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3.4 PALAEOENVIRONMENTS

In this section, I will provide a detailed analysis of palaeoenvironmental and

palaeogeographical data for the SE Asian region. The information is literature-based

and aims to provide a more complete picture of the palaeoenvironmental changes,

especially regarding the presence of land bridges between the present-day islands of

Java, Sumatra, and Borneo, and the Asian mainland. I also address to a lesser extent

Sulawesi and the Philippines. The resulting palaeoenvironmental and

palaeogeographical maps are presented in Chapter 3.5.

General patterns of vegetation changes

Tropical rain forests occur in areas typically receiving more than 2,000 mm of rain

annually, with not more than four consecutive months with less that 100 mm of

precipitation in two years out of three, with the coldest mean temperature not falling

below 18ºC, with small annual temperature variations and essentially frost-free

(Morley 2000). Further away from the equator, the nature of tropical rain forest

changes into what has been termed paratropical rain forest by Wolfe (1979 in Morley

2000). This type of evergreen rain forest is delimited by the 20–25ºC mean annual

temperature isotherms, in still essentially frost-free areas. Moving further away from

tropical rain forests along a gradient of increasing seasonality and decreasing rainfall,

forest eventually grades into savannah. The forest/savannah boundary is usually

abrupt and maintained by fire (Hopkins, 1974 in Morley 2000), occurring over a few

hundred metres, approximately where annual rainfall drops below 1,500 mm.

From the start of the Miocene until now, Sundaland is thought to have undergone

significant changes in its land/sea distribution and predominant vegetation types. It is

thought that in tropical Asia, precipitation amount and the length of the dry period

play a more important role than air temperature in changing vegetation patterns.

Temperature changes of 1–3 ºC will not seriously affect the vegetation in the

lowlands, although in highland areas such changes may shift vegetation zones

(Urushibara-Yoshino & Yoshino 1997). Fig. 3.3 shows the distribution of the forest

types in SE Asia before the 19th and 20th century forest destruction started.

Solving mammalian riddles Chapter 3. The palaeoenvironmental scene based on data mining

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Figure 3.3. Original forest cover (before large-scale destruction) of SE Asia (after MacKinnon 1997). Green areas represent tropical everwet forest types; beige areas are deciduous and dry forest types; brown areas are montane forest types; and blue areas are mangroves.

In the Early Miocene the Sundaland area was mostly covered in dry, seasonal

vegetation which, from 21 Mya ago, started to change into much wetter forests,

including peat swamps (Morley 1999). At 20 Mya the rainforest area covered all of

Sundaland and extended as far north as southern China (Song et al., 1984 in Heaney

1991), southern Japan (Tsuda et al., 1984 in Heaney 1991), and northern India

(Mathur, 1984 in Heaney 1991). Between the late Middle Miocene and early Late

Miocene (before 15 Mya) there was a period of drier climates with maxima of

Gramineae pollen (Morley 1999), probably due to global glaciations (Moss & Wilson

1998) and global cooling which led to continental climate conditions in Sundaland

(Batchelor 1979). Tropical rainforest was pushed south as far as Malaysia (Smith et

al., 1994 in Morley 1999), and a seasonal savannah may have run through Late

Miocene Sundaland (Batchelor 1979). It is unlikely that the Late Miocene savannas in

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Sundaland consisted of grasses only, more likely they would have been woodland

savannas. The rest of the Late Miocene was once more warmer and wetter and

rainforest could be found as far north as Vietnam and Thailand (Morley 1999). Dry

and continental conditions dominated again in the Pliocene (Batchelor 1979; Morley

2000), and rainforest did no longer occur north of the Natuna islands and the Malay

Peninsula (Morley 2000).

The Pleistocene was characterised by much climatic variation and very pronounced

changes in sea-levels and accompanying changes in vegetation cover (van den Bergh

et al. 1996). In the final 300 Kyr of the Pleistocene, pollen records from island SE

Asia show a vegetation and climatic response to glacial cyclicity with interglacials

being substantially wetter and warmer than glacials (Kershaw et al. in press).

Superimposed on this cyclicity is strong evidence for a general and possibly stepwise

decline in forest types relative to more open sclerophyllous or grassland vegetation

(Kershaw et al. in press). Data from several sites in SE Asia suggest that, during the

LGM, montane vegetation components expanded into or near to the lowlands.

Precipitation was reduced substantially during the LGM, extending to the beginning of

the Holocene. There was a strong expansion of grasses at many sites indicating that

lowland rainforest was at least reduced in surrounding areas (Dam et al. 2001). It has

been suggested that during the LGM, and other glacial phases of the later part of the

Pleistocene, present-day lowland and montane elements were intermixed, forming

associations with no modern analogues (Colinvaux et al. 2000).


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Palaeoenvironments of Java

Geology and palaeogeography of Java and the Java Sea

The land area of Java came into being sometime in the Miocene (Whitten et al. 1996).

Initially ‘Java’ was present as volcanic islands, which eventually coalesced to form a

single island. Fig. 3.4 and the legend in Fig. 3.7 provide an overview of the main

geological features of Java, including the major basins and platforms.

Figure 3.4. Main geological features of Java (after Steinshouer et al. 1997).

West Java

Van Bemmelen (1970) provided the following description of onshore West Java. This

area can be divided into four E–W trending belts: The Southern Mountains of West

Java, the Bandung Zone, the Bogor Zone, and the lowland plain of Jakarta. Some

emergent land was already present in the Southern Mountains area during the Eocene,

which was further uplifted during the Oligocene. This Oligocene geanticline was a

non-volcanic range or a row of islands, which became submerged during the Early

Miocene. In the end of the Middle Miocene, the Southern Mountains Zone was arched

up and became emergent, only to subside once more during the rest of the Miocene,

although some higher areas, like those in the central part of the Southern Mountains,

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south of Pegalengan, remained subaerial during the Late Miocene. During the

Pliocene, the western and eastern Southern Mountains probably remained low land,

with higher areas in the central parts. Other land areas remained in the Mt. Kancana

area, east of Sukabumi, and the Rongga Plateau, southeast of Cianjur. These areas

were further uplifted during the Pleistocene. In the Middle Pleistocene, the Cianjur-

Bandung Valley consisted of lakes and swamps, containing fossil of the Jetis (=Trinil)

fauna. Further west, the mountains of South Bantam formed a link between the

geanticlines of Sumatra and Java. They were already emergent, probably as a row of

islands during the Early Eocene, as part of the non-volcanic outer arc of the Sunda

Mountains System. This land disappeared during the Oligocene, and only reappeared

in the Late Miocene, although van Bemmelen mentions that the Bayah area (NW of

Pelabuhanratu) remained above sea-level during the Early Miocene.

Figure 3.5. Legend to geological maps used in Figs. 3.4, 3.7, 3.10, 3.12 and 3.13.

Black dots represent oil field center points and red dots gas field center points.


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Volcanic activity during the Pliocene brought about a general subsidence with a

marginal transgression of the sea. The Malang, Endut, and Halimun volcanoes are of

Middle or Late Pleistocene age. Further west still, the Mt. Honje and Ujung Kulon

areas form the southeastern end of the Barisan Range of Sumatra. The Lampung High,

and upland area that joined the areas of Java and Sumatra, collapsed and subsided

below sea-level, accompanied by enormous outbursts of tuffs (van Bemmelen 1970);

van Bemmelen thought that this happened during the Pliocene, but Ninkovich (1979 in

Ninkovich et al. 1982) dated these tuffs at ca. 7 Myr which indicates a Late Miocene

minimum age of the destruction of the Lampung High (and also a minimum age for

the opening of the Sunda Strait.

In the northwest of Java an island was formed in Early Miocene times after which a

transgression took place from the southeast, and only the Tangerang High remained

above sea-level. While transgressions continued in the Middle Miocene the Tangerang

High was eventually submerged as well. This lasted until the Pliocene when the whole

area of northwest Java emerged (Sujanto & Sumantri 1977). The east side of West

Java (east of Bandung town) had a marine to littoral environment until the Late

Pliocene after which it was covered by Quaternary volcanic and fluvial deposits. In

the Early Pleistocene, fluvial sedimentation occurred in a meandering river system and

in a swamp environment (Zaim 1999).

The Bogor Zone is an anticlinorium extending from Rangkasbitung in the west to

Bumiayu in west Central Java. The western part is characterized by Neogene and

Early Pleistocene breaks in sedimentation, suggesting that land and forests (there are

many remains of silicified wood) emerged and submerged several times. The Danau

Complex in the Mt. Karang area remained emergent after the Strait Sunda area

collapsed during the Pliocene. The central and eastern part of the Bogor Zone became

emergent at the end of the Pliocene, when brackish conditions developed, and more so

in the Early Pleistocene. The area between Purakarta and Cirebon was emergent

during most of the Pliocene–Early Pleistocene, and contains one of the thickest non-

marine successions on Java (Setiadi 2001).

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Central Java

Central Java can be divided into the following structural units: The south coast plain

with the Karangbolong Mountains, the South Serayu and West Progo Mountains, the

Serayu depression, the North Serayu Range, and the north coast plain (van Bemmelen

1970). The ‘Southern Mountains’ zone is almost absent in Central Java, as most of

these deposits slumped southward in the Late Miocene and Early Pliocene, and only

isolated remnants crop out, i.e. Nusakambangan, Selok Hill, and the Karangbolong

Mountains (De Goffau & van den Linden 1982). In the Early Miocene, the only

emergent land in this area was the Menoreh Range, west of Yogyakarta, which was

mostly submerged again, probably during the Middle Miocene transgression. During

this transgression, the central and western parts of the Progo Mountains probably

remained emergent. After the end of the Miocene, the area of the West Progo

Mountains remained low land, in which the Menoreh Range formed a ridge of

maximum 400 m height (van Bemmelen 1970). The South Serayu Range probably

became emergent in the Middle Miocene when the area was uplifted, but was

submerged again, possibly by the Mid-Miocene transgression. By the end of the

Miocene or Early Pliocene, this area was uplifted again, and the Serayu Range became

subaerial (van Bemmelen 1970). This was accompanied by extensive volcanism in the

Early Pliocene (de Goffau & van den Linden 1982). At this time, the Serayu Basin

north of these mountains was still marine. The north Serayu Range was uplifted during

the Pliocene and Pleistocene. Initially, a number of volcanoes accumulated during the

Pliocene, and partly emerged above sea-level, forming islands in a shallow sea. The

whole range probably became emergent during the Early Pleistocene (Kali Glagah

beds of ca. 2 Mya). There are, however, several sedimentation series that change

between a freshwater and marine character, which indicates that the land was probably

low-lying and influenced by sea-level changes (van Bemmelen 1970).

The area of south-central Java did not emerge until Late Miocene–Early Pliocene

times when some highs were exposed. However, as a new transgression occurred

around that time, these areas were rapidly submerged again. Only in the Late Pliocene

was the northern part of south-central Java uplifted in connection with the forming of

the Java geanticline and some areas were exposed and acted as sediment sources for

shallow seas nearby (Sujanto & Sumantri 1977). In the Sangiran area, from Middle


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(2.99 ± 0.47 Mya) to Late Pliocene (app. 2.2–1.8 Mya), the depositional environments

gradually changed from marine, to shallow open marine, to coastal and finally

brackish. From Late Pliocene to late Early Pleistocene (app. 1.2–1.0 Mya), the

brackish water basin gradually changed to a fresh water environment (also see

reconstructions in Huffman 1999). Here in the Solo Basin, an abrupt change from

lacustrine to fluvial sedimentation marks the contact between the Pucangan and the

overlying Kabuh formation (Larick et al. 2001). From late Early Pleistocene onwards,

this coastal environment was followed by lacustrine to fluvial environments, which

were covered by a large-scale lahar (probably from a volcano east of Sangiran) at ca.

600 Kya and another one at ca. 400 Kya, resulting in a dry land environment. The

sedimentation of this last period seems to have been characterised by cycles in which

firstly erosion occurred in a dry land environment, followed by volcanism and ending

with deposition in a fluvial environment (Itahara et al. 1985). The area of middle

central Java did not emerge until the end of Miocene time when a regional uplift

occurred in the south and some areas were exposed (see above for south central Java).

The arching up of the Java geanticline again occurred in Plio-Pleistocene times,

uplifting most of the area.

The North Serayu Mountains of Central Java became terrestrial in the Late Pliocene

(Kali Glagah beds), while to the east on the western and central parts of the Kendeng

Ridge the change comes a bit later in the Early Pleistocene (van Bemmelen 1970). The

Kendeng Ridge probably became emergent at the end of the Pliocene, as the volcanic

breccias directly on top of the marine beds have been dated at 1.87 Mya (Bandet et al.

1989).

East Java

In northeast Java, the northern parts (Kujung area) emerged in Middle Miocene times

(van Bemmelen 1970) but were submerged as a stable high during transgression in

Middle and Late Miocene times. The entire area of north-east Java remained

submerged until Pliocene-Pleistocene times (Sujanto & Sumantri 1977). In the Early

Pleistocene, the giant volcanos of East Java (Tengger-, Ijang, Ijen-complexes) were

built up (van Bemmelen 1970). Note that van Bemmelen uses a different definition of

the Pleistocene, and his Early Pleistocene probably correlates to the Mid-Pleistocene

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used in this thesis. Madura probably became emergent after the Early–Middle

Miocene, but after this the sea transgressed over most of the land. In the Early

Pliocene, several areas on the south coast of eastern Java were emergent (Schiller et

al. 1994), but the entire eastern Java area probably only re-emerged in the Late

Pleistocene (van Bemmelen 1970), and during the Pleistocene the area between

central Java and Bali was probably a chain of volcanic islands.

In Early–Mid Pleistocene times (ca. 1.3 Mya) a fresh water lake existed between the

Willis Volcano in the East, and Solo in the West. The lake was bordered in the north

by the Kendeng Hills, which were in turn bordered by a sea-strait formed by the

Randublatung Zone (see p. 565 in van Bemmelen 1970). In the black clays that were

deposited in this lake many fossils belonging to the Trinil fauna (see Chapter 4.1) have

been found, which would point to an age of about 0.9 Mya rather than the above-

mentioned 1.3 Mya (see van den Bergh 1999). The Willis Volcano was already active

in the early Middle Pleistocene (Bandet et al. 1989), before the Mid-Pleistocene uplift

of the Java geanticline (van Bemmelen 1970). The eastern part of the Kendeng Ridge

probably remained submerged until the Late Pleistocene or Holocene, while even

further east, in the Madura Strait, the environment is still marine. North of the ridge,

the Randublatung Zone probably remained marine until the Mid-Pleistocene or later.

North of the zone, the northern part of the Rembang Hills emerged above sea-level

during the Late Miocene–Early Pliocene, but the southern parts remained marine until

the Pleistocene. The northern parts and the adjacent parts of the Java Sea were uplifted

twice during the Pleistocene, but later the Java Sea parts subsided again. This area is

an extension of the Pulau Laut centre of diastrophism that also included the island of

Bawean. Its southwestern part was uplifted during the Miocene and again during the

Pleistocene; the latter leading to establishment of the Lasem and Muriah Volcanoes.

According to van Bemmelen (1970) the southern part of this area only subsided below

sea-level during the Late Pleistocene–Holocene. During the Holocene sea-level

highstand, a sea strait was also formed between Semarang and Rembang, isolating the

Muriah volcano on an island; silting up of this strait continued into the Middle Ages,

when ships could still navigate the strait.


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During a major Mid-Pleistocene glacial (possibly at ca. 800 Kya), the shallowed parts

of the southern Java Sea became subaerial for the first time, while in the deeper parts

islands emerged. These land areas became submerged during the following

interglacial. During the Late Pliocene–Middle Pleistocene, the eastern part of the

southern sea was folded eventually resulting in the present Kendeng Hills between

Semarang and Surabaya. These hills initially emerged during the Mid-Pleistocene

regression, to be submerged again in the following transgressive period. Only in Mid-

Late Pleistocene times became this area permanently emergent (Smit-Sibinga 1947b).

Two other small islands were formed during Early–Middle Pleistocene regressions,

i.e. the small Pucangan anticlinal northwest of Mojokerto, and an island on present-

day Madura.

Java Sea

After the late Cretaceous, most of south-eastern Borneo and the Java Sea seem to have

been above sea-level, and eroded to a surface of low relief, with occasional hilly

remnants remaining. Subsequently, this surface was submerged (Cater 1981). In the

Late Oligocene, the shorelines of the Sunda highlands in the western Java Sea were

roughly parallel with the present-day north coast of Java, while the relatively high

areas of the Eastern Shelf (north of the coast between Cirebon and Semarang) and the

Seribu Platform were separated by the subsiding Ardjuna Basin (Ponto et al. 1988).

Palaeodepositional maps published by Ponto et al. (1988) show the migration of the

Sunda Shelf shoreline in Late Oligocene to Early Miocene times toward the north (but

note that the land was to the north (Sundaland) rather than to the south of this

coastline, as it is today). During Early Miocene times, a marine transgression from the

south flooded much of the south Sunda Shelf including all of the Northwest Java

Province (see Fig. 2 in Bishop 2000). Widespread marine highstands in the Miocene

deposited marine shales and marls over much of the western Java Sea, while a major

regression in the Middle Miocene resulted in the deposition of clastic shallow marine,

shoreline, deltaic, and continental deposits. Posamentier (2001) found evidence of

Miocene river channels, flowing towards the southeast just north of the present-day

north coast of Java (north of Cirebon). These appeared to have been established

directly on the palaeo-sea floor, indicating that they developed in response to a rapid

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sea-level fall; possibly these river systems were related to the Middle Miocene sea-

level lowstand.

The Late Miocene to Plio-Pleistocene Cisubuh Formation coarsens and shallows up

representing regression to the south with sediment input from the east and west.

During the Pleistocene, the Sundashelf was exposed intermittently, and shales, fluvial

clastics, and volcanics were deposited (Pertamina, 1996 in Bishop 2000). In the

northeast Java Basin, a sea-level drop towards the end of the Pliocene resulted in a

shallowing upward sequence of varies limestone facies, eventually leading to

emergence of the present-day onshore areas of east Java (Musliki & Suratman 1996).

The Sunda and Seribu Platforms, high areas in the Java Sea (see Fig. 3.6), remained

emergent in earliest Miocene times until the start of the Middle Miocene (Sudiro

1973), although parts of the ridge are covered by lower Miocene reefoid limestone

(Ben-Avraham & Emery 1973). An even higher area, the Karimun Jawa Arch (Fig.

3.6) probably remained emergent throughout the entire Tertiary (Koesoemadinata &

Pulunggono 1975; Nayoan 1975). Also, van Bemmelen (1970) suggested that the

Karimun Jawa Arch was emergent during much of the Pliocene, and only became

submerged sometime during the Pleistocene. There appears to be a conflict in the data

on submergence/emergence of the Karimun Jawa Arch, maybe due to poor dating; this

might suggest that a low ridge existed that was particularly sensitive to sea-level

changes; this is important for the region’s zoogeography as it may have provided a

landbridge between Sundaland and Java. Similar lower Miocene limestone horizons

occur in the East Java Sea, where other highs, the Meratus and Pulau Laut Ridges, and

possibly the Bawean Arch, were uplifted after the deposition of the limestones (Ben-

Avraham & Emery 1973). Clastic sediments in the Bawean Basin also indicate that in

Early and Middle Miocene the interior of Kalimantan, and perhaps the Bawean Arch

were above sea-level (Cater 1981); for the latter area Manur and Barraclough (1994)

suggested that it remained an island until Early Miocene times.

An analysis of the stratigraphical evolution in the East Java Sea by Bransden and

Matthews (1992) showed that some areas were exposed during an Early Miocene sea-

level fall, including the north Lombok carbonate system, and Madura, which went

through a number of clastic cycles. The Sundaland shoreline was probably located


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near the present-day island of Bawean. During the subsequent Late Miocene

transgression these areas were mostly submerged. This Late Miocene is transgressive

overall, causing retreat of the shelf margin from its maximum position in Middle

Miocene. The East Java Sea Basin, which is located east of Madura and north of Bali,

Lombok, and Sumbawa, was a marine environment during the Early to Middle

Miocene highstand (Matthews & Bransden 1995). In the late Middle Miocene to

earliest Pliocene, much of the East Java Sea Basin was marine, while on the northern

platform between East Java and Sulawesi carbonates developed around subaerial

highs.

During an Early Pliocene lowstand, the Sunda shield to the west and north-west of the

East Java Sea Basin underwent fluvial incision leading to sandstone deposition in the

basin. At that time, the Sundaland coastline was located approximately north of

Madura and north-west of Kangean, where the deposits also indicate the location of

Figure 3.6. Main geological features in the North-West Java Basin (after Bishop 2000).

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incised river valleys (see Figure 10 in Matthews & Bransden 1995). At this time, the

exposed highs on the northern platform could have provided potential stepping stones

between Sundaland and Sulawesi. Finally, in the Early Pliocene the shelf edge once

more started to move south and eastwards because of a sea-level fall, probably

running just north of Madura and the Kangean area. Bransden and Matthews (1992)

suggest that the resulting incised river system would be similar to the submerged

Holocene drainage pattern on the Sunda Shelf as mapped by Tjia (1980) and

Verstappen (1980). Finally, in Plio-Pleistocene times, the shelf area in the East Java

Sea showed the effects of repeated progradation and transgression, but the detailed

sequence stratigraphy remains poorly understood (Bransden & Matthews 1992).

Interestingly, some of the exposed carbonate platforms in the north East Java Sea are

located very close to the south-western edge of Sulawesi and these would have

provided possible stepping stones for animal dispersal between Sundaland and

Sulawesi (see Figure 22 in Bransden & Matthews 1992).

Smit-Sibinga (1947b) described further geomorphological features of the Java Sea at

the start of the Pleistocene. Beside, the above-mentioned Karimunjawa land bridge, a

land area (island) existed, situated in the central part of the present Java Sea, which

included Madura, the Rembang Hills of NE Java, and Bawean. According to Smit-

Sibinga, the Pleistocene Java Sea consisted of 3 distinct parts:

1. A southern basin, including the northern part of the present island of Java, and

the southern part of the present Java Sea

2. A land area in the central part of the present Java Sea

3. A northern basin, including the northern part of the present Java Sea and a part

of the coastal plain of Borneo. The detritus, rich in radiolarites, carried by the

rivers from Borneo, has not been found in the Pleistocene sediments of the

southern basin, which suggests that the central land area presented a watershed

between the northern and southern basins.

Smit-Sibinga therefore envisaged that during the LGM, the Java Sea consisted of two

basins separated by a low dividing ridge. The southern basin was drained by the rivers


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of northern Java, the northern basin by the rivers of southern Borneo. Detailed

reconstructions of Pleistocene palaeochannels near the Masalembo Islands (south of

SE Borneo) by Ilahude and Situmorang (1994) possibly support Smit-Sibinga’s

theory. These reconstructions show that, during the LGM, several major rivers

(including the Barito and Riam) deviated to the east at about 6º S, whereas the delta of

the main river channel that collected its waters from Java was located at about 7º S. It

appears, however, that at a later stage (post LGM, Holocene), the northern rivers no

longer deviated to the east but continued south. It is therefore possible that at the start

of the LGM there were two parallel river system running west to east in the Java Sea

area, but that these were joined at a later stage.

Other river valleys in the western Java Sea were described by Posamentier (2001), a

Late Pleistocene, 3-4 km wide, 200 km long valley running south-southwest towards

the Sunda Straits, and one which flowed from west to east some 20 km north of the

present day coast of northwest Java; Posamentier thought that the latter river was

possibly associated with the sea-level lowstand at 160 Kya. The nature of these rivers

suggested to Posamentier that only the sea-level drop during the latest glacial exposed

the entire shelf area; those before that (from Middle–Late Pleistocene) did expose land

but not the entire shelf area. This also included the lowstands between 105 and 20

Kya, which did lead to the emergence of land in the western part of the Java Sea, but

did not expose the entire shelf. Posamentier’s data also suggest that most of the

Pleistocene can be considered lowstand time, with brief (app. 10–15 Kya) periods of

sea-level highstand. The same Late Pleistocene river valley was described by Gresko

and Lowry (1996); it is located in a large present-day bathymetric trough, 20–30 km

wide, more than 300 km long and with over 100 meters of relief. The trough was the

primary drainage path for rivers from Northwest Java and southeastern Sumatra into

the Indian Ocean during sea-level lowstands. During subsequent rises in sea-level, the

trough filled with sediment except in localized areas where strong currents kept the

channel relatively free of sediment. Finally, Butlin (1989) speculated that during the

LGM a very large freshwater lake existed in the western Java Sea, which he named

Lake Baalzephon; I found no supporting evidence for such a lake, but it could be that

it existed in the lower lying areas east of the Pulau Seribu ridge, where a basin appears

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to exist with water depths >50m (Peta Lingkungan Laut Nasional, 1:500,000, LLN-

08).

Sunda Strait

Motion along the Sumatran Fault System began at about 13 Mya in northwest

Sumatra, which, as a result of the northwestward movement of the Southwest Sumatra

Block, eventually led to the opening of the Sunda Strait (Lee & Lawver 1995). During

the Miocene and Pliocene, this area between Java and Sumatra, which included the

Lampung and Bantam areas, formed an elevated part of the basement complex, which

separated the basins of East Sumatra and North Java. Coward et al. (2000) suggested

that initially the landmass of this area formed a high plateau some 3 km above sea-

level, of which the ridge underlying the Pulau Seribu island group of north West Java

marks the northeastern edge (van Bemmelen 1970). At the end of the Pliocene–Early

Pleistocene, this area was at first domed up, and thereafter the central part collapsed

and was engulfed (van Bemmelen 1970). This seems to fit the data by Nishimura and

Suparka (1997), who asserted that the Sunda Strait began to open at 3 Mya, and is

continuing to do so to the present. Lelgemann et al. (2000) described the stratigraphy

of a small part of the Sunda Strait. The absence of Pleistocene sedimentary cover on

the higher areas in the Sunda Strait suggest that these areas were emergent. It remains,

however, unclear when in the Pleistocene this happened; presumably at least until the

Late Pleistocene as otherwise there would have been some sedimentary cover. This

fits the data by Gingele et al. (2002), who, on the basis of sediment composition in the

area offshore SW Sumatra, deduced that the Sunda Strait became emergent at 74 Kya

and did not become inundated until 12 Kya. This means that, for 62 Kyr, Java and

Sumatra were connected by land, although the Sunda Straits River would still have

been a partial barrier to animal movement. Finally, according to Obdeyn (1941–1944

in van Bemmel 1949), the Sunda Strait only became navigable for ships after ca. 1,175

AD, which suggests that the sea between Sumatra and Java was still narrow during

most of the Holocene.

Land bridges

Van Bemmelen (1970) mentioned that the Karimunjawa Islands in the Java Sea were

the highest summits of the surrounding Sundaland, which still formed a highland in


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the Neogene. This elevated area supplied part of the clastic sediments of the adjacent

geosyncline of northern Java. In the Late Neogene and early Quaternary this general

pattern was reversed by the subsidence of Sundaland, so that the river systems in the

Java Sea could develop. Bawean Island on the other hand cannot be considered as an

element of the stable Sunda Land, and it belongs to the unstable marginal areas that

were subjected to considerable vertical movement during the Neogene and

Quaternary. Van Bemmelen (1970: 301) suggested that these areas formed a Pliocene–

Pleistocene land bridge between Malaya and Central Java, via the Riau-Lingga

Archipelago, Bangka, Belitung, and the Karimunjawa Islands to the North Seraju

Range of Central Java. This land bridge slowly eroded during the Pleistocene, and

probably completely disappeared below sea-level only during the last interglacial. The

similarity between the basement of Bangka and Belitung and that of the Karimunjawa

Ridge is also pointed out by Ben-Avraham and Emery (1973). The present shape of

the Java Sea is therefore likely to be younger than ca. 100 Kya (Smit-Sibinga 1947a).

According to Ben-Avraham and Emery (1973), the Madura Ridge and Rembang

anticlinorium emerged from the sea during the Pleistocene. Raven (1947) estimated

that this land area may have been hilly country with a maximum altitude of 100 m.

Furthermore, a southern sea existed, comprising the northern part of the present island

of Java and the southern part of the present Java Sea, and a northern sea, comprising

the northern part of the present Java Sea and part of the coastal plain of south Borneo

(Smit-Sibinga 1947b). From this it appears that in the Early–Middle Pleistocene

Central and East Java were linked to Asia via a land bridge. West Java was separated

by a narrow Sunda Strait from Sumatra, while a largish island existed in the Bawean–

Madura area. The rivers of southern Borneo did not pass the land area in the central

part of the Java Sea, as discussed earlier, and did not reach the southern basin (Smit-

Sibinga 1947b).

Vegetation of Java and the Java Sea

Morley (1991) pointed out that pollen finds in the Java Sea in the earliest Miocene

suggest a markedly seasonal climatic regime, with species indicating a swamp or

mangrove setting. Pollen grains of Mimosaceae, Compositae and Loranthaceae are

also observed (Cater 1981). Morley (2000) further reported on the vegetation of the

65

Early Miocene Talang Akar formation in the Java Sea, which was characterized by

swamp thickets and peat swamps. This suggests that the climate of Java was wet and

warm during most of the Miocene and land would have been covered mostly in

rainforest and peat-swamps (Whitten et al. 1996). Finds of fossil pollen in East Java

indicate that during the Middle Miocene there were alternating cycles of brackish

swamp forest, mangrove and back mangrove forest, probably related to sea-level

changes (Yulianto & Rahardjo 1998). On top of this formation pollen indicates

changes from a dry land forest to forest dominated by riparian vegetation or

mangroves and back mangroves (Rahardjo et al. 1998). During the Late Miocene and

Pliocene, R.J. Morley et al. (2000) found a very marked increase in Monoporites

annulatus (Gramineae) and Casuarina in the Java Sea area, which they believed to be

related to a marked climatic change at that time leading to much cooler and drier

conditions during the Pliocene (also see Morley 1999). It is possible that the

occurrence of Casuarina savannah in Java relates to phases of forest destruction as a

result of volcanic activity, since Casuarina junghuhniana is presently dominant on

volcanoes of East Java and Nusa Tenggara, where it forms a fire-climax forest.

Rahardjo (1999) described the changes in climate and vegetation around the Bakung

River in the Mojokerto area of East Java at the Pliocene-Pleistocene boundary. Based

on palynological analysis, climatic drying occurred at that time, which is characterised

by a distinct increase in Gramineae and by the reduction of arboreal pollen. Judging

from the pollen diagrams provided by Rahardjo (1999), there are two peaks of

Gramineae dominance around the Pliocene-Pleistocene boundary, with a period of low

Gramineae occurrence (25% of all pollen) at the boundary. The last appearance of

planktonic foraminifera was observed at about 50 meters below the Plio-Pleistocene

boundary, indicating a Late Pliocene emergence of the area, possibly during the 2.7

Mya low stand. Within the Early Pleistocene, palynological and lithological data from

the Kaliglagah Formation also suggest dry climates, with abundant charred Gramineae

cuticles indicating the expansion of savanna in the Late Pliocene–Early Pleistocene in

the Central Java region. A similar palaeoenvironment was suggested for the Early

Pleistocene Mojokerto region, where pollen indicated extensive grassland with hardly

any trees (R. Morley, pers. comm., 12 April 2003).


66

Interestingly, and in apparent contrast to the findings in the previous paragraph,

species such as Chiropodomys gliroides indicate a forested environment at the time of

the Satir fauna, in the Early Pleistocene. However, the presence of wide-toothed

species of Mus also indicates the presence of grasslands (van der Meulen and Musser

1999). It therefore seems that during the Late Pliocene–Early Pleistocene, open

vegetation existed with grasslands in the drier parts and forest areas along rivers and

presumably on mountain sides.

Dubois (1892) was one of the very early scientists to make assumptions about the

Pleistocene palaeoenvironments of Java. Based on his fossil finds of crocodile,

hippopotamus, and river turtles he suggested that lakes covered the large plains of

Central Java at the times of the Kendeng fauna (= Early Pleistocene). Tokunaga et al.

(1985) described the palaeoenvironment in the Sangiran area in Central Java based on

the analysis of pollen records. At about 2.0–1.6 Mya, grassland containing Cyperaceae

and Gramineae dominated the area, while ferns were also common. Furthermore,

pollen records of tropical trees such as Casuarina and Florschuetzia indicate the

presence of mangrove swamps, while marine and fresh-water algae suggest marine or

lacustrine marshes or lake environments. At 1.51±0.25 Mya grasslands with wet

conditions occurred, suggested by the domination of Gramineae and Typha, and the

disappearance of ferns and tropical trees. At about 1.4–1.1 Mya, grasses mostly from

the Gramineae family dominated the site, with some ferns and no evidence of trees.

This environment gradually changed into one in which Cyperaceae were dominant in

relation to Gramineae, with many ferns, indicating a marshy, sea-shore environment.

Following these periods, a very wet environment developed with peat formation and

ferns. Pollen found just below the Grenzbank Zone suggests that the surroundings at

that time was almost entirely covered in ferns, while grasses almost disappeared.

Schuster (1911, in van Heekeren 1972) found a fossil arboreal flora at Trinil in Java,

just above the chief fossil-bearing bone bed. The flora comprised 54 species of which

24 still occur in Java, but at an altitude of 600–1,200 m a.s.l. Taken at face value, this

would mean that Java in the Middle Pleistocene had a climate cooler and damper than

at present with a temperature of 6–8 °C lower, although it cannot be excluded that

plant remains had been washed down from higher sites in which case no

67

palaeoenvironmental conclusions can be drawn. Medway (1971) described the Middle

Pleistocene environment of Java as very diverse with tall evergreen forests, shrubby or

woodland vegetation, and extensive grasslands, reminiscent of the mosaic of forest,

scrub and grassland, such as seen today in wetter, hillier parts of East Africa (e.g.,

western Uganda) or India.

During the Pleistocene climatic fluctuations probably increased, leading to more

seasonality and more extremes in precipitation and temperatures. De Goffau and van

der Linden (1982) described buried andosols at low altitude in south Central Java. In

this area, under the present climate, these soils only form at altitudes >600 m, and de

Goffau and van der Linden suggested that this indicates cooler and wetter Pleistocene

conditions, and a vegetation of tropical rainforest, but they could not determine when

these conditions prevailed. Furthermore, the presence of oxisols, indicated that during

the glacial periods there was also increased seasonality, which is required for the

formation of these soils (De Goffau & van den Linden 1982).

Van der Kaars and Dam (1995, 1997) investigated the Late Pleistocene

palaeoenvironments of the Bandung area in West Java, where today annual rainfall

amounts to between 1,500 and 2,000 mm (Whitten et al. 1996). Around 135 Kya,

annual precipitation, inferred from palaeosol development, is estimated to have been

750–1,000 mm with strong seasonality in the Bandung area. Between 126 and 81 Kya,

very warm and humid conditions prevailed on Java with up to 2,000 mm of annual

precipitation. At around 81 Kya the vegetation in the Bandung plain changed from

closed fresh water swamp forest to open herbaceous swamp, dominated by grasses and

sedges, suggesting a reduction in annual precipitation to ca. 1,000 mm; between 74

and 47 Kya, slightly warmer conditions existed. Finally, between 47 and 20 Kya,

cooler and possibly drier conditions existed in Bandung area.

During the LGM, the lowering of forest altitudinal zones in the Javan and Sumatran

highlands and increased abundance of gymnosperms at moderate elevation imply a

considerable fall of annual temperature down to 7ºC lower than at present, although no

increased dryness was observed (Stuijts et al. 1988). By contrast, a clear indication for

cooler and drier climates in the lowland of West Java is provided by a lowering of the

montane forest vegetation boundaries by 1,200 m, associated with changes from


68

closed freshwater swamp forest to open herbaceous-swamp dominated by Poaceae and

Cyperaceae (van der Kaars, 1998 in Sun et al. 2000). Also, Late Pleistocene black

alkaline clays (presumably vertisols) in East Java (Smit-Sibinga 1947a), indicate a

pronounced seasonal, dry monsoon climate. However, Gingele et al. (2002) suggested

that such ‘seasonal’ scenarios for the last glacial were not seasonal at all, but dominant

during most of the year. They concluded that the ‘dry season’ may have prevailed

from 74–70 and 20–12 Kya, whereas the ‘wet season situation may have prevailed

from 70–55 and 35–20 Kya. This fits the data by van der Kaars et al. (2001) who

provided more details on the Late Pleistocene vegetation of a lowland swamp in West

Java.

The presence of Early Holocene peats at 30 m below sea-level in the eastern Java Sea

between Madura and Kalimantan suggests that peats occurred before the Java Sea was

inundated after the LGM (Situmorang et al. 1993). Peats are widespread in this area,

but unfortunately, the data provided in the abstract of Situmorang et al.’s paper do not

indicate whether they were only present after or also during the LGM.

Palaeoenvironments of Sumatra

Geology and palaeogeography of Sumatra

At the present time Sumatra represents part of the Sundaland continental plate.

Oceanic crust flooring the Indian Ocean is being consumed at the western margin of

this plate (Curray et al., 1979, in Cameron et al. 1980). This subduction has given rise

to extensive volcanism, which is generally considered to form the NW extension of

the Sunda Volcanic Arc of Java, Bali and adjacent islands. In Sumatra, this arc has a

classic morphology of trench, accretionary prism, outer-arc ridge, forearc and volcanic

chain. Simeulue, Nias, the Batu islands, the Mentawai islands (Siberut, Sipora, and

North and South Pagai) and Enggano form the north-west to south-east sub-aerial

expression of the Sumatran outer-arc ridge (Samuel et al. 1997). Stresses resulting

from the subduction in the Sumatra region have been released periodically by dextral

movement parallel to the plate margin, and these have produced the major Sumatra

Fault System, which runs the entire length of the island (Rock et al. 1982). This

system is the only Cenozoic plate boundary that may separate part of Sumatra from

the Malay Peninsula (Lee & Lawver 1994). The displacement along this strike-slip

69

fault has been estimated as being no more than 100 km, with apparently all the

observed motion having occurred since the Middle Miocene (13 Mya) (Huchon and

Le Pichon, 1984 in Lee & Lawver 1994).

Structurally Sumatra can be divided into six main basins: West Aceh, Northwest

Aceh, North Sumatra, West Sumatra, Central Sumatra, and South Sumatra, while also

the Barisan Geanticline and the Nias group form distinct geological units (Cameron et

al. 1980). De Coster (1977) considered the Central Sumatra and South Sumatra Basins

as one large basin, which has, however, been separated from the northern basins

through most of history by the Asahan Arch (see Fig. 3.7). This Asahan Arch and also

the Lampung High at the south and east side of the South Sumatra Basin have been

emergent throughout much of the Tertiary, apart from when they were submerged in

the Early and Middle Miocene (de Coster 1977). The North, Central, and South

Sumatra Basins are probably of Miocene and younger age (Lee & Lawver 1994). The

South Sumatra Basin received most of its sediments in the Miocene from the Sunda

Plate to the north and from the Lampung High in the east (Sitompul et al., 1992 in

Bishop 2001).

From the Eocene onwards, Sumatra was the site of a periodically active volcanic arc

and widespread sedimentation (Cameron et al. 1980). The finding of freshwater fish in

Eocene deposits of Central Sumatra (Sanders, 1934 in Kumazawa & Nishida 2000)

suggests that a considerable land area was emergent at that time. In the Oligocene,

Sumatra consisted of an elongate landmass in the south giving way along the axis of

Sumatra to a chain of periodically volcanic islands in the north, through which there

appear to have been localised seaways open to the Indian Ocean, and the modern

geography of Sumatra only took shape following uplift in the Early Pleistocene. These

Early Pleistocene movements were followed by the deposition of transgressive, mostly

non-marine clastics in the basins at the foot of the Barisan Mountains (Cameron et al.

1980).

At the end of the Miocene the environments in the Central and South Sumatra Basin

gradually changed from neretic (see Glossary) to continental, and by the start of the

Pliocene, areas of swampland and marsh were widespread (de Coster 1977;

Wongsosantiko 1976). Sumatra was rotated 20º clockwise during the period 5–1 Mya


70

(Nishimura & Suparka 1997). The current subduction system, located offshore west of

Sumatra and south of Java, began in the Oligocene. Uplift of the Barisan Mountains,

resulting from the subduction, began in the Late Miocene but primarily occurred in the

Plio–Pleistocene (Sudarmono et al., 1997 in Bishop 2001).

At the beginning of the Christian era the present alluvial lowland of Sumatra’s east

coast did not yet exist, whereas the Malay Peninsula might have been much longer,

forming a narrow promontory via the Riau Archipelago to Bangka and Belitung

(Obdeijn 1942b). For instance, in the 16th century, the strait between Singapore and

the rest of the Malay Peninsula was so narrow that tree branches on either side

touched a ship’s masts (Giovanno Botero, 1595 in Obdeijn 1942b), while now that

same strait is several kilometers wide. Quite likely, the extension of the Malay

Peninsula towards Bangka and Belitung only gradually broke up during the Holocene.

North Sumatra

The North Sumatra Basin (Fig. 3.7) is a northwesterly trending Tertiary basin bounded

by the Malacca Platform to the east; the Barisan uplift to the west; the Asahan Arch to

the south; and the Mergui terrace to the north (Wang et al. 1989). In Early Miocene,

tectonic uplift led to complete marine withdrawal from the whole of north Sumatra,

except the middle of the North Sumatra Basin. Still most of the area of north Sumatra

remained submerged in shallow waters, apart from a volcanic chain near present-day

Kutacane and Blangkejeren (Cameron et al. 1980). It is unclear whether the Central

and North Sumatra basins were interconnected at this time, or whether the Asahan

Arch basement high was already in existence (Cameron et al. 1980), although Aldiss

and Ghazali (1984) claim that the Asahan Arch was emergent during most of the

Tertiary, and Riadhy et al. (1998) suggested that the Malacca Platform-Asahan Arch

contributed most of the clastic sediment to the North Sumatra Basin from middle–late

Early Miocene to late Middle Miocene. After the late Middle Miocene, the rising

Barisan Range started to supply most of the clastic sediments. The Lakota High, a

basement feature to the southwest of Lake Toba, was probably an area of non-

deposition as early as the Oligocene and it was only inundated offshore, during the

Pliocene. It seems likely that the Lakota High is a south-westernly extension of the

71

Asahan Arch, which has been dextrally displaced along the Sumatra Fault System

during the later Caenozoic (Aldiss & Ghazali 1984).

Figure 3.7. Main geological features of north Sumatra (after Steinshouer et al. 1997); for legend see Fig. 3.5

From early to late Middle or Late Miocene a widespread transgression occurred,

which at its peak in Middle Miocene resulted in marine sedimentation over most of

northern Sumatra. Open marine conditions prevailed in the centre and west of the

North Sumatra Basin, while westwardly transported fluviatile and deltaic sediments

were deposited along the eastern and southeastern margins of the North Sumatra basin

(Kamili and Naim, 1973; Adinegoro and Hartoyo, 1974; both in Cameron et al. 1980),

suggesting emergent land on the Malay Peninsula. However, the numerous outcrops of

Miocene volcanics show successions that include lavas, tuffs, and thin marine, paralic

or fluviatile sediments (e.g. sandstones, mudstones, and coals) (Rock et al. 1982), and

presumably a chain of volcanic islands existed at this time in northern Sumatra. At this

time, there was sedimentation on the Asahan Arch showing that the North and Central

Location of Asahan Arch


72

Sumatra basins were interconnected and that the north Sumatra land area was no

longer connected to Peninsular Malaysia (Cameron et al. 1980), but it is likely that

after this marine highstand the Asahan Arch once more connected northern Sumatra

and Malaya.

Rock et al. (1982) noted that there is a marked increase of Tertiary volcanic outcrops

from the north to the south, particularly south of Lake Toba. This trend continues into

southern Sumatra. Also there seems to have been a shift of the Tertiary volcanic axis,

which was located west of the Sumatra Fault System in the Tertiary, and east of it in

the Quaternary (Rock et al. 1982). A pronounced Late Pliocene unconformity in all

the higher parts of the North Sumatra Basin and the Malacca Platform suggests that

during a major fall of sea-level, most, if not all, the land between north Sumatra and

the Malay Peninsula was emergent (Wang et al. 1989). This land became inundated

during the Pleistocene (Wang et al. 1989).

From the late Middle Miocene to the Early Pleistocene, north Sumatra experienced a

period of regression. In the east, sedimentation was essentially continuous, but the rest

of the mainland was emergent, with sedimentation only recommencing from the

beginning of the Pliocene (Cameron et al. 1980). In the north Sumatra Basin, marine

withdrawal to the north-west (Keutapang Delta) started in late Middle Miocene

(Cameron et al. 1980), although in the Pliocene the north Sumatra Basin shortly

reverted to marine conditions (Hartoyo et al., in Cameron et al. 1980), possibly during

the Early Pliocene highstand. In the southeast of the basin, non-marine sedimentation

occurred throughout the Pliocene, while the basin shows unstable marine conditions

elsewhere. The north-west Aceh Basin also shallowed in this period, but probably

remained submerged (Cameron et al. 1980).

A Mio-Pliocene sea strait through the Barisan Range may have existed near Sipirok,

SE of Sibolga (van Bemmelen 1970, p. 677). Marine beds of Early Miocene age are

found in this transition between the central and the northern part of the Barisan Range

(Durham, 1940 in van Bemmelen 1970), which points to the possibility that here a

passage existed between the backdeep of NE-Sumatra and the Indian Ocean. This is

also shown in the transition from the Late Oligocene non-marine Sibolga Formation to

the Early Miocene predominantly marine Barus Formation, which was found

73

southeast of Sibolga town (Koesoemadinata & Sastrawiharjo 1988), although it does

not provide proof that a sea connection existed between the Indian Ocean and the

Central Sumatra Basin. Later on, in Plio–Pleistocene times, this area was capped by

the young volcanic Lubukraya-Bualbuali complex (Helbig, 1940 in van Bemmelen

1970), which presumably closed off the gap between northern and central Sumatra.

There are further indications that the Sibolga-Sipirok area of Sumatra was a relatively

narrow land area until quite recently, or possibly even consisted of a sea strait splitting

off a large part of Sumatra as a separate island. Historic Chinese and Arabic sources

mention that ca 1,000 years ago one could walk from the east coast of Sumatra to

Angkola on the west coast in 7 days (Obdeijn 1942a), which would suggest that the

eastern coastline of Sumatra was near the mountains in the Gunung Tua region (the

two coast are now located at ca. 200 km from each other, and it would take at least 2

weeks to cross the rugged terrain between the coasts). This is supported by Malay oral

tradition that claims that in earlier times the Batak Plateau bordered the eastern sea

shore (Hoekstra in Obdeijn 1942a). Obdeijn (1942a) further speculates on the island of

Uńgūdya, which was mentioned in Arabic text by, among others, Ibn Sa’ïd (1214–

1286). Obdeijn hypothesized that this mountainous ‘island’ with a 2000 miles

circumference (Dimaski, 1325 in Obdeijn 1942a) consisted of the present Batak

territories, which at time included all of northern Sumatra down to where the Indragiri

River flows in the South Sumatra Bay (which was the border between Java Menor

(=northern Sumatra) and Java Major (=southern Sumatra + Java)). Thus, in Obdeijn’s

opinion, Uńgūdya was not a proper island, but only identified ethnologically. It could,

however, also be that Uńgūdya was a proper island, separated from the rest of Sumatra

by a sea strait that took half a day to cross (as mentioned by Ibn Sa’ïd in Obdeijn

1942a); this sea strait may have been located in the narrowest and lowest part of the

Sumatra mountains, which is just south of Lake Toba. This would fit historic

descriptions that mention Sumatra as divided into several islands (Obdeijn 1942a),

possibly up to the Holocene highstand.

Central and South Sumatra

In South Sumatra, during the most widespread marine transgression of the Early and

early Middle Miocene, seas are interpreted to have covered the northern extension of


74

the Lampung High, which separated the South Sumatra and Sunda Basins (Bishop

2001), and to have joined with the marine seas of the Sunda basin to the east. On the

north flank of the Central Sumatra Basin shallow seas covered the Asahan Arch. To

the west, the Central and South Sumatra Basins, which were divided at the Tigapuluh

Mountains (Fig. 3.8), were in communication with the open sea through broad

seaways. Islands probably persisted on the west borders of the basins, although their

exact position is as yet unclear (de Coster 1977). At the end of the Middle Miocene a

rapid drop of the sea-level occurred (Hartanto et al., 1991 in Bishop 2001). In the west

of the South Sumatra Basin, coal-bearing units were formed in the Late Miocene–

Early Pliocene as a result of the uplift of the Barisan Mountains (Friederich et al.

1999), while the northeast of the Basin presumably remained submerged.

The South Palembang Sub Basin (Fig 3.8), in the southern part of the South Sumatra

Basin, shows a number of sea-level changes in Early to Middle Miocene (Sitompul et

Figure 3.8. Map of the South Sumatra Basin (after Bishop 2001)

75

al. 1992). A sequence from the Early Miocene indicates sedimentation in a fluvial

environment, after which a rise in sea-level resulted in deltaic sedimentation

(Sitompul et al. 1992). Further eustatic sea-level rises resulted in marine sedimentation

during Middle and Late Miocene, and only in the Early Pliocene deposition in fluvial

and terrestrial environments occurred after the area had been uplifted (Sitompul et al.

1992).

A similar process occurred in the Rambutan area in the central section of the South

Sumatra Basin (Suhendan 1984). When sea-levels were high at 18 Mya, shallow

marine and deltaic environments developed, with sediments probably originating from

the Lampung High. Between 14.5 and 9.8 Mya the general depositional cycle became

regressive, and a delta plain environment developed. These eventually turned into a

marshy environment (Suhendan 1984). Finally, in the Central Sumatra Basin, the Duri

delta, occupying the north of the Basin, retreated north-westwards in Early Miocene to

late Middle or Late Miocene times (Wongsosantiko, 1976, in Cameron et al. 1980).

These patterns of generally retreating seas during the Miocene and Pliocene are

confirmed by the geological work conducted in the Indragiri area by Musper (1928),

who described the slow change during the Miocene of a marine environment to a

lacustrine one.

Van Bemmelen (1970) claimed that during most of the Miocene, and locally even into

the Pliocene a sea strait existed between the present-day provinces of Lampung and

South Sumatra (between Manna and Krui on the west Sumatran coast). Initially, this

strait was located between the Garba and Gumai Mountains, which was closed after

the Mid-Miocene sea-level highstand. In Late Miocene, the strait was located between

the Tembesi-Rawas and Gumain Mountains. During the Late Pliocene–Early

Pleistocene, however, all sea connections were apparently cut off by the rise of the

Barisan Zone and the Lampung High.

Mentawai Islands Group

Samuel et al. (1997) hypothesized that Middle Miocene to Pliocene subsidence of the

forearc led to submergence of the outer-arc ridge islands, after which a Pliocene

unconformity indicates that more of the area had become sub-aerial again. This


76

unconformity represents the initiation of a major phase of uplift that continues today.

The western part of Nias was sub-aerial in the Early Miocene, as shown by major

unconformities in the stratigraphic records. Because of the uplift of the Barisan

Mountains, parts of the continental shelf west of Sumatra temporarily emerged in the

Late Miocene (Karig et al., 1980; Mertosono and Nayoan, 1974 both in Cameron et al.

1980), and Cameron et al. (1980) suggested that new emergent land west of Sumatra

was found as far as Nias. Hopper (1940 in van Bemmelen 1970) provided a detailed

stratigraphic analysis of Nias. He also reported the major unconformity between Late

Miocene and Pliocene sediments, which indicate that much land around Nias emerged

at this time, probably because of the sea level lowstand. Hopper also reported Pliocene

marly clays and limestones on parts of the island, which suggested that during the

Pliocene a coral reef was built up around the borders of a hilly island considerably

smaller than the Nias of today; the limestones were uplifted during the Pleistocene. It

is therefore safe to assume that indigenous terrestrial faunas on the Sumatran outer-arc

ridge islands have an age of no older than the Late Miocene. Van Bemmelen (1970)

correlated Pliocene sediments on Simeulue with those on Niah, which suggests that

both islands were partly inundated, possibly during the Early–Middle Pliocene

highstand.

Enggano Island was visited by Pontoppidan (1915a in van Bemmelen 1970), who

reported the presene of folded tuffogenous Neogene, which were unconformably

covered by Plio-Pleistocene deposits (mainly reef-limestones). This indicates that the

island was submerged (or at least parts of it) during the Quaternary. No such deposits

were reported from Sipura, but on Siberut there is again evidence of Late Miocene–

Pliocene shallow or moderately deep water sediments unconformably overlying older

Tertiary sediments (van Bemmelen 1970).

Whitten et al. (1987a) stated that it is more than half a million years since the

Mentawai Islands had a land connection via the Batu Islands to the mainland, while

two islands, Simeulue and Enggano, have probably never had land connections with

the mainland. However, because sea depths between the mainland and islands such as

Nias, Siberut, Sipura, and the Pagai Islands is presently less than 120 m (presumably

77

the lowest sea-level during the LGM), it is possible that these islands were connected

to the Sumatran mainland during Pleistocene sea-level lowstands.

Vegetation of Sumatra

By the Tertiary many of the plants now found in Sumatra’s forests had developed

there. For example, an imprint of a Dipterocarpus leaf and a fossil dipterocarp fruit

have been found in southern Sumatra dating from that era (Whitten et al. 1987a).

Morley (2000) reported that Laurasian conifers were common in Early Miocene

Sumatra, with scattered Alnus, Pterocarya, Juglans, Carya and Betula, which,

together with pollen types characterizing seasonal freshwater swamp and the absence

of peat swamp taxa, suggest a markedly seasonal climate (Morley 1991).

A pollen analysis by Finger and Drugg (1992) indicated an Early Miocene deltaic

environment, similar to the present-day Mahakam delta in East Kalimantan, in the

area of the Minas oil field in Central Sumatra. Mangrove associates, such as

Barringtonia, Brownlonia, and Nypa are common, as are ferns. The vegetation of

south Sumatra was at this time also characterised by swamp thicket vegetation, and

possibly kerapah, or watershed peats (Morley 1999). Coal formation during the Late

Miocene–Early Pliocene in the South Sumatra Basin indicates fluvial, deltaic and

coastal plain environments, probably similar to the modern peat-forming environments

of Sumatra. These coals indicate that they were formed under a climate of year-round

rainfall, allowing the formation of ombrogenous or domed peats (Friederich et al.

1999).

Maloney and McCormac (1996) described the palaeoenvironments for an upland site

in northern Sumatra. At 32 Kya or older, the pollen diagrams suggest warm and wet

conditions similar to those at present, although the low percentage of pteridophytes

may suggest somewhat drier conditions. Between 32 and 24 Kya there appear to be

signs of increasing cooling, and at 24 Kya there are clear signs of climatic change,

certainly to colder conditions and perhaps to increased effective precipitation.

Between 24 and 21 Kya, the pollen diagrams are suggestive of further declining

temperatures, but with conditions remaining fairly wet.


78

One of the greatest volcanic eruptions of the Quaternary occurred 74 Kya ago

(recently revised to 70 Ka by Schulz et al. 2001) when Lake Toba (Fig. 3.9) in central

northern Sumatra was formed (Ninkovich et al. 1978; Rampino & Self 1993). The

duration of eruption has been estimated as up to two weeks, and eruption cloud

heights may have been as high as 40 km (Rampino & Self 1992, 1993). The ejected

tuffs spread over 20,000–30,000 km2, and were 600 m thick in some areas (Whitten et

al. 1987a); they reached as far as southwest Sumatra where a distinct Toba ash layer

was found (Gingele et al. 2002). The Toba caldera measures 2,500 km2, while, in

comparison, those of Krakatau and Tambora are both less than 50 km2 (Aldiss &

Ghazali 1984). In fact, the basalt volume of the Toba eruption was several orders of

magnitude larger than that of well-known eruptions such as St Helens, Krakatau,

Pinatubo, or Tambora (Rampino & Self 1993).

The direct effect of the eruption may have been a 3–5 ºC drop in air temperature in the

year following the eruption. In addition, the eruption occurred at a transition of the

oxygen isotope record, a time of rapid ice growth and falling sea-level. It has therefore

been suggested that the Toba explosion greatly accelerated the shift to glacial

conditions (Rampino & Self 1992), although Schulz et al. (2001) considered it

unlikely that the Toba eruption caused a long-lasting volcanic winter or triggered the

inception of the penultimate ice age by starting a positive feedback mechanism.

79

Figure 3.9. Lake Toba from space (data from Tropical Rainforest Information Center, University of Michigan).

A second explosion probably occurred as recently as 30 Kya ago (Francis, 1983 in

Whitten et al. 1987a), while ash layers offshore southwest Sumatra and dated at 67–65

Kya and 56 Kya suggest further volcanic eruptions in the region (Gingele et al. 2002).

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