AN EVALUATION OF PLATE TECTONICS MODELS.pdf

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Gondw ana Research,

L?

6, No.

I p p .

1-28;

003

Internationa l Association fo r G ondwan a Research, Japan.

ISSN:

1342-937X

G R E Y

An

Evaluation

of

Plate

Tectonic

Models for

the

Development

of

Sumatra

A. . Barber1and M.J. Crow2

Southeast Asian Research Gro up, Department of Geology, Royal Holloway, U niversity

of

London, Egkam, S urrey,

TW20 OEX,

U K

Formerly British Geological Survey, Keyw orth, Nottingkam

NG1 2

5GG,

UK

Present address:28A Lenton Road, The Park, Nottingkam N C 7 ZDT, UK

(Ma nusc ript received M arch 3,2002; accepted July 5,2002)

Abstract

Over the past two decades models have been developed which suggest that the Asian continent has been formed

since the Late Palaeozoic by the accretion of continental blocks derived from the northern margin of Gondwana. Sumatra,

forming the southwestern margin of the Southeast Asian promontory (Sundaland), is considered to be composed

of

fragments

of

continental plates and magmatic arcs which were derived from Gondwana during the Late Palaeozoic and

Mesozoic. The Indochina Block forms the core of Sundaland, extending into the eastern part of the Malay Peninsula. The

greater part of Sumatra is considered to form part of the Sibumasu Block which accreted to the Indochina Block along

the Bentong-Raub Suture in the Triassic.

A

model has been proposed in which the southern part of the Sibumasu Block

in the western part

of

the Malay Peninsula, and Sumatra, is divided into Malacca and Mergui Microplates by the Mutus

Assemblage which represents another Triassic suture.

A

review of the Permo-Triassic stratigraphy of Malaya and Sumatra

provides no support for

this

model.

Comparison of the Permo-Carboniferous stratigraphy and palaeontology of northern Sumatra with that of the Malay

Peninsula and Peninsular Thailand, and in particular the occurrence of tilloids, links Sumatra fimdy to the rest

of

the

Sibumasu Block to the north. Comparison with the Permo-carboniferousstratigraphyof Bonaparte Gulf region

of

northwest

Australia shows a mirror image relationship, suggesting hat the Sibumasu Block separated fromths part of the Gondwanan

margin in the mid-Permian. On the other hand Permo-Carboniferous rocks in Central Sumatra contain a Cathaysian

fauna and flora, which relates

this

area

to

the Indochina Block rather than to Sibumasu.

This

anomaly was recognised

early in the study of the geology of Sumatra and led to the proposal of the Djambi Nappe, thrust over Sumatra from the

east. The Cathaysian fauna and flora isassociated with an Early Permian Volcanic arc. It has been suggested tha t this was

an independent island arc accreted to the western margin

of

Sibumasu, but from the relationships

of

the volcanic rocks

to Permian sediments and the underlying basement, it is most probable that this arc was developed on the margin of

the Cathaysian Block and was emplaced in its present position outboard of Sibumasu by strike-slip faulting. The most

recently accreted pre-Tertiary tectonic unit on Sumatra is the Woyla Group, a Jurassic-Early Cretaceous oceanic

volcanic arc which, together with its associated accretionary complex of oceanic crustal material, was thrust over the

western margin of Sumatra in the mid-Cretaceous. Earlier plate models for the development of Sumatra are reviewed,

and a revised model

is

proposed. However there are still many difficulties in interpreting the stratigraphy and the

tectonic development of Sumatra which will require further detailed study, aimed at resolving the many outstanding

problems.

K e y words:

Gondwana, Sibumasu, Permo-Carboniferous, Permo-Triassic, Cretaceous.

Geological Map Sheets covering the whole of Sumatra.

This mapping is at a reconnaissance level only, and

requires detailed follow-up studies of critical areas. The

availability of a tremendous wealth of new data provided

by the mapping programme, supplemented by the follow-

up palaeontological studies by Father Henri Fontaine and

his colleagues Fontaine and Gafoer, 1989)has stimulated

Introduction

Geological mapping of Sumatra by the Indonesian

Directorate of Mineral Resources and the Geological

Research and Development Centre GRDC), Bandung, in

collaboration with the United States Geological Survey

USGS) and the British Geological Survey BGS),

culminated in the publication of a set of 1:250,000


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2

A J BARBER AND M.J. CROW

a re-assessment of the stratigraphy, structure and tectonic

evolution of the island of Sumatra.

As

the results of the

new survey accumulated they were placed in the context

of the developing plate tectonic synthesis for the origin

of Southeast Asia, as a collage of terranes separated

from the northern margin of eastern Gondwana and

accreted to the southern margin of Asia during the

Palaeozoic and Mesozoic see Metcalfe, 1996 and

references therein).

The purpose of this communication is to examine the

basic geological data from Sumatra, in order

to

distinguish

factual data from interpretation, and to direct attention

to those problems which should be the focus of future

work.

Stratigraphy

of

Sumatra

Pre-Tertiary basement rocks in Sumatra outcrop mainly

along the central spine of the Barisan Mountains, which

extend along the length of the island parallel to the southwest

coast Fig. 1).To the northeast and southwest the basement

is overlain by Tertiary sedimentary and volcanic rocks, and

wth ntheB a r i s m , by the products of Quatemaryvolcanicity.

Rock units of all ages are transected by the currently active

Sumatran Fault System which follows the

NW-SE

trend

of the Barisans along the whole length of the island.

As a result of the Integrated Geological Survey of

northern Sumatra Cameron et al., 1980) a general

stratigraphic scheme was developed which has been used

98" 100"

96

96" 96"

100"

\ 1020 104 106"

Fig. 1. Simplified geological map

of

Sumatra. Inset: the position

of

Sumatra

in

Southeast Asia.

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PLATE TECTONIC MODELS FOR SUMATRA

3

on the 1:250,000 Geological Quadrangle Sheets of the

region (Fig. 2). Pre-Tertiary rocks in northern Sumatra

were classified into three major geological units: the

Carboniferous-Early Permian Tapanuli Group, the Permo-

Triassic Peusangan Group and the Jurassic-Cretaceous

Woyla Group.

At

the same time a mapping programme

was carried out by GRDC in the southern part of Sumatra

(Gafoer and Purbo-Hadiwidjoyo, 1986) and the three-fold

classification was extended to central and southern

Sumatra (McCourt et al., 1993).

I t

has been suggested that some of the occurrences of

high-grade metamorphic rocks in Sumatra represent a

Precambrian metamorphic basement, but there is as yet

no isotopic evidence to support this suggestion. There is

little doubt, however, that Sumatra is underlain by a

continental crystalline basement a t depth, as the chemistry

of plutonic intrusions and extrusions, including

ignimbrites, indicates that magma sources lay within a

continental margin (McCourt et al., 1996) and the

occurrence of tin-bearing granite plutons is generally

regarded as indicating their derivation from an underlying

continental basement. No outcrops of Lower Palaeozoic

rocks have so far been identified in Sumatra. The

occurrence of a thick sequence of Lower Palaeozoic rocks

in the Langkawi Islands only a few hundred kilometers to

the north, suggests that rocks

of

this age may yet be

discovered. The oldest rocks in Sumatra were found in

boreholes in the Malacca Strait where sediments yielded

palynomorphs indicating an age near the Carboniferous-

Devonian boundary (Koning and Darmono, 1984).

CENOZOIC

CRETACEOUS

TRIASSIC

ERMIAN

CARBONIFEROUS

DEVONIAN

LOWER

PALAEOZOIC

and

PRECAMBRIAN

BASEMENT

Sediments and Volcanics

Volcanics -Bentaro

Reef limestones

-

Lamno

Serpentinites, pillow lavas:

cherts, greywackes

-

Geumpang, Lam Minet

Sandstones and shales,

cherts

-

Kualu, Tuhur

Limestones

-

Situtup,

B atumilmil

Volcanics, sandstones,

limestones, shales

-

Palepat,

S

ilungkang,Mengkarang

Tillites

-

Bohorok, Mentuli

Limestones

-

Alas, Kuantai

Sandstones and shales -

- ? in boreholes

I

i

Kluet, Kuantan

ntrusive tin granites imply

an underlying continental

basement

Fig. 2. Pre-Tertiary stratigraphic units

in

Sumatra as given by Cameron et al. (1980) and on GRDC Map Sheets.

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4

A J BARBER AND M.J. ROW

Northern

Sumatra

Tapanuli Group

In northern Sumatra the area to the northeast of the

Sumatran Fault is underlain by the Tapanuli Group. From

northeast to southwest, outcrops of this group have

been classified as the Bohorok, Alas and Kluet Formations

(Fig. 3).

The Bohorok Formation to the northeast near Medan

is composed mainly of sandstones and shales, but also

contains pebbly mudstones, poorly-sorted breccio-

conglomerates, composed of subangular to rounded clasts

of a variety of rock types, scattered in a fine grained clay

or silt matrix. The pebbly mudstones have been

interpreted, by comparison with the similar deposits of

the Singa Formation in the Langkawi Islands, offshore

West Malaya, as diamictites of glacio-marine origin. N o

macrofossils which could be used to determine the age

have yet been found in the Bohorok Formation, although

an Early to Middle Carboniferous microflora has been

reported from a borehole along the Malacca Strait in the

eastern part

of

Sumatra which bottomed in this formation

(Koning and Darmono, 1984).

The Alas Formation is composed characteristically of

massive limestones, locally oolitic and current-bedded.

The limestones are interbedded with sandstones and

shales, indistinguishable from those of the Bohorok

Formation. The limestones have yielded corals,

brachiopods and conodonts

of

Lower Carboniferous,

Vis6an age (FoGaine and Gafoer, 1989; Metcalfe, 1983).

As mapped, the Alas Formation also includes high-grade

metamorphic rocks. These have been attributed to the

effects of contact metamorphism of the sediments

(Cameron et al., 1980), but some of the metamorphic

rocks are garnetiferous and could have resulted from

regional metam orphism. The association of

unmetamorphosed fossiliferous limestones adjacent to

high-grade metamorphic rocks suggests that there may

Fig.

3.

Dismbution

of Pre-Tertiary stratigraphic units in northern Sumatra based

on

GRDC Map Sheets).

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PLATE

TECTONIC MODELS FOR

SUMATRA

5

be an unconformity between the limestones and a

metamorphic basement within the area mapped as Alas

Formation.

The Kluet Formation is composed of alternating quartz-

wackes, siltstones, shales and some limestones. The shales

are largely converted to slates, but on the west coast near

Tapaktuan again higher grade metamorphic rocks occur

within the area mapped as Kluet Formation Cameron et

al., 1982) Fig.

3).

No age-diagnostic fossils have yet been

recovered from this formation. Low-grade metabasic rocks,

some with phenocrysts, are recorded from both the Alas

and Kluet Formations Cameron et al., 1982).

Because of poor exposure, scattered outcrops and the

presence of large numbers of faults which disrupt the

sequence, it has not yet proved possible to determine the

stratigraphic relationships of the three formations which

make up the Tapanuli Group.

As

has been pointed out

above, the Alas Formation of Visean age is the only unit

for which there is direct fossil evidence of age. The

Bohorok and Kluet Formations have also been presumed

to be in part of Carboniferous age because they are closely

association with the Alas Formation in the field, and

because all three formations contain similar lithologies,

and in general show the same degree of deformation

Cameron et al., 1980).

On the western shore of Lake Toba Fig. l) , o the south

of Medan, an outcrop of decalcified argillaceous limestone

is found associated with lithologies of the Bohorok or Kluet

Formations Aldiss et al., 1983). All the rocks in this area

show the development of slaty cleavage. The limestone

has been called the Pangururan Bryozoan Bed, as it

contains large numbers of fenestellid bryozoa.

Unfortunately, the fenestellids are deformed and their

internal structure has been destroyed, so that they cannot

be used to determine the age precisely However, the

fenestellids and members of other fossil groups have been

determined at the British Natural History Museum, as

of either Late Carboniferous or Early Permian age Aldiss

et al., 1983).

The recognition of the Pangururan Bryozoan Bed

encouraged the surveyors to correlate this unit with the

Bryozoan Bed of the Phuket region of Peninsular Thailand,

which is identified as

of

Lower Permian age by its fossil

content Cameron et al., 1980; Young and Jantaranipa,

1970) Fig. 4) . Aldiss et al. (1983) included the

Pangururan Bryozoan Bed within the Kluet Formation,

but there is no evidence of the precise stratigraphic

position of the Pangururan Bed within the Kluet, nor of

its relationship to the Bohorok Formation. In Thailand

the Bryozoan Bed is underlain by pebbly mudstones and

is both underlain and overlain by sandstone and shale

units. If a similar sequence occurs in Sumatra it is possible

that the sandstones, shales and pebbly mudstones of the

Kluet/Bohorok Formation extend into the Lower Permian.

It is likely that the Bohorok Formation

is

at least in part

of Early Permian age, as brachiopods of this age have

been found in sediments interbedded with tilloids in

northwestern Peninsular Malaysia Metcalfe, 2000). It

should be emphasised that no Upper Carboniferous, let

alone Lower Permian fossils have been identified with

certainty in the Tapanuli Group of Sumatra.

Comparable Carboniferous to Early Permian

stratigraphic sequences, seen in Sumatra, northwest Malay

Peninsula and southern Thailand, and as far north as

Baoshan in China indicate that these areas were

contiguous at that time and all are considered to form

part of the Sibumasu Terrane Metcalfe, 1984,1988; Wang

et al., 2001) Fig. 4).

I t has been suggested that the Tapanuli Group

represents a continental margin sequence developed along

a rifted passive margin Cameron et al., 1980). The

reduction in clast sizes in the mudstones and

conglomerates of the Bohorok and Kluet formations, with

a decrease in the frequency and grain size of sandstone

units in a southwesterly direction, suggest tha t in

Carboniferous-EarlyPermian times an open ocean lay in

this direction. In this scenario the turbiditic sandstones

and shales of the Kluet Formarion were deposited in rift

basins, and limestones of the Alas and Kluet Formation

were deposited on horst blocks of uplifted basement,

represented perhaps by the associated high grade

metamorphics.

Comparisons can be made between the stratigraphic

sequence in Sumatra and the Permo-Carboniferous

sequence in the Bonaparte Gulf region of northwest

Australia Roberts and Veevers, 1973) Fig. 4). Unlike

Sumatra the rocks in Australia are unmetamorphosed.

Here, a limestone and turbiditic sandstone-shale unit, the

Bonaparte Beds, resembling the Kluet, is followed by

limestone and shale of the Tanmurra Formation.

Limestones in the Tanmurra Formation are similar to the

Alas limestones and contain Visean fossils. The Upper

Carboniferous is poorly represented in this area, but

elsewhere in northwest Australia is composed of terrestrial

sediments passing into marine deposits offshore. Onshore,

tillites, resembling the pebbly mudstones of the Bohorok,

overlie the Tanmurra Formation unconformably, and are

followed by shales, siltstones and coal deposits of the

Lower Permian Kulshill Formation, again passing into

marine deposits in boreholes offshore to the northwest

Fig.

5).

The Bonaparte Gulf succession is a passive continental

margin sequence representing the mirror image of the

Tapanuli Group of Sumatra, but showing the opposite

Gondwana Research,

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6

A.J. BARBER

AND

M.J. CROW

GONDWANA

WEST NORTHWEST

SIBUMASU

MALAY S1A AUSTRALIA

NORTHERN

Metcalfe

2000

B ~ ~ ~ ~ ~ ~ ~ ~ y

oberts

and Veever5

(1973)

SUMATRA

Cameron

et al(1980) THAILAND

GRDC

Map\

Mount Godwin

I I

I

angururan Bryozoan

Bed

7 L Permian)

Bohorok

(tilloids)

. . . I

_ I

.

.

. . .

.

.

.

. .

. .

. .

.

. L . .

.

. . . . . . . .

c l

?

-~(?TO

u naisi an)FC*Td

Semanggol

(Part)

Ratburi Lst

Chuping Lst

Upper Formation

Bryozoan Bed

Phuket

Singa

(tilloids)

Lower

Formation

.

.

. . . . .

I

Hyland Bay

Sugar oaf

F Kulshill

with

coals)

o o c ) o c ) . D Tillites

r1

Bonaparte Beds

g. 4. Comparison of the Carboniferous, Permian and Triassic sequences in the Sibumasu Terranes of northern Sumatra (after Cameron et al.,

1980; and GRDC Map Sheets), West Malaysia and Thailand (after Metcalfe,

2000)

and the Gondwana Terrane in northwest Australia (after

Roberts and Veevers, 1973).

polarity towards an ocean lying towards the northwest

(Fig.

5).

The similarity of the faunas of northwest Australia

and northern Sumatra suggests that Sumatra, and

Sibumasu as a whole, were derived from the northern

continental margin of Australia and once formed part of

Gondwana (Metcalfe, 1992, 1993). The separation of

Sibumasu probably commenced with extension and rifting

during the Carboniferous.This was followed by separation

of

the continental blocks, with the development of

complementary passive margins. Complete separation

may have occurred during the Early Permian.

If

the pebbly

mudstones of the Bohorok Formation represent Late

Carboniferous-Early Permian glacial deposits, Sibumasu

could not have moved very far from the Gondwana glacial

environment at this stage.

Peusangan Group

During the mapping of northern Sumatra pre-Tertiary

rocks, mainly limestones, lying to the northeast of the

Sumatran Fault, but apparently less deformed than the

Tapanuli Group, were distinguished as the Peusangan

Group (Fig.

3).

Outcrops of these rock units are isolated,

so that during the survey each occurrence was given a

separate formation name. Some of these occurrences have

yielded fossils of Permian or Triassic age, and sometimes

both (Fontaine and Gafoer, 1989), others are apparently

unfossiliferous, eithe r because they are too highly

recrystallised or have not been examined thoroughly. The

relationships between the Carboniferous Tapanuli Group

and the Permo-Triassic Peusangan Group are assumed to

be unconformable, but no localities where an

unconformity may be seen have yet been described.

No fossils which could be of Early Permian age have

so

far been found in northern Sumatra, apart from those

of

the Pangururan Bryozoan Bed already discussed. Middle

to Late Permian fossils have been described from the

Situtup Formation (fusulinids), near Takengon (Cameron

et al., 1983), the Kaloi Formation (trilobite), south of

Langsa (Bennett et al., 198 l ), and the Batumilmil

Formation (foraminifera) to the southwest of Medan

(Fontaine and Vachard, 1984). The Permian limestones

of northern Sumatra can be correlated with the Chuping

Limestone Formation of the northwest Malay Peninsula

and the Ratburi Limestone

of

southern Thailand (Fig. 4).

N o fossils diagnostic of latest Permian or of Early Triassic

age have been found in northern Sumatra.

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PLATE TECTONIC MODELS

FOR

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7

Fossils of Middle to Late Triassic age occur in the

Situtup Formation (foraminifera) (Cameron et al., 1983),

the Kaloi Formation (conodonts) and the Batumilmil

Formation (conodonts) (Metcalfe, 1984) (Fig. 3). Some

of these formations, Situtup, Kaloi and Batumilmil, have

yielded both Permian and Triassic fossils, but the

relationships between the Permian and Triassic rocks,

whether a continuous sequence or separated by an

unconformity, have not yet been established.

Metavolcanics, slates and phyllites are described as part

of the Peusangan Group in Aceh, associated with

limestones in the Uneuen, Situtup and Tawar (Toweren

Member) Formations (Cameron et al., 1983) (Fig. 3) .

These outcrops are much affected by thrusts, and it is

possible that these lithologies belong to either the Kluet

Formation or the Woyla Group, occurring in the same area,

which have been imbricated together with the limestone

units by thrusting.

A distinctive unit, the Kualu Formation outcrops to the

south of Medan, composed of thin-bedded sandstones,

siltstones and mudstones (rhythmites). A unit,

distinguished

as the

Pangunjungan Member, composed

of thin-bedded radiolarian cherts outcrops to the south

of Lake Toba (Fig.

1)

(Clarke et al., 1982). As far as the

authors are aware no identifiable radiolaria have yet been

described from the Pangunjungan Member. The upper part

of the Kualu Formation is composed of sandstones. The

formation is fossiliferous and contains the distinctive thin-

Fig.

5.

The Lower Permian palaeo-

geography

of

the Bonaparte Gulf

regionof Northwest Australia after

Roberts and Veevers,

1973).

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8

A J BARBER NDM.J.

CROW

shelled pelecypod Halobia sp. and ammonoids which

indicate a Middle to Late Triassic age.

A

limestone unit, the

Sibaganding Member, near Lake Toba Fig.

1)

has yielded

early Late Triassic conodonts Metcalfe, 1986).

The Kualu Formation is directly comparable in

lithological sequence and fossil content to the Semanggol

Formation in the northwestern part of the Malay Peninsula

and the Sibaganding Member can be correlated with

the Kodiang Limestone Metcalfe, 1992) of the same area

Fig.

4).

The deposits of the Peusangan Group show that in the

Permian northern Sumatra Sibumasu Block) formed part

of a shallow water continental platform or shelf, facing

the Meso-Tethys, far from any terrigenous source, on which

mainly carbonate sediments were deposited. As already

reported, the uppermost Permian and earliest Triassic

rocks are missing, but by the Middle Triassic the Sibumasu

Block formed part of a continental margin, which from

Middle to Late Triassic underwent extension, leading to

the development of deep faulted grabens, in which the

pelagic cherts of the Kualu Formation were deposited.

The grabens were separated by intervening horsts on

which shallow water carbonates were deposited. This

tectonic and sedimen tary environment extended

eastwards into Peninsular Malaysia. However, cherts

which form part of the lower Semanggol Formation in

NW

Peninsular Malaysia were found to contain Middle to

Late Permian radiolaria Sashida et al., 1995). On the

other hand Spiller and Metcalfe 1995) reported Middle

to Late Triassic radiolaria from cherts in the same area.

Metcalfe (2000) points out that while the Permian cherts

are deformed, the Triassic cherts are relatively

undeformed, and suggests that the Permian and Triassic

cherts are not part of the same sequence. The Permian

cherts formed part of an accretionary complex, involved

in the Late Permian to early Triassic collision, while the

Middle to Late Triassic cherts were deposited in a foredeep

basin. Metcalfes (2000) interpretation is not incompatible

with the interpretation given here. Conglomerates occur

in the upper part of the Semanggol Formation in the Malay

Peninsula, signalling post-orogenic uplift of the source

region in Malaysia towards the end of the Triassic.

In

Sumatra uplift in the Malay Peninsula is indicated by the

influx of sands in the upper part of the Kualu Formation.

Woyla Group

In northern Sumatra pre-Tertiary rocks of the Woyla

Group lie largely to the southwest of the Sumatran Fault

System, although some units extend across the fault.

The group has been divided into two units: an arc

assemblage and an oceanic assemblage Cameron et al.,

1980) Fig. 3).

The arc assemblage, which lies on the west coast of

Sumatra to the south of Banda Aceh Fig. 3), is made up

of basaltic to andesitic volcanics and volcaniclastics

associated with massive or bedded limestones, interpreted

as fringing reefs to volcanic islands Cameron et al., 1980).

Fossils from the limestones give a Late Jurassic-Early

Cretaceous age for the arc assemblage. The oceanic

assemblage, which is dissected by the Sumatran Fault, is

composed of serpentinites, amphibolitised gabbros, pillow

basalts, hyaloclastites, cherts and red manganiferous

shales, interpreted as imbricated segments of ocean floor

and its underlying mantle Cameron et al., 1980).

No

age-diagnostic fossils have yet been obtained from

the oceanic assemblage, not even the cherts, but since

both the arc and oceanic assemblages are intruded by the

Late Cretaceous Sikuleh Batholith they are considered to

be of approximately the same age. Both Woyla

assemblages are interpreted as representing a Jurassic-

Early Cretaceous oceanic arc, with associated fringing reefs

developed on oceanic crust, overthrust on to the Sumatran

margin in the mid-Cretaceous, and subsequently intruded

by a batholith forming part of a Late Cretaceous Andean

magmatic arc Barber, 2000).

Central Sumatra

The Pre-Tertiary rocks of central Sumatra have been

correlated with the stratigraphic units defined in northern

Sumatra, with stratigraphic units corresponding to the

Permo-CarboniferousTapanuli Group, the Permo-Triassic

Peusangan Group and the Jurassic-Cretaceous Woyla

Group McCourt et al., 1993) Fig. 6).

Tapanuli Group

Quartzites and shales, identified in oil company

boreholes along the Malacca Straits have been used to

define a Quartzite Terrain Eubank and Makki, 1981).

Similar quartz-rich rocks of Carboniferous age in Malaya,

on the northeastern side of the strait in the Kubang Pasu

and Kenny Hill formations, are considered to be

stratigraphically equivalent to the Tapanuli Group of

Sumatra. In the Malay Peninsula these quartz-rich

sediments are described as having an easterly provenance

Fontaine and Gafoer, 1989).

Rock units which have been correlated with the

Bohorok Formation of northern Sumatra outcrop in

the

Tigapuluh Mountains on the northeastern side of the

Barisan Mountains Suwarna et al., 1991; Simandjuntak

et al., 1991) Fig. 6). The Mentulu Formation, in the

northeastern part of the outcrop, consists of pebbly

mudstones, identical to those in the Bohorok Formation,

interbedded with quartz sandstones and shales, the latter

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PLATE TECTONIC MODELS FOR

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9

Fig.

6.

Dismbution of Pre-Tertiary tratigraphic

un ts

in Cental Sumatra (based on

GRDC

Map Sheets).

N.B. The

Jurassic-Cretaceous

Rawas

Formation

outcrops with the Peneta and Asai Formations about

25 km

south

of the southern edge of the map

commonly as slates. The Mentulu Formation passes

southwestwards, by

loss

of the pebbly mudstones, into

greywacke sandstones and shales of the Pengabuhan

Formation, and then by the increase in the argillaceous

component into the Gangsal Formation.

A

volcanic unit,

of andesitic and basaltic tuffs, the Condong Member,

outcrops along the northeastern margin of the mountains.

The Kuantan Formation (Silitonga and Kastowo, 1975),

outcropping in the Barisan Mountains to the east of Bukit

Tinggi (Fig.

61,

and consisting of quartzites and quartz

sandstones with intervening shales, commonly altered to

slates or phyllites, with the local development of

limestones, has been correlated with the KIuet Formation

of

northern Sumatra (Rock et al., 1983). The Kuantan

Formation is

so

similar to the Kluet Formation, that during

the mapping of the Padangsidempuan Sheet to the north

of the area where the Kuantan Formation had been defined

previously an arbitrary boundary was drawn between the

two

formations (Aspden et al., 1982). Limestone units of

the Kuantan Formation in the Agam River and at the type

locality in the Kuantan Gorge, contain a wide range of

fossil groups, including algae, foraminifera, corals and

conodonts indicating a Carboniferous, VisCan, age

(Fontaine and Gafoer, 1989; Metcalfe, 1983).

Intensely folded, anomalous, muscovite, tremolite,

chlorite, carbonate and quartz schists forming a

NW-SE

trending belt to the east of Lubuksikaping were identified

as Pawan and Tanjung Puah members and included within

the Kuantan Formation during the DMR/GRDC/BGS

mapping programme (Clarke et al., 1982). Pulunggono

and Cameron (1984) alternatively correlated these

tectonised units with the Triassic Tuhur Formation, which

occurs along strike to the southeast. However, neither of

these correlations provides a convincing interpretation of

this anomalous occurrence.

The Quartzite Terrain can be interpreted as littoral

and shelf deposits derived from the Malay Peninsula to

the east. The Mentulu and Kuantan Formations of Central

Sumatra can be interpreted in the same way as the

Tapanuli Group to the north,

as

representing a continental

margin sequence, with pebbly mudstones, sandstone and

shale units in basins, deposited by turbidity flows, while

limestones were deposited on horst blocks.

On the face of it the VisCan limestone units of the

Kuantan Formation could be correlated directly with the

Visean Alas Formation

of

northern Sumatra (Fig. 7).

However, Fontaine and Gafoer (1989) relate the faunas

in the

two

limestone units to different faunal provinces.

Gondwana

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10

A J BARBER AND M.J. CROW

Fontaine and Gafoer (1989) maintain that the fauna in

the Alas Formation indicates a cool-temperate shallow

water environment, and is related to the Vis6an faunas of

Peninsular Thailand and elsewhere in the Sibumasu

Terrane, while the fauna of the Kuantan limestones

indicates a tropical shallow water environment, and is

related to that of East Malaya and the Indochina Block in

Laos, Vietnam and eastern Thailand. Also the glacio-

marine environment in which the Mentulu pebbly

mudstones were deposited is completely incompatible

with tropical shallow water environment indicated by the

algal mats and reef corals of the Kuantan limestones. This

contrast in environments of deposition and the faunal

affinities of the Kuantan Formation suggest that the

western part of Central Sumatra represents part of a

Cathaysian terrane, separate from both the Kluet

Formation to the north and the Bohorok/Mentulu

Formation to the northeast (Fontaine and Gafoer, 1989)

(Fig. 6). Conveniently, the boundary between these two

terranes may be drawn through the outcrops of the

chlorite and tremolite schists of the Pawan and Tanjung

S

BU

MASU

NORTHERN

Cameron et al (1980)

CENTRAL

SUMATRA

GRDC

Maps SUMATRA

Puah members representing the Medial Sumatra Tectonic

Zone of Hutchison (1994).

Peusangan

Group

In Central Sumatra, Permian and Triassic rocks outcrop

in a broad belt extending

NW-SE

between the Sumatran

Fault Zone and the outcrop of the Kuantan Formation

(Fig. 6).There are no descriptions of the contacts between

the Carboniferous rocks of the Kuantan Formation and

the overlying Permian or Triassic rocks, but the

relationships are presumed to be unconformable.

The Menkarang Formation, famous for its Jambi Flora,

outcrops in the Menkarang River and adjacent river

sections to the southwest of Bangko (Fig.

6).

The

formation consists of conglomerates, sandstones,

siltstones, claystones, often carbonaceous, and some

limestones (Suwarna and Suharsono, 1994). Fossils,

including algae, fusulinid foraminifera, corals and

brachiopods, from calcareous beds underlying or

interbedded with the plant-bearing horizons, show that

the Mengkarang Formation is of Early Permian, Late

INDOCHINATERRANE

EASTERN MALAY Hutchison

(1994)

GRDC Maps

PENINSULA Metcalfe

2000)

Linggiu

S

umalayang

Dohol

Sagor

Redang Beds

(continental

red beds)

Panching

Limestone

Charu

Fig.

7.

Comp arison of the Carboniferous, Permian and Triassic sequences

of

the northern Sumat ra Sibumasu Terrane (after Cameron et al., 1980 and

GRDC Map Sheets) and the Indochina (Cathaysian) Terranes of Central Suma tra (after GRDC Map Sheets) an d the eas tern Malay Peninsula

(after Hutchison, 1994 and Metcalfe, 2000).

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PLATE

TECTONIC MODELS FOR

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Asselian to Sakmarian age (Fontaine and Gafoer,

1989).

The marine fauna and flora, with an absence of annual

rings in tree trunks in the Menkarang Formation, indicate

that these rocks, like the Kuantan limestones, were

deposited in a tropical environment. The Jambi Flora was

described by Jongmans (1937) and has more recently been

re-assessed by Asama et al. (1975) and Vozenin-Serra

(1989) who conclude that the flora consists entirely of

Euramerican and north Cathaysian species, with no

Gondwanan forms. The fauna and flora of the

Mengkarang Formation indicates that these Lower

Permian rocks, together with the underlying Kuantan

Formation, form part of a distinct Cathaysian Terrane

The Palepat Formation, south of Muarabungo (Fig. 6),

is composed of andesitic and rhyolitic lavas and tuffs

interbedded with siltstones and limestones (Katili, 1969).

The limestones contain a rich fauna of brachiopods and

fusulinid foraminifera which indicate an Early Permian

age (Fontaine and Gafoer, 1989) . The Silungkang

Formation to the east of

Solok

(Fig. 6) consists of a

volcanic lower member, similar to the Palepat Formation,

with andesitic lavas and tuffs interbedded with limestone,

shale and sandstone, passing up into an upper limestone

member. The rich fauna of large foraminifera and corals

indicates a Permian, Artinskian to Kazanian age (Katili,

1969; Fontaine and Gafoer, 1989). The Barisan Formation

of Rosidi et al. (1976) to the southeast, consists of

sandstone, shale, limestone and chert, in which the shales

are commonly converted to slates or phyllites. The eastern

part of this formation a t Bukit Cermin includes a massive

limestone member containing fusulinids of Early Permian

age. Fontaine and Gafoer (1969) recommend that the use

of the term Barisan Formation should be discontinued

as the area of outcrop shown on the GRDC map (Rosidi

et al., 1976) includes units of various ages. Rosidi et al.

(1976) also defined the Ngaol Formation in the Tabir River

in the southeast of the Painan Sheet, which includes a

limestone member rich in Middle Permian fossils,

including fusulinids (Fontaine and Gafoer, 1989) (Fig. 6).

Since the outcrops mapped as Ngaol Formation also

include metamorphic rocks and sediments of Jurassic age,

Fontaine and Gafoer (1989) recommend that this name

should be abandoned. The Permian limestones and shales

of both the Barisan and Ngaol formations should be

regarded as part

of

the Silungkang Formation.

The Permian sequence of Central Sumatra can be

compared with that of the eastern part of the Malay

Peninsula which contains volcanics and a Late Permian

Cathaysian flora (Fig. 7).

As in northern Sumatra, no rocks of latest Permian or

earliest Triassic age have been reported from Central

Sumatra.

The Triassic Tuhur Formation outcrops extensively to

the southeast of Solok and extends northwards across the

equator (Fig. 6). Silitonga and Kastowo (1975)

distinguished Slate and Shale and Limestonemembers.

The former consistsof grey or black argillaceous sediments

with brown cherts and thin greywacke limestones. Similar

lithological units to the north of the equator (e.g., Cubadak

Formation

-

Rock et al., 1983) have yielded Late Triassic

ammonoids and the characteristic Middle to Late Triassic

pelecypod, Halobia. The Tuhur Formation may be directly

compared to the Kualu Formation of northern Sumatra.

The limestone member includes bedded sandy limestones

and massive conglomeratic imestones.The conglomerates

contain limestone clasts with fusulinid foraminifera of

mid- to Late Permian age, indicating a Late Permian to

Middle Triassic unconformity (see also Turner, 1983).

Evidently, previously deposited Late Permian carbonates

were eroded from uplifted horst blocks and incorporated

into Middle Triassic conglomerates, which accumulated

on, or adjacent to, the horst blocks.

The absence of latest Permian and Early Triassic rocks

in Central Sumatra, together with the incorporation of

Middle Permian clasts in Triassic limestone conglomerates,

indicate that a period of uplift and erosion occurred during

this time interval. When deposition resumed

in

the Middle

Triassic it followed the same pattern as in northern

Sumatra, with shallow water limestones and deep water

cherts and shales, suggesting a period of extension, with

the development of horst blocks and intervening rift

basins. In the Triassic the distinction between the

Sibumasu Terrane of northern Sumatra and the

Cathaysian Terrane of Central Sumatra, seen in the

Carboniferous and the Permian had disappeared,

suggesting that the juxtaposition of these ter ranes

occurred between the Middle Permian and the Middle

Triassic and that subsequently they both formed part of a

continuous terrane.

Woyla

Group correlatives

The Indarung Formation, outcropping

in

West Sumatra

near Padang (Fig.

6),

is composed of basic volcanics,

sometimes pillowed, volcanic breccias and volcaniclastic

sediments closely associated with massive limestones and

chert (Yancey and Alif, 1977). Fossils in the limestones

indicate Late Jurassic to Early Cretaceous age and the

cherts have yielded radiolaria of Aalenian (Middle

Jurassic) age (McCarthy et al., 2001). These rocks have

been correlated with the Woyla Group of northern

Sumatra on the basis of lithological similarity and age

(Cameron et al., 1980) and they include components

which could belong to both volcanic arc and oceanic

assemblages.

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PLATE TECTONIC MO DELS

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13

1986). These limestones are the same age as those of the

Silungkang Formation in Central Sumatra which lies to

the northwest along strike.

De Roever (1951) reported that fusulinids in a sample

from Bangka in the Geological Museum in Bandung,

together with his subsequent field investigations, ndicated

that some of the shales and sandstones on the island were

of Permian age.

Permo-Carboniferous rocks also occur on the island of

Billiton to the east of Bangka. Jongmans (inVan Overeem,

1960) tentatively dentified vague plant remains found

in the SE part of the island as belonging to the Cathaysia

(Glossopteris)

flora of Late Carboniferous (Stephanian)

age. Van Overeem (1960) reports some ill-preserved

Fusulinae, possibly

---

Fusulina schwagerina (sic) from

limestones in an offshore borehole to the

NW

of the island,

Strimple and Yancey, 1976) reported that he had seen

illustrations of these fusulinids which are almost certainly

schwagerinids of Early Permian age. It is neccessary for

the fusulinids from both Bangka and Billiton to be

identified at least at generic level before a definite age

can be ascribed to them (personal communication, Ueno,

2002). However, Lower Permian rocks certainly occur

on

Billiton, as a cassiterized Early Permian ammonoid,

Agathiceras sundaicurn Haniel, was reported by Van

Overeem (1960).

On Bangka the Triassic Tempilang Formation is composed

of alternating sandstone-mudstones, comparable to the

rhythmites in the upper part of the Kualu Formation of

northern Sumatra. These rocks also outcrop in the Lingga

islands to the north, and together with bedded cherts are

encountered in boreholes in a belt parallel to the northeast

which would indicate a Middle Permian age. Yancey (in

coast of Sumatra, near Jambi (Fig.

8).

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14 A J BARBER

AND M.J.

CROW

Woyla Group correlatives

In southern Sumatra, rock units which have been

correlated with the Woyla Group of northern Sumatra

have been described from the Gumai and Garba

Mountains and from the neighbourhood of Bandar

Lampung Fig.

8).

In the Gumai Mountains, to the east of Bengkulu, pre-

Tertiary units, occupying the core of an anticline

in

Tertiary

rocks, are described as the Saling, Lingsing and

Sepingtiang Formations Gafoer et al., 1992). Both the

Saling and Lingsing Formations include andesitic and

basaltic lavas, breccias and tuffs but the Lingsing

Formation also includes clastic sediments, calcilutites and

cherts. Age determination on dykes associated with the

lavas gave a K/Ar age of 11 6+ 3 Early Cretaceous)

Gafoer et al., 1992). The basaltic lavas are tholeiitic in

composition and are associated in the field with

serpentinised peridotites and cherts,

so

that the

assemblage has been interpreted as representing a

disrupted ocean floor sequence Gafoer et al., 1992). The

presence of andesitic lavas suggests that fragments of a

volcanic arc are also present. The Saling and Lingsing

Formations are overlain discordantly by limestones of the

Sepingtiang Formation which contain fossils indicating a

Late Jurassic to mid-Cretaceous age. As in northern

Sumatra these limestones have been interpreted as

representing fringing reefs related to the arc volcanics

thrust over the other units. These rocks are cut by granites,

regarded as of Late Cretacous age Gafoer et al., 1992).

The pre-Tertiary rocks of the Gumai Mountains can be

correlated directly with the arc volcanic and oceanic

assemblages in the Woyla Group of northern Sumatra,

both in terms of their lithology and their age.

An inlier of pre-Tertiary rocks forms the Garba

Mountains to the southwest of Baturaja Fig.

8).

Here

the Tarap Formation of low-grade metasediments, which

have been correlated with the Tapanuli Group of northern

Sumatra, are imbricated with andesitic and basaltic lavas,

sheared serpentinite and bedded chert of the Garba

Formation Gafoer et al., 1994). The Garba Formation

also includes lenticular bodies of melange, the Insu

Member, with blocks of volcanic rocks, chert, fine grained

clastics and massive limestones in a scaly clay matrix.

More continuous outcrops of bedded chert are

distinguished as the Situlangang Member. These rocks

are intruded by the Late Cretaceous Garba pluton. The

outcrops of these rock units are too small to be

distinguished in figure 8, but are shown on the Baturaja

Quadrangle Sheet Gafoer et al., 1994). Although no

direct age determinations have been made on the

volcanics, and no fossils have been reported from the

cherts or the limestones, the lithological assemblage, the

contrast with the metamorphics of the Tarap Formation,

together with the age constraint provided by the pluton

make a direct correlation with the Woyla Group of

northern Sumatra highly probable.

Rocks similar to those of the Gumai and Garba Mountains

are encountered

inoil

companyboreholes beneath the Tertiary

South Sumatra Basin in a broad area extending to the

northeast of the mountains towards Palembang Fig. 8).

Pre-Tertiary rocks form scattered outcrops among

Tertiary and Quaternary sediments and volcanics in the

region of Bandar Lampung in the far south of Sumatra

Fig.

8).

The greater part of these outcrops are migmatitic

gneisses and metasedimentary schists of the Gunungkasih

Complex. The gneisses have been interpreted as deformed

granitic and basaltic intrusions which formed the basal

parts of a Late Cretaceous magmatic arc Barber, 20001,

while the metasediments, in which the intrusions were

emplaced, have been correlated with the Tarap Formation

of Garba and the Tapanuli Group of northern Sumatra

Amin et al., 1994; Andi Mangga et al., 1994). The

Gunungkasih Complex is thrust southwestwards over

unmetamorphosed sandstones, shales, manganese

nodules, cherts, limestones, with occasional basalts, of

the Menanga Formation. The limestones contain

Orbitulina sp. a diagnostic mid-Cretaceous fossil. The

assemblage is interpreted as a deep water marine

sequence, with volcanic detritus derived from a volcanic

arc, deposited in a forearc environment. Again, the

Menanga Formation has been equated with the Woyla

Group of northern Sumatra Amin et al., 1994; Barber,

2000) and constitutes part of the Woyla Nappe.

Critical

R e v i e w

of

P l a t e

Tectonic

M o d e l s

Pulunggono and Cameron I 984) Model

Following the completion of the Integrated Geological

Survey of Northern Sumatra the new data were integrated

with pre-existing data from the literature, and information

from boreholes acquired during petroleum exploration,

to compile a plate model to explain the distribution of

stratigraphic units in Sumatra and the adjacent part of

Malaysia Pulunggono and Cameron, 1984; Pulonggono,

198s) Fig. 9).

In this synthesis Sumatra and the Malay Peninsula are

considered to be composed of a series of microplates. The

East Malaya Microplate to the east, characterised by

Permo-Triassicmagmatism, is separated from the Malacca

Microplate, forming the western part of the Malay

Peninsula, by the Bentong-Raub Line, marked by a zone

of basic and ultrabasic rocks and melanges, which

represents the suture where the two microplates collided

in the Triassic see Metcalfe, 2000).

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PLATE TECTON IC MODELS

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15

To the west and southwest the Malacca Microplate is

limited by the Mutus Assemblage characterised by

radiolarian cherts, red-mauve shales and rhythmic thin-

bedded sandstone and shale sequences with Late Triassic

fossils. Basalts, chlorite schist, gabbro and serpentinite

encountered in boreholes in the southeastern extension

of this zone suggested to Pulunggono and Cameron (1984)

that the Mutus Assemblage represented another suture,

marking the zone of collision between the Malacca Plate

and the Mergui Plate to the west. However, the

characteristic rock types

of

the Mutus Assemblage are not

restricted to this narrow zone, but are widespread across

Sumatra, being identical to those of the Middle-Late

Triassic Kualu and Tuhur Formations. These rock units

have been interpreted

in

the present account as deep water

deposits laid down in rifts developed during a Triassic

phase of extension. The concept of separate Malacca and

Mergui Plates, as proposed by Pulunggono and Cameron

1984), can no longer be supported.

The Mergui Microplate, characterised by the Permo-

Carboniferous pebbly mudstones and a Permian arc

assemblage, is shown extending across the greater part

of Sumatra, including the outcrops

of

the Bohorok, Alas,

Kluet and Kuantan Formations Fig. 9). The Permian

volcanic arc, represented by the Palepat and Mengkarang

Formations with the Cathaysian flora, is shown overlying

the southwestern margin of the Mergui Plate. In the

northern part of Sumatra the Situtup Formation near

Takengon is shown as a tectonic outlier of this arc, on the

basis of the volcanics associated with limestones. The

limestones contain the Middle Permian fusulinids

Pseudodoliolina sp. and Neoschwagerina sp. Fontaine and

Gafoer, 1989) which are considered to be typical

Cathaysian forms personal communication, Ueno,2002)

which supports this interpretation.

Oceanic and Arc assemblages of the Jurassic-Cretaceous

Woyla Group, described as the Woyla Terrains, are shown

along the west coast of Sumatra, thrust under rather

than over) the Permian arc and southwestern margin

of

the Mergui Plate Pulunggono and Cameron, 1984)

Fig. 9). These terranes include areas in Sikuleh, Natal

and Bengkulu not named) identified as microcontinental

INDONESIA

(after Pulunggono & Cameron, 1984)

JURASSIC-

Arc Association

CRETACEOUS

Mutus

Assemblage

CARBON FEROUS Pebbly Mudstone

Fig. 9. Microplates in western Indonesia

from

Pulonggono 198S), after Pulunggono and

Cameron

1984).

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BARBER AND M.J.

CROW

blocks. Wajzer et al. 1991) demonstrated by isotopic

dating that the supposed Natal Terrane is a fragment of

an Eocene-Oligocene magmatic arc, and more recently

Barber 2000) has argued that the Sikuleh Terrane is part

of a Jurassic-Cretaceous intra-oceanic volcanic arc, other

fragments of which can be identified throughout western

and southern Sumatra. Pre-Tertiary rocks are not exposed

in the Bengkulu area, so that the whole concept of

microcontinental terranes accreted along the western

margin of Sumatra is in doubt.

Fontaine and Gafoer

I 989) Model

Comprehensive palaeontological studies of the Perrno-

Carboniferous stratigraphic units in Sumatra by Fontaine,

Gafoer and their colleagues Fontaine and Gafoer, 19891,

prompted a reassessment of their age, environment of

deposition and their provincial affinity Fig. 10) Fontaine

and Gafoer 1989) interpreted the Carboniferous rocks

in the northern part of Sumatra as a series of

contemporaneous sedimentary facies formed on a

continental margin, with littoral and shelf facies sands in

the east, represented by the Kubang Pasu and Kenny Hill

Formations in the western part of the Malay Peninsula,

and quartzites and quartz sandstones encountered in oil

company boreholes along the Malacca Straits. In this

model pebbly mudstones of the Bohorok Formation

represent deposits from a melting floating ice-shelf or

icebergs, which are interbedded with turbiditic sands and

shales, passing into distal turbidites and deep water shales

further offshore in the Kluet Formation. The limestones

of the Alas Formation, with oolites and current bedding,

as described in the foregoing account, represent shallow

water carbonates deposited on a high, perhaps a horst

block, in the continental shelf environment.

Fontaine and Gafoer 1989) relate the fauna and algal

flora of the Visean Alas limestones to those found

elsewhere in the Sibumasu Block, in western Peninsular

Malaya, Thailand and Burma. On the other hand, they

relate the fauna and algal flora of the limestones in the

VisCan Kuantan Formation to those of the eastern

Peninsular Malaya and the Indochina Block in Thailand,

Laos and Vietnam. While the Alas limestones could have

been deposited in a cool environment, the fauna and flora

of the Kuantan limestones clearly indicate a tropical

environment of deposition. Since the the Alas and Kuantan

Formations are contemporaneous, they must have been

deposited on separate plates, and were only been brought

together in Sumatra by post-Carboniferous movements.

This relationship is indicated on the Carboniferous

palaeogeographic reconstruction of Sumatra Fontaine

and Gafoer, 1989) Fig. 10) by an arbitrary

WNW-ESE

boundary, which has no present structural expression,

Fig.

10.

Carboniferous palaeogeography after Fontaine and Gafoer,

1989 .

separating the Kuantan Formation from the outcrops of

the Kluet, Alas and Bohorok Formations to the north.

As reported above Vozenin-Serra 1989) reviewed the

Jambi flora of Central Sumatra and confirmed its

Cathaysian affinity. Fontaine and Gafoer 1989) were also

able to date the Jambi flora very precisely as earliest

Permian, from the fusulinid fauna in the marine sediments

interbedded with the plant beds. The presence of a

Cathaysian flora, together with Permian volcanics in

Central Sumatra led the geologists of the Netherlands

Indies Geological Survey, who mapped the area in the

1920s and 1930s, to draw a comparison between the

sequence in this area and the Permian sequence of the

Malay Peninsula, and to contrast this sequence with that

of the rest of Sumatra. In order to account for this anomaly

they proposed that the Permian sequence constituted the

Jambi Nappe which was derived from a root zone which

lay in the Riau Islands to the east. In this interpretation

unmetamorphosed Permian rocks of the nappe rest on a

thrust plane above metamorphic rocks of the Schiefer

Barisan Zwierzycki,

1930)

Fig.

11 .

The low angle fault

shown by Zwierzycki 1930) as the base of the Jambi

Nappe was subsequently re-interpreted by Katili 1970)

as a strike-slip fault Fig. 11).

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PLATE TECTONIC MODELS FOR SUMATRA

17

sw LEMATANG NE

Permo-Carboniferous

erangin

LINE

from Katili, 1970

I-

JAMB1 NAPPE

Vorbarisan

JAMB1THRUST

(LEMATANG LINE)

sw I

3

50km

from Zwierzycki, 1930

Fig. 11. The Jambi Nappe and the Lematang Line from Pulunggono and Cameron, 1984, after Zwierzycki, 1930 and Katili, 1970).

Nevertheless, the Cathaysian flora and the similarities (1989), between the Kuantan Formation and Carboniferous

of the Permian sequence to that of the eastern part of the rocks of the Tapanuli Group to the north.

Malay Peninsula shows at that time Central Sumatra Metcalfes (1996) ma p shows a group of

formed part of the Cathaysian continental block.

It

also microcontinental blocks, the Woyla Terranes, on the

shows that the affinities of northern Sumatra to the

Sibumasu Terrane and of Central Sumatra to Cathaysia

continued from the Carboniferous into the Middle

Permian, so that the the two blocks can only have come

together after this period.

Metcalfe

I

996 Model

Metcalfe has published many versions of his

interpretation of the distribution of tectonic blocks in

southeast Asia, of which that published in the Geological

Societys volume on the Tectonic Evolution of Southeast

Asia may be taken as representative (Hall and Blundell,

1996). In this model, although Sumatra is not discussed

in the text, the map showing the terranes and sutures in

East and Southeast Asia includes the major part of

Sumatra in the Sibumasu Terrane (Metcalfe, 1996)

(Fig.

12).

However, the Bentong-Raub Line, which

separates the Indochina/East Malaya from the Sibumasu

Terrane in the Malay Peninsula is shown continuing into

Central Sumatra as proposed by Tjia (1989) following

the Tertiary Bengkalis Graben identified in oil company

seismic data, and then turning sharply to the NW,

following the boundary, proposed by Fontaine and Gafoer

Fig.

12.

Accreted terranes in Southeast Asia after Metcalfe 1996).

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8

A.J.

BARBER AND M.J.

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southwestern margin of the Sibumasu Terrane. Metcalfe

1996, his Fig.2), following Cameron et al. 1980),

identifies hese terranes as

the

Sikuleh, Natal and BengMu

Terranes.As already discussed the microcontinental nature

of these terranes is in doubt.

Hutchison

(1994)

Model

The whole problem of the distribution, relationships

and tectonic history of the Gondwana and Cathaysian

Terranes in Sumatra and the Malay Peninsula has been

reviewed by Hutchison 1994). He recognises three

terranes in the Malay Peninsula and Sumatra Fig. 13 . The

East Malaya Terrane in the east, linked to Indochina and

South China, is characterised by limestones with fusulinids

in the Lower Permian, Mid-Late Permian arc volcanics

and an Upper Permian Cathaysian flora at Jengka Pass

and Linggiu Fig. 7). East Malaya is separated from the

Sinoburmalaya Terrane to the west by the Medial Malaya

Line = Bentong-Raub Suture).

To the east, Sinoburmalaya cf. Sibumasu of Metcalfe,

1996) is characterised by quartz sandstones, occupying

the western part of the Malay Peninsula and the Malacca

Strait, and tilloid pebbly mudstone)-bearing = Singa

and Bohorok) formations to the west. Hutchison 1994)

I I I

b-

100" 1b2 104" 106" 1064

VEST SUMATRAm SINOBU RMALAYA EAST MALAYA

8

6

49

2"

0

2

I 1uL

400km

4

>

>

Lower to Mid Permian

a

ith volcanic arc

IIIIII]

Carboniferous

NEST SUMATRA

Q

Medial Sumatra Line

Mid

to

Upper Permian

with volcanic arc

Carboniferous

Medial Malaya Line

Bentong-Raub Suture)

AST MALAYA

Kluang Limestone

of

unknown age)

m age unknown)

arboniferous-Permian without

Diamictite

sand

dominant)

Carboniferous-Permianwith Mutus Mutus Assemblage

Diamictite Pebbly mudstone)

....

iINOBURMALAYA

Fig

13. Tectonic u ni ts whic h have amalgamated to

make up Sumatra an d the Ma lay Peninsula,

after Hutchison (1994).

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PLATE TECTON IC MODELS FOR SUMATRA

19

(Fig.

13)

shows the Bentong-Raub Suture following a

sinuous course through southern Sumatra, following

reported occurrences of basic and ultrabasic rocks. This

course leaves the islands of Bangka and Billiton in the

East Malayan Terrane, consistent with the presence of

Permian sediments containing schwagerinid fusulinids in

the northern part of the Bangka (De Roever, 1951) and

offshore Billiton (Strimple and Yancey, 1976) and the

presence of poorly preserved plant remains, tentatively

identified as belonging to the Cathaysian flora (van

Overeem, 1960). However, as has already been reported,

in the southern part of the island, where De Roever (1951)

described an arkosic conglomerate, KO (1986) identified

a pebbly mudstone which may be correlated with the

Bohorok Formation. Hutchison (1994) acknowledges the

uncertainty of the course adopted in his model by a liberal

sprinkling of question marks. If KOs (1986) identification

of the pebbly mudstone is correct the Bentong- Raub

Suture must pass through Bangka, where it had been

placed in several earlier syntheses (Hutchison, 1975,

1983; Mitchell, 1977; Pulunggono and Cameron, 1984).

As yet no distinct lineament marking the trace of the

Bentong-Raub Suture has been identified in Bangka.

If the pebbly mudstones in southern Bangka and the

Mentulu and Bohorok Formations are correctly identified

as glacial deposits then the whole Sinobunnalaya Terrane

is clearly related to Gondwana (Northern Australia).

In Hutchisons (1994) synthesis, Sinoburmalaya is

separated to the southwest from the West Sumatra Terrane

by a Medial Sumatra Line (Fig. 13) . In identifying the

West Sumatra Terrane, Hutchison (1994) follows Fontaine

and Gafoer (1989) who related the limestone fauna of

the Vis6an Kuantan Formation in Central Sumatra to those

of East Malaya, Laos, Vietnam and eastern Thailand. While

acknowledging that the limestones of the Visean Alas

Formation do not contain the same fauna as the Kuantan,

and that during mapping the surveyors had concluded

that there were sedimentary facies transitions between

the Bohorok, Kluet and Alas Formation (Cameron et al.,

1980), nevertheless, Hutchison (1994), extends the West

Sumatra Terrane northwards to include the outcrops

of

the Kluet and Alas Formations (Fig. 13).He suggests that

the Medial Sumatra Line is a major strike-slip fault,

parallel to the Main Sumatran Fault, which brought the

Alas and Kluet Formations into juxtaposition with the

Bohorok Formation during the Cenozoic (Hutchison,

1994). Movement along the Medial Sumatra Line must

have occurred much earlier than the Cenozoic, as the Middle

to Late Triassic Kualu and Tuhur Formations, with similar

lithologies and faunas, occur on either side

of

the fault.

Hutchison (1994) strengthens his case for the

recognition of a Cathaysian West Sumatra Terrrane by

incorporating the Lower Permian Jambi Series

(Zwierzycki, 1930) (cf. tTambi Nappe refered to above),

which includes the Menkarang Formation containing a

tropical Cathaysian flora and interbedded fusulinid-

bearing limestones, in this terrane. The Mengkarang

Formation is associated with volcanic rocks of the Palepat

and Silungkang Formations forming a NW-SE trending

belt along the southwestern margin of the West Sumatra

Terrane (Fig.

13).

Following Pulunggono and Cameron

(1984), Hutchison (1994) identifies an outlier of this

volcanic belt in the volcanics

of

the Situtup Formation

near Takengon, where the limestones have yielded mid-

Permian fusulinids

of

Cathaysian type.

As Hutchison (1994) points out, the West Sumatra

and East Malaya Terranes have similar volcanic arc

characteristics, are rich in fusulinid limestones and contain

a Cathaysian flora, but all these features are of different age.

The West Sumatra Terrane

is

not therefore demonstratively

a detached part of the East Malaya Terrane, although both

were evidently once part of Cathaysia.

Hutchison (1994) follows Pulunggono and Cameron

(1984) in identifymg the Mutus Assemblage, here shown

as separating the quartzites and pebbly mudstones

through Central and southern Sumatra (Fig.

13 .

Reasons

have been given earlier in this account for interpreting

this assemblage as a zone

of

deeper water sediments

occupying the site of a Triassic extensional rift.

Also,

in

southern Sumatra Hutchison (1994) illustrates the

subcrop of the Kluang Limestone identified from borehole

records (De Coster, 1974). De Coster (1974) suggested a

Cretaceous age for this massive limestone formation.

Hutchison (1994) by analogy with the Kuala Lumpur

Limestone in Malaya suggests a Silurian age. From the

position of this occurrence, along strike to the southeast

of the outcrop of the Kuantan Formation. As suggested

earlier, a more reasonable correlation is with limestone

units of the Carboniferous Kuantan Formation.

Models f o r the Woyla Group

Cameron et al. (1980) interpreted the Jurassic-

Cretaceous Woyla Group in northern Sumatra as an arc

assemblage, composed of basaltic and andesitic volcanics

with surrounding reef limestones, an oceanic assemblage,

composed of ocean

floor

materials, serpentinite, gabbro,

pillow basalts, volcaniclastics and ribbon-cherts,

imbricated into an accretionary complex. They suggested

that the arc had been constructed on continental crust,

as it is intruded by the Sikuleh granitoid batholith which

is associated with tin anomalies. They suggested that the

continental sliver, on which the arc was built, had separated

from the mainland

of

Sumatra by the development

of

a

short-lived narrow marginal sea, similar to the Andaman

Gondw ana Research,

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20

A J BARBER

AND M.J. CROW

Sea. Subsequently the arc collapsed back against the

continental margin, deforming the ocean floor materials

to form the oceanic assemblage. All these events occurred

within the Late Jurassic to Early Cretaceous, as the deformed

rocks are intruded by the mid-CretaceousSikuleh Batholith.

This model was extended to the south where similar

rocks are found in the Natal area Rock et al., 1983).

However, when the supposed volcanic arc and its

associated granitoids were dated, as already mentioned,

it was found that they were of Oligocene-Eocene age and

did not correspond to the volcanic arc of northern Sumatra

Wajzer et al., 1991). In addition a Late Triassic

foraminifer was found in a limestone block within the

oceanic assemblage at Natal, extending the age of the

ocean floor back into the Triassic. Wajzer et al. 1991)

argued that the abundance of cherts in the oceanic

assemblage indicated that the ocean floor sediments had

been deposited far from land and that the oceanic

assemblage constituted part of a major ocean basin, rather

than a narrow marginal sea. In this model the volcanic

arc was interpreted as a mid-oceanic, rather than a

continental arc, which collided with Sumatra following

the subduction of the intervening ocean floor. A similar

model has been proposed by Mitchell 1992) for the

emplacement of a Late Jurassic-Early Cretaceous volcanic

arc in western Myanmar which could be the northern

extension of the Woyla arc.

In a recent study, using correlations proposed by earlier

authors, Barber (2000) has shown that rock units

equivalent to the arc and oceanic assemblages of the Woyla

Group in northern Sumatra occur thoughout the western

part of Sumatra, and that the model proposed by Wajzer

et al. 1991) can be extended to explain all these

occurrences. In this account it is proposed that the oceanic

island arc constitutes a Woyla Nappe, which was thrust

over the southwestern margin of Sundaland, composed

of the amalgamated Sibumasu and West Sumatra blocks,

during the mid-Cretaceous.

Revised Tectonic

Model

for Sumatra

A revised plate tectonic model, modified from earlier

models in the light of the data and the discussion above,

is presented as figures 14 and 15.

In figure 14, the East Malaya Block, characterised by a

Cathaysian flora and fauna, as proposed by Hutchison

1994) and Metcalfe 1996) lies to the east, limited to

the west and south by the Bentong-Raub Suture which

separates it from the Sibumasu Block, characterised by a

temperate Visean fauna in the Alas Formation and the

occurrence of pebbly mudstones in the Bohorok

Formation. The Sibumasu Block is extended into the island

of

Bangka to include the pebbly mudstone occurrence

described by Ko 1986), leaving Billiton and the northern

part of Bangka in the East Malaya Block. The Bentong-

Raub Suture marks the junction along which the East

Malaya and Sibumasu blocks were amalgamated.

As proposed by Hutchison 1994), to the southwest of

the Sibumasu Block, and separated from

it

by the Medial

Sumatra Tectonic Zone lies the West Sumatra Block,

characterised by a tropical Visean fauna in the Kuantan

limestones, Early Permian volcanics in the Palepat and

Silungkang Formations and an Early Permian Cathaysian

flora Jambi Flora) in the Mengkarang Formation. The

block also includes the fossiliferous Middle Permian

limestones of Silungkang, Ngaol and Pendopo. In figure 14

the block is shown extending to the northwest to include

the Sibolga Granite, considered to form part of the Early

Permian magmatic arc. The West Sumatra Block is not

considered to extend further to the northwest to include

the Carboniferous Alas and Kluet Formations as proposed

by Hutchison 1994). The continuity of the sediments of

the Bohorok, Alas and Kluet Formations, as described by

the surveyors Cameron et al., 1980), is taken to indicate

that all these units form part of the Sibumasu Block.

However, the Situtup Formation with its typical Middle

Permian Cathaysian fusulinids may be a tectonic outlier

Mippe) of the West Sumatra Block as Pulonggono and

Cameron 1984) and Hutchison 1994) have already

proposed.

Further to the southwest, and occupying the whole of

the western part of Sumatra, is the volcanic island arc and

imbricated ocean floor materials of the Jurassic Cretaceous

Woyla Group, thrust over the western margins of the

Sibumasu and West Sumatra blocks in the Woyla Nappe.

In figure 1.5

A-D

a series of cartoons represents the

major tectonic events in the development of Sumatra.

According to Sengor et al. 1988) and Metcalfe 1996)

the blocks which constituted Cathaysia, North and South

China and Indochina/East Malaya separated from the

northern margin of Gondwana with the development of

the Palaeo-Tethys in the Devonian. By the Early

Carboniferous Cathaysia, with the West Sumatra Block

forming part of its southern continental margin, lay in

tropical latitudes. The continental margin sediments are

represented by the Kuantan Formation with its tropical

Visean coral-algal fauna and flora.

Figure 15A shows the situation in the Early Permian with

West Sumatra Block attached

to

Cathaysia. At this stage

subduction of the Palaeo-Tethys commenced beneath the

southern margin of Cathaysia in the Early Permian, generating

an Andean-type magmatic arc in the West Sumatra Block.

The arc

is

represented by intrusive granites, volcanic rocks

and associated sediments with their tropical faunas and

floras, of the Palepat, Mengkarangand SilungkangFormations.

Subduction with related volcanism also commenced in

Gondwana Research

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PLATE TECTONIC MODELS FOR SUMATRA 21

Fig.

14.

Pre-Tertiary ectonic blocks in Sumatra and Peninsular Malaysia, modified from Hutchison (1994). The Situtup klippen n northern Sumatra

are regarded

as

tectonic outliers of the West Sumatra Block.

P

and Tp: the locationof the Pawan and Tanjungpuah members of the Kuantan

Formation (Clarke et al., 1982), along the Medial Sumatra Tectonic Zone. No allowance has been made for post-Cretaceous movements

along the Sumatran Fault System. These amount to a maximum movement of only 200 lan in northern Sumatra.

the Early Permian along the section of the Cathaysian

margin represented by the East Malaya, but it is unlikely

that the West Sumatra Block lay adjacent to East Malaya,

as in East Malaya volcanism continued into the Late

Permian, but in West Sumatra ceased in the mid-Permian.

Figure

1 5 B

illustrates the separation of the Sibumasu

Block from Gondwana in Northwest Australia during Late

Carboniferous and Early Permian times by extension,

rifting and the formation

of

new oceanic crust on the floor

of the opening rift. This new ocean crust formed part of

Meso-Tethys. Volcanism related to this extension may be

represented by metabasics in the Kluet and Alas

Formations. The separation of Sibumasu occurred at a

time when northern Gondwana was covered by

continental glaciers and ice sheets. It is visualised that

ice sheets extended as ice shelves across the opening gulf.

As the ice shelves and icebergs melted they released

boulders and finer grained materials, to form tillite

deposits on the developing continental shelves in the

Bonaparte Gulf area of northwest Australia and the pebbly

mudstones of the Bohorok Formation in Sibumasu. During

the Permian Sibumasu drifted northwards into a more

temperate environment as Meso-Tethys expanded (Shi and

Archbold, 1995).

The sequence of events postulated by Sengor et al.

(1988) and Metcalfe (1996) for the separation of

continental blocks from Gondwana implies that West

Sumatra, like the other Cathaysian blocks, lay originally

Gondwan a Research,

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22

A J BARBER

NDM.J. CROW

A. EARLY PERMIAN

Palaeo-Tethys subducting beneath the margin

of

Cathaysia

Palepat Magmatic Arc Continental block

volcaniclastics and carbonates

Silunkang and Mengkarang

including Indochina and

ondwana

I

I

I I

I

I

I

I

I I

I I 1

I I I I

Site of future

strike-slip ault

(Median Sumatra Tectonic Zone)

6. LATE CARBONIFEROUS- EARLY PERMIAN

Separation

of

Sibumasu from Gondwana

Transient Ice Shelf

Transient Ice Shelf

MESO-TETHYS

C. END PERMIAN - EARLY TRIASSIC Collision

of

West Sumatra and

Sibumasu with East Malaya (Indochina) Block

Magmatic Arc

Volcanics

No

sedimentatv record

D.

MID-CRETACEOUS Collision of Woyla Oceanic Arc with the margin

of Sundaland

Sundaland

Woyla Nappe Inverted Triassic Grabens

Overthrust with folding of Jur-Cret

continental marain sediments Semanggol Basin Semantan Basin

Tectonic Zone

Meso-Teth

ys

B-R

S : Bentong-Raub Suture

Fig.

15.

Cartoons illustrating the plate tectonic evolution of Sumatra according to the model presented

in

this account. For justification see text.

to the north of Sibumasu. The problem to be addressed

is: How did the West Sumatra Block arrive in its present

position on the southern side of Sibumasu? The only

plausible explanation is that proposed by Hutchison

(1984)

:

that West Sumatra arrived in its present position

outboard

of

the Sibumasu Block by strike-slip faulting

along the Median Sumatra Tectonic Zone. The position

of this zone is indicated in figures 15A,C and D),

A

model

for the translation of continental blocks along active

continental margins is provided by the history of

Wrangellia translated along the Pacific margin of North

America by oblique subduction during the Late Mesozoic

and Cenozoic (e.g. Coneyet al., 1980). Hutchison (1994)

suggested that translation of the West Sumatra Block also

occurred during the Cenozoic, but the continuity of Middle

to Late Triassic sediments across the West Sumatra Block,

Sibumasu and East Malaya indicates that these block had

their present relationships by Middle Triassic times. The

translation of West Sumatra to its present position must

therefore have occurred in Late Permian or Early Triassic

times; as pointed out earlier in this account there is no

record of rocks of this period in Sumatra.

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PLATE

TECTONIC

MODELS

FOR SUMATRA

23

In figure 15C it is suggested that also during the Late

Permian and Early Triassic period the segment of Palaeo-

Tethys, which lay between the combined West Sumatra

and Sibumasu blocks and East Malaya, was subducted

beneath East Malaya until the blocks collided. The

Bentong-Raub Suture in Malaya represents the site of the

collision, much modified by later strike-slip faulting.

Following collision, the collision zone was invaded by

granite plutonism, accompanied by tin mineralisation.

During the Middle and Late Triassic, the whole of

Sumatra and Peninsular Malaya were subjected to E-W

extension, with the formation of several N-S graben

structures, such as the Kualu and Tuhur Basins in Sumatra

and the Semantan and Semanggol Basins in Malaya,

separated by intervening horst blocks.

As

the result of

extension the whole area subsided below sea level.

Carbonates were deposited on the horst blocks, while the

graben, cut off and far from sources of terrigenous

sediment, accumulated bedded cherts and thin shales.

The record of Middle to Late Permian cherts in the

Semanggol Formation Sashida et al., 1995) which

suggests that the Semanggol Basin originated at an earlier

stage than envisaged here, has been explained by Metcalfe

(2000) who reports that these cherts were deformed by

the collision event, while Middle to Late Triassic cherts in

the same area show only tilting and open folding. Towards

the end of the Triassic, uplift of the eastern part of the

Malay Peninsula, perhaps associated with the intrusion

of the granites, provided a sourceof terrigenous sediments.

Turbiditic sands and shales were deposited in the graben,

the sands becoming coarser and more conglomeratic in

the more easterly of the graben towards the end of the

Triassic.

During the later part of the Mesozoic the southwestern

margin of the combined East Malaya-Sibumasu-West

Sumatra continental block, which can now be regarded

as Sundaland, formed a passive continental margin facing

the Meso-Tethys Ocean. In the Late Jurassic a subduction

zone developed within the ocean, outboard of Sumatra,

forming an oceanic island arc above the subduction zone.

Arc volcanoes, rising above sea level, were surrounded

by carbonate reefs. In the mid-Cretaceous, as the

intervening ocean floor Meso-Tethys)was subducted, the

island arc collided with the continental margin of

Sundaland Fig. 15D). The island arc with its forearc and

accretionary complex formed the Woyla Nappe which was

thrust over the Sundaland continental margin. Jurassic-

Cretaceous continental margin sediments of the Asia,

Peneta and Rawas Formations, were folded and

metamorphosed to the slate grade. It is suggested that

inversion of the Triassic basins, with folding of the basin

sediments, resulted from the far-field effects of the

collision of the Sundaland continental margin with the

Woyla Arc Fig. l5D) . A mid-Cretaceous compression

event, identified by the structural studies of Harbury

et al. 1990) in the central Malay Peninsula, shows that

compression affected the whole of Sumatra and the Malay

Peninsula at this time.

Following the collision with the Woyla Arc it is

considered that the polarity of the subduction system

reversed, so that subduction of oceanic crust outboard of

the collided arc beneath Sumatra generated a Late

Cretaceous magmatic arc Barber, 2000).

Outstanding Problems in Interpreting the

Tectonic Development

of

Sumatra

I. The age of the

Tapanuli Group

The only unit in the Tapanuli Group of northern

Sumatra which has beeen dated reliably is the Early

Carboniferous Alas Formation. Since the Bohorok and

Kluet Formations both contain sandstones and shales

similar to those interbedded with the Visean limestones

of the Alas Formation, so that no clear boundaries can be

drawn between the three formations, it has been presumed

that these units are also of Carboniferous age, and broadly

contemporaneous. This presumption, together with the

observation that pebbly mudstones and conglomerates

decrease in frequency, with a reduction in clast size

towards the southwest, led to the deduction that the

formations in the Tapanuli Group represent different facies

of a continental margin sequence which faced an ocean

to the southwest Cameron et al., 1980).0wing to the

apparent absence of fossils in the Bohorok and Kluet

Formations it has not even been established with certainty

that they are of Carboniferous age; it is possible that the

Tapanuli Group extends down into the Devonian and even

into the Lower Palaeozoic.

The upper limit to the age of the Tapanuli Group is

also uncertain. No fossils of Late Carboniferous or of

undoubted Early Permian age have been identified in

northern Sumatra, but it is probable that the Tapanuli

Group extends into the Early Permian. Although its

position within the stratigraphic succession has not been

established, the Bohorok Formation could be the youngest

formation within the Tapanuli Group. As already

described, the pebbly mudstones of the Bohorok

Formation have encouraged direct correlation, with the

pebbly mudstones of the Peninsular Thailand, the

Langkawi Islands and NW Malaysia, where tillites are

interbedded with sediments containing Early Permian

fossils. In Australia glacial deposits indicate that glaciation

commenced in the Namurian, reached its peak in the

Gondwana

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A J

BARBER

AND M.J.

CROW

Stephanian and Sakmarian and had ceased by the

Artinskian (Quilty 1984). These correlations suggest that

the Bohorok Formation is possibly of Late Carboniferous

to Early Permian age.

I t

is unfortunate that the fossiliferous

Pangaruran Bryozoan Bed has

so

far failed to yield precise

age-diagnostic f

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