Sedimentation on the north shelf of the East China Sea

Marine Geology, 81 (1988) 123-136 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

123

SEDIMENTATION ON THE NORTH SHELF OF THE EAST CHINA SEA E R Q I N Z H U a n d QI W A N G

Shandong Collegeof Oceanography, Qingdao (P.R. China) (Received September 8, 1986; revised and accepted October 30, 1987)

Abstract Zhu, E. and Wang, Q., 1988. Sedimentation on the north shelf of the East China Sea. Mar. Geol., 81: 123-136. A reconnaissance study has determined the overall pattern of sedimentation on the northern continental shelf of the East China Sea. In the study area, modern and Pleistocene terrigenous sediments and bioclastic accumulations dominate, but biochemical and volcanic deposits are locally developed. The northern shelf shows the following sedimentary divisions: (1) A modern terrigenous subdivision, including the nearshore and Hupijiao muddy subdivisions, (2) the relict Changjiang River delta (inner shelf), and (3) a palimpsest subdivision, including two subdivisions which differ in terms of the effects of modern biogenic and terrigenous sedimentation (middle and outer shelf). These divisions reflect the variation in geological history, sediment supply, hydrodynamic conditions and biological productivity on the shelf.

Introduction F o u r e x t e n s i v e cruises in the n o r t h e r n E a s t C h i n a Sea w e r e c a r r i e d o u t by t h e R.V. Dongfanghong in 1981 a n d 1982. This p a p e r describes r e s u l t s b a s e d on t h e surficial sedim e n t s a m p l e s dredged d u r i n g t h e s e cruises. T h e E a s t C h i n a S e a is a m a r g i n a l sea w i t h a n a r e a of 7.52 x 105 k m : . T h e c o n t i n e n t a l s h e l f o c c u p i e s the n o r t h e r n two thirds of the sea, a n d slopes v e r y g e n t l y s e a w a r d to a d e p t h of 150-160 m at the o u t e r edge (the shelf-break). T h e i n n e r s h e l f lies w e s t of t h e 50 m isobath. Here, n o r t h o f t h e C h a n g j i a n g (Yangtze) R i v e r m o u t h , the relict C h a n g j i a n g R i v e r d e l t a is t h e d o m i n a n t g e o m o r p h i c u n i t a n d t h e 50 m isobath marks the approximate seaward boundary of the d e l t a (Fig.l). T h e middle s h e l f lies b e t w e e n a p p r o x i m a t e l y 50 a n d 100 m a n d the o u t e r s h e l f lies e a s t o f t h e 1 0 0 m isobath. S e v e r a l s u b m a r i n e t e r r a c e s a r e p r e s e n t on the middle a n d o u t e r shelves. 0025-3227/88/$03.50

S e v e r a l studies of t h e s e d i m e n t s of the E a s t C h i n a Sea h a v e b e e n m a d e (Shepard, 1932; S h e p a r d et al., 1949; N i i n o a n d E m e r y , 1961; Qin, 1963; Zhu, 1983a, 1986a). T h e s e d i m e n t t y p e s and d i s t r i b u t i o n d i a g r a m s w e r e given, a n d t h e possible s o u r c e a r e a s of the s e d i m e n t s w e r e analyzed. Based on t h e s e e a r l i e r res e a r c h e s a n d the p r e s e n t study, this p a p e r will discuss the t y p e s a n d d i s t r i b u t i o n s of the surficial sediments, a n d t h e i m p l i c a t i o n s for s e d i m e n t a t i o n on the n o r t h p a r t of t h e E a s t C h i n a Sea shelf.

Sedimentation T h e s e d i m e n t s in t h e n o r t h p a r t of the E a s t C h i n a Sea s h e l f a r e of different o r i g i n s a n d include t e r r i g e n o u s detritus, bioclasts, authigenic c o m p o n e n t s a n d v o l c a n i c debris. T h e f o r m e r t w o a c c o u n t for 95% o r m o r e of the t o t a l volume, w h e r e a s the l a t t e r t w o a r e m u c h less in a m o u n t , b u t a r e c o n c e n t r a t e d locally.

© 1988 Elsevier Science Publishers B.V.

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Fig.1. Types of surficial sediments on t h e n o r t h shelf of the East C h i n a Sea.

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According to the relative contents of the terrigenous and biogenic clastics, the surficial sediments may be classified as follows: terrigenous sediments (CaCO3 < 10 wt.%; the carbonate content may principally represent bioclasts), terrigenous-bioclastic sediments (1025 wt.% CaCO3) and bioclastic-terrigenous sediments (25-50 wt.% CaCO3). All these types may further be subdivided into sand-silt and clay-types. The terrigenous sediments principally consist of silty clay and fine sand and are generally distributed on the inner shelf. The terrigenous-bioclastic sediments are made up of silty clay, muddy sand and fine sand, and are most common on the middle shelf but locally extend across the outer shelf. The bioclastic-terrigenous sediments are mainly of foraminiferal sands and are found on the outer shelf and beyond the shelf break (Fig.l).

Terrigenous sedimentation The distribution of textural types of terrigenous sediments in the study area is unusual, i.e., silt and clay occur on the inner shelf and medium-fine sand occurs on the outer part of the inner shelf and on the middle shelf (Fig.l). Based on the time of formation, these terrigenous sediments may be divided into modern and relict sediments, wherein the relict sediments were chiefly formed in the Pleistocene. The modern sediments have come to textural equilibrium with their present hydrodynamic environment, although the relict sediments have not yet reached this equilibrium. Thus, the modern influx of muddy terrigenous sediment supplied by the Changjiang River is mainly restricted to the nearshore an d ~ extensive shelf area to the east is still dominated by the relict Pleistocene terrigenous deposits with subordinate accumulations of recent suspended matter. Of the terrigenous sediments, the sand and silt-size fractions consist dominantly of quartz (52-79%), with somewhat less feldspar (16-34%) and few (1-17%) lithic fragments from volcanic and low-grade metamorphic rocks (Fig.2). Clay minerals are the major

Q 95

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F

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Fig.2. Compositional diagram of terrigenous components of the sand-_size fraction in s~rficia] sediments. (Sand classification accordin 8 to Folk (1968)}~ Closed square - modern terrigenous se~'unent (T); circle - - relict Changjiang River delta sediment (R}i cross - - middle shelf palimpsest sediment (PB); triangle - - outer shelf palimpsest sediment (PTB); Q - - quartz; F - Feldspar; R - - rock fragments. For explanation of the abbreviations in brackets, see Fig.5.

component of the mud-size fraction, in which illite (68-75%), kaolinite (13-19%) and chlorite (8-18%) are the main constituents.

Biogenic sedimentation In the area, especially on the outer shelf, there is a high rate of biogenic sedimentation, shown by the high concentration of bioclasts in the sediments. However, deposits of biochemical origin occur only locally.

Bioclastic sedimentation Skeletal detritus of various phyla in 1 g of dried sample were identified and counted in the > 63/zm fraction. Calcareous skeletal material dominates over the siliceous. The principal organisms contributing to the carbonate sedimentation on the shelf are mollusks, forams, ostracods, corals, echinoderms and bryozoans. Calcareous bioclasts may range from a few tens up to 90,000 grains in 1 g of dried sample of the sand fraction, and they average 12,399

126

grains in a total of 46 samples. The distribution pattern of the skeletal fragments shows that the relict delta of the Changjiang River has a low abundance; less than 1000 grains. The outer shelf, except in the northwest, displays a high concentration of generally greater than 10,000 grains. The middle shelf generally shows intermediate values which gradually increase both to the east and to the northwest (Fig.3). 12

The abundance of calcareous bioclasts in the bulk samples ranges from 1.97 to 69.93 grain %. The CaCO 3 content of the sediments reflects the abundance of biogenic components. Although such content also includes that of chemogenetic carbonate formed by diagenetic processes (Zhu, 1986c), the latter was rarely observed. The carbonate content was estimated in the bulk sample, in the coarse-medium sand frac-

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127 TABLE 1 Range of CaCO 3 c o n t e n t in t h e bulk sample and in the different fractions (wt.%)

Maximum Minimum Range Average N u m b e r of samples

Bulk sample

Coarse-medium sand fraction

F i n e - v e r y fine sand fraction

silt-clay fraction

51.23 2.06 49.17 12.24 46

87.83 0.95 86.88 40.56 30

38.89 1.30 37.59 11.37 44

43.26 3.51 39.75 13.75 46

tion (grain size > 0.25 mm), the fine-very fine sand (0.25-0.063 mm) and the silt-clay fraction (< 0.063 ram). Table 1 shows that the average carbonate content in the coarse-medium sand fraction is higher (40.56 wt.%) than that of the bulk samples, but that the other two fractions (11.37-13.75 wt.%) are close to that of the bulk samples. In the coarse-medium sand fraction, the samples containing more than 30 wt.% carbonate account for 63.3 wt.% of the total carbonate. In the fine sand and silt-clay fractions and bulk samples however, carbonate contents are mostly lower than 15 wt.% (Table 2). The siliceous detritus is contributed by

diatoms, sponges and radiolaria (Table 3). The radiolaria detritus exists only on the outer shelf where the Kuroshio current passes. Diatoms are concentrated at several stations in the middle shelf, and range from several hundreds to about 1000 grains per l g of sample. Sponge spicules are distributed extensively, but the greatest abundances are near 127°E. Multiple cluster analysis of the skeletal detritus data (Zhu et al., 1986, fig.3) shows that four bioclastic assemblages exist in the study area. These assemblages are restricted to the following bioclastic provinces (Fig.4). A pteropod-dominated division, a foram division con-

TABLE 2 Frequency distribution of carbonate c o n t e n t in the different fractions and bulk samples CaCO 3 content range (wt.%)

<5 5-9 10 14 15-19 20-24 25 29 30-34 35 39 > 40 Total

Bulk samples

Fractions

No.

Coarse-medium sand

F i n e - v e r y fine s a n d

Silt-clay

N

I

N

I

N

I

1 2 3 1 1 3 2 2 15 30

3.3 6.7 10.0 3.3 3.3 10.0 6.7 6.7 50.0 100.0

22 8 2 1 3 2 3 3 0 44

50.0 18.2 4.5 2.3 6.8 4.5 6.8 6.8 0 99.9

3 16 14 4 4 2 1 0 2 46

6.5 34.8 30.4 8.7 8.7 4.3 2.2 0 4.3 99.9

10 18 8 3 2 0 1 2 2 46

Percentage of sample

21.7 39.1 17.4 6.5 4.3 0 2.2 4.3 4.3 99.8

N - - Number of samples. I - - Percentage of total sample number.

12~ TABLE3 Abundance of siliceous skeletal materials (46 stations)

Number of stations Percentage of the total No. stations Average content (grain %) Average content in entire area (grain %)

Radiolaria

Diatoms

Sponges

Total

10 22

16 35

41 89

41 89

0.28

4.36

3.30

5.07

0.06

1.52

2.94

4.52

sisting of planktonic and benthonic foramdominated subdivisions, a mollusk-dominated division, and a nearshore division dominated by echinoids. It should be pointed out that the bioclastic sedimentation of the area includes not only the skeletal materials but also the fecal pellets which consist of quartz, feldspar and clay cemented by CaCO 3. Such pellets account for 5-8 wt.% of the sand fraction at several stations. Biochemical sedimentation The products of biochemical sedimentation in the area are organic carbon and phosphate. These components have been formed either at the time of deposition or during early diagenesis. The organic carbon content (Table 4) of the different fractions is commonly low except in the silt-clay fraction. Organic carbon abun-

dance higher than 1 wt.% in the bulk sample was found only at two stations. However, the silt-clay fraction with > 1 wt.% organic carbon accounts for 87.0% of total number of stations. Thus, the organic carbon is mainly in the silt-clay fraction and the stations showing the highest amounts in organic carbon are dominated by the silt-clay fraction. The organic carbon is composed mostly of organic compounds, such as humus and lipids derived from the planktonic metabolites such as proteins, amino acids, carbohydrate and phenol. The abundance of the organic carbon is mainly controlled by preservation and shows no direct relation to present biogenic productivity. A weak phosphatization which forms during early diagenesis exists in the sediments of the East China Sea. Polski (1959) reported that some shells were replaced by phosphate. The authigenic phosphate in the area occurs in two forms: phosphatized tests and phosphatic

TABLE 4 Range of organic carbon content in the bulk sample and the different fractions (wt.%)

Maximum Minimum Range Average Number of samples

Bulk sample

Coarse-medium sand fraction

Fine-very fine fraction

Silt-clay fraction

1.24 0.16 1.08 0.57 46

0.80 0.03 0.77 0.24 30

0.30 0.06 0.24 0.16 36

2.79 0.81 1.98 1.40 39

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Fig.4. Provinces of bioclastic components. I = pteropod division; H = f o r a m division: H1 = planktonic foram-dominated subdivision, II2 =benthonic foram- and ostracod-dominated subdivision; III=mollusk division; IV=nearshore division, characterized by echinoids.

pellets. Electron microprobe analysis reveals that the yellow-brown phosphatized tests are composed chiefly of CaO and P205. The mineral present is francolite which was identified by X-ray diffraction and infrared spectroscopy. Brown phosphatic pellets, containing 2.542.94% P205 (equal to 6-7~/o francolite), were

found primarily on the relict Changjiang River delta. The P205 content of the sediments consists of authigenic and allogenic phosphate. The chemical data show that the P205 content ranges from 0.11 to 0.58~/o, and averages 0.27%. Phosphorus has been concentrated slightly in

130

the present areas of slow deposition (i.e., in the relict Changjiang River delta and on the middle shelf). Although phosphatization is likely on the outer shelf, it may be diluted by the existing high rate of biogenic sedimentation.

Volcanogenic sedimentation Pyroclastics (lithic, crystal and vitric ashes) directly derived from submarine volcanism are present in the sediments on the outer shelf. The lithic fragments consist of pumice, and were found at just one station (29°59.5'N, 127°46.3'E) (Zhu et al., 1983). The vitric ash present at the same station constitute 2 wt.~/o of the sand fraction, and have three forms: colorlesstransparent, white fibrous (sheaf-like) and dark yellow-brown semitransparent. Their refractive indices are 1.493, 1.511 and 1.508 respectively (Table 5). Within the pyroclastic part, the crystal ashes make a greater contribution than the others, and are composed of aquartz, fl-quartz (a pseudomorph), sanidine, andesine and clinohypersthene. The andesine composition is An48 (N~=1.562, Nm=1.553, Np=1.555, 2V=82°). When the particles of crystal ashes are large, they generally possess the bubble-wall texture. However, the small grains are also locally coated with volcanic glass. The pyroclastic crystals appear to be from different lavas which range from rhyolitic (quartz and sanidine) to andesitic (andesine and clinohypersthene).

Sedimentary divisions The characteristics and pattern of sediments on the north shelf of the East China Sea form three sedimentary divisions: (1) a modern terrigenous division, (2) a relict Changjiang River delta and (3) a palimpsest division (Table 6, Fig.5). The characteristics of these provinces reflect the differences in their geological history, geomorphic position, hydrodynamic condition, sediment supply and biological productivity.

Modern terrigenous division The modern terrigenous sediments on the north shelf of the East China Sea are of two textural types. One settles from the suspended load delivered by the Changjiang River or from the resuspended sediments of the relict Huanghe River delta, dispersed by longshore currents. The other is a coarse clastic material (sand or gravel) occurring along the shoreline and around the islands in the area.

Nearshore muddy subdivision (TN) This zone is undergoing a high rate of sedimentation of the suspended load from the Changjiang River. Under the influence of longshore currents, the sediments are distributed in a narrow belt parallel to the coastline. There is a normal tendency for the grain size to become finer offshore. These sediments mainly consist of silty clay with more than 30% clay,

TABLE 5 Characteristics of the vitric ash in the sand fraction at the station at 29°59.5'N, 127°46.3'E Sample No.

Characteristic

Refractive index

Primitive lava

1

Colorlesstransparent White fibrous (sheaflike) Dark yellow-brown, semitransparent

1.493

Rhyolitic

1.511

Andesitic

1.508

Dacitic

2 3

70-100 m deep; affected by the H u a n g h a i coldwater mass

20-60 m deep

Hupijiao muddy subdivision (TH)

Relict Changjiang River delta division (R)

110-160 m deep; affected by the Kuroshio current

Olive gray (4Y 4/1) and dark yellow-brown (SYR 3.5/3); silty clay-fine sand Light olive gray (5Y 5/1) or dark yellow-green (5GY 6/2); medium-fine sand

Dark yellowbrown (10YR 5/2); fine s a n d - s i l t y clay

Dark yellowbrown (10YR 4/2); silty clay dominant Various colors; silty clay

Color and texture of sediments

Intermediate abundance of skeletal detritus (5-30%); benthonic population dominant Very high a b u n d a n c e of skeletal detritus (30-50%); p l a n k t o n (foram and pteropod) dominant

Low abundance of skeletal detritus (1.5%); echinoidsand mollusks dominant Intermediate abundance of skeletal detritus (20-30%); benthonic forams and ostracods dominant Low abundance of skeletal detritus ( < 5%); mollusks dominant

Skeletal detritus

-

-

Corg organic carbon. Skeletal detritus expressed as grain %; Core and CaCO 3 expressed as wt.%.

Subdivision intensely affected by modern biogenic sedimentation (PB)

Subdivision affected 5 0 - 1 0 0 m d e e p by modern terrigenous and biogenic sedimentation (PTB)

(P)

Palimpsest division

12-20 m deep

Physical conditions

Nearshore muddy subdivision (TN)

Modern terrigenous division (T)

Sedimentary division

-

-

Glauconite (4-7%); Mg-calcite concretion

Glauconite (2-4%) and pyrite (trace)

0.11-0.30

1.00-5.40

Modern sedimentation rate (cm/yr)

Lithic, vitric Low or intermediate Fe3+/Fe 2+ ratio (0.5and crystal 1.0); high CaCO 3 ashes (25-50%)

0.10-0.16 Intermediate CaCO 3 (10-25%), Cor~ (0.5-0.7%) and Fe 3 +/Fe 2 + ratio ( < 1)

Low Con (<0.5%) and CaCO 3 (3-8%); high Fe3+/Fe 2+ ratio ( > 1)

Low Fe3+/Fe 2+ ratio (0.5); high Cors (1.24%); intermediate CaCO 3 (12.53%)

and CaCO 3 (9%)

(0.5); low Cor~(0.5%)

Low Fe3+/Fe 2÷ ratio

Volcanogenic Geochemistry sedimentation

Glauconite a b u n d a n t (7.4%); calcareous and ferric concretions

Pyrite in small amounts

Pyrite in small amounts

Authigenic mineral

Characteristics of the different sedimentary divisions on the n o r t h e r n East China Sea shelf

TABLE 6

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and h a v e a m e a n grain size o f > 6~b w i t h a dark y e l l o w - b r o w n c o l o r (10YR4/2). A l t h o u g h the a m o u n t of silt and clay is high, t h e o r g a n i c carbon a b u n d a n c e ( a v e r a g e 0.5%) is l o w bec a u s e of t h e h i g h d e p o s i t i o n rate. T h e sedim e n t s h a v e a l o w c o n c e n t r a t i o n (approxi-

m a t e l y 9 grain %) o f s k e l e t a l d e t r i t u s - m a i n l y b i v a l v e s and e c h i n o i d s . In t h e sand f r a c t i o n of t h e sediments, quartz (72%) d o m i n a t e s o v e r feldspar ( a v e r a g e a p p r o x i m a t e l y 25%). T h e s e s e d i m e n t s markedly differ from t h o s e delivered by the

133

Huanghe River (63% quartz and 31-34% feldspar).

Hupijiao muddy subdivision (TH) The fine sediments in this area are carried in by the currents in spite of the distance separating the subdivision (31°45'N, 126°30'E) and the river mouth. The sediments are silty clay, characterized by 60% clay (Mz=8.4, S k i = - 0 . 2 6 , Kg=0.83) (Folk (1968)) and an olive gray color (5Y 5/1). This zone exhibits clay-like characteristics such as a high organic carbon content, a low Fe3+/Fe 2+ ratio and a high concentration of P205. The CaCO3 content is intermediate (average 12.53 wt.%). Bioclastic sedimentation is chiefly contributed by forams, in which benthonic species account for 69.1 grain % of total skeletal detritus. A weak southward circulation of the Huanghai Sea cold-water mass (Jin and Sui, 1983) enters the area of the subdivision from the southern Huanghai Sea. It is an important dynamic factor resulting in the formation of the Hupijiao sedimentary zone. Based on the study of satellite images, it is known that the weak circulation carries muddy material eastward along the northern Jiangsu coast after discharge by the Huanghe River. Moreover, the sediment is rich in montmorillonite (13.59%) and thus corresponds to the sediment containing 21-22% montmorillonite from the Huanghe River. The sediments from Changjiang River contain only 6-9% montmorillonite.

Relict Changjiang River delta division (R) The relict delta was formed about 35,000 yrs B.P., and did not experience typical shelf conditions until the Late Pleistocene transgression (12,000 yrs B.P.; Zhu (1983a, 1986b)). Holocene marine sediments have accumulated locally, but a considerable part of the area (32°-31°N) still comprises exposed Pleistocene deposits. The relict sediments are usually dark yellow-brown in color (8YR 3/2), but are also olive gray (5Y 4/2) on the southern and northern boundaries. Fine sand dominates, with subordinate silt and silty clay. The grain

size parameters of the fine sand range widely (Mz = 2.41-4.03, Ski = 0.39-0.85, Kg = 1.873.31). The cumulative probability curves have either river or beach characteristics (Visher, 1969) which suggest that the hydrodynamics changed greatly between the time of deposition and subsequent formation. In the fine sand, the quartz (70-73%) and feldspar (25-28%) contents are close to those of the sand size fraction of the present Changjiang River. The quartz in the medium sand fraction in this area exhibits the lowest degree of roundness (Mp of mainly 0.17) in the study area, and is strongly stained by Fe oxides. Chemically, the sediments are characterized by a low organic carbon content (mostly <0.5 wt.%) and a high ratio (>1) of Fe 3+/Fe 2÷. The CaCO3 content is low (2.80-7.56 wt.%) because of a low bioclastic abundance. The fossils are mostly mollusks and echinoids, of which some species are extinct. Many calcareous concretions have formed in the relict delta sediments. Isotopic analysis (613C = 7.93 to - 9.98%0; 6180 = - 4.8 to -6.9%~ PDB) indicates that they formed under continental conditions, and 14C analysis dates them at about 11,850-23,700 yrs B.P. (Zhu, 1983a). Furthermore, the authigenic glauconite (average 7.4% of the >63 #m fraction) is concentrated in the division because of the present low accumulation rate of terrigenous material. In general, the characteristics of the sediments in the division obviously reflect formation in a transitional fluvial-coastal environment with strong reworking by marine processes after deposition.

Palimpsest division (P) In general, this division occupies the middle and outer shelf where Pleistocene deposits have been mixed with modern sediments.

Subdivision intensely affected by modern biogenic sedimentation (PB) The sediments in this subdivision mainly consist of medium and fine terrigenous-bioclastic sand (10-25 wt.% CaCO3) or bioclastic terrigenous sand (25-50 wt.% CaCO3) owing to

134 minor traction population, and a minor viscous s u s p e n s i o n p o p u l a t i o n (Fig.7). T h e m e a n g r a i n size of t h e t r a c t i o n p o p u l a t i o n s is g e n e r a l l y c o a r s e r t h a n 2 ¢, w i t h t h e c o a r s e r t r u n c a t i o n p o i n t s l o c a t i n g a t 1.5-2.0 ~b. T h e i n t e r m i t t e n t s u s p e n s i o n p o p u l a t i o n is w e l l s o r t e d , a n d h a s two or more curve segments, the inflection p o i n t o f w h i c h l o c a t e s n e a r 2.5 gb. T h e c o a r s e r segment predominates. These features suggest

F3 ol

20

,I

3

~0

j~370I

60

e30~

TABLE 7

oo 2

402 207

202

F) 103 3O9 2O3

106

Matrix of factor analysis (Q-mode) of grain size data (35 CaCO3-free samples)

206

Fig.6. Factor analysis of CaCO3-free grain size data (numbers indicate sampling stations). See Table 7 for explanation of F 1, F 2 and F 3.

t h e h i g h a m o u n t of m o d e r n b i o g e n i c p r o d u c t i o n d o m i n a t e d by p l a n k t o n . F a c t o r a n d corres p o n d e n c e a n a l y s i s (Fig.6) o f g r a i n size d a t a (CaCO3-free) s h o w t h a t t h e s a m p l e s c o n c e n t r a t e i n t h e v i c i n i t y o f t h e F1 f a c t o r ( T a b l e 7). These s e d i m e n t s are fine-or m e d i u m - f i n e g r a i n e d a n d t h e c u m u l a t i v e p r o b a b i l i t y freq u e n c y c u r v e s for t h e s e d i m e n t s e x h i b i t f o u r five t r u n c a t e d p o p u l a t i o n s d o m i n a t e d b y a n intermittent suspension population with a

lO8A

0 0 1 1 2 2 3 34 45 5 6 6 7 7- 8 8 9 9-10 > 10

F1

F2

F~

0.0540 0.0448 0.1017 0.7944 0.4249 0.0781 0.0921 0.1117 0.1136 0.0984 0.1341 0.3247

- 0.0572 -0.0529 -0.1124 - 0.5341 0.2792 0.0218 0.2116 0.2023 0.2215 0.1812 0.2548 0.6206

- 0.0792 - 0.0738 - 0.1286 0.2480 - 0.8060 - 0.1687 - 0.0860 0.0815 0.1207 0.1039 0.1552 0.4107

F1 = samples consisting mainly of 2-4¢. F 2 = samples consisting mainly of > 9¢. F 3= samples consisting mainly of 7 > 10¢.

9o.8

108B

8s8

98

9~

90

.90

70

70

50 3O

3O

I0

I0

1

0.5

0,5

0.1 , ,

o

I

,

.5

L

~_ .25

,

~

_

325

l

~ .067

0.01

03

o

I

i

,, .5

l

I .25

,

.125

oo

.067

Fig.7. Cumulative frequency probability curve of a sample closely related to F I at station 108 (31°00.0'N, 127°44.9'E).

108.4= bulk sample; I08B = CaCO3-free sample.

135 that the sediments may have been produced by swash and backwash action with varying velocity. Therefore, factor F~ represents the strong wave action. The sediments close to the end-member (F1) are mostly the products generated in the beach environment. Stations with samples that plot close to F~ usually lie on the outer shelf, but a few are on the middle shelf. It may be concluded that the terrigenous detritus of the sediments in this subdivision are chiefly of relict beach deposits. Within this division at a depth of 100 m, we collected some mollusk remains, such as Ostrea gigas, O. pestigris, Cardita spp., Gari spp., Neverita ampla and Oliva mustelina. These fossils also provide reliable evidence that the sediments on the outer shelf primarily accumulated in the nearshore. The 14C data reveal that these fossils are 8900-30,000 yrs old. The sand in the subdivision contains 67-74% quartz and 25-31% feldspar (arkosic). The roundness (Mp = 0.18-0.20) of detrital quartz is the greatest in the study area. Both the Vshaped depressions and etch pits on the quartz grain surfaces as seen using a SEM are more common than those in other divisions.

Subdivision affected by modern terrigenous and biogenic sedimentation (PTB) This subdivision occupies the extensive outer shelf. Some of the sediments are part of the nearshore deposits of the glacial epoch, and are basically similar to those of the PB subdivision. However, the sediments in the PB subdivision differ from those of the PTB subdivision in being strongly overprinted by modern planktonic sedimentation, whereas the sediments in the PTB subdivision have been overprinted by reworked suspended matter from the Huanghe and Changjiang rivers. As a result, the sediment texture ranges from muddy sand to silty clay. The sand fraction of the sediments in the subdivision contains a large amount of feldspar (31-36%) and relatively little quartz (61-68%). The abundance of authigenic glauconite is only 2-4°//0 in the > 63 #m fraction, probably because the rate of recent terrigenous accumulation is higher

than that in the PB subdivision and in the relict Changjiang River delta. Discussion

The predominant factors controlling the types and patterns of sedimentation are the geological and geomorphic conditions of the surrounding land mass, the hydrodynamics of the marine environment and the geological history of the north shelf of the East China Sea. The geological and geomorphic condition of surrounding land mass determines the composition and texture of the terrigenous detritus. The hydrodynamics of the marine environment control the dispersal of the terrigenous materials, and biological productivity. Sea level fluctuations, caused by glacial advances and retreats during the Late Pleistocene (since 15,000-30,000 yrs B.P.), determine the sediment types and sedimentary pattern on the shelf. The western part of the area (the inner shelf) emerged earlier during regression and submerged later during the last transgression (15,000 yrs B.P.). Thus, the sediments in the relict Changjiang River delta display characteristics similar to those of continental facies, such as a yellow-brown color due to Fe staining of skeletal and mineral grains, and extensive calcareous and ferric concretions (Zhu, 1983a, b, c, 1984, 1986b). By contrast, the eastern part of the shelf experienced a shorter emergence and was submerged for a longer period under deep water, so that these relict sediments have preserved most of their original features. On the middle shelf, the original characteristics of the relict sediments gradually decrease in distinctness from east to west under the influence of modern terrigenous sedimentation. Conclusion

(1) The north shelf of the East China Sea is dominated by Pleistocene and modern terrigenous and bioclastic sedimentation. Biochemical and volcanic deposition exists locally (Figs.l-4).

136

(2) There are three major sedimentary divisions on the northern shelf: nearshore terrigenous, relict Changjiang River delta and palimpsest (Fig.5). (3) The modern terrigenous sediments mainly consist of silty clay, derived from the Changjiang River load and are mostly deposited in the nearshore. (4) The relict Changjiang River delta on the inner shelf is mainly covered with Pleistocene terrigenous deposits. Calcareous concretions and authigenic glauconite develop widely because of the low present rates of accumulation. (5) The middle and outer shelf comprise the palimpsest division which consists of Pleistocene nearshore sediments mixed with modern bioclastic and terrigenous materials. References Folk, R.L., 1968. Petrology of Sedimentary Rocks. Hemphills, Austin, Texas, 170 pp. J i n Qingming and Sui Liangren, 1983. Modern sedimentation process on the continental shelf of the East China Sea. In: Acta Oceanologica Sinica (Editor), Proc. Int. Symp. Sediment. Cont. Shelf, Special Ref. East China Sea. China Ocean Press/Springer, Hangzhou, China, pp.497--512. Niino, H. and Emery, K.O., 1961. Sediments of shallow portions of the East China Sea and South China Sea. Bull. Geol. Soc. Am., 72:731 762. Polski, W., 1959. Foraminiferal biofacies off the north Asiatic Coast. J. Paleontol., 33:1164 1182. Qin Yunshan, 1963. Primitive research on geomorphology

and sediment types of the shelf sea along China. Oceanologia Limnogia, 5(1): 71 86 (in Chinese). Shepard, F.P., 1932. Sediments of continental shelves. Bull. Geol. Soc. Am., 43:1017 1040. Shepard, F.P., Emery, K.O. and Gould, H.R., 1949. Distribution of sediments on the East Asiatic continental shelf. Univ. Calif., Allan Hancock Found. Occas. Pap. No.9, 64 pp. Visher, G.S., 1969. Grain size distribution and depositional processes. J. Sediment. Petrol., 39:1074 1106. Zhu Erqin, 1983a. Calcareous concretions in the Northern East China Sea. Sci. Sin. B, 26(10): 1088 1098. Zhu Erqin, 1983b. Discovery of ferric concretions in the north part of the East China Sea. Mon. J. Sci. 28(11): 1572 1573. Zhu Erqin, 1983c. A study of ferric concretions in the n o r t h e r n part of the East China Sea. In: Acta Oceanologica Siniea (Editor), Proc. Int. Syrup. Sediment. Cont. Shelf, Spec. Ref. East China Sea. China Ocean Press/ Springer, pp.705 714. Zhu Erqin, 1984. Mossbauer spectroscopic characteristics of ferric concretions in the East China Sea. Collect. Oceanic Works, 7(1): 116 124. Zhu Erqin, 1986a. Marine sediments. In: Comprehensive Survey and Research Report on the Water Areas Adjacent to the Changjiang River Estuary and Chejudo Island. J. Shandong Coll. Oceanol., 16(2): 88 172 (in Chinese). Zhu Erqin, 1986b. Characteristics and origin of calcareous concretions in the Huanghai Sea and the East China Sea. Acta Oceanol. Sin., 5(1): 99 124. Zhu Erqin, 1986c. Mineralogy of the high-Mg calcite concretions collected from the East China Sea. Sci. Bull., 31(23): 1628 1633. Zhu Erqin, Wang Qi, Hang Xiaomei and Cheng Tianchun, 1983. The bubble-wall texture of volcanic crystalline fragments in the East China Sea. Mon. J. Sci., 28(12): 1662 1665. Zhu Erqin, Wang Qi, Li J i a n h u a , Xia Mingjie and Cheng Qing, 1986. Formation of carbonate in the surface sediments in the n o r t h e r n part of the East China Sea. Acta Sedimentol. Sin., 4(3): 44-56 (in Chinese).