Quaternary Science Reviews, Vol. 10, pp. 523-526, 1991. 0277-3791/91 $0.00 + .50 Printed in Great Britain. All rights reserved. ~) 1991 Pergamon Press plc

DEVELOPMENT OF THE YANGTZE AND YELLOW RIVER ESTUARIES DURING THE HOLOCENE

Chen Jiyu, Wu Lichen and Liu Cangzi Institute of Estuarine and Coastal Research, East China Normal University, Shanghai, China

INTRODUCTION

As is well known, sea-level first rose and then entered a phase of relative quiescence during the postglacial period. The transgressive sedimentary sequences of the early Middle Holocene, and the regressive sedimentary sequences of the mid-Late Holocene are widely distributed in the estuarine and deltaic areas of the world, forming a transgression- regression sedimentary cycle. However, the thickness and the sedimentary fill was different in areas of different initial relief. The model of estuary and delta development varies with both boundary and dynamic conditions. The Yangtze and Yellow River estuaries are of two different types and may be considered typical of large estuaries throughout the world.

HOLOCENE SEDIMENTARY SEQUENCES

According to core analyses in the Chongming and Hengsha Islands, the Holocene strata are 55-66 m thick and the sedimentary facies sequence, from the bottom to top, is as follows: submerged valley --->

estuarine bay ---> pre-delta ~ delta front ---> mouth bar ---> deltaic plain (Fig. 1). An erosion surface exists between the Lower Holocene submerged valley facies deposit and the Upper Pleistocene littoral sand layer.

Quaternary research in the Yellow River delta area shows that the Holocene strata are 25-28 m thick. From bottom to top, the Holocene facies sequence is as follows: river and lake ~ tidal flat and salt marsh ---> shallow sea ~ subaqueous delta ~ deltaic plain (Fig. 2) (Cheng, 1987).

The Holocene sedimentary sequences of both the Yangtze and Yellow River estuaries reflect a trans- gression-regression sedimentary cycle and deltaic sequence. Nevertheless, owing to the difference in their initial relief before the Holocene transgression, there is a difference in thickness and in the evolution of the sedimentary facies. The Yangtze River is a large river with abundant runoff and a stable river course. When sea level was at its lowest stand at the end of the Late Pleistocene, the Yangtze river incised its channel by 30-40 m. As a result, the Lower Holocene sediment is deposited directly on to the littoral facies sediment of the sub-interstadial (W/inn I-II correlative) of the Upper Pleistocene. The 14C dating shows that the age

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Facies

Delta plain

Sand bar

ALternating silty clay and silt beds

Fine sand with siR~ silty cloy and shell debris; horizontal, wavy and cross bedding

DeLta front ALternating siR-fine sand and sandy clay, silty clay with siLt., horizontaL, wavy beddingi burrowed

ProdeRa SiLty clay and clayey siLt

Estuarine bay ALternating silt. cloy and clayey siLt with siLt beds ; horizontaL, cross bedding; burrowed

Submerged valley ALternating siLt.y clay and siR-sand; horizontaL, wavy and cross bedding; fine sand and sand with gravels and sheLL debris near bat:Lore, beLonging tO channel Lag deposit

F I G . 1. S e d i m e n t a r y s e q u e n c e o f t he Y a n g t z e R i v e r e s t u a r y .

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524 J. Chen et al.

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DeLta pLain

TidaL fLat

Subaquatic delta

ALternating silt and clayey siLt bedsl horizorrtoL and climbing rlppW bedding

ALternating silt and clayey silt, bedsi horizontaL1 smaLL-scaLe cross bedding

SiLt, fine sand with cLayey siLt and sheLL debrisi paraLLeL v cross bedding

ShaLLow- sea CLayey silt, siLt with sheLL debris i horizorrtoL, bedding

TidaLfLat-marsh. ALternating sit, and clayey silt beds with carboniferous siLt bed i horizonlu=L1 fraser, smaLL-scaLe cross bedding; highly burrowed

River and Lake SiLt I clayey siLt with fine sand

FIG. 2. Sedimentary sequence of the Yellow River estuary.

of the silty clay, layer buried to a depth of 55 m and containing a few foraminifera, is 11210 + 100 a BP. Thus, at the beginning of the Holocene, sea water intruded landwards along the bottom of the river channel, and the present Yangtze River estuarine area was a submerged channel. Then, as the sea level rose, the submerged valley environment gradually changed into an estuarine bay environment as indicated by the sedimentary sequence.

The Yellow River is characterized by its high sediment load and frequent shifting of the lower channel. By the end of the Upper Pleistocene, a high and flat initial surface was formed in the modern deltaic area owing to strong sediment accumulation. When the modern Yangtze River estuary (ancient river channel) was still affected by sea water at the beginning of the Holocene, the environment of the Yellow River delta area was still continental. The t4c age of the bottom carbonaceous clay of the marine facies at a depth of 23 m (collected in two places) is 8835 +_ 100 a BP and 8870 _ 105 a BP (Cheng, 1987). This indicates that sea water did not reach the Yellow River deltaic area until 8.8 ka BP. Because the topography of the old Yellow River delta area is flat, sea water gradually overflowed the deltaic plain when the sea level rose, forming widely distributed tidal flats and salt marshes on the delta in the early stages of the Holocene. Such a sedimentary environment lasted for a comparatively long period because of the favourable conditions of the rising sea-level combined with ample sediment supply. The tidal flat and salt marsh facies sediments reached 1-4 m thick. Then, during the Middle Holocene, as the rate of sea-level rise exceeded that of the sediment supply, this area was turned into a shallow sea environment.

DEVELOPMENT OF THE YANGTZE AND YELLOW RIVER ESTUARIES

The Yangtze River is the largest river in China. About 7 ka BP, when sea level reached or approached the modern position, the Yangtze River estuary was a funnel-shaped estuarine bay with intrusion of sea water (Fig. 3). The head of the bay was located near Yangzhou. The north bank of the bay extended along the line Yangzhou-Taizhou-Haian, while the south bank followed the Zhenjiang-Jiangying-Caojing line. As the sediment discharge at that time was relatively lower, the Yangtze River estuary was maintained as an estuarine embayment until 2-3 ka BP. However, human activity had a tremendous effect on the develop- ment of the estuary after the 3rd century A.D. As a result of the increasingly intensive land use in the mountain areas of the Yangtze River basin, soil erosion and consequently the river's suspension load were greatly increased, causing rapid sediment deposition in the estuary and accelerating the growth of the Yangtze River delta (Chen e t a l . , 1979).

The development of the Yangtze River estuary shows its own peculiar pattern. Generally, the north part of the estuary was developed by the merging of sand bars into the north bank. During the past 1 ka or more, there were at least six important phases of merger of the sand bars into the bank. About the 7th century, the Dongbu sandbars merged with the bank, as did the Gua sandbar in the 8th century, the Matuo sandbar in the 17th century, the Haimen sandbars in the 18th century, and the Qidong sandbars at the end of the 19th century and in the early 20th century. In the 1920s, the Changyin sandbar was connected to the south bank by artificial engineering works. The cause

The Yangtze and Yellow River Estuaries 525

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Rudong

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I. Gua I. 4. Dongbu L

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FIG. 3. Historical changes in the Yangtze River estuary.

of the accretion of sandbars on to the north bank was that the main current naturally deflected from north to south under the action of the Coriolis force (Chen et al., 1979).

On the other hand, the south bank progressed seaward by the growth of shoals. Owing to changes in the sediment supply, coastline accretion alternated with coastline erosion in the progradation of the south bank. Several cheniers exist on the coastal plain, forming the chenier plain of the southern delta (Liu et al., 1985).

Thus, the Yangtze River estuary was gradually narrowed by the accretion of sandbars on to the north bank and the growth of the southern shoal. For example, the river width between Yangzhou and Zhenjiang was about 12 km in the 8th century, and is now only 2.3 km. The fiver width near Nantong decreased from 18 to 7.5 km over the same period. The width of the fiver mouth has narrowed from 180 to 90 km. With the narrowing of the estuary and deepening of the river channel, the fluviatile reach extended downstream to near Xulujing and the present estuarine situation of the three-order bifurcations and four outlets into the sea was formed.

The Yellow River is well known for having the highest sediment discharge in the world. The channel of its lower reach is characterized by rapid deposition, frequent breaching and shifting. In historical times, the lower reach of the river migrated between the Haihe and Huaihe rivers. The old Yellow River delta which built up in Jiangsu Province from 1185 to 1855 A.D. is clearly discernible. The modern Yellow River delta was formed after the old one shifted into the Bohai Sea in 1855. The delta, with an area of 5900 km 2, takes Ninghai as its apex and is bounded by the Tuhai River in the west and Nanwang River in the south (Fig. 4). In

fact, it consists of two parts: the first is the subdelta with the apex at Ninghai, and the second is the subdelta with the apex at Yuwa.

Since 1855, the lower reach of the Yellow River has burst its banks more than 50 times, causing 10 large migrations of the lower reach. Each time a small delta lobe formed near the mouth of the channel, there being 10 lobes in all. Six lobes were built during the period 1855-1934 when the deltaic apex was at Ninghai and 4 lobes were formed after 1934 when the apex moved down to Yuwa.

The development stages of the lobes are as follows: (1) Fan-shaped sheet deposition. The river was in a

sheetflow and anastomosing stage. The sediments diffused and accumulated in the mouth, forming 2--4 m of thick fan-shaped deposits.

(2) Lengthening of the estuarine sandbar. The river was in a single straight or slightly curved state. The sediment, carrying capacity was so strong that a large amount of sediment was transported into the mouth, building up the sand spit out into the sea.

(3) Transverse expansion. The channel was in a branching and shifting condition. The mouth bar was widened by the joining of deposit bodes from the various branches, forming a small delta lobe.

(4) Reworking. While a new lobe was being built in another place because of river channel migration, the old lobe was eroded by ocean waves because the sediment supply was cut off, causing the retreat of the coastline there, and the chenier was built up near the high tide shoreline.

Thus, it appears that the Yangtze and Yellow River estuaries belong to two different categories. Since they had different boundary and dynamic conditions, their models of development are very different. The Yellow River delta progressed seaward in the form of a

526 J. Chen et al.

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FIG. 4. Changes of channels and lobes in the modern Yellow River delta. Dashed line is the first subdeltic (1855-1934) channels and lobes. Dark line is the second (1934-present). The age of the lobes shown are as follows: (1) 1855.6-1889.3, (2); 1889.3- 1897.5, (3) 1897.5-1904.6, (4) 1904.6-1926.6, (5) 1926.6-1929.8, (6) 1929.8-1934.8, (7) 1934.8-1953.7, (8) 1953.7-1964.1, (9)

1964.1-1976.5 and (10) 1976.5-present (from Cheng, 1987).

prograding series of deltaic fans. It is composed of several small delta lobes built up at the mouths of the shifting channels. The Yangtze River delta, on the other hand, grew by infilling of an estuarine bay by means of sandbar accretion along the north bank and shoal growth along the south bank. The former is a complex delta including two subdeltas and 10 small deltaic lobes, in some way similar to the Mississippi delta. The latter is a single delta formed in an estuary. The course of the Yangtze River is stable, although there is deflection of the main current, and the growth and decline of sandbars, as well as the progradat ion of the coastline. Although the filling of the Yangtze River estuary and the development of the delta has clearly

evident stages, it has been confined to the estuarine trench, so that subdeltas do not exist as they do at the mouth of the Yellow River.

REFERENCES

Chen, J. et al. (1979). The model of development of the Yangtze estuary during the last 2000 years. Acta Oceanologia Sinica, 1(1), 103-111 (in Chinese).

Cheng, G. (1987). Evolution and framework of the modern Huanghe river delta. Marine Geology & Quaternary Geology, 7 (Supple- ment), 7-19 (in Chinese).

Liu, C. et al. (1985). Depositional features, origin and ages of ancient sand ridges (cheniers) in the southern part of the Yangtze delta, Acta Oceanologia Sinica, 7(1), 55-66 (in Chinese).