Asia, Eastern, Coastal Geomorphology
The shores of eastern Asia, extending from Thailand and Indonesia to eastern Siberia and including the Philippines, Taiwan, and Japan, largely follow the tectonically active zones along the eastern and northern sides of the Eurasian plate (Inman and Nordstrom 1971). Large stretches of coastal area along Indonesia, the Philippines, Taiwan, Sakhalin, Kamchatka, and the East Asian island chains are therefore tectonically unstable. In these areas the structural trends are generally parallel to the coast, which was distinguished by Suess (1892) as the Pacific type. Outside these areas, away from the tectonically active collision zones, the coastal regions are generally more stable and the structural trends are usually not parallel to the coast, which corresponds to Suess’ Atlantic type. This type is present along most of the Asian mainland from Thailand to Siberia.
Comparatively straight coasts, situated along mountain chains, sometimes with river deltas and local alluvial foreland, are found mainly in western Sumatra, southern Java, northern Vietnam, and the Pacific coast of Siberia. A drowned older topography with an irregular coastline is present along parts of southern Vietnam, the mainland coast north of the Red River, on the islands of eastern Indonesia, on northern Kalimantan (Borneo), the Philippines, the Shandong Peninsula in China, Japan, southern Sakhalin, and the eastern end of Siberia. These coasts are usually somewhat altered by the sea, with bays containing beaches, spits, and barriers, sometimes being filled up with sediment and partially surrounded by alluvial foreland. The main exceptions are the large river deltas, the west coast of Kamchatka that consists largely of sand barriers, some beach barriers in Japan and on Sakhalin, and some areas like eastern Taiwan where tectonic uplift matches the Holocene sea-level rise. Elsewhere the coast is predominantly depositional, consisting of beaches, spits, barriers, tombolos, mudflats, marshes, mangrove swamps, and coral reefs.
The general direction of beaches, spits, and barriers is related to the direction of the swell. Between northern Japan and Indonesia the swell comes chiefly from northern directions, and the beaches and spits face largely E–NE. Where the swell is southerly, as in the Indian Ocean and in the Pacific north of 45°N, the beaches face mainly SE–SW. A major distinction can be made between the coasts north and south of about 40°N (Davies 1973). North of this latitude storm waves are predominant and pebbles form an important part of the beach material. During the winter, sea ice (pack-ice) borders the coast as far south as Vladivostok and northern Japan. Further north, along the northern shore of the Sea of Okhotsk and north of the Kamchatka peninsula, permafrost reaches the sea, and from the eastern Siberian coast ice cliffs have been reported. South of 40°N the ocean swell is more important, and large areas from southern Japan to Malaysia are affected by tropical storms. In this area, especially south of 25°N, sandy beaches are predominant while pebbles are relatively unimportant as beach material.
Coral reefs and beach rock are confined to the tropical and subtropical zones – they are not present north of 35°N. Mangrove grows south of 31°N (on the Asian mainland coast south of 26°N) and dominates large stretches of coast in Indonesia, Malaysia, and Thailand, especially in areas that are sheltered from the ocean swell. An important part of the East Asian coastline consists of several large river deltas (those of the Mekong, Yang-tse-Kiang or Chang Jiang, and the Hoang-Ho or Yellow river) and a large number of smaller deltas often of still appreciable dimensions such as the deltas of the Chao Phraya, the Red River, and the Si Kiang. This preponderance of deltas is due to the high mechanical erosion on the mainland between Thailand and Korea (>60 ton per km2 per year), which, associated with the presence of large and numerous rivers, has resulted in extensive deltaic and estuarine deposition.
Tidal effects are especially important along the mainland coast from Taiwan to Kamchatka, where the tides have an appreciable range. Usually the tidal range in eastern Asia is less than 4 m (and around Japan mostly less than 2 m) but in the northern Sea of Okhotsk, in the Yellow Sea, and along the Chinese coast it reaches more than 6 m. Tidal marshes have been formed locally, as, for example, along the Gulf of Bohai, in southern Korea, and in Hangzhou Bay.
A striking feature on many coasts from Japan through southeast Asia to Indonesia is the evidence of raised coastlines at 0.5–1 m, about 2 m, and 5–6 m above present sea level. These are indicated by flat, sandy terraces, beach ridges, coral reefs, mollusks and/or barnacles, notches, benches, platforms, and beachrock. Similar raised coastlines are known from many other coasts in Asia, Africa, South America, and Australia (for references, see Tjia 1975; Clark et al. 1978). Daly (1934) and many others therefore assumed that during the Holocene, about 6,000–4,000 years ago, sea level stood about 5 m higher than at present. Others have contended that these features are related to temporary high water during storms, or to local uplift. Tjia (1975) has suggested that glacioeustatic uplift of the areas in the Northern Hemisphere that were glaciated during the Pleistocene, may account for the absence of any indications for high sea levels during the Holocene. C14 dates of raised reefs and mollusk shells from the Pacific and Indian Ocean, Southeast Asia, and Australia point to a slight Holocene transgression as well as to local uplift. Clark et al. (1978) indicate that regional differences in the Holocene sea-level rise are related to differences in crustal rebound after the Pleistocene through ice cap melting and the increasing water load of the oceans.
The Siberian coast east of the Taymir Peninsula is flat up to the Kolyma River estuary and shows a drowned topography with only a higher coast at the Laptev Straits, where bedrock is exposed. The flat coast is locally raised a few meters and here small cliffs are formed by intensive thermal abrasion. This is caused by the regular melting of ice and frozen deposits (permafrost) along the cliffs and removal by waves of the loose sediment. The coastal flats are flooded over large distances (up to 30 km) during wind-induced surges. During offshore winds the muddy nearshore seafloor can be exposed over similar distances. Several large rivers reach the sea in this area such as the Lena, Indigirka, and Kolyma rivers, but only the Lena has a vast delta, which is largely a relict feature. The other rivers have long, partly filled-in estuaries and insignificant deltas. The islands off the coast (the New Siberian Islands and Wrangel Island) have a bedrock nucleus surrounded by loose but frozen sedimentary deposits. Thermal erosion is large and can annually remove up to 50 m from the cliffs.
East of the Kolyma River mouth the coast is partially rock, where the mountain ranges between Kolyma and the Bering Strait come near to the sea. There is usually a narrow coastal plain, several kilometers wide, and at Cape Billing a sand barrier extends for about 50 km into the sea with an up to 10 km wide lagoon on the landward side. Transverse sandbars divide the lagoon into several rounded lakes; their formation is related to the development of wind-induced circulation cells in the lagoon. The sand barrier has been formed in an area that is protected against pack-ice (probably by Wrangel Island). In general, longshore sediment drift is limited by the short period that the coast is free of ice. The sediment is mainly derived from reworked glacial deposits. The easternmost part of Siberian coast along the Bering Strait is characterized by high cliffs and fjords where the Chukhotsk range meets the coast (Zenkovich 1985).
The Pacific coast of Siberia down to Korea extends from the arctic into the temperate zone and has complex tectonics with stable areas alternating with areas of emergence and subsidence. The coastal area is usually mountainous with rather short rivers (except the large Amur River) and narrow coastal plains. Waves and tides are variable: storm waves are up to 12 m in the Bering Strait and in the Sea of Okhotsk, while lower waves (normally up to 2–3 m, and up to 7 m during the winter) dominate in the other areas. Most of the coast is mesotidal, but locally, as in Penzinskaya Bay, becomes macrotidal (up to 12 m). There is a relatively large sediment supply to the coast, because of a wet, cold, or temperate climate in combination with easily weathered rock, such as volcanic deposits and Pleistocene fluvioglacial sediments. Barriers, coastal lagoons, and tombolos are common features, separated by promontories of rock or (unconsolidated) sediment (Kaplin 1985). The largely drowned topography has for a large part been filled in during the postglacial period. Along Sikhote Alin the coast is relatively straight and uninterrupted because of a low mountain range that lies parallel to the coast.
The shores of Sakhalin and Japan consist of large stretches of ria coast, alternating with many small and several large stretches of beaches, spits, barriers, sand dunes, lagoons, and river outlets. Especially on northern Sakhalin, but also on Hokkaido and along Honshu, large parts of the coast are depositional alternating with rocky promontories. Locally, volcanism close to the coast has resulted in cliffs and beaches of volcanic material. Most of the coastal lowland in Japan is intensively used so that a large proportion of the coast is man-made: less than 50% has remained natural. Coastal terraces are widely distributed in northern Japan; in the south they occur mainly on the promontories along the Pacific coast. The terraces are predominantly present on the more easily abraded Tertiary and Quaternary formations, whereas the indented ria coasts have been developed mainly in folded Palaeozoic and Mesozoic rock. The highest terraces have been found in southwestern Hokkaido at +585 m, but usually they reach to +150 m (Kosugi 1971). Below present sea level submerged flat surfaces have been found to depths of −150 and −190 m.
The terraces as well as raised coral reefs (Yabe and Sugiyama 1935) indicate that the Quaternary eustatic changes in sea level may have had an important effect on coastal development in Japan, but there is also much evidence for differential uplift, tilting, and local subsidence. Consequently, in an appreciable number of papers by Japanese authors the combined effects of both eustatic sea-level changes and crustal movements have been discussed (Richards and Fairbridge 1965; Yoshikawa et al. 1965; Richards 1970). The presence of ria coasts is generally seen as evidence of an overall subsidence since the Pleistocene, but narrow terraces scattered along the rias suggest a more recent emergence. Some terraces (at +40 – +60 m) are widely distributed along the Japanese coast and are seen as the result of eustatic sea-level changes. Coastal terraces, formed at the same sea level, have also been found at different altitudes within short distances, even locally disappearing below present sea level, as a result of crustal movements. From this complex situation it has been deduced that the Holocene sea-level rise started around 18,000 BP and that between 6,000 and 3,000 BP sea level was 3–5 m higher than at present, which is indicated by former wave-cut notches, sea caves, benches, small terraces, marine deposits, and neolithic remains (Tada et al. 1952). Clark et al. (1978) have shown that these features can also be explained by crustal rebound after the Pleistocene and the increasing water load of the ocean.
A marked feature along the Japanese and Sakhalin coasts is the wide-spread occurrence of shore platforms (Takahashi 1974). These are most conspicuous along the coasts of the Pacific and the East China Sea, less on the Sea of Japan coasts, and least along the Seto Inland Sea. Their maximum width varies from 530 m along the Pacific to 80 m on the Sea of Japan coasts, which roughly corresponds to the wave conditions on each coast. The average width is 30–80 m and the maximum length about 4 km; they occur on all types of rock, but tend to be wider on Neogene mudstones, sandstone, and tuff-breccia.
The East Asia Mainland, Korea to Malaya
The mainland coast from Korea to the Malayan Peninsula is largely a drowned or ria-type coast, with abrasion at the headlands, accumulation (with sandy beaches, spits, and beach/ ridges) in the bays and, especially in China and southern Korea, numerous islands. Large estuaries and river deltas are present where major rivers reach the coast. In West and South Korea tidal flats have been formed in the bays and in the shallower parts between the islands (Guilcher 1976), which have been partly reclaimed. Coastal terraces are present along the East Korea coast up to 100–150 m and along the West Korea coast up to 30–60 m, which reflects the emergence of the east coast and the submergence of the west coast. Also from the area around Hong Kong (Williams 1971) coastal terraces and old beaches have been reported: they occur at +5 and + 14 m (Berry 1961) and are probably also present elsewhere. At higher levels, up to 70 m, benches are found, below present sea level flat surfaces can be distinguished down to 60 m. Especially the terraces at +5 and + 14 m are thought to be caused by eustatic sea-level changes. Flat surfaces at +130 and + 230 m are probably not of marine origin.
Around the Gulf of Bohai, between the Liaodong and Shandong peninsulas, the coast has been filled with recent alluvium from the Huang He (Yellow River), Luan He, Liao, and a number of smaller rivers. Long stretches of tidal marshland have been formed. Along the northwestern shore, however, up to 80 km of alluvial deposits were eroded again in historical times (Von Wissmann 1940), probably because the Huang He then had a southerly course and no new sediment from that river reached the Gulf of Bohai at that time. The present situation dates from 1853, when the Huang He again took a northerly course. Between 1448 and 1853 it had been reaching the sea south of the Shandong Peninsula after having joined the Huai He River. While at present the silt from the Huang He moves northward anticlockwise through the Gulf of Bohai, the silt flowing out of the southern mouth before 1853 was transported further south as far as the Yang-tse (Chang Jiang) River mouth, which was deflected southward. In 400 years the coastline in this area moved 36–62 km eastward.
South of the Yang-tse-Kiang (Chang Jiang) a ria coast ends at the delta of the Si-Kiang, while further south the coast is interrupted by the deltas of the Red River, the Mekong, the Chao Phraya, and other smaller rivers. Tidal flats of variable widths have been formed along Hangzhou Bay, Wenzhou Bay, and numerous smaller bays and estuaries. Salt-marshes along the upper parts of the flats have mostly been reclaimed. South of Hong Kong mangrove swamps are present and fringing coral reefs become increasingly numerous; in particular around the Gulf of Thailand extensive coastal barriers and sandy beaches have been formed. From Dong Hoi to Cape Varella in Vietnam the coast is formed by a 15–40 km wide belt of barrier complexes, lagoons, and sand dunes; from Cape Varella to Cape Padaran the coast is again a typical ria coast. Terraces have been found in Vietnam and Cambodia between +2 and + 80 m (Carbonnel 1964; Saurin 1965). The terrace at 2 m and raised reefs, mollusks (oysters), and conglomerates at the same level have been found all along the coasts of Vietnam and Cambodia. Other terraces occur more regionally; the 4 m terrace only in Vietnam, the +15 and + 25 m terraces in southern Vietnam and Cambodia, and the terrace at 80 m only near Cape Padaran.
Compared to the mainland coast, Taiwan is very different. Situated in the tectonically active zone it has been slowly uplifted during the Quaternary. This uplifting continues at present, although there is some evidence for regional temporary subsidence. Eustatic sea-level changes are considered to have had little effect on the coast, since the changes in sea level have been more than balanced by the amount of uplift (Hsu 1962). On the Pacific side, the coast is steep and erosion predominates (as also on the southwestern and northern coasts), but on the west coast, which is sheltered from the ocean swell and the tropical storms as well as from the strong northeastern monsoon, a flat, marshy coast is formed with tidal flats. Coral reefs are present on the Pacific side as well as round the Ryukyu Islands further north and along the Pacific coasts of Kyushu and Shikoku.
The Philippine coasts, situated like those of eastern Taiwan in the tectonically active zone, are comparatively steep but intersected by river outlets, broad valleys, and inlets with beaches, surrounded by coral reefs. Although the area has been drowned during the Holocene transgression, the islands have been rising during the Pleistocene and Holocene and there is evidence of differential uplift. Numerous and often extensive marine terraces are present; on Sabtan Island and on Luzon the highest are at +180 m, on Bataan at +275 m, and in northern Mindanao at 360 m (Van Bemmelen 1970), but no correlations have been made.
The Malay Peninsula, together with the eastern part of Sumatra, western and central Kalimantan (Borneo), and the Sunda-Java Sea, belongs to the Sunda landmass, which has probably been tectonically stable since the middle Pleistocene. The presence of beach ridges several meters above present sea level has been interpreted as being the result of recent uplift, but can also be explained by crustal rebound after the Pleistocene and the increasing water load of the oceans (Clark et al. 1978).
The east coast of Malaya is relatively smooth with shallow bays and few indentations. More than 80% of this coast is formed by sandy beaches interrupted by river outlets and small deltas; cliffs form about 10% of the coast (Nossin 1965; Swan 1968). Tidal swamps with mangrove are usually found landward of the beaches and sandbars. They are sheltered against the swell of the strong monsoon that comes from the northeast between October and April. During this period there is more erosion than accretion, especially between late October and January, when spring tides are at their highest. Accretion is widespread between February and September, when the tides are less effective and the wind blows mainly offshore. A minor period of erosion occurs from May to July when the spring tides reach secondary maxima.
The west coast of Malaya is for a large part covered with mangrove, which continues northward to the Irrawaddy delta and from there to the mouths of the Ganges. Locally, this mangrove belt is interrupted by cliffs, river outlets, and beaches. The islands around Singapore have been studied in detail by Swan (1971), who has drawn attention to the fact that in this sheltered, low-energy coast the coastal forms are very different from those supposed to be characteristic for such coasts in the humid tropics. Mangrove swamps, tidal flats, beaches, and fringing reefs, are present as well as cliffs, caves, shore platforms, and beach conglomerate (ironstone). This diversity is due to the intense and continuous chemical weathering, which makes possible subaerial and marine erosion by small waves, and to differences in rock type and exposure. Coastal sediment is produced through cliff erosion, which takes place along both sheltered and exposed parts of the coast.
In Indonesia, mangrove swamps are present along the coasts of Sumatra, northern Java, along most of Kalimantan (Borneo), along northern Borneo (Sarawak), and at the southern end of Sulawesi (Celebes). All these areas are relatively sheltered against the ocean swell. Cliffs and beaches are locally present (Wall 1964), but although the rivers of Sumatra and Kalimantan carry large amounts of sediment to the sea, only the Kapuas and Pawan rivers, the Rajang and Baram rivers on the northern coast, and the Mahakam River in the east have built up sizeable deltas. Elsewhere, where sediment supply is abundant, broad alluvial lowlands have been formed. The coasts of Java are predominantly beaches interrupted by river outlets and deltas and locally by cliffs. However, along eastern Java, western Sumatra, and on most of the eastern Indonesian islands, the coasts are relatively steep. A few river deltas with associated swamps and alluvial foreland are present on the west coast of Sumatra and on the northeast coast of Java.
The main feature in western Indonesia is the large, drowned Sundaland (Molengraaff 1921). Two large submarine valley systems, being the continuation of the present rivers in Sumatra and Kalimantan, are present on the continental shelf. During the Pleistocene periods of low sea level, one system drained Sumatra and east Kalimantan, discharging into the South China Sea. The other system, draining Java and south Kalimantan, discharged south of Makassar Strait. A divide crossed the Sunda Sea between Sumatra and Kalimantan across Billiton and the Karimata Islands. The Sundaland plain has been dissected at least twice during the Pleistocene as a consequence of the eustatic lowering of sea level (Van Overeem 1960). In the whole area, including Malaya, eustatic terraces are present from 50 to 90 m (Tjia 1970).
On the former east coast of the Sundaland, a large barrier reef has been formed, stretching from Balikpapan on Kalimantan to the island of Sumbawa in the south. It is interrupted in many places and shows a large gap of about 100 km wide facing a deep embayment in the former Sundaland. The main river outlet was probably situated here. The reef began as a late Pleistocene fringing reef and has grown upward with the gradual rise in sea level. Locally, it reaches the sea surface as separate coral islands. Similar fringing reefs of smaller dimensions are present around Sulawesi and the smaller islands in eastern Indonesia, along the west coast of Sumatra (where they show gaps in front of the river deltas), and further north along the Nicobar and Andaman Islands. In the Sunda Sea, coral growth is restricted to a number of isolated areas away from muddy river outlets (Kuenen 1933; Umbgrove 1947). In eastern Indonesia, numerous atolls and barrier reefs have grown upward from gradually subsiding submarine ridges and platforms, rising abruptly and steeply from a depth of 1,000–2,000 m. The effects of winds and waves on the reefs are conspicuous, especially in eastern Indonesia, where the reefs grow more vigorously on the windward side. Sea currents cause erosion and may shape a whole group of reefs. Solution of coral occurs within the tidal range. On the former northern coast of the Sundaland no barrier reef has been formed. Here the former coast was flat and gradually merged into an extensive sandy and probably muddy shelf. The water in this area was presumably too turbid for coral reef growth. This is also the case along the south coast of Irian Jaya (New Guinea), where large alluvial plains have been formed, covered along the coast by mangrove.
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