N 126–26.53E
16–17
B 94-20 PN-5GC 3-2, 16–17
104
Subbottom depth (cm)
B 94-20 PN-4GC 3-2, 98
Section depth (cm)
21–27
Core
B 94-20 PN-4GC 3-2, 15–21
Cruise
Table 1 List of the radiocarbon dates determined by this study
220 Y. Saito et al. / Sedimentary Geology 122 (1998) 217–232
Y. Saito et al. / Sedimentary Geology 122 (1998) 217–232
221
ness is approximately 200 m in the Yellow Sea (Qin et al., 1989). At least nine marine units have been discriminated since 1.7 Ma in the southwestern Yellow Sea and the western East China Sea (Zheng, 1988; Jin, 1992; Yang, 1994).
3. Methods
Fig. 3. Map showing the distributions of sand ridges, and grainsize of sediments (after Yang and Sun, 1988; Saito and Yang, 1994). Closed circles show core locations with radiocarbon ages of older than 10 ka within 1 m from the seabottom (Saito and Yang, 1994).
in the middle to outer shelf areas, where accumulation rates are very low in comparison with muddy sediments (DeMaster et al., 1985; Alexander et al., 1991). These surficial sandy sediments are thought to be transgressive sediments deposited since the last glacial maximum at ¾20–18 ka BP (Suk, 1989; Bartek and Wellner, 1995). In a NW–SW transect from the inner shelf off the Changjiang River mouth to the outer shelf, ridge-and-swale topography is recognized. These sand ridges are tidal sand ridges that formed during the last post-glacial sea-level rise (Yang and Sun, 1988). In response to the rise in sealevel, the zone of active tidal sand ridges migrated shoreward and older sand ridges in the offshore region were progressively abandoned (Yang and Sun, 1988; Chen and Tang, 1996). The sand ridge zone is located in a valley-like depression on the shelf that was formed by the paleo-Changjiang River during the last lowstand of sea-level. Quaternary sediments in the Yellow Sea and the East China Sea consist of alternating marine and fluvial sediments formed in response to glacio-eustatic sea-level changes (Qin et al., 1987; Yang, 1994; Bartek and Wellner, 1995). Their maximum thick-
Field surveys were conducted during the BO 94-20 cruise from September 26 to October 12, 1994, with the R.V. Bosei-Maru of Tokai University. The total length of the seismic reflection survey was approximately 1700 km. Hull-mounted 3.5-kHz subbottom profiling (SBP) was carried out on all lines. A 200 J EG and G model 240 subtow-type Uniboom was only used along the PN line (Changjiang to Naha on Okinawa Island), which had a total length of approximately 300 km (Fig. 2). The PN line was located in the northern part of the sand ridge zone (Fig. 3). Ship speed was about 6 knots for the Uniboom survey and 6, 10, or 11 knots for the 3.5-kHz SBP survey. Subsurface depths and thicknesses are based on a sound velocity of 1500 m s 1 . Sediment cores were taken using a gravity corer with a length of 3 m and multi-corer with a length of 0.6 m (Table 1). Cores were cut vertically, logged (slabs of 25 ð 5 ð 1 cm), and X-radiographed. Radiocarbon dating was performed using Accelerator Mass Spectrometry (AMS) methods by Beta Analytic Inc. (Geo-Science Laboratory, Japan). All dates were calculated using the Libby half-life of 5568 years and a reference of AD 1950.
4. Results 4.1. Seismic reflection survey Seismic reflection profiles are shown in Figs. 4–7. A prominent flat-lying reflector (R) that truncates reflectors of underlying strata, is found below the seafloor in the study area, at 10–50 m. Strata above the erosional surface R are divided into two units. The Lower Unit is characterized by seaward-dipping, progradational tangential clinoforms. The unit is 30– 40 m thick in the middle shelf environment and less than 30 m in the outer shelf. Its depocentre is located
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in the 100–110 m isobath area. The Upper Unit is characterized by a smooth and flat-lying basal reflector and a ridge and swale top. The seismic reflection profiles indicate that the Upper Unit is composed of transparent or slight dark-tone structureless features. The lower boundary surface of the Upper Unit is an erosional surface that truncates clinoforms of the Lower Unit. This bounding surface is named reflector H. The thickness of the Upper Unit is less than 12 m at middle shelf depths. The thickness of the Upper Unit is almost 0 m in the outer shelf. It thins in a seaward direction. Surficial ridge topography in the middle shelf environment has a smooth uppermost surface and consists of the Upper Seismic Unit. However, the surface topography of ridges at outer shelf depths is irregular and the ridges comprise the Lower Seismic Unit. Internal structures of the Lower Unit are truncated by the seafloor. 4.2. Core analysis Sediment cores taken from the shelf and slope areas in the East China Sea are shown in Figs. 8 and 9. Core locations are shown in Figs. 2 and 4. Radiocarbon dates are indicated in Table 1. Sediment facies of cores are divided into two facies: sandy facies and muddy facies. PN-8GC, and PN-6GC samples from the shelf area and PN-4GC sample from the upper slope consist of sandy sediments, particularly PN-6GC sample which is composed of well-sorted fine sand. Five radiocarbon dates of shells from these sandy cores indicate ages of less than 11 ka BP. PN6GC and PN-8GC are located in the Upper Seismic Unit. Sample PN-5GC was taken from a sand ridge on the outer shelf, where the Lower Seismic Unit almost crops out. This core is divided into two parts by a sharp erosional contact between an upper shelly sand with a thickness of 29 cm and ages of 7.8–11.4 ka BP, and a lower part that consists of dark olivegrey marine silt, intercalated with thin sandy layers. Radiocarbon dates from this part range mainly from 20 to 38 ka BP, although ages of more than 40 ka BP were determined for molluscan shell samples which look old and weathered, and also ages of less than 10 ka BP for molluscan shell and foraminifer samples. The preparation for diatom analysis was done for the muddy part of PN-5GC; however, the muddy
sediments do not contain any diatoms. The muddy lower part in the core is correlated with the Lower Unit described from seismic reflection profiles.
5. Discussion and conclusion 5.1. Shelf sequence Results of seismic sequence and sediment core analysis are summarized in Table 2. The erosional surface R was probably formed during the fall of sea-level of the last glacial. Strata overlying reflector R are divided into two units. The Lower Unit is characterized by seaward-dipping progradational clinoforms in seismic reflection records and consists of muddy sediments with radiocarbon ages of 20–38 ka BP. The Upper Unit is characterized by sandy sediments with radiocarbon ages of less than 11 ka BP and is formed into a ridge and swale topography. Yang and Lin (1996) showed the chronostratigraphy of the DZQ-4 borehole taken from the location at 29º240 4500 N and 125º210 5100 E with a water depth of 88.7 m, and located near the PN line between PN8MC and PN-6GC (Figs. 2 and 7). They indicated that sediment facies changes at 28.1 m below the seafloor from underlying shallow marine facies with TL ages of 87–105 ka BP to overlying deltaic facies with TL ages of 50–27 ka BP. This boundary and the overlying deltaic sediment are correlated with reflector R and the Lower Unit, respectively. Radiocarbon ages from the Lower Unit of PN5GC indicate a wide range from about 4 ka BP to 42 ka BP. Moreover, these ages are stratigraphically reversed and=or random. Yim et al. (1990) showed that radiocarbon dates of pre-Holocene marine deposits in Hong Kong (22–46 ka BP) indicate younger ages than the uranium-series dates (130–142 ka BP) because of subaerial exposure during the lowstand of sea-level in the last glacial and that old radiocarbon dates are likely to be minimum age estimates. Moreover, ages of the lower muddy part of PN-5GC must be younger than the oldest ages of the Lower Unit in the DZQ-4 borehole because PN-5GC and DZQ-4 are located at the seaward and landward margins of the seaward-dipping progradational clinoforms. Therefore, estimated radiocarbon ages of the lower muddy part of PN-5GC are thought to be 30–40 ka
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Fig. 4. A 3.5-kHz sub-bottom profiler record in the middle to outer shelf of the central East China Sea (horizontally compressed). Location is shown in Fig. 2. PN-7GC (gravity core) was not recovered.
Fig. 6. A high-resolution seismic reflection record (Uniboom) in the middle to outer shelf of the central East China Sea. Location is shown in Fig. 2.
pp. 223–224
pp. 225–226
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Fig. 5. A 3.5-kHz sub-bottom profiler record of the continental slope to the Okinawa trough in the central East China Sea. Location is shown in Fig. 2.
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227
Fig. 7. Interpretation of seismic records in the East China Sea shelf. The upper one from a 3.5-kHz record, the lower one from a Uniboom record. Arrow shows the location of deep borehole DZQ-4.
BP, though there is still the question of why the lowest part of PN-5GC shows young ages of less than 10 ka BP. Similar lithologies and stratigraphic interpretations in the survey area are shown by the Editorial Board for Marine Atlas (1990). Their core logs also show that the upper sandy facies has radiocarbon ages of less than 8 ka and that the lower muddy facies has radiocarbon ages of 27–29 ka, except for older radiocarbon dates from shelly lag deposits at the base of the Upper Unit. The thickness of the upper sandy facies is 0.2 to 2.7 m, and that of the lower muddy facies is 0.7 to 3 m (Editorial Board for Marine Atlas, 1990). A composite of all radiocarbon dates and TL ages for the Lower Seismic Unit indicates a range of 4–50 ka BP, mostly 25–50 ka BP, coinciding with the epoch of the last glacial and of the lowstand
in sea-level before the lowest sea-level. Estimated sea-level during the 25–50 ka BP is 70–90 m below the present level (Shackleton, 1987; Chappell et al., 1996). The age of reflector R at the DZQ-4 site is between 87 and 50 ka BP. These cores do not indicate direct evidence of emergence at this time. However, there is no good core indicating continuous marine deposition through the last glacial maximum (18–20 ka BP). The formation of reflector R is considered not to be a fluvial incision surface, but instead is interpreted as a marine erosion surface, because reflector R is a flat-lying erosional surface without fluvial sediments. If it is assumed that the formation of the reflector is linked to the fall of sea-level during the oxygen isotope stage 4 with an age of about 65 ka BP, or the lowering of sea-level during oxygen iosotope stage 2 between 25 and 50 ka BP, then the nearshore deposition of the Lower Unit
Table 2 Summary of seismic units and sediment facies in the middle to outer shelf of the East China Sea
Seismic facies Thickness Surface topography Sediment ages
14 C
Upper Unit
Lower Unit
Transparent <10–12 m 0m Ridge and swale
Seaward-dipping clinoform 30–40 m <30 m Smooth, flat Ridge and swale Mud with sand 20–38 ka BP Late Pleistocene
Sand (gravel) <10 ka BP Holocene
Middle shelf (PN-8MC to PN-6-GC) Outer shelf (PN-6GC to shelf edge) Middle shelf (PN-8MC to PN-6-GC) Outer shelf (PN-6GC to shelf edge)
228
Y. Saito et al. / Sedimentary Geology 122 (1998) 217–232
Fig. 8. Core logs taken from the East China Sea. Core locations are shown in Fig. 2 and Table 1. Materials for radiocarbon ages are shown in Table 1.
is contemporaneous with offshore marine erosion on the shelf. Fig. 10 shows a schematic depositional model of the study area. In relation to the fall and lowstand of sea-level during the last glacial with ages
of 25–50 ka BP, the paleo-Changjiang delta prograded seaward and formed the Lower Unit (stage A). The ensuing fall in sea-level leading up to the last glacial maximum and related changes of the paleo-Changjiang delta are not clear. The subse-
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229
Fig. 9. X-radiographs (negative) of core samples. From left to right: PN-5GC 8–28 cm (sandy facies: shelly lag deposits), 33–43 cm (muddy facies: intercalated 1–2-cm-thick silt–sand layers, s) and 43–63 cm (muddy facies: silt–sand layer, s, bioturbated, some burrows in the lower part). Each slab is 5 cm ð 20 cm.
230
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Fig. 10. A schematic model for the East China Sea shelf sequence.
quent rise in sea-level induced erosion in nearshore environments, at which time the bounding surface, reflector H, was formed (stage B). Related to the post-glacial transgression, sandy sediments were deposited forming a sand ridge topography after 8–11
ka (stage C). At 9 ka BP sea-level was 25–40 m below the present sea-level and rose to 10–20 m at 8 ka BP (Zhao et al., 1979; Chen et al., 1985). Therefore the water depth at site PN-5GC during the early stage of sand ridge deposition was 88–103 m
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deep at 9 ka BP and 108–118 m deep at 8 ka BP. The shoreline was located about 400 km westward. Yang and Sun (1988) thought that these tidal sand ridges were formed at river mouths or nearshore areas of the paleo-Changjiang River. However, radiocarbon ages of subsurface sediments presented in this study suggest that the tidal sand ridges were formed in offshore areas on the middle to outer shelf. The sea-level reached the present level at about 6 ka BP and has been essentially stable for the last 6 ka (Zhao et al., 1979; Chen et al., 1985). Transgressive sandy sediments have been partly covered by modern muddy sediments as shown in Fig. 3 (south of Cheju Island). Radiocarbon ages of the base of the muddy sediments show less than 8 ka BP (Yang et al., 1995). Therefore these muddy sediments are interpreted as late transgressive systems tracts to highstand systems tract (stage D). The Lower Unit and Upper Unit are interpreted as lowstand systems tract and transgressive to highstand systems tract, respectively (Van Wagoner et al., 1988; Vail et al., 1991). Reflectors R and H are interpreted as bounding surfaces formed during phases of regressive and transgressive marine erosion, respectively. The sequence boundary of reflector R was formed during the lowstand of sea-level between 25 and 50 ka BP, but not during the lowest sea-level. Finally this study is based on only one seismic reflection line, offshore from the Changjiang River. Further work using a 3-D seismic survey and an intensive coring program will enable the determination of the precise route of the paleo-Changjiang River as well as elucidating the position of paleo-shorelines and the geometry of each unit related to the sea-level changes.
Acknowledgements We would like to thank A. Sato and K. Kawabe of Kokusai Kogyo Co. Ltd. for the operation of the seismic surveys, and N. Araki, S. Sato, M. Hanada and the crew of the R.V. Bosei-Maru for their support during the cruise. We thank Scott Nodder and an unknown reviewer for critical reviews of the manuscript and Bob Cater and Tim Naish for encouraging us to publish in this special volume, and for their helpful reviews. This study was done as part
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of the Marginal Sea Flux Experiment in the West Pacific (MASFLEX), supported by Special Coordination Funds for Promoting Science and Technology of the Science and Technology Agency of Japan.
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