- Email: [email protected]
Yangtze Shoal—a modern tidal sand sheet in the northwestern part of the East China Sea
ELSEVIER
Marine Geology 137 ( 1997) 321-330
Yangtze Shoal-a
modern tidal sand sheet in the northwestern part of the East China Sea Zhen Xia Liu
First Institute of Oceanography, SOA, Qingdao 266003, China
Received 4 July 1995; accepted 29 March 1996
Abstract
From an analysis of other’s data on tidal currents, bedforms, and sediment grain sizes, it is Yangtze Shoal in the northwestern East China Sea is a typical modem tidal sand sheet rather than submarine delta or a relict sedimentary accumulation on the continental shelf formed during postglacial transgressive sea. The Yangtze Shoal and the Jiangsu offshore tidal ridges of the northwest jointly comprise the active tidal geomorphic system off the Changjiang Estuary.
concluded that the a paleo-Changjiang a stillstand of the Subei Shoal to the
Keywords: East China Sea; Jiangsu tidal ridges; tidal sand sheet; Subei Shoal; Yangtze Shoal
1. Introduction The Yangtze Shoal lies east of the mouth of the Changjiang River (i.e., Yangtze River) (Fig. 1). The topographic and geomorphic characters and sediment types of this shoal differ from those of the surrounding sea area, so that the Yangtze Shoal has attracted much attention and led to different inferences about its formation. Because the sediments are dominated by fine sand, of which molluscan and foraminiferal fossils constitute 26%, and because the latter were much worn and stained by iron, it had been interpreted as the remains of the paleo-Changjiang River delta (Jin, 1992). Other workers considered it consisting of relict sands of the continental shelf or paleobeach sands, deposited during a stillstand of the postglacial transgressing sea, that have not been covered by modern sediments (Liu, 1987; Chen et al., 1986; Emery, 1968). In contrast, Li (1990) 0025-3227/97/$17.000 1997Elsevier Science B.V. All rights reserved PII SOO25-3227(96)00026-6
considered it together with the paleo-Yellow River delta in the north as a complex, paleo-submerged delta formed jointly by the paleo-Changjiang and paleo-Yellow Rivers, based on its irregular fanshaped relief (50 m contour) and the slight decline from northwest to southeast (see map of submarine geomorphology, scale 1:5,000,000 and illustration). Qing ( 1987) considered that the Yangtze Shoal is a submarine delta of the paleo-Changjiang River. They interpreted the Yangtze Shoal as the foreset of the submarine delta, a “relict prodelta sequence” composed of regressive sediments deposited at the end of the late Pleistocene (see Qin et al., 1987, pp. 134-135, fig. 113). In this scenario, the sediments were reworked during the transgression, but thought to retain their original depositional characters despite a transgression. Beginning in the early 198Os, a joint SinoAmerican study of the Changjiang River mouth and its adjacent continental shelf synthesized data
322
2.X.
122
Liu / Marine Geology 137 (1997) 321-330
124”
126” E
Fig. 1. The location and bathymetry of the study area as well as uniboomer profile-lines. Ehl-Eh6 Contours are depth in meters below sea level.
about the hydrology, depositional process, topography, and upper most strata. Although the tidalcurrent action in this area is very strong, it reformed only the deposits of the paleo-Changjiang River delta, but did not significantly alter or mask recognition of the relict deposits. Beginning in the early 1990s a joint Sino-French research on tidal-current deposition in the eastern part of the Bohai Sea produced a different hypothesis on the cause of formation, sediment supply, and age of formation of the Liaodong Shoal and Bozhong Shoal. The previous conclusion, that the Liaodong and Bozhong Shoals consist of relict sediments and relict geomorphology, was refuted by Liu et al. ( 1994). Therefore, doubts were raised about the proposed relict origin of sediments in shoals on the Chinese continental shelf to the south. The present study reanalyses the earlier data, especially the Sino-American data obtained off the Changjiang Estuary, and proposes a new interpretation of the formation of the Yangtze Shoal.
2. Tidal deposition on the continental the Holocene
shelf during
The relevant contents of the studies of tidalcurrent deposition on the continental shelf during
are current-measurement
stations.
the Holocene is briefly summarized. Tidal deposits accumulate through tidal-current action, and their sedimentary characteristics can reflect the properties of tidal-current dynamics. Since the Holocene transgression, tidal-current action has prevailed on the continental shelf, so that both tidal-current erosion and accumulation have been very active. These have formed longitudinal and transversal tidal bedforms of different dimensions as well as tidal shoals of various sizes. For the Bohai Sea, the Sino-French data show that the Liaodong Shoal and the Bozhong Shoal are the areas where the sediments are thick and the deposition is rapid, less than in the area of the submarine delta of the Yellow River (Liu et al., 1994). The two factors influencing tidal deposition are tidal-current action and the rate of sediment supply. The waters of the Bohai Sea, the Yellow Sea, and the East China Sea are wide and shallow, and tidal-current action is fairly strong. The sea floor is rich in loose sediments. Therefore, geomorphic features formed by tidal currents are varied, and tidal deposition is dominant. The tidal deposits on the continental shelf can be divided into a tidal sand-ridge (bank) facies and a tidal sand-sheet facies (Stride, 1982), each with a corresponding morphology. It is the particular geometry of the tidal sand ridges that initially
Z. X. Liu / Marine Geology 137 (1997) 321-330
aroused interest. However, it is only recently that the processes maintaining the tidal sand sheet were understood by Chinese workers. Both tidal sand ridge and tidal sand sheet depend on tidal-current properties. Generally speaking, when maximum tidal-current velocity is 2-3 knots (about 100-l 50 cm/s), tidal sand ridges often form under reciprocating current action (Liu and Xia, 1985; Xia and Liu, 1986). In contrast, the tidal sandsheet bedform often forms under rotating current action with a velocity of about 2 knots (about 100 cm/s). The absolute value of M2 tidal ellipticity that represents a value of the short axis vs. the long axis is less than 0.4 for the tidal sand ridges, and greater than 0.4 for the tidal sand sheets (Liu et al., 1997a). Sediment supply is also important. North of the study area, the eastern part of the Bohai Sea is a geomorphic system dominated by tidal currents. That area contains scour trenches of the Laotieshan Channel, tidal sand ridges of the Liaodong Shoal, and a tidal sand sheet of the Bozhong Shoal. The sediment of the shoals comes from the eroded Laotieshan Channel, where tidal currents are the main agent of erosion, with maximum speeds of 4-5 knots. The sediment constituting the sand ridges and the sand sheet is reworked Late Pleistocene sediments eroded from the Laotieshan Channel and redeposited in the shoals. Therefore, some of its original sedimentary characters are inherited. Nevertheless the depositional tidal processes forming the sand ridges and sand sheet have persisted since the mid-Holocene, so that the deposits are modem (Liu et al., 1997b).
3. Evidence that the Yangtze Shoal is an active offshore tidal sand sheet The Yangtze Shoal lies east of the Changjiang River mouth (Fig. 1). It extends from 122.5” to 125”E longitude and from 30.7” to 32.6”N latitude. It is 270 km wide E-W, 200 km long N-S, and occupies an area of about 30,000 km2. Water depth is from 25 to 55 m. Bottom sediments are dominated by sand. Through a synthesis of previous studies of tidal currents, bedforms, sediment grainsize distribution, and thickness of the Holocene
323
series, the author concludes that the Yangtze Shoal is a modem tidal sand sheet rather than relict paleobeach sands or relict sediments of the paleoChangjiang River delta submerged on the continental shelf by the postglacial rise of sea level. 3.1. Tidal-current action The Yangtze Shoal is affected by the Pacific tidal wave which transmits tidal energy from southeast to northwest and belongs to a progressive wave. Based on the current data of six stations (Fig. 1), the maximum tidal-current velocity of the bottom water is 55-70 cm/s in the NW-SE direction (Table 1). Ye et al. ( 1983) considered that the tidal-current direction west of 124”3O’E long. is NW-SE to NNW-SSE, whereas that east of 124”3O’E long. is NNW-SEE with a maximum current velocity of 1.0-2.4 knots. These velocities are enough to move fine sand and medium-fine sand which the Yangtze Shoal consist of and whose threshould velocities are about 20-30 cm/s. Because of the very strong rotation of the Yangtze Shoal tidal current, Tang (1989) thought that the sea area near 32”N, 124”E is an area of large tidal ellipticity of M, tidal component and that the tidal ellipsoid is nearly round with clockwise rotation and an absolute value of ellipticity of about 0.7-0.8 (Fig. 2). The present author thinks that the maximum current velocities and the ellipticity are favorable for the development of a tidal sand sheet. 3.2. Bedform features Based on the data of side-scan sonar, submarine photography, and echo sounding during the joint Sino-American study, the transverse bedforms formed by tidal currents are widespread throughout the Yangtze Shoal. They were named megaripples and sandwaves previously (Ye et al., 1983). According to the classification suggested at the SEPM meeting in Austin, Texas, USA in 1987 (Ashley, 1990), they are subaqueous dunes. These subaqueous dunes occupy a total area of about 20,000 km2. These dunes constitute two groups based on dimensions. One group consists of medium and small subaqueous dunes with crestline spacings of 2.3-13.6 m and their heights of
Z.X. Liu / Marine Geology 137 (1997) 321-330
324
Table 1 Maximum tidal-current velocities and types of bedforms in the Yangtze Shoal (from Ye et al., 1983) Station no.
Ehl Eh2 Eh3 Eh4 Eh5 Eh6
Water depth (m)
38 36 40 47 32 34
120
Bottom maximum ebb and flood current
Wmls)
Direction
70 65 58 55 60 65
NW-SE NNW-SSE NW-SE WNW-ESE NW-SE NNW-SSE
12P
Median diameter of sediments (mm)
0.1592 0.1919 0.1805 0.0842 0.2681 0.2179
124”
12~ -il
E
Bedform
Crestline
Direction
straight-crested medium and small dunes chequer-crested medium and small dunes branch-crested medium and small dunes honeycomb-crested medium and small dunes straight-crested medium and small dunes large and very large dunes
80” 80 80 irregular 90” 13”
120”
122
124”
126”
32”
30”
Fig. 2. Ellipsoid (A) of M2 tidal component and its ellipticity (II), based on the data of numerical modelling of the M, constituent on the East China Sea. Positive value represents anticlockwise rotation, whereas negative is clockwise rotation (from Tang, 1989).
0.20-0.97 m. Their crests are straight, branched, chequered, and honeycomb shaped (Table 1). The other group consists of large and very large subaqueous dunes with crest-line spacings of 70-1265 m and their heights of 0.5-2.6 m, and is distributed in a small area (Fig. 3). Locally, dunes of the first group are superimposed on dunes of the second group. Most of crest lines of subaqueous dunes are approximately perpendicular to main tidalcurrent direction, i. e., long-axial directions of tidal ellipses obtained by numerical modeling of the M2 constituent (Figs. 2 and 3).
The steep slopes of subaqueous dunes often reflect the direction of net sand transport (Ye et al., 1983; Liu et al., in ms). The steep slopes of dunes in the profile 31”45’N (from 123”44’E to 123”59’E) face mostly east; whereas those in the profile 123”16’E (from 31”24.5’N to 31”26.8’N) face mostly south, evidence that net transport direction of sands is toward the southeast (Ye et al., 1984). Based on two echo-sounding investigations along 32”N from 122”47’ to 122”57.5’E in June 1975 and from 122”50.5E to 123”01.8’E in June
Z.X. Liu 1 Marine Geology 137 (1997) 321-330
122”
325
126” E
124 I
I
l:
4
cl
Q5
I
30”N
Fig. 3. Distribution of subaqueous dunes. Legend: I = directions of the crests of dunes; 2 = directions of the bottom maximum ebb and flood tides; 3 = well-developed medium and small subaqueous dunes; 4 = partially developed medium and small subaqueous dunes; 5= large and very large subaqueous dunes (from Ye et al., 1983).
1980, 23 transverse subaqueous large and very large dunes migrated seaward 6.88 km in 5 years. Ye et al. ( 1984) estimated that the mean migration rate is 1375 m/a. Because of no bearing time on results, possible misidentification of individual large dunes, or growth of some dunes and diminishing of others, or navigation error, such high rates of translation may be incorrect. At least, the existence and migration of the subaqueous dunes is evidence supporting modern tidal-current action. Subaqueous dunes are transverse bedforms developed in fluvial, intertidal, and marine environments (Ashley, 1990). In consideration of strong tidal-current action, and actively migrating subaqueous dunes (scale: small/med/large/v. large) covering the Yangtze Shoal are product of tidal currents. Research on tidal sand ridges which extend approximately parallel to main tidal currents in the North Sea indicate that subaqueous dunes often are superimposed on the surface of active sand ridges, whereas there are generally no subaqueous dunes over moribund sand ridges (Belderson et al., 1982; Stride, 1982). Therefore, this criterion can be used to suggest that the Yangtze Shoal is an active sand sheet rather than a moribund sand sheet.
3.3. Sediment grain size Although the Yangtze Shoal is dominated by sand, the sediments range from local gravel and coarse sand to silty fine sand and clay-silt-sand (Fig. 4). Most of the shoal is well-sorted fine sand and medium-fine sand in which sand reaches 92% by weight. The content of sand in silty fine sand is 70% (Butenko et al., 1983). The median diameter of the bottom sediment of the six hydrological stations in Table 1 (see Fig. 1) ranges from 0.08 to 0.26 mm, basically very fine to fine sand (0.063-0.25 mm). The sediments contain remains of land mammals and of brackish-water and shallow-marine mollusks. Ages of the mollusks range from 4000 to 30,000 Cl4 years (Butenko et al., 1983; Qin and Zhen, 1982; Emery, 1969; Niino and Emery, 1961). Because the shells are broken and have undergone strong frictional erosion, these previous workers thought that sands of the Yangtze Shoal represent paleo-beach sands formed by waves during the postglacial period of lower sea level. The distribution map of bottom-sediment sizes (Fig. 4) shows the wide distribution of gravel (Li, 1990), which indicates a high-energy environment. In fact, both tide and wave environments
Z. X. Liu / Marine Geology 137 (1997) 321-330
326
32
6 7 llmll l!zzl
8 9
Fig. 4. Distribution of sediment sizes. Legend for lithologyic: I = gravel; 2 = medium-fine sand; 3 = fine sand; 4 = silty fine sand; 5 = clayey fine sand; 6= fine-sandy silt; 7= clayey silt; 8 = silty clay; 9= clay-silt-sand (from Li, 1990).
can have high energy. In the subsequent processes of tidal-current erosion, transportation, and redeposition, the sediments have undergone protracted washing and sorting, so that they are rather well-sorted, and the biological shells have undergone strong abrasion. In order to understand better the migration of bottom material, a sediment dynamic system (SDS) was set up at 31”29’N, 123”27’E (Eh2) in the relict sand area during the period of SinoAmerican joint investigation. The maximum current velocity 218 cm above the sea bottom, measured from 4 to 28 June 1980 was 53.2 cm/s with a net direction toward the north. Hence, the annual transport rate was calculated to be 54 t kn- l yr-’ (Sternberg et al., 1983, 1985) northward. This conclusion does not agree with the eastward direction of transport inferred by Ye et al. ( 1984) from bedform features in the same part of the Yangtze Shoal. Perhaps this difference results from differences in observation time and place. If so, the
migration direction of bottom sands was not fixed, and may vary due to lateral shifts in the position of strong tidal currents. Sands are seen as the most significant product of the offshore tidal-current environment (Stride, 1982, p. 8). Most tidal deposits of the Holocene are from the subbottom of the local or neighboring continental shelf. They are the Late Pleistocene sediments, reaccumulated after the tidal current erosion and transportation in a new marine environment. It has been proven that the Pleistocene sediments on the Northwestern European continental shelf were subjected to erosion, transportation and redeposition and became modem tidal deposits (Houbolt, 1968; Stride, 1982). The same case occurred also on the Bohai Sea (Liu et al., 1994). Thick Late Pleistocene loose sediments on the continental shelf of the Yellow Sea and the East China Sea provided enough material for the tidal current action. As the sediments of the Yang&e Shoal sheet were derived mainly from the
2.X. Liu 1 Marine Geology 137 (1997) 321-330
Late Pleistocene series, it is normal that the sediments could contain many sorts of fossils which do not correspond to the present environment and whose ages (from 4 to 30 ka B.P) should be a mixture of ancient and modern. This is similar to conditions on the Liaodong Shoal of the Bohai Sea where the ages of surficial sediments of tidal sand ridges range from 26 to 75 ka B.P. based on ESR dating. These ages reflect the time when the sediments were deposited in the Laotieshan Channel, prior to redeposition on the Liaodong Shoal (Liu et al., 1994). Because older sediments are again eroded, transported, and reaccumulated in a new environment, we cannot determine whether they are relict sediments or modem sediments, based on the absolute ages of the sediments. The basis for discriminating modem from older sediments can only be the time of final deposition. 3.4. Thickness of Holocene sediments Early researchers considered the Yangtze Shoal to be relict sediments on the continental shelf or a paleo-Changjiang submarine delta, with the surficial layers interpreted as the Pleistocene Series and
the thickness of the Holocene Series as minimal (Huang et al., 1985; Gao et al., 1982). The uniboomer records used in the Sino-American joint investigation revealed the presence of a large number of channels in a rough erosional surface buried under 8-25 m of sediment (Butenko et al., 1983). Judging from the interpretive sections of longitudinal and transverse profiles of the shallow stratigraphy (Figs. 5 and 6), the Holocene marine strata range from several to 20 m thick (thickest in the channels) but mostly range from 5-10 m in thickness. Butenko et al. (1983) could not explain why such thick Holocene strata exist in the relict sediment area, so they placed many question marks on their interpretive map. However, the interpretive map would be clear and reasonable if the Late Pleistocene strata were cut by rivers to form an undulating erosional surface, subsequently filled by fluvial and coastal sands as sea level rose in the early Holocene. A similar sequence is commonly observed in subbottom profile data from the eastern part of the Bohai Sea (Liu et al., 1994; Marsset et al., 1996). During the Holocene transgression, the sea submerged river courses, and the action of the
31 o
32” N
m
30°47’
Fig. 5. North-south
m
0
25-
-
321
uppar unit II
31”
32” mkrn
interpretive profiles based on uniboomer data. A. 122.5”E; B. 123.5”E (modified from Butenko et al., 1983).
Z.X. Liu / Marine Geology 137 (1997) 321-330
328
122O30’ m
122’30’
123” I
123”
124”
124”
125”
,
125” I I 0 I
126” I
1 1 I I 30 6okm I I
126”30’ E
126”
100’B Fig. 6. East-west interpretive profiles based on uniboomer data. A. 32”N; B. 31”N (modified from Butenko et al., 1983).
tidal currents reworked and redistributed the Pleistocene sediments to accumulate on the undulating erosional surface, to form Holocene deposits 8-25 m thick. Through the above synthesis of tidal dynamics, geomorphology, and sedimentation, it is likely that the Yangtze Shoal is an active offshore tidal sand sheet on the continental shelf.
4. Tidal geomorphic system off the Changjiang Estuary A complete tidal geomorphic system is often composed of several erosional and accumulative geomorphic units, such as in the eastern part of the Bohai Sea mentioned above. The tidal-current geomorphic system off the Changjiang Estuary is composed of the tidal sand sheet in the Yangtze Shoal and the tidal sand ridges off Jiangsu Province (i.e., Subei Shoal, Fig. 7). Although each has a different geometry, they have the same cause of formation and are affected by the Pacific tidalwave system from the southeast. In addition, the northern part of the Subei Shoal is still influenced by the rotating tidal wave in the western part
of the south Yellow Sea. These two groups of tidal waves meet approximately at a point off Qianggang Port. Influenced by both coastal outline and topography, the reciprocating tidal current increases. The direction of the reciprocating tidal current in the southern part of the Subei Shoal is mainly WNW-ESE, whereas that in the northern part of the Subei Shoal is mainly NNW-SSE and NNE-SSW. Sand ridges were developed in a radial orientation, approximately parallel to the main tidal-current direction (Liu et al., 1989). Viewed as a whole, the Yangtze Shoal and Subei Shoal are two tidal geomorphic units which are affected by the Pacific tidal-wave system.
5. Conclusions (1) In the Yangtze Shoal area, tidal-current action is strong, with a maximum velocity of 1.0-2.4 knots, and the tidal-current ellipsoid is nearly round, evidence that the tidal current has very strong rotation. (2) Subaqueous dunes of different dimensions are distributed widely on the surface of the Yangtze
2.X: Liu J Marine Geoiogy 137 (1997) 321330
329
30” Fig. 7. Tidal geomorphic system off the Changjiang Estuary.
Shoal, and are active bedforms formed by modern tidal currents. (3) In the Yangtze Shoal, sandy sediments are dominant, and their depositional characters reflect a high-energy environment. The sediments came from the Late Pleistocene series of the local and adjacent continental shelf. (4) According to the data from a uniboomer, the Holocene sediments of Yangtze Shoal are 8-25 m thick. (5) Yangtze Shoal is a typical active offshore tidal sand sheet, being neither a paleo-Changjiang River submarine delta nor relict sediments on the continental shelf. (6) The tidal sand sheet in Yangtze Shoal and the Jiangsu tidal sand ridges (Subei Shoal) to the northwest together constitute jointly the modern tidal geomorphic system off the Changjiang Estuary. They belong to the same tidal depositional environment created by the Pacific tidal-wave system.
Acknowledgements
The author wishes to thank Prof. R.Q. Oaks Jr. and Prof. A.H. Stride for their careful reading of the manuscript and valuable comments and revisions, which helped greatly to improve the manuscript.This work was funded by the National Natural Science Foundation of China.
References Ashley, G.M., 1990. Classification on large-scale subaqueous bedforms. J. Sediment. Petrol., 60(l): 160-172. Belderson, R.H., Johnson, M.A. and Kenyon, N.H., 1982. Bedforms. In: A.H. Stride (Editor), Offshore Tidal Sands Processes and Deposits, pp. 27-57. Butenko, J., Ye YinCa and Milliman, J.D., 1983. Morphology, sediments and Late Quaternary history of the East China Sea. Proc. Int. Symp. Sedimentation Cont. Shelf, East China Sea (SSCS), pp. 762-774. Chen ZhongYuan, Zhou ChangZhen, Yang WenDa and Wu
330
Z.X. Liu / Marine Geology 137 (1997) 321-330
ZhiGuo, 1986. Subaqueous topography and sediments off modern Changjiang estuary (in Chinese). Donghai Mar. Sci., 4(2): 28-37. Emery, K.O., 1968. Relict sediments on continental shelves of world. Bull. AAPG, 52(3): 445-464. Emery, K.O., 1969. Geological structure and some water characteristics of the East China Sea and Yellow Sea. ECAFE Tech. Bull., 2: 343. Gao MingDe, Mu GuangZhi, Han YouSong, Wang ShaoHong, Li ChengZhi and Zhang MingHan, 1982. The basic characteristics of the subaqueous delta of the Changjiang River. In: Geology of the Yellow Sea and the East China Sea. Science Press, Beijing, pp. 208-219 (in Chinese). Houbolt, J.J.H.C., 1968. Recent sediments in the Southern Bight of the North Sea. Geol. Mijnbouw, 47: 245-273. Huang HuiZhen, Shen BangPei and Wu QiangSheng, 1985. Geological implication of geophysical data in Holocene Changjiang subaqueous delta. Mar. Geol. Quat. Geol., 5(4): 81-94 (in Chinese). Jin XiangLong (Editor), 1992. Marine Geology of the East China Sea. China Ocean Press, p. 120, 194 (in Chinese). Li QueXing (Editor), 1990. Marine Atlas of Bohai Sea, Yellow Sea and East China Sea: Geology and Geophysics. China Ocean Press. Liu XiQing, 1987. Relict sediments in China continental shelf. Mar. Geol. Quat. Geol., 7(l): l-14 (in Chinese). Liu ZhenXia and Xia DongXing, 1985. A Preliminary study of tidal current ridges. Chin. J. Oceanol. Limnol. 3( 1):126-133. Liu ZhenXia, Huang YiChang and Zhang QiNian, 1989. Tidal current ridges in the southwestern Yellow Sea. J. Sediment. Petrol., 59(3): 432-437. Liu ZhenXia, Xia DongXing, Tang YuXiang, Wang KuiYang, S. Beme, T. Marsset and J.F. Bourillet, 1994. Tidal deposit system in the Eastern Bohai Sea in the Holocene. Sci. China (Ser. B), 24 (12): 1331-1338 (in Chinese). Liu ZhenXia, Beme, S., Wang KuiYang, Marsset, T., Xia DongXing, Bourillet, J.F. and Tang YuXiang, 1997a. Tidal bedfonns in the eastern part of the Bohai Sea. Mar. Geol., in press. Liu ZhenXia, Xia DongXing and Wang KuiYang, 1997b. Tidal depositional systems of China’s continental shelf. Mar Geol., in press.
Marsset, T., Xia, D., Beme, S., Liu, Z., Bourillet, J.F. and Wang, K., 1996. Stratigraphy and sedimentary environment since the Late Quatemary in the Eastern Bohai Sea. Mar. Geol., 135: 97-114. Niino, H. and Emery, K.O., 1961. Sediments of shallow portions of East China Sea and South China Sea. Geol. Sot. Am. Bull., 72: 731-762. Qin YunShan (Y.S. Chin) and Zhen TieMin, 1982. A study of distribution patterns of sediment on the continental shelf of the East China Sea. In: Geology of the Yellow Sea and the East China Sea. Science Press, Beijing, pp. 39- 51 (in Chinese). Qin YunShan, Zhao YiYang, Chen LiRong and Zhao SongLing (Editors), 1987. Geology of the East China Sea. Science Press, Beijing, pp. 134-135 (in Chinese). Stemberg, R.W., Larsen, L.H. and Miao YuTian, 1983. Near bottom flow conditions and associated sediment transport on the East China Sea continental shelf. Proc. Int. Symp. Sedimentation Cont. Shelf, East China Sea SSCS. China Ocean Press, pp. 522-535. Stemberg, R.W., Larsen, L.H. and Miao, Y.T., 1985. Tidally driven sediment transport on the East China Sea continental shelf. Cont. Shelf Res., 4( l/2): 105-120. Stride, A.H. (Editor), 1982. Offshore Tidal Sands Processes and Deposits. Chapman and Hall, London, p. 8, 95-126. Tang YuXiang, 1989. Numerical modeling of the M2 constituent in the East China Sea. Donghai Mar. Sci., 7(2): l-12 (in Chinese). Xia DongXing and Liu ZhenXia, 1986. Formation mechanism and developmental conditions of tidal current ridges. Acta Oceanol. Sin., 5(2): 247-255. Ye YinCan, Jin ChangMao, Lei ZhiYan and Chen XiTu, 1983. Bedform morphologies of the continental shelf off Changjiang River mouth and their environmental conditions. Proc. Int. Symp. Sedimentation Cont. Shelf, East China Sea SSCS, pp. 762-774. Ye YinCan, Song LingQing and Chen XiTu, 1984. An analysis of geotechnical hazards of the East China Sea floor. Donghai Mar. Sci., 2(3): 28-35 (in Chinese).
Marine Geology 137 ( 1997) 321-330
Yangtze Shoal-a
modern tidal sand sheet in the northwestern part of the East China Sea Zhen Xia Liu
First Institute of Oceanography, SOA, Qingdao 266003, China
Received 4 July 1995; accepted 29 March 1996
Abstract
From an analysis of other’s data on tidal currents, bedforms, and sediment grain sizes, it is Yangtze Shoal in the northwestern East China Sea is a typical modem tidal sand sheet rather than submarine delta or a relict sedimentary accumulation on the continental shelf formed during postglacial transgressive sea. The Yangtze Shoal and the Jiangsu offshore tidal ridges of the northwest jointly comprise the active tidal geomorphic system off the Changjiang Estuary.
concluded that the a paleo-Changjiang a stillstand of the Subei Shoal to the
Keywords: East China Sea; Jiangsu tidal ridges; tidal sand sheet; Subei Shoal; Yangtze Shoal
1. Introduction The Yangtze Shoal lies east of the mouth of the Changjiang River (i.e., Yangtze River) (Fig. 1). The topographic and geomorphic characters and sediment types of this shoal differ from those of the surrounding sea area, so that the Yangtze Shoal has attracted much attention and led to different inferences about its formation. Because the sediments are dominated by fine sand, of which molluscan and foraminiferal fossils constitute 26%, and because the latter were much worn and stained by iron, it had been interpreted as the remains of the paleo-Changjiang River delta (Jin, 1992). Other workers considered it consisting of relict sands of the continental shelf or paleobeach sands, deposited during a stillstand of the postglacial transgressing sea, that have not been covered by modern sediments (Liu, 1987; Chen et al., 1986; Emery, 1968). In contrast, Li (1990) 0025-3227/97/$17.000 1997Elsevier Science B.V. All rights reserved PII SOO25-3227(96)00026-6
considered it together with the paleo-Yellow River delta in the north as a complex, paleo-submerged delta formed jointly by the paleo-Changjiang and paleo-Yellow Rivers, based on its irregular fanshaped relief (50 m contour) and the slight decline from northwest to southeast (see map of submarine geomorphology, scale 1:5,000,000 and illustration). Qing ( 1987) considered that the Yangtze Shoal is a submarine delta of the paleo-Changjiang River. They interpreted the Yangtze Shoal as the foreset of the submarine delta, a “relict prodelta sequence” composed of regressive sediments deposited at the end of the late Pleistocene (see Qin et al., 1987, pp. 134-135, fig. 113). In this scenario, the sediments were reworked during the transgression, but thought to retain their original depositional characters despite a transgression. Beginning in the early 198Os, a joint SinoAmerican study of the Changjiang River mouth and its adjacent continental shelf synthesized data
322
2.X.
122
Liu / Marine Geology 137 (1997) 321-330
124”
126” E
Fig. 1. The location and bathymetry of the study area as well as uniboomer profile-lines. Ehl-Eh6 Contours are depth in meters below sea level.
about the hydrology, depositional process, topography, and upper most strata. Although the tidalcurrent action in this area is very strong, it reformed only the deposits of the paleo-Changjiang River delta, but did not significantly alter or mask recognition of the relict deposits. Beginning in the early 1990s a joint Sino-French research on tidal-current deposition in the eastern part of the Bohai Sea produced a different hypothesis on the cause of formation, sediment supply, and age of formation of the Liaodong Shoal and Bozhong Shoal. The previous conclusion, that the Liaodong and Bozhong Shoals consist of relict sediments and relict geomorphology, was refuted by Liu et al. ( 1994). Therefore, doubts were raised about the proposed relict origin of sediments in shoals on the Chinese continental shelf to the south. The present study reanalyses the earlier data, especially the Sino-American data obtained off the Changjiang Estuary, and proposes a new interpretation of the formation of the Yangtze Shoal.
2. Tidal deposition on the continental the Holocene
shelf during
The relevant contents of the studies of tidalcurrent deposition on the continental shelf during
are current-measurement
stations.
the Holocene is briefly summarized. Tidal deposits accumulate through tidal-current action, and their sedimentary characteristics can reflect the properties of tidal-current dynamics. Since the Holocene transgression, tidal-current action has prevailed on the continental shelf, so that both tidal-current erosion and accumulation have been very active. These have formed longitudinal and transversal tidal bedforms of different dimensions as well as tidal shoals of various sizes. For the Bohai Sea, the Sino-French data show that the Liaodong Shoal and the Bozhong Shoal are the areas where the sediments are thick and the deposition is rapid, less than in the area of the submarine delta of the Yellow River (Liu et al., 1994). The two factors influencing tidal deposition are tidal-current action and the rate of sediment supply. The waters of the Bohai Sea, the Yellow Sea, and the East China Sea are wide and shallow, and tidal-current action is fairly strong. The sea floor is rich in loose sediments. Therefore, geomorphic features formed by tidal currents are varied, and tidal deposition is dominant. The tidal deposits on the continental shelf can be divided into a tidal sand-ridge (bank) facies and a tidal sand-sheet facies (Stride, 1982), each with a corresponding morphology. It is the particular geometry of the tidal sand ridges that initially
Z. X. Liu / Marine Geology 137 (1997) 321-330
aroused interest. However, it is only recently that the processes maintaining the tidal sand sheet were understood by Chinese workers. Both tidal sand ridge and tidal sand sheet depend on tidal-current properties. Generally speaking, when maximum tidal-current velocity is 2-3 knots (about 100-l 50 cm/s), tidal sand ridges often form under reciprocating current action (Liu and Xia, 1985; Xia and Liu, 1986). In contrast, the tidal sandsheet bedform often forms under rotating current action with a velocity of about 2 knots (about 100 cm/s). The absolute value of M2 tidal ellipticity that represents a value of the short axis vs. the long axis is less than 0.4 for the tidal sand ridges, and greater than 0.4 for the tidal sand sheets (Liu et al., 1997a). Sediment supply is also important. North of the study area, the eastern part of the Bohai Sea is a geomorphic system dominated by tidal currents. That area contains scour trenches of the Laotieshan Channel, tidal sand ridges of the Liaodong Shoal, and a tidal sand sheet of the Bozhong Shoal. The sediment of the shoals comes from the eroded Laotieshan Channel, where tidal currents are the main agent of erosion, with maximum speeds of 4-5 knots. The sediment constituting the sand ridges and the sand sheet is reworked Late Pleistocene sediments eroded from the Laotieshan Channel and redeposited in the shoals. Therefore, some of its original sedimentary characters are inherited. Nevertheless the depositional tidal processes forming the sand ridges and sand sheet have persisted since the mid-Holocene, so that the deposits are modem (Liu et al., 1997b).
3. Evidence that the Yangtze Shoal is an active offshore tidal sand sheet The Yangtze Shoal lies east of the Changjiang River mouth (Fig. 1). It extends from 122.5” to 125”E longitude and from 30.7” to 32.6”N latitude. It is 270 km wide E-W, 200 km long N-S, and occupies an area of about 30,000 km2. Water depth is from 25 to 55 m. Bottom sediments are dominated by sand. Through a synthesis of previous studies of tidal currents, bedforms, sediment grainsize distribution, and thickness of the Holocene
323
series, the author concludes that the Yangtze Shoal is a modem tidal sand sheet rather than relict paleobeach sands or relict sediments of the paleoChangjiang River delta submerged on the continental shelf by the postglacial rise of sea level. 3.1. Tidal-current action The Yangtze Shoal is affected by the Pacific tidal wave which transmits tidal energy from southeast to northwest and belongs to a progressive wave. Based on the current data of six stations (Fig. 1), the maximum tidal-current velocity of the bottom water is 55-70 cm/s in the NW-SE direction (Table 1). Ye et al. ( 1983) considered that the tidal-current direction west of 124”3O’E long. is NW-SE to NNW-SSE, whereas that east of 124”3O’E long. is NNW-SEE with a maximum current velocity of 1.0-2.4 knots. These velocities are enough to move fine sand and medium-fine sand which the Yangtze Shoal consist of and whose threshould velocities are about 20-30 cm/s. Because of the very strong rotation of the Yangtze Shoal tidal current, Tang (1989) thought that the sea area near 32”N, 124”E is an area of large tidal ellipticity of M, tidal component and that the tidal ellipsoid is nearly round with clockwise rotation and an absolute value of ellipticity of about 0.7-0.8 (Fig. 2). The present author thinks that the maximum current velocities and the ellipticity are favorable for the development of a tidal sand sheet. 3.2. Bedform features Based on the data of side-scan sonar, submarine photography, and echo sounding during the joint Sino-American study, the transverse bedforms formed by tidal currents are widespread throughout the Yangtze Shoal. They were named megaripples and sandwaves previously (Ye et al., 1983). According to the classification suggested at the SEPM meeting in Austin, Texas, USA in 1987 (Ashley, 1990), they are subaqueous dunes. These subaqueous dunes occupy a total area of about 20,000 km2. These dunes constitute two groups based on dimensions. One group consists of medium and small subaqueous dunes with crestline spacings of 2.3-13.6 m and their heights of
Z.X. Liu / Marine Geology 137 (1997) 321-330
324
Table 1 Maximum tidal-current velocities and types of bedforms in the Yangtze Shoal (from Ye et al., 1983) Station no.
Ehl Eh2 Eh3 Eh4 Eh5 Eh6
Water depth (m)
38 36 40 47 32 34
120
Bottom maximum ebb and flood current
Wmls)
Direction
70 65 58 55 60 65
NW-SE NNW-SSE NW-SE WNW-ESE NW-SE NNW-SSE
12P
Median diameter of sediments (mm)
0.1592 0.1919 0.1805 0.0842 0.2681 0.2179
124”
12~ -il
E
Bedform
Crestline
Direction
straight-crested medium and small dunes chequer-crested medium and small dunes branch-crested medium and small dunes honeycomb-crested medium and small dunes straight-crested medium and small dunes large and very large dunes
80” 80 80 irregular 90” 13”
120”
122
124”
126”
32”
30”
Fig. 2. Ellipsoid (A) of M2 tidal component and its ellipticity (II), based on the data of numerical modelling of the M, constituent on the East China Sea. Positive value represents anticlockwise rotation, whereas negative is clockwise rotation (from Tang, 1989).
0.20-0.97 m. Their crests are straight, branched, chequered, and honeycomb shaped (Table 1). The other group consists of large and very large subaqueous dunes with crest-line spacings of 70-1265 m and their heights of 0.5-2.6 m, and is distributed in a small area (Fig. 3). Locally, dunes of the first group are superimposed on dunes of the second group. Most of crest lines of subaqueous dunes are approximately perpendicular to main tidalcurrent direction, i. e., long-axial directions of tidal ellipses obtained by numerical modeling of the M2 constituent (Figs. 2 and 3).
The steep slopes of subaqueous dunes often reflect the direction of net sand transport (Ye et al., 1983; Liu et al., in ms). The steep slopes of dunes in the profile 31”45’N (from 123”44’E to 123”59’E) face mostly east; whereas those in the profile 123”16’E (from 31”24.5’N to 31”26.8’N) face mostly south, evidence that net transport direction of sands is toward the southeast (Ye et al., 1984). Based on two echo-sounding investigations along 32”N from 122”47’ to 122”57.5’E in June 1975 and from 122”50.5E to 123”01.8’E in June
Z.X. Liu 1 Marine Geology 137 (1997) 321-330
122”
325
126” E
124 I
I
l:
4
cl
Q5
I
30”N
Fig. 3. Distribution of subaqueous dunes. Legend: I = directions of the crests of dunes; 2 = directions of the bottom maximum ebb and flood tides; 3 = well-developed medium and small subaqueous dunes; 4 = partially developed medium and small subaqueous dunes; 5= large and very large subaqueous dunes (from Ye et al., 1983).
1980, 23 transverse subaqueous large and very large dunes migrated seaward 6.88 km in 5 years. Ye et al. ( 1984) estimated that the mean migration rate is 1375 m/a. Because of no bearing time on results, possible misidentification of individual large dunes, or growth of some dunes and diminishing of others, or navigation error, such high rates of translation may be incorrect. At least, the existence and migration of the subaqueous dunes is evidence supporting modern tidal-current action. Subaqueous dunes are transverse bedforms developed in fluvial, intertidal, and marine environments (Ashley, 1990). In consideration of strong tidal-current action, and actively migrating subaqueous dunes (scale: small/med/large/v. large) covering the Yangtze Shoal are product of tidal currents. Research on tidal sand ridges which extend approximately parallel to main tidal currents in the North Sea indicate that subaqueous dunes often are superimposed on the surface of active sand ridges, whereas there are generally no subaqueous dunes over moribund sand ridges (Belderson et al., 1982; Stride, 1982). Therefore, this criterion can be used to suggest that the Yangtze Shoal is an active sand sheet rather than a moribund sand sheet.
3.3. Sediment grain size Although the Yangtze Shoal is dominated by sand, the sediments range from local gravel and coarse sand to silty fine sand and clay-silt-sand (Fig. 4). Most of the shoal is well-sorted fine sand and medium-fine sand in which sand reaches 92% by weight. The content of sand in silty fine sand is 70% (Butenko et al., 1983). The median diameter of the bottom sediment of the six hydrological stations in Table 1 (see Fig. 1) ranges from 0.08 to 0.26 mm, basically very fine to fine sand (0.063-0.25 mm). The sediments contain remains of land mammals and of brackish-water and shallow-marine mollusks. Ages of the mollusks range from 4000 to 30,000 Cl4 years (Butenko et al., 1983; Qin and Zhen, 1982; Emery, 1969; Niino and Emery, 1961). Because the shells are broken and have undergone strong frictional erosion, these previous workers thought that sands of the Yangtze Shoal represent paleo-beach sands formed by waves during the postglacial period of lower sea level. The distribution map of bottom-sediment sizes (Fig. 4) shows the wide distribution of gravel (Li, 1990), which indicates a high-energy environment. In fact, both tide and wave environments
Z. X. Liu / Marine Geology 137 (1997) 321-330
326
32
6 7 llmll l!zzl
8 9
Fig. 4. Distribution of sediment sizes. Legend for lithologyic: I = gravel; 2 = medium-fine sand; 3 = fine sand; 4 = silty fine sand; 5 = clayey fine sand; 6= fine-sandy silt; 7= clayey silt; 8 = silty clay; 9= clay-silt-sand (from Li, 1990).
can have high energy. In the subsequent processes of tidal-current erosion, transportation, and redeposition, the sediments have undergone protracted washing and sorting, so that they are rather well-sorted, and the biological shells have undergone strong abrasion. In order to understand better the migration of bottom material, a sediment dynamic system (SDS) was set up at 31”29’N, 123”27’E (Eh2) in the relict sand area during the period of SinoAmerican joint investigation. The maximum current velocity 218 cm above the sea bottom, measured from 4 to 28 June 1980 was 53.2 cm/s with a net direction toward the north. Hence, the annual transport rate was calculated to be 54 t kn- l yr-’ (Sternberg et al., 1983, 1985) northward. This conclusion does not agree with the eastward direction of transport inferred by Ye et al. ( 1984) from bedform features in the same part of the Yangtze Shoal. Perhaps this difference results from differences in observation time and place. If so, the
migration direction of bottom sands was not fixed, and may vary due to lateral shifts in the position of strong tidal currents. Sands are seen as the most significant product of the offshore tidal-current environment (Stride, 1982, p. 8). Most tidal deposits of the Holocene are from the subbottom of the local or neighboring continental shelf. They are the Late Pleistocene sediments, reaccumulated after the tidal current erosion and transportation in a new marine environment. It has been proven that the Pleistocene sediments on the Northwestern European continental shelf were subjected to erosion, transportation and redeposition and became modem tidal deposits (Houbolt, 1968; Stride, 1982). The same case occurred also on the Bohai Sea (Liu et al., 1994). Thick Late Pleistocene loose sediments on the continental shelf of the Yellow Sea and the East China Sea provided enough material for the tidal current action. As the sediments of the Yang&e Shoal sheet were derived mainly from the
2.X. Liu 1 Marine Geology 137 (1997) 321-330
Late Pleistocene series, it is normal that the sediments could contain many sorts of fossils which do not correspond to the present environment and whose ages (from 4 to 30 ka B.P) should be a mixture of ancient and modern. This is similar to conditions on the Liaodong Shoal of the Bohai Sea where the ages of surficial sediments of tidal sand ridges range from 26 to 75 ka B.P. based on ESR dating. These ages reflect the time when the sediments were deposited in the Laotieshan Channel, prior to redeposition on the Liaodong Shoal (Liu et al., 1994). Because older sediments are again eroded, transported, and reaccumulated in a new environment, we cannot determine whether they are relict sediments or modem sediments, based on the absolute ages of the sediments. The basis for discriminating modem from older sediments can only be the time of final deposition. 3.4. Thickness of Holocene sediments Early researchers considered the Yangtze Shoal to be relict sediments on the continental shelf or a paleo-Changjiang submarine delta, with the surficial layers interpreted as the Pleistocene Series and
the thickness of the Holocene Series as minimal (Huang et al., 1985; Gao et al., 1982). The uniboomer records used in the Sino-American joint investigation revealed the presence of a large number of channels in a rough erosional surface buried under 8-25 m of sediment (Butenko et al., 1983). Judging from the interpretive sections of longitudinal and transverse profiles of the shallow stratigraphy (Figs. 5 and 6), the Holocene marine strata range from several to 20 m thick (thickest in the channels) but mostly range from 5-10 m in thickness. Butenko et al. (1983) could not explain why such thick Holocene strata exist in the relict sediment area, so they placed many question marks on their interpretive map. However, the interpretive map would be clear and reasonable if the Late Pleistocene strata were cut by rivers to form an undulating erosional surface, subsequently filled by fluvial and coastal sands as sea level rose in the early Holocene. A similar sequence is commonly observed in subbottom profile data from the eastern part of the Bohai Sea (Liu et al., 1994; Marsset et al., 1996). During the Holocene transgression, the sea submerged river courses, and the action of the
31 o
32” N
m
30°47’
Fig. 5. North-south
m
0
25-
-
321
uppar unit II
31”
32” mkrn
interpretive profiles based on uniboomer data. A. 122.5”E; B. 123.5”E (modified from Butenko et al., 1983).
Z.X. Liu / Marine Geology 137 (1997) 321-330
328
122O30’ m
122’30’
123” I
123”
124”
124”
125”
,
125” I I 0 I
126” I
1 1 I I 30 6okm I I
126”30’ E
126”
100’B Fig. 6. East-west interpretive profiles based on uniboomer data. A. 32”N; B. 31”N (modified from Butenko et al., 1983).
tidal currents reworked and redistributed the Pleistocene sediments to accumulate on the undulating erosional surface, to form Holocene deposits 8-25 m thick. Through the above synthesis of tidal dynamics, geomorphology, and sedimentation, it is likely that the Yangtze Shoal is an active offshore tidal sand sheet on the continental shelf.
4. Tidal geomorphic system off the Changjiang Estuary A complete tidal geomorphic system is often composed of several erosional and accumulative geomorphic units, such as in the eastern part of the Bohai Sea mentioned above. The tidal-current geomorphic system off the Changjiang Estuary is composed of the tidal sand sheet in the Yangtze Shoal and the tidal sand ridges off Jiangsu Province (i.e., Subei Shoal, Fig. 7). Although each has a different geometry, they have the same cause of formation and are affected by the Pacific tidalwave system from the southeast. In addition, the northern part of the Subei Shoal is still influenced by the rotating tidal wave in the western part
of the south Yellow Sea. These two groups of tidal waves meet approximately at a point off Qianggang Port. Influenced by both coastal outline and topography, the reciprocating tidal current increases. The direction of the reciprocating tidal current in the southern part of the Subei Shoal is mainly WNW-ESE, whereas that in the northern part of the Subei Shoal is mainly NNW-SSE and NNE-SSW. Sand ridges were developed in a radial orientation, approximately parallel to the main tidal-current direction (Liu et al., 1989). Viewed as a whole, the Yangtze Shoal and Subei Shoal are two tidal geomorphic units which are affected by the Pacific tidal-wave system.
5. Conclusions (1) In the Yangtze Shoal area, tidal-current action is strong, with a maximum velocity of 1.0-2.4 knots, and the tidal-current ellipsoid is nearly round, evidence that the tidal current has very strong rotation. (2) Subaqueous dunes of different dimensions are distributed widely on the surface of the Yangtze
2.X: Liu J Marine Geoiogy 137 (1997) 321330
329
30” Fig. 7. Tidal geomorphic system off the Changjiang Estuary.
Shoal, and are active bedforms formed by modern tidal currents. (3) In the Yangtze Shoal, sandy sediments are dominant, and their depositional characters reflect a high-energy environment. The sediments came from the Late Pleistocene series of the local and adjacent continental shelf. (4) According to the data from a uniboomer, the Holocene sediments of Yangtze Shoal are 8-25 m thick. (5) Yangtze Shoal is a typical active offshore tidal sand sheet, being neither a paleo-Changjiang River submarine delta nor relict sediments on the continental shelf. (6) The tidal sand sheet in Yangtze Shoal and the Jiangsu tidal sand ridges (Subei Shoal) to the northwest together constitute jointly the modern tidal geomorphic system off the Changjiang Estuary. They belong to the same tidal depositional environment created by the Pacific tidal-wave system.
Acknowledgements
The author wishes to thank Prof. R.Q. Oaks Jr. and Prof. A.H. Stride for their careful reading of the manuscript and valuable comments and revisions, which helped greatly to improve the manuscript.This work was funded by the National Natural Science Foundation of China.
References Ashley, G.M., 1990. Classification on large-scale subaqueous bedforms. J. Sediment. Petrol., 60(l): 160-172. Belderson, R.H., Johnson, M.A. and Kenyon, N.H., 1982. Bedforms. In: A.H. Stride (Editor), Offshore Tidal Sands Processes and Deposits, pp. 27-57. Butenko, J., Ye YinCa and Milliman, J.D., 1983. Morphology, sediments and Late Quaternary history of the East China Sea. Proc. Int. Symp. Sedimentation Cont. Shelf, East China Sea (SSCS), pp. 762-774. Chen ZhongYuan, Zhou ChangZhen, Yang WenDa and Wu
330
Z.X. Liu / Marine Geology 137 (1997) 321-330
ZhiGuo, 1986. Subaqueous topography and sediments off modern Changjiang estuary (in Chinese). Donghai Mar. Sci., 4(2): 28-37. Emery, K.O., 1968. Relict sediments on continental shelves of world. Bull. AAPG, 52(3): 445-464. Emery, K.O., 1969. Geological structure and some water characteristics of the East China Sea and Yellow Sea. ECAFE Tech. Bull., 2: 343. Gao MingDe, Mu GuangZhi, Han YouSong, Wang ShaoHong, Li ChengZhi and Zhang MingHan, 1982. The basic characteristics of the subaqueous delta of the Changjiang River. In: Geology of the Yellow Sea and the East China Sea. Science Press, Beijing, pp. 208-219 (in Chinese). Houbolt, J.J.H.C., 1968. Recent sediments in the Southern Bight of the North Sea. Geol. Mijnbouw, 47: 245-273. Huang HuiZhen, Shen BangPei and Wu QiangSheng, 1985. Geological implication of geophysical data in Holocene Changjiang subaqueous delta. Mar. Geol. Quat. Geol., 5(4): 81-94 (in Chinese). Jin XiangLong (Editor), 1992. Marine Geology of the East China Sea. China Ocean Press, p. 120, 194 (in Chinese). Li QueXing (Editor), 1990. Marine Atlas of Bohai Sea, Yellow Sea and East China Sea: Geology and Geophysics. China Ocean Press. Liu XiQing, 1987. Relict sediments in China continental shelf. Mar. Geol. Quat. Geol., 7(l): l-14 (in Chinese). Liu ZhenXia and Xia DongXing, 1985. A Preliminary study of tidal current ridges. Chin. J. Oceanol. Limnol. 3( 1):126-133. Liu ZhenXia, Huang YiChang and Zhang QiNian, 1989. Tidal current ridges in the southwestern Yellow Sea. J. Sediment. Petrol., 59(3): 432-437. Liu ZhenXia, Xia DongXing, Tang YuXiang, Wang KuiYang, S. Beme, T. Marsset and J.F. Bourillet, 1994. Tidal deposit system in the Eastern Bohai Sea in the Holocene. Sci. China (Ser. B), 24 (12): 1331-1338 (in Chinese). Liu ZhenXia, Beme, S., Wang KuiYang, Marsset, T., Xia DongXing, Bourillet, J.F. and Tang YuXiang, 1997a. Tidal bedfonns in the eastern part of the Bohai Sea. Mar. Geol., in press. Liu ZhenXia, Xia DongXing and Wang KuiYang, 1997b. Tidal depositional systems of China’s continental shelf. Mar Geol., in press.
Marsset, T., Xia, D., Beme, S., Liu, Z., Bourillet, J.F. and Wang, K., 1996. Stratigraphy and sedimentary environment since the Late Quatemary in the Eastern Bohai Sea. Mar. Geol., 135: 97-114. Niino, H. and Emery, K.O., 1961. Sediments of shallow portions of East China Sea and South China Sea. Geol. Sot. Am. Bull., 72: 731-762. Qin YunShan (Y.S. Chin) and Zhen TieMin, 1982. A study of distribution patterns of sediment on the continental shelf of the East China Sea. In: Geology of the Yellow Sea and the East China Sea. Science Press, Beijing, pp. 39- 51 (in Chinese). Qin YunShan, Zhao YiYang, Chen LiRong and Zhao SongLing (Editors), 1987. Geology of the East China Sea. Science Press, Beijing, pp. 134-135 (in Chinese). Stemberg, R.W., Larsen, L.H. and Miao YuTian, 1983. Near bottom flow conditions and associated sediment transport on the East China Sea continental shelf. Proc. Int. Symp. Sedimentation Cont. Shelf, East China Sea SSCS. China Ocean Press, pp. 522-535. Stemberg, R.W., Larsen, L.H. and Miao, Y.T., 1985. Tidally driven sediment transport on the East China Sea continental shelf. Cont. Shelf Res., 4( l/2): 105-120. Stride, A.H. (Editor), 1982. Offshore Tidal Sands Processes and Deposits. Chapman and Hall, London, p. 8, 95-126. Tang YuXiang, 1989. Numerical modeling of the M2 constituent in the East China Sea. Donghai Mar. Sci., 7(2): l-12 (in Chinese). Xia DongXing and Liu ZhenXia, 1986. Formation mechanism and developmental conditions of tidal current ridges. Acta Oceanol. Sin., 5(2): 247-255. Ye YinCan, Jin ChangMao, Lei ZhiYan and Chen XiTu, 1983. Bedform morphologies of the continental shelf off Changjiang River mouth and their environmental conditions. Proc. Int. Symp. Sedimentation Cont. Shelf, East China Sea SSCS, pp. 762-774. Ye YinCan, Song LingQing and Chen XiTu, 1984. An analysis of geotechnical hazards of the East China Sea floor. Donghai Mar. Sci., 2(3): 28-35 (in Chinese).