Cai Qiurong
Abstract: Based on a large amount of bathymetry and side sonar data, the characteristics of seafloor micro-landforms in the northern South China Sea were analyzed, and the dynamic changes of micro-landforms in modern times were discussed. Coastal landforms mainly include estuary bays, underwater shoals, underwater bank slopes, underwater deltas, tidal deltas, submarine sand waves, tidal sand ridges, sea cauldrons, etc.; inland shelf landforms mainly include continental shelf accumulation plains, underwater terraces, ancient Coastlines, ancient river channels, hills and pothole groups, etc.; outer continental shelf landforms mainly include shelf plains, ancient shoals, ancient deltas, shallow troughs, sand waves, sand dunes, buried ancient river channels, submarine landslides and other residual landforms. The hydrodynamic effects in coastal areas are strong, and coupled with human factors, the landforms change greatly. The hydrodynamic effects on the inner shelf and outer shelf are weak, and the landforms are basically relatively stable.
Keywords Dynamic changes in seabed landform characteristics, northern South China Sea
1 Introduction
Developing the ocean and seeking resources from the ocean has become a powerful trend in the world today and is a solution to the problem. The primary approach to the three major problems facing human beings: population, resources, and environment. With the exploration and development of marine oil and gas resources and solid sand deposits, the construction of coastal ports and the regulation of waterways, the reclamation and utilization of tidal flats, aquaculture and fishing, the development of coastal tourism and the construction of bathing beaches, the characteristics of seabed micro-landforms and modern The study of change becomes increasingly important. Based on a large amount of bathymetry and side sonar data, this article analyzes and discusses the characteristics of seafloor micro-topography in the northern South China Sea, and makes a preliminary discussion of its recent changes.
2 Types and distribution of seabed landforms
2.1 Coastal landforms
Mainly refers to landforms within a water depth of 15m.
Etuaries and harbors There are 130 estuaries and harbors along the coast of Guangdong. According to the landforms, they can be divided into mountainous valley ports, such as Guanghai Bay, Zhenhai Bay, Hailing Bay, etc.; platform drowned valley ports, such as Zhanjiang Bay, Liusha Bay, etc.; lagoon ports, such as Shapa Port, Shuidong Port, and Wushi Port. etc.; estuary ports, such as Pearl River Estuary, Hanjiang Estuary, Moyangkou, Fujiang Estuary, etc. With large scale and good water depth conditions, it is a transportation hub between the ocean and inland and an important gateway for the development of marine economy.
Underwater shoals and bank slopes Due to different geographical environments, the width of underwater shoals varies from place to place. Areas where underwater shoals develop often include estuary fans, tidal gullies, tidal flats, underwater sand embankments, sand bars, etc. The underwater shoal outwards is the underwater bank slope, and the lower limit water depth can reach 20-40m. The nearshore side is steep, with an average slope drop of 750×10-3, and gradually becomes gentle toward the sea, with a slope drop of 60×10-3. The material composition changes regularly from coarse to fine from shore to sea.
Taiwan Shoal is located on the southwest side of Taiwan Island. It is surrounded by a 40m water depth line and covers an area of ??8,800km2. It is dotted with hundreds of underwater sand dunes and sand ridges. The sand dunes are generally distributed in a northeast-southwest direction, which is basically consistent with the trend of the isobaths within 200m. Some sand dunes are evenly arranged, with similar heights and basically the same width. Some sand dunes are unevenly arranged and vary greatly in height and width, but their shapes are basically similar. The dunes are steep in the north and gentle in the south. Although they are still affected by wind and waves, they are obviously not formed in modern times.
Underwater deltas: Underwater deltas are developed in the mouths of the Pearl River, Hanjiang River, Moyang River, Jianjiang River, Lianjiang River, etc., generally 1 to 8km wide, and protruding in the south-west direction in the shape of a tongue. Underwater deltas generally develop runoff gullies, tidal gullies and sand ridges.
The Pearl River is the largest water system in South China, and its underwater delta is relatively well developed. Due to the drop in sea level in the Late Pleistocene, multiple underwater paleo-deltas have developed on the seabed.
The Phase I underwater ancient delta is located south of the Shangchuan Island-Wanshan Islands-Danggan Islands line, with a water depth of 30-62m. The ancient delta is 50km long from north to south and 92km wide from east to west. The surface of the ancient delta is flat with an average slope of 14.5×10-3. Several sand bars are developed along the bank on both wings, with a height of 0.5 to 1.5 m and a distance between sand bars of 1 to 1. It varies from 3km. There are 6 shallow troughs developed in this delta, 2 to 3 meters deep and 2000 to 5000 meters wide. It is the ancient drowning valley of the Pearl River. The Phase II underwater paleo-delta is located at a water depth of 80-100m in the Pearl River Mouth Basin. This phase of the paleo-delta is large in scale, divided into east and west parts. The isobaths clearly protrude to the south, with a length of 58km, a width of 110km, and an average slope drop of 0.4× 10-3～0.7×10-3. There are sand banks or sand bars on the outer edge of the ancient delta, 2 to 3 meters high, and there are many shallow troughs in the valley. Among them are two larger ancient river channels, which run through the entire ancient delta. The water depth of the Phase III ancient delta is 100-160m, and the top is connected or overlapped with the edge of the Phase II ancient delta. It consists of a large delta and a small delta, 47km long and 128km wide, with an average slope drop of 0.8×10-3～1.2× 10-3. However, at a water depth of 140 to 150m, there is a steep slope extending to the northeast, with a relative height difference of 2 to 4m. The topography of the ancient delta front is complex, the slope is steep, and there is a large submarine landslide zone, which is composed of landslide walls, collapse valleys, landslide bodies, and platforms. It is about 3km wide and has a relative height difference of 10 to 25m. The three phases of ancient delta have their own characteristics, with the phase II paleodelta being the most spectacular.
Tidal Deltas There are four tidal deltas in the northern part of the South China Sea, namely outside the entrance of Zhanjiang Bay, outside the entrance of Zhapokou and the east and west entrances of Qiongzhou Strait.
The shape of the delta at the west mouth of the Qiongzhou Strait is single. Four troughs and alternate shoals extend westward in a finger-like shape. The water depth gradually becomes shallower from east to west. The shallowest shoal is only 6.5m deep. There are no major rivers flowing into the sea on both sides of the west entrance, and there is not much terrestrial debris supply, so the delta formed is small and stable.
The tidal delta at the east entrance of Qiongzhou Strait consists of two overlapping deltas, the inner and outer deltas, with a complex shape. The inner delta is composed of four shoals and troughs, which are small in scale. The relative height difference between the shoals and troughs is large and they extend in a finger-like shape. The base of the shoal is bedrock, the surface is covered with sand, and there is a small amount of gravel accumulation at the bottom of the trough. The outer delta is composed of four shoals and troughs, which are large in scale, have small relative height differences in the shoals and troughs, and are distributed in a northeast or nearly east-west direction. The water depth of the shoal is small, generally 0.2 to 0.5m, and the Luodousha shoal has emerged from the water. The delta is generally stable except for changes in local areas affected by seasonal hydrodynamic factors.
Tidal sand ridges are shaped by strong tides, such as Qishui-Jianghong Offshore, Haikang-Dongli Offshore, Yamenkou, Modaomen and Lingdingyang at the Pearl River Estuary, Guangdong Zhelin Bay in the east and so on. The tidal sand ridges are arranged in alternating uplifts and depressions along the direction of the tide. The height ratio between uplifts and depressions is generally 3 to 5m, and in some areas it can reach 15 to 25m. The tidal sand ridge is 3-7km long and 0.7-3.2km wide. Because its shape is often changed by modern hydrodynamic forces, it is unstable.
Haifu is only found in the Qiongzhou Strait. Due to the strong tidal erosion and erosion of the bottom of the strait, a "deep water basin" is formed. It is about 60km long, 1~3km wide, and has a maximum water depth of 127m. It is mainly composed of basalt rock mass. composition. There are scattered basalt blocks in the western section, while there is a small amount of gravel or gravel in the eastern section, and secondary small sea cauldrons are developed. They are oval or egg-shaped and live at the maximum water depth.
Undersea sand waves: Sand waves are distributed over a wide range of areas, such as the eastern and western sea areas of Leizhou Peninsula and Taiwan shoal areas. Due to relatively active hydrodynamic conditions, various sand waves develop, with complex shapes, different sizes, and different directions. They are the result of waves, wave currents, tidal currents, ocean currents, etc. Waves within a water depth of 10m play a dominant role in the shape of sand waves. The shape of sand waves is basically parallel to the coastline. Generally, the wavelength is 8 to 10 m, the wave height is 0.5 to 2.0 m, and the waves are closely arranged. For sand waves with a water depth less than 10m, sea waves and waves play an important role; for sand waves with a water depth greater than 10m, the shaping effects of longshore currents, tidal currents and wave currents are significantly enhanced. Large-scale undersea sand ridges can also be seen. The distance between sand ridges is 30-40m. The peak-to-valley ratio of the sand ridges is 3-4m high, and the shape is asymmetrical. The early sand ridges were relatively straight, and later gradually developed into curves and further developed into crescent-shaped sand dunes. The shape of sand waves is unstable, different in winter and summer, and different before and after a typhoon. When tropical storms pass through, they destroy the original landforms. The original sand waves and sand ridges were washed away and moved, and the depressions were quickly filled. Submarine cables or oil pipelines should avoid sandy slope areas, otherwise they may shift or break, and the drilling platform pile feet may also shift, forcing the drilling project to be interrupted or even the drilling scrapped.
2.2 Inner shelf landforms
The inner shelf generally refers to the water depth range of 50m, and is the sea area with the strongest modern marine sedimentation.
Inner Shelf Accumulation Plain The Inner Shelf Accumulation Plain is a modern marine accumulation landform. The seabed is flat, with an average slope of 0.7×10-3, and there are no obvious sudden uplifts or depressions on the seabed. There are abundant sources of terrestrial materials and strong modern sedimentation. They are mainly silty clay or clayey silt with a thickness greater than 3m and a maximum thickness greater than 25m. At present, marine sedimentation is still in progress and the sediments are continuously thickening. The accumulation plain range is Gradually expand.
Underwater Terraces There are four levels of underwater terraces developed in the northern inland shelf area of ??the South China Sea. The water depth of the first-level underwater terraces is 15-20m. They are found in the eastern sea area of ??Leizhou Peninsula, the Beibu Gulf sea area, and the Shantou sea area. The ground is flat, with an average slope of about 0°01′20″ and a width of about 10 to 20km. The submarine terrain on the outer edge of the terrace becomes steeper, with the slope increasing to 0°06′37″. The water depth of the second-level underwater terrace is about 50m. It is found in the western Guangdong sea area, the eastern Guangdong sea area, and the Beibu Gulf sea area. The terrace surface is extremely flat. This terrace is covered by fluvial sand and mud outside the large river mouth. The water depth of the third-level underwater terrace is about 80m. It is the most typical underwater terrace in the northern continental shelf shape of the South China Sea. It is widely distributed and has a width of 30km. There are some sand ridges, sand bars, hills, depressions and underwater ancient rivers distributed on the terrace. and other remaining landforms. The fourth-level underwater terrace is only found on the outer edge of the continental shelf south of the Pearl River Estuary. The terrace surface is wide, generally 7 to 15km.
There are many buried ancient rivers on the seafloor in the ancient river area, which appear intermittently and spread out in the form of dendritic branches on the continental shelf plain. Some are covered or are being covered by modern sediments, becoming drowned valleys or buried valleys. .
There are often groups of hills and depressions on the seafloor of the northern continental shelf of the South China Sea. Some exist alone, some are distributed in groups, some are sparse and some are dense. They are usually called It is a group of pockmarks. Pockets are round, oval, meniscus-shaped, dish-shaped, basin-shaped, etc. They are often developed in a certain area and are relatively dense. They are mainly found on the seafloor in oil and gas field basin areas. It may be that during the formation of oil and gas fields, part of the gas rises to the seafloor along the pores, fissures, and fault interfaces of the formation to form hillocks and depressions.
2.3 Outer continental shelf landforms
The residual accumulation plain of the outer continental shelf in the northern part of the South China Sea is wide, with a maximum width of up to 128km in the west, about 100km in the middle, and about 65km in the east, from west to east. It gradually becomes narrower, with an average slope drop of 0.47×10-3～0.91×10-3. It is an accumulation-type continental shelf plain formed during the low sea level during the Yumu Ice Age and the rising sea level during the post-glacial period. Although later affected by the large amounts of fine-grained sediments carried by water systems such as the Pearl River and Hanjiang River and the influence of modern dynamic factors, this area still has a residual landform. As the main body, the material components of the landforms are mostly fine sand and medium-fine sand, which are residual sediments from the low sea level period. Residual landforms such as shallow troughs, ancient shoals, ancient deltas, sand waves, sand dunes, buried ancient river channels, and submarine landslides are developed on the broad residual accumulation plains of the outer continental shelf.
The outer edge slope of the continental shelf is located in the steeper area outside the continental shelf. It is distributed in a strip shape, extending in the northeast or northeast direction, with a width of 5 to 40 km and an average slope of 0°. Between 15′ and 0°30′, it is 5 to 10 times the average slope of the outer shelf plain. The slope topography of the outer edge of the continental shelf changes more complexly. In some places, stepped terrain appears. As the depth increases, the slope becomes steeper and gradually transitions to the continental slope. The surface sediments are mainly residual sediments such as fine sand, coarse and medium sand, and Holocene strata are scattered sporadically, and sand banks, sand dunes, landslides, slump valleys, and small sea mounds are developed.
2.4 Continental Slope Landform
The continental slope starts from the break line of the outer edge of the continental shelf in the northwest, and the water depth gradually increases to about 3400m in the southeast direction. The land slope terrain in this area is highly undulating and complex, and it descends in a step-like manner toward the southeast. There are complete types of landforms. According to the morphological characteristics and arrangement of the inland slopes of the area, the three-level landforms are divided into continental slope platforms, continental slope troughs, continental slopes, continental steep slopes, etc.
Continental slope sea platform is located between 300 and 350m in water depth and has a flat terrain, such as Dongsha sea platform, with Beiwei Beach, Nanwei Beach and Dongsha Island distributed on the platform. The platform looks like an inverted delta, with a width of about 160km from east to west and a length of about 105km from north to south. Its southern part is the slope of the outer edge of the sea platform. The slope becomes obviously steeper and there are radial valleys.
Continental Slope Trough The area only involves the eastern part of the Xisha Trough. The trough spreads nearly east-west, and the trough bottom is relatively flat. The turning point between the trough bottom and the trough wall is very obvious. The trough bottom tilts slightly from west to east, and the width gradually narrows, reaching only 11km wide at the east entrance. The Xisha Trough is a rift created on the basis of the Proterozoic massif due to Cenozoic tension. It experienced early extension and rupture, forming the prototype of the rift. It settled stably in the middle stage and accepted sedimentation. In the late stage, the rift developed further and the sea The groove gradually forms.
Continental slopes Continental slopes are dominated by accumulation, with large sediment thickness and relatively gentle terrain. Most of their slopes range from 2° to 5°. It is divided into east and west parts with the Pearl River Valley as the boundary.
The western continental slope is steep at the top and gentle at the bottom. The shape is relatively simple, the water depth is about 200-1000m, and the slope is steep. Among them, small landslides, collapses and erosion gully groups occurred locally at a water depth of about 200 to 500m. The water depth of 1000-1800m is dominated by accumulation, the slope is relatively gentle, and the landform type is relatively simple.
On the eastern continental slope, the landform type is more complex, and the morphological characteristics of the upper and lower parts are quite different. There are multiple landslide areas in the upper part, especially in the area where the outer edge of the continental shelf and the continental slope meet. This is because the sediments in this area are continuously advancing from the continental shelf area to the continental slope area, the sediments are relatively loose, and the terrain slope has become significantly larger, making it an unstable zone. The terrain in the lower part is relatively gentle, dominated by accumulation landforms. However, the terrain near the water depth of 1200-2800m becomes obviously gentle, and has the geomorphological characteristics of a continental rise and abyssal fan. There are many seamounts and seahills distributed in the southeast. Large seamounts include pinnacle seamounts, etc., and large and small faults are everywhere. Their extension directions are variable, but they are mainly northeast and northwest, with a small amount near east-west distribution.
Continental slope scarp The continental slope scarp is a sea area with steep slope and the most complex terrain changes in the continental slope. It can be divided into two areas.
One of them is located on the northern upper continental slope of the Xisha Trough. The Xisha Trough is based on the Proterozoic block. Due to the Cenozoic tension, a rift valley was formed. It extends in the northeast direction and has strong fault activity on both sides and in the trough. A series of tensile faults develop on the northern slope of the trough. The fault zone is 200-300km long and the fault distance can reach 1000-2000m. Due to the step-like faults that form the zone, the area descends in steps towards the bottom of the trough, forming a series of fault valleys, edge trenches, cliffs and steep slopes. As the strata are squeezed, a series of various landform types such as landslides, collapses, and turbidity current accumulations appear.
The second one is located at the junction between the north trough slope of the Xisha Trough and the continental slope to the east and the deep sea basin. It is mainly controlled by the northeast-northeast-trending fault structure. The seafloor magma material intrudes and erupts along the fault zone, forming seamounts and sea mounds of various shapes and sizes.
3 Dynamic evolution of seabed landforms in modern times
Due to the influence of various factors, seabed landforms have been changing. Generally speaking, seafloor landforms change relatively greatly in sea areas with strong hydrodynamic effects in estuaries and bays, while in sea areas with weak sea water dynamic effects, the changes are relatively small.
3.1 Pearl River Estuary Sea Area
In the past century, due to changes in the natural environment, the landforms of the Pearl River Estuary Sea Area have also changed accordingly. The degree of change varies from region to region and is explained separately.
The seabed landforms in Lingdingyang were relatively stable from 1900 to 1960. The East Shoal and Hongshi Channel basically remained unchanged, except that the Lingding Channel became shallower and the 10m isobath retreated southward to the inner Lingding Island. Southwest side. There have been major changes since 1960. A large amount of sediment has accumulated in strips on the west side of Chuanbi Waterway and Lingding Waterway, and the seabed has increased by 4 to 5m. Sediment is also rapidly accumulating in the northwest waters of Qi'ao Island, and the seabed is increasing at a rate of 50mm every year. The northern section of the eastern shoal in Lingdingyang silted up slowly, while in the southern section, on the contrary, the seafloor eroded and the shoal retreated. At the same time, the shoals that have suffered erosion and retreat include the Floodstone Shoal and the Lingding River Barrage. The southern section of the Lingding Waterway is silted up to a height of 1 to 3 meters, the northern section is eroded, and the east side of the Humen River Nose Channel is subject to strong erosion.
Modaomen: From 1900 to 1960, the seabed landforms changed greatly. The area around Baiteng Lake was originally a shallow sea and became a tidal flat. The Niwanmen-Longshiku flume and the deep groove of the gate were silted up and filled up. Hezhou-Jiaobei sand silted up the fastest and was exposed to the water surface. The newly appeared lantern sand also shows the modern changes in the seabed landforms. Since 1960, siltation has been common in the Modaomen sea area, and the seabed has increased significantly. Hezhou Jiaobei Shoal, Baiteng Lake-Sanzao Bay, and the sea area south of Sanzao have silted up 0.5m, and the sea area south of the Modaomen Barrier Sand Bar has accumulated the fastest. , the seabed sedimentation height is 2.0~2.5m.
Huangmao Sea: A hundred years ago, Huangmao Sea was originally a vast ocean. By 1960, it had formed a trumpet-shaped estuary. The original 2-3km wide Hutiaomen was silted up into a waterway on both sides. There is obvious sedimentation on the seabed in this area, and the 5m isobath and 10m isobath have moved southward significantly to the south of Hebao Island. Since 1960, the beach in the southern part of Huangmao Sea has been silted up by 0.5m, and the deep trough between Dayu Island and Nanshui Island has been partially eroded.
In short, under the action of Kodi force, the west side of the Pearl River Estuary Sea Area has obvious sedimentation. In the past century, the Pearl River Delta has advanced seaward, especially the west side of Lingding Yang, Modaomen to Dahaihuan area at a rapid rate. Wanqingsha advances 50m to the sea every year, and the ring from Modaomen to the sea moves 40 to 140m every year, with a maximum of 160m.
3.2 Hanjiangkou Sea Area
The beaches from the mouth of Yifeng River to the north of Laiwu Island have eroded and retreated. The beaches south of the mouth of Yifeng River have also been eroded and retreated. The side beach was eroded, the elevation of a large shoal dropped by 0.5m, and a new trough 2m deep was washed out at the mouth of the east stream. The shoal from the south of Wuyu to Dongxikou has obvious siltation and tidal flats, and the 2m isobath has moved outward, while the 5m isobath from Fengyu to the north of Laiwu Island has moved outward more. From Laiwu to Xinjin Creek, beaches are eroded, tidal flats are washed away, and the elevation generally drops by 1.0 to 2.0m. The Daiyijin Shoal outside the mouth of Xinjin Creek was washed down by 0.5m. Undersea erosion, the 2m and 5m isobaths advance to a large extent shoreward. On the south side of Haishan Island, the beach is eroded, and the 0m isobath is advancing toward the shore, while the underwater shoals have not changed much, and the 5m isobath is basically stable.
3.3 Zhanjiang Bay and Leizhou Bay Sea Area
Zhanjiang Bay Zhanjiang Bay’s submarine landform is relatively stable, with slight erosion in deep troughs, underwater bank slopes, and bay edge platform scarps. A small amount of siltation occurred in and near the branch watercourse.
Leizhou Bay Compared with 1978, the seabed landforms of Leizhou Bay changed greatly. The top beach of the Western Bay has silted up 300m to the south, 1,800m to the north, and 900m near Beitan. However, the Houge area has been eroded, and grooves have appeared in the underwater shoals. Coastal erosion has retreated 900m, and the Maiqi Deep Channel Siltation is shallow 1.2~2.4m. The western section of the middle trough is eroded, the trough depth is increased by 0.5m, and the trough bed is widened by 500m. The middle section of the middle tank is silted up, and the tank depth is reduced by 1.2 to 2.5m. The slot bed is narrowed by 300m. In the triangle area between the south and north of the middle trough, shoal sedimentation increased, with the largest increase reaching 2m. The erosion of the shoal opposite Longwan Village has been reduced, and the general water depth of the shoal has increased by 1.5 to 2.0m compared with the original water depth.
4 Conclusion
The seabed landforms in the northern South China Sea are complex, with complete types and numerous micro-landforms. Coastal landforms within a water depth of 15m mainly include estuaries, harbors, underwater shoals, bank slopes, underwater deltas, tidal deltas, tidal sand ridges, sea cauldrons, and submarine sand waves; inland shelf landforms with a water depth of 50m include Inner shelf accumulation plains, underwater terraces, ancient coastlines, ancient river channels, and groups of hills and depressions; outer shelf landforms with water depths greater than 50m include outer shelf residual accumulation plains, on which shallow troughs, ancient shoals, ancient deltas, Residual landforms such as sand waves, sand dunes, buried ancient river channels, submarine landslides, and continental shelf outer edge slopes; continental slope landforms include continental slope platforms, continental slope troughs, continental slopes, continental slopes, etc. The seafloor geomorphology changes greatly in sea areas with strong hydrodynamic effects in estuaries and bays, while the landforms in sea areas with weak dynamic effects of sea water are basically stable.
References
[1] Cong Hongwen et al. 1992. Topographic map of China’s sea areas and adjacent areas (1:5 million). Beijing: Geology Press
< p>[2] Wang Wenjie. 1995. Dynamic effects of the development of the geomorphological system of the Qiongzhou Strait tidal channel. Special collection on sea level changes and coastal erosion, National Key Laboratory of Coastal and Island Development, Nanjing University. Nanjing: Nanjing University Press, 234～242< /p>[3] Mao Shuzhen et al. 1982. Seafloor topographic characteristics of the central and northern South China Sea. Comprehensive research report on the South China Sea area (1). Beijing: Science Press
[4] Feng Wenke et al. Preliminary study on the submarine landforms in the northern South China Sea. Acta Oceanographica Sinica, 1982, 4 (4), 462～472
[5] Chen Junren et al. 1983. Preliminary study on the -50m ancient coastline in the northern South China Sea. Acta Geographical Sinica, 38 ( 2), 176～187
[6] Chen Junren et al. 1985. Research on the -20m ancient coastline in the northern South China Sea. Proceedings of the Academic Symposium on China’s Quaternary Coastline. Beijing: Ocean Press, 230～240
[7] Xie Yixuan, Mao Shuzhen. 1985. Undersea topography of the South China Sea area. Comprehensive survey and research report of the South China Sea area (2). Beijing: Science Press, 13～34
[8 ] Xie Yixuan. 1985. Undersea landforms in the northeastern part of the South China Sea. Tropical Oceans, 2 (3), 182～190
[9] Xie Yixuan. 1986. Continental margin expansion landforms of the South China Sea. Tropical Oceans, 5 (2) ,12～19
[10] Bao Caiwang. 1987. Topographic Map Manual. Editor-in-chief He Liansheng. South China Sea Geological and Geophysical Atlas. Guangzhou: Guangdong Map Publishing House, 3～12
MICRO.GEOMORPHIC FEATURES AND THEIR DYNAMIC CHANGE IN NORTHERN SOUTH CHINA SEA
Cai Qiurong
Abstract: Based on a large amount of bathymetric and side-scan data, the micro-geomorphic features and their recent changes were discussed in the present paper. There exist the different geomorphic types including the branching bay, submarine shoal, subfluvial bank, subaquatic delta, tidal current delta, tidal sand ridge, sand wave and caldron in the littoral zone. In the inner shelf, the main geomorphic types have the accumulational plain, shoreface terrace, ancient costal line, ancient stream channel, mamelon and pit-and-pots. However the main geomorphic types in the outer shelf include shelf plain, ancient shoal, ancient delta, slack,sand wave,sand dune,buried ancient stream channel and submarine slumping.The geomorphic shapes changed greatly because of the strong hydrodynamic action and the anthropic factor in the littoral zone,whereas the geomorphic shapes are relatively stable because of the weak hydrodynamic action in the inner and outer shelf.
Key words: geomorphic feature, dynamic change, northern South China Sea