Directory UMM :Data Elmu:jurnal:J-a:Journal of Asian Earth Science:Vol18.Issue6.Dec2000:

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Stratigraphic constraints on suture models for eastern Indonesia

J. Milsom

Department of Geological Sciences, University College London, London WC1E 6BT, UK Received 10 May 2000; accepted 29 June 2000

Abstract

Although collision in eastern Indonesia is now accreting the Australian continent to Southeast Asia, the small North and South Banda oceanic basins within the suture zone are interpreted as Late Cenozoic extensional features. Stratigraphic columns from the surrounding islands conform to one of three generalised patterns, two of which can be related to the margins of SE Asia (Sundaland) and the Australian continent, respectively. The third system, which is dominant in the outer Banda Arc and eastern Sulawesi, is associated with a microcontinent that was rifted from Australia in the Jurassic, drifted northwards ahead of Australia in the Cretaceous and collided with the Sundaland Margin in the Paleogene. Subsequent collapse of the resulting collision orogen led to rapid extension and the formation of the Banda Sea behind the Outer Banda Arc thrust belt. Eastern Indonesia thus duplicates a pattern familiar in the Mediterranean. The Tertiary compressional structures of the region cannot be explained solely in terms of the most recent collision, which began only in the Pliocene.q_{2000 Elsevier Science Ltd.} All rights reserved.

Keywords: Eastern Indonesia; Banda Arc; Eurasian Plates

1. Introduction

Suturing of northern Australia to SE Asia takes place within a diffuse and still poorly understood region (Fig. 1) where relative motions between the Indo-Australian and Eurasian Plates are absorbed by subduction beneath the Sunda Arc and collision around the strongly curved Banda Arc. The plate boundary, which south of the Sunda Arc is marked by a deep trench, is replaced in the Banda Arc by a series of relatively shallow troughs. There is no significant offset in the line of active and recent volcanoes but the two forearcs are very different. Except in the area to the west of Sumatra, the Sunda forearc ridge is entirely submarine, but the Banda forearc (Outer Banda Arc) is capped by large islands such as Timor, Tanimbar, Seram and Buru (Fig. 1). The origin of the back-arc Banda Sea is still unclear, with some authors (e.g. Silver et al., 1985) regarding the oceanic parts as trapped slices of Indian Ocean or Molucca Sea crust, while others (Hamilton, 1979; Re´hault et al., 1994) have argued in favour of Neogene extension. A pattern of local extension in an overall collisional environ-ment suggests analogies with the Mediterranean, where continental collision has produced deep basins floored by attenuated continental crust in the Alboran and Aegean seas and by oceanic crust in parts of the Tyrrhenian Sea (Dewey,

1988). The Mediterranean basins resemble the Banda Sea, not merely in size, but also in being partly enclosed by orogens with total curvatures approaching 180₈(Fig. 2).

Many of the allochthonous terranes which make up east-ern Indonesia are of Australasian origin. This is true not only of the large landmass of New Guinea, which is still linked to Australia, but of many of the smaller islands on the Asian side of the collision suture. Some of this material has been transferred from Australasia to Eurasia during the Pleistocene and continuing Banda Arc collision, but other fragments must have been accreted earlier. Two important ‘Australian’ elements are Buton Island, southeast of Sula-wesi, and the Banggai and Sula Islands, which form the Sula Spur (Fig. 1). There is a wide measure of agree-ment (Hamilton, 1979; Milsom, 1985; Silver et al., 1985) that the Sula Spur is a fragment of New Guinea which was transported west along transcurrent faults during the late Tertiary. Observations made in the course of oil exploration programmes have dated its collision with the East Arm of Sulawesi to between 5.2 and 3.8 Ma (Davies, 1990).

The history of Buton, which collided with the Sundaland Margin much earlier, in the Early or Middle Miocene, is more controversial (Davidson, 1991). One school of thought considers it to have also come from the Birdshead (Smith and Silver, 1991), but the correlations with New Guinea are less convincing, whereas the Mesozoic sediments are

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virtually identical to those on the northern Banda Arc islands of Buru and Seram.

The Australian-derived fragments listed above all contrast strongly with Sumba, the westernmost island in the Outer Banda Arc (Fig. 1), which resembles SW Sula-wesi. Thus, and in very simple terms, the geology of eastern Indonesia can be summarised by three generalised associa-tions of sedimentary, metamorphic and igneous rocks, of which two are related to the continental margins of South-east Asia (Sundaland) and Australasia, respectively. The third, Banda, association is dominant in and around the Banda Sea. The stratigraphic data, while not defining the entire history of the suture zone, can be used to constrain the range of acceptable hypotheses.

2. Sundaland Margin Association

Subduction at the Sundaland Margin can be traced back into the Cretaceous, and the exposed metamorphic rocks are thought to represent Cretaceous accretionary complexes. Conditions changed in the Oligo-Miocene as a result of collision with a microcontinent and many of the younger rocks record extension rather than compression.

2.1. SW Sulawesi

SW Sulawesi is the end product of a series of volcanic episodes that began in the late Mesozoic, when the block was joined to eastern Borneo, but continued after the Eocene opening of the Macassar Straits (Polve´ et al., 1997). Meta-morphic basement complexes exposed in the Barru and Bantimala areas of SW Sulawesi (Wakita et al., 1996) and farther north in the Latimojong area (Bergman et al., 1996) are in thrust or depositional contact with weakly metamor-phosed deep marine clastics of the Upper Cretaceous Balangbaru Formation. Carbonates were deposited in two main periods, in the Eocene–Oligocene and Miocene (Wilson and Bosence, 1996). Volcanogenic sediments are widely distributed, especially in the fault-bounded Walanae depression that developed following the Late Oligocene or Early Miocene collision, which sutured western and eastern Sulawesi. Volcanic rocks with ages ranging from 2 to 18 Ma, but concentrated around 8 Ma, were interpreted by Bergman et al. (1996) as evidence for orogenic collapse and extension, their chemistries being consistent with partial melting at the base of an extending, collision-thickened and possibly delaminating lithosphere. This conclusion was endorsed by Polve´ et al. (1997), who noted the J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761–779

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122 Eo

126 Eo

130 Eo

134 Eo

2 So

6 So

10 So

Timor Wetar Alor Flores

Buton

Misool

Onin Peninsula

Komewa Peninsula

TBJ-1X

Buru

Tanimbar Ridges

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Flores Sea Islands

BANDA

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BASIN

BASIN WEBER Bay

U.P.

Bone SULAWESI

Seram

BIRDSHEAD (Irian Jaya)

Sumba

Sula Is. Banggai

Kai

Aru Obi

Aru Trough Seram

Trough

Timor Tr ough

NORTH WEST SHELF

La w_an

opo Fault

7 1

400 km 200

4a 5a

5b 4b

NORTH

9b 3a

6b 8b

Fig. 1. The East Indonesia suture zone. Numbers enclosed in triangles, inverted triangles and circles refer to figure numbers for stratigraphic columns for, respectively, stratigraphies of the Australian Margin association, the Banda association and the Sundaland Margin association. The thick line with triangles (on upper plate) indicates the approximate present-day location of collision suture. UPUjung Pandang.

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comparative scarcity of conventional subduction-type calc-alkaline rocks with Neogene ages. The geological evolution of western Sulawesi is summarised by the stratigraphic column of Fig. 3a, which does not, however, include the important but controversial Lamasi Complex. The date of emplacement of this deformed, metamorphosed and thrust bounded ophiolite (Bergman et al., 1996) is not known but the mid-Tertiary orogenic phase is an obvious possibility.

2.2. Flores Sea Islands

Bouguer gravity levels indicate that the northern part of the Flores Sea, south of Sulawesi, is underlain by thinned continental crust, but that there is oceanic crust further south (Silver et al., 1986). The small and scattered islands within the sea have been described by Guntoro (1995) as closely related to the longitudinally corresponding areas of Sula-wesi, with acid igneous rocks in the west and more basic igneous rocks in the east. A volcanic sequence (Old Volca-nic Breccia) in the western islands is equivalent to the Langi Volcanics of SW Sulawesi, and the unconformably over-lying bioclastic limestones, reliably dated as Oligocene, are equivalent to the upper, bioclastic, units of the Tonasa Limestone described by Wilson and Bosence (1996). Wide-spread calc-alkaline and alkaline, granitic to rhyolitic pluto-nic and volcapluto-nic rocks have not been dated, and their

contacts with other rocks have not been seen, but they contain dioritic xenoliths interpreted as belonging to the Old Volcanic Breccia. These suggest an age no greater than Eocene and a probable correlation with the Early to Middle Miocene granites of SW Sulawesi.

Volcanic activity recommencing in the Pleistocene, produced the Young Volcanic Breccia, which consists of conglomerate, volcanic tuff and volcanic breccia of andesi-tic and basalandesi-tic composition. Alkaline andesiandesi-tic and basalandesi-tic dykes and sills intrude all units except the Quaternary coral limestones. As in western Sulawesi, the combination of calc-alkaline and alkaline chemistries suggests both subduc-tion and extension.

2.3. Sumba

The Flores Sea Islands form a partial link between SW Sulawesi and the Outer Banda Arc island of Sumba (Fig. 1), which lies to the west of the region of current arc-continent collision. The position of Sumba, and the absence there of any Australasian material, is evidence that the large islands of the Outer Arc do not owe their existence solely to accre-tion in the course of the present-day collision.

The oldest rocks exposed on Sumba are Cretaceous open marine sediments of the Lasipu Formation, described by Wensink (1997) as identical to the Balangbaru of SW J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761–779 763

Tethyan Oroclines

Banda Sea

Aegean

Alboran

Carpathian

Tyrrhenian

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MIO-CENE CENE

MIO-

OLIGO-CENE OLIGO-CENE

PALEO-CENE PALEO-CENE

EOCENE EOCENE

100 100

50 50

0 0

BALANGBARU FORMATION LASIPUFORMATION

BARRU, BANTIMALA AND LATIMOJONG COMPLEXES

LANGI VOLCANICS

JAWILA VOLCANICS

SW SULAWESI

SUMBA

MALA WA

FORMATION MASU

FORMATION TONASA

LIMESTONE FORMATIONPAUMBAP A

TACIPI MEMBER

QU ATERNARY REEF

CAMBA FORMATION

KANANGGAR WAIKABUBAK

WALANAE FORMATION

Tectonic melange; sandstones, shales,

cherts, basalt ultramafics and

schists Deep marine

clastic sediments Turbidites andsubmarine fan

deposits Tholeiitic and

calc-alkaline volcanics

Tholeiitic and calc-alkaline

volcanics Carbonate platform with

redeposited marginal facies including platform carbonatesShallow marine sediments

Volcaniclastics Carbonate

platform

Shallow marine

clastics Reef limestones

Shales Coals

? ? ?

Neritic Sediments

Chalk and reef limestones

Volcaniclastics

3a 3b

Fig. 3. (a) Stratigraphic column for SW Sulawesi, after Bergman et al. (1996) and Wilson and Bosence (1996). (b) Stratigraphic column for Sumba, after Fortuin et al. (1997) and Wensink (1994). Numbers in inverted triangles refer to locations shown in Fig. 1. Vertical scale in m.y.

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Milsom

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BATH U R ST ISLAND GR OU P ASHMOR E LIMESTONE

Marine siltstone and shales Marine shales and sandstones Marine glauconitic shales

Continental redbeds Non-marine siliciclastic Paralic to shallow marine clastics

and carbonates Marine clastics deposited at a wide variety of depths

OUGHT

OUP

TIM O R G AP

IRIAN JAYA

MT. GOODWIN FOR MATION PLOVER FOR MATION

WONWOGI SANDSTONE

EKMAI SANDSTONE

KOPAI FOR MATION

NEW GU INEA LIMESTONE

MALITA FOR MATION CAPE LONDONDER RY

FOR MATION FLAMINGO GR OU P

TIPU MA FOR MATION

AIFAM GR OU P

KEMBELANGAN

OUP

IMSKIN

Thickly-bedded micaceous and glauconitic orthoquartzite Massive to thickly-bedded quartz sandstone and siltstone

Dense, well bedded calcilutite

Micaceous glauconitic and micaceous sands and silts

Argillaceous, glauconitic and calcarous quartz sandstone

and silty mudstone

Continental redbeds

Nonmarine, lacustrine and paralic sediments, some coals

Shallow water platform carbonates with isolated reefs

Sandstones and clays STEENKOOL FOR MATION

Semi-consolidated pelagic and nanno chalks

Radiolarian chalk

MIO-CENE

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PALEO-CENE EOCENE

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PLIOCENE

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300 100

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PALEO-CENE EOCENE

OZOIC

PLIOCENE

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JURASSIC

TRIASSIC

PERMIAN

MESOZOIC

CENOZOIC

EARL

MIDDLE

M L

4 a 4 b

?

PINYA MU DSTONE

Fig. 4. (a) Stratigraphic column for the Timor Gap region of the Northwest Shelf, after Brown (1992). (b) Stratigraphic column for Irian Jaya, after Pieters et al. (1983). Numbers in triangles refer to locations shown in Fig. 1. Vertical scale in m.y.

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J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761 – 779 766 CRYSTALLINE BASEMENT BANGGAI GRANITE TANAMU FOR MATION BU YA FOR MATION Continental redbeds Paralic to shallow marine

and fluvial clastics Sandy paralic to nearshore clastics Deepwater clays Clays (limestones on Misool) Slates, schists and gneisses Acid to intermediate intrusives Calcilutite/calcarenite ? ? ? ? ? ? ? ? ? ? ?

SU LA SPUR

_MISOOL

MANGOLE

VOLCANICS TIPU MA

FOR MATION

AIFAM GR OU P KABAU W

FOR MATION

BOBONG FOR MATION _{POLYSEQU ENCE}INANWATAN YEFBI

SHALE R OABIBA

POLYSEQU ENCE SEBYAR POLYSEQU ENCE PELENG FOR MATION

PANCORAN FOR MATION SALODIC FOR MATION

Nonmarine, lacustrine and paralic sediments, some coals FAFANLAP FOR MATION

DARAM SANDSTONE Z AAG LIMESTONE KASIM MAR LSTONE

ATKAR I LIMESTONE Fine-grained calcarenite

Calcarenite with minor oolite

FACET LIME-STONE BOGAL LIME-STONE Calcarenite and coralgal limestone LIGU META-MOR PH ICS

Sandstone, calcareous siltstone Siltstone Tuff-aceous calci-lutite and clayey lime-stone Intrus iv es

Restricted shallow marine anoxic and highly fossiliferous shales H ighly fosilliferous pelagic

limestones Shallow water platform carbonates with isolated reefs

Shallow marine carbonates with localised reefs

Subaerial acid volcanics Continental redbeds

Paralic to shallow marine clastics and carbonates JASS POLYSEQU ENCE MIO-CENE OLIGO-CENE PALEO-CENE EOCENE PA L EOZOI C PLIOCENE EARL Y CRET A CEOUS JURASSIC TRIASSIC PERMIAN M ES OZO IC C EN OZO IC EARL Y EARL Y MIDDLE LA TE LA TE LA TE M L E MIO-CENE OLIGO-CENE PALEO-CENE EOCENE PA L E O Z O I C PLIOCENE EARL Y CRET A CEOUS JURASSIC TRIASSIC PERMIAN MESOZOIC CENOZOIC EARL Y EARL Y MIDDLE LA TE LA TE LA TE M L E 100 200 300 100 200 300 5 b 5 a

Fig. 5. (a) Stratigraphic column for the Sula Spur, after Garrard et al. (1988). (b) Misool. In the Mesozoic and Paleozoic, the formations and polysequences identified on the left are after Fraser et al. (1993). All other data from Rusmana et al. (1989). Numbers in triangles refer to locations shown in Fig. 1. Vertical scale in m.y.

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Sulawesi. Mid-Cretaceous dinoflagellates suggest a North Tethys affinity (Fortuin et al., 1997). The upper age limit of the Lasipu is uncertain, but there was widespread igneous activity in the Late Cretaceous and Paleogene. The Jawila Volcanics, originally thought to be Early Miocene, have now been dated as Late Eocene (Fortuin et al., 1997) and can be regarded as part of a belt that includes the Langi Volcanics of Sulawesi and the Old Volcanic Breccia of Tanahjumpea. Also in the Eocene, a platform developed and, as with the Tonasa of SW Sulawesi, remained a site of carbonate sedimentation (Paumbapa Formation) into the Early Miocene (Fortuin et al., 1997). The Paleogene sedi-ments are truncated by a Middle Miocene angular uncon-formity above which reef carbonates, chalks and volcanoclastic turbididites were deposited. The stratigraphy of Sumba is summarised in Fig. 3b; the similarities to SW Sulawesi are clear and are enhanced in both areas by the presence of Eocene granodioritic intrusions.

Extensive paleomagnetic work (Wensink, 1997) has provided additional support for a Late Mesozoic position of Sumba close to western Sulawesi, followed by detach-ment and a complicated series of rotations, the net effect of which has been some 908 of clockwise rotation. This contrasts with the mounting evidence for counter-clockwise rotation of Kalimantan and western Sulawesi (Fuller et al., 1999).

2.4. Sundaland Margin summary

The diagnostic features of the Sundaland Margin strati-graphy include Upper Cretaceous–Paleogene deep-water clastic sediments, volcanics which are of island arc type in the Paleogene but extensional in the Neogene, and the development of large carbonate platforms in the Eocene– Early Miocene. The type area is the South Arm of Sulawesi and, in particular, the region northeast of Ujung Pandang (Fig. 1). Similar, although not always complete, Mesozoic and Paleogene sections can be recognised in the Flores Sea Islands and Sumba. Sediments above the mid-Miocene angular unconformity, which is a feature of the association, show fewer common characteristics, which is unsurprising if dispersion began during the unrecorded interval. Disper-sion, and the generation of oceanic crust in the Flores Sea, must predate the Late Neogene development of the eastern Sunda/Banda volcanic arc, because this lies to the north of Sumba. Since the volcanic islands from Flores to Wetar separate two Sunda-related blocks, it is possible, and perhaps even probable, that they are themselves built on Sundaland basement, although this is nowhere exposed.

3. Australasian Margin Association

Sediments were deposited along the Australian Margin under terrestrial or marginal marine conditions in the Trias-sic and JurasTrias-sic and in deeper water during and after the Cretaceous. Basement rocks in the east belong to a Late

Paleozoic orogenic belt and in the west to a craton covered by Paleozoic platform sediments. Anomalously, ‘eastern’ granitic basement crops out in the Banggai Islands (Fig. 1), the most northwesterly Australasian fragment. Neogene sediments vary widely due to differences in setting in rela-tion to the collision orogenies.

3.1. The Northwest Shelf

During the Paleozoic, north-western Australia formed part of the interior of the Gondwana super-continent, but rifting in the Triassic and Jurassic detached India and other blocks and created new passive margins along the Northwest Shelf. Sediments deposited at these margins are almost nowhere seen in outcrop but are known from numer-ous wells. The stratigraphy of the shelf to the south of Timor has been described and compared with stratigraphies in adjacent areas by Brown (1992). Three major sedimentary groups were recognised, these being the Triassic–Jurassic Troughton Group, the Jurassic–Cretaceous Flamingo Group and the Middle and Upper Cretaceous Bathurst Island Group (Fig. 4a). Troughton group sediments are predomi-nantly siliciclastic and include red beds in the Malita Forma-tion, which is of latest Triassic and earliest Jurassic age. Marine transgression followed, with deposition of the fluvio-deltaic sediments of the Jurassic Plover Formation. There are no sediments which can be unequivocally assigned to a rift/break-up setting before the Late Jurassic when, as a result of rifting, an unconformity developed on which the sandstones and shales of the Flamingo Group were deposited under deeper marine conditions. Generally similar sediments characterise the Bathurst Group, depos-ited following an Early Cretaceous hiatus, but the sea had evidently deepened still further. Radiolarian shales were deposited in the Aptian to Early Albian and a black clays-tone with high gamma-ray signature represents a condensed sequence in the Turonian to Coniacian. Chalks were then deposited, which were dominantly radiolarian in the Late Cretaceous and foraminiferal in the Paleocene through to the Pliocene (von Rad and Exon, 1983). Harris (1991) noted strong similarities between these sediments and the Lower Cretaceous Kolbano Series of Timor.

3.2. New Guinea

Island arcs, which accreted to the northern margin of Australia during the Tertiary, now form mountain ranges along the north coast of New Guinea (Dow, 1977). These terranes can be tentatively correlated with Halmahera, where Indonesia borders on the Pacific, but are remote from the Banda Arc and are therefore not further considered here. Southern and central New Guinea have also been inter-preted in terms of large numbers of allochthonous or ‘suspect’ terranes (Struckmeyer et al., 1993), but most of the geological features of northern Australia can be traced at least as far north as the watershed in the central ranges. The western peninsula (Birdshead), which forms the link J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761–779 767

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between the main body of New Guinea and the smaller islands of eastern Indonesia, presents particular problems. It is widely thought to have moved independently for much of its history (cf. Hamilton, 1979) and some movement relative to New Guinea continues to the present day (Punto-dewo et al., 1994). The core of the peninsula is formed by the Birdshead/Kemum Terrane of Struckmeyer et al. (1993), in which metamorphosed Siluro-Devonian turbidites have been intruded by Carboniferous and Permo-Triassic grani-toids and are overlain by Late Paleozoic shallow marine clastics and Triassic to Lower Jurassic continental redbeds. Marginal marine conditions were re-established in the early Middle Jurassic and continued throughout the Mesozoic and into the early Tertiary, interspersed with periods of erosion and non-deposition (Dolan and Hermany, 1988). Sediments deposited during this long interval have traditionally been assigned to the ‘Kembelangan Formation’ (Visser and Hermes, 1962) but the term has been used in such a variety of contexts that it has become virtually meaningless (Fraser et al., 1993). The stratigraphic column of Fig. 4b is based on the more westerly of the New Guinea stratigraphies presented by Pieters et al. (1983).

Shelf carbonates (New Guinea Limestone, Visser and Hermes, 1962) dominate the Tertiary throughout New Guinea but deposition on the Birdshead was interrupted by a period of folding and erosion in the Late Oligocene.

3.3. The Sula Spur

The Banggai and Sula Islands, which lie immediately to the north of the North Banda Basin and the northern limb of the Banda Arc, were transported from the New Guinea region to their present position by transcurrent movements along strands of the Sorong Fault System (Hamilton, 1979; Pigram et al., 1985). Basement consists of poorly known metamorphics but there are also granitic rocks of assumed Paleozoic age. The Triassic is dominated by the acid Mangole Volcanics and by probably co-magmatic Permo-Triassic granites. Sedimentation, generally in marine basins with restricted circulation and water depths of less than 200 m, was almost continuous throughout the Jurassic but a ‘break-up unconformity’ (Garrard et al., 1988) occupies much of the Cretaceous. Following this break, bathyal sedi-ments were deposited during the Late Cretaceous and Paleo-cene (Garrard et al., 1988). A second hiatus occupied much of the Eocene but thereafter carbonate platform sedimenta-tion continued almost uninterrupted until the onset of colli-sion with East Sulawesi in the latest Miocene (Davies, 1990). Davidson (1991), amongst others, has suggested correlations between the Sula Spur and Buton and the Outer Banda Arc but the statigraphic sequence described above and summarised in Fig. 5a has virtually nothing in common with either of these areas.

3.4. Kai Besar

Seismic reflection surveys near the Kai islands (Fig. 1)

have shown that the plate suture runs between Kai Besar and Kai Kecil, rather than through the Aru Trough (Milsom et al., 1996). The oldest rocks exposed on Kai Besar, on the Australian side of the suture, are Eocene flat-bedded calcilutites and marls. Shallow-water carbonates were deposited from the Oligocene almost to the present day. This stratigraphy is not significantly different from the end-Cretaceous to Late Miocene succession on the Sula Spur.

3.5. Misool–Onin–Komewa

The Late Oligocene compression widely observed in the Birdshead was interpreted by Struckmeyer et al. (1993) as due to a collision with a Misool–Onin–Komewa Terrane. On Misool island a Paleozoic basement of folded and meta-morphosed turbidites is overlain by an almost complete Mesozoic passive margin sequence of Triassic turbidites, Upper Triassic shallow-water limestones and Lower Juras-sic to Upper Cretaceous bathyal clastics and carbonates (Rusmana et al., 1989). Outcrop information on the geology of Misool and the related Onin and Komewa peninsulas of the New Guinea mainland (Fig. 1) has been supplemented by drilling, and Fraser et al. (1993) used subsurface data from both the Misool–Onin–Komewa province and adja-cent parts of the Birdshead to develop a new scheme to replace the Kembelangan nomenclature. In this scheme the Mesozoic sediments were divided into a Lower to Middle Jurassic shallow marine to fluvial ‘Inanwatan Poly-sequence’, a Middle to Upper Jurassic paralic to nearshore ‘Roabiba Polysequence’, an Upper Jurassic–Lower Cretac-eous deepwater open marine ‘Sebyar Polysequence’and an Upper Cretaceous to Paleocene, ‘Jass Polysequence’, sepa-rated by major unconformities (Fig. 5b). In the crucial TBJ-1X well (Fig. 1) off the Onin Peninsula (and therefore within the Misool–Onin–Komewa Province), section is missing from the Middle Triassic to the base of the Toarcian, from the top of the Bajocian to the base of the Oxfordian, from the Lower Kimmeridgian to the mid Tithonian and from the Lower Valangian to the Cenomanian. Jass sediments have also largely been removed from the well section by erosion. Elsewhere, the Cenomanian base of the Jass is marked by a volcanic event and the overlying sediments are mainly deepwater clays, although shallow water rudists outcrop in the Misool archipelago.

Fig. 5b shows that there are considerable differences between the rocks outcropping on Misool and those inter-sected in TBJ-1X. Whereas the well section fits into the pattern of the Australian Margin Association, Misool has much in common with the Banda Association described below, although it reportedly lacks the characteristic Juras-sic unconformity. Moreover, palaeomagnetic data indicates that the island was more than 1000 km north of Australia in the Cretaceous (Wensink et al., 1989). There is thus a clear possibility that Misool was detached from the Australian Margin as an independent fragment in the Mesozoic and J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761–779

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J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761 – 779 769 LAKANSAI FOR MATION TOBELO FOR MATION TOBELO FOR MATION

Dark shales, limestones, occasional massive sandstone

Fossiliferous red calcareous mudstone Argillaceous limestone

Bathyal fine-grained limestones and argillaceous limestone

Pelitic phyllite and slate with subordinate quartzose

and micaceous sandstone White to pink

nannofossil-microfossil pelagic limestones, some cherts Well laminated calcilutite

with local clastic detritus Reef Limestone Pelagic foraminiferal marly chalk

Coarse to fine grained terrigenous clastics SAMPOLAKOSA FORMATION

WAPULAKA FORMATION

Po s s ib le h ia tu s

? ? ? ? ?

BUTON

WINTO FOR MATION

R U MU FOR MATION

OGENA FOR MATION TONDO FORMATION

H igh - very high grade schists and gneisses h i a t u s ?

h i a t u s ?

h i a t u s ? erosional hiatus

LATE CR ETACEOU S NIEF BEDS PALAEOGENE

NIEF BEDS

KOBIPOTO COMPLEX TAU NU SA COMPLEX

Coralli-genous limestones Chert-rich limestones, locally bioclastic Clays, shales, graywackes, some limestones

Weakly metamorphosed shales, graywackes, some limestones

Phyllites with graywackes, some limestones

Neritic shales Argillaceous calcilutites Dense, brittle calcilututes,

some cherts Cream and white calcilutites

red and green marls

Medium - high grade schists and gneisses. POSSIBLE GRADATIONAL CONTACT

Neritic sediments Bathyal sediments _Tectonite Grey argillaceous calcilututes

SAKU FOR MATION

TEH OR U FOR MATION

EARLY NEOGENE NIEF BEDS WAHAI BEDS SALAS

BLOCK CLAY

SERAM

KANIKEH FOR MATION SAMAN SAMAN LST EAR LY NIEF BEDS

MANU SELA FOR MATION

h i a t u s ? KOLA SHALE TOBELO FOR MATION

FUFA FORMATION MIO-CENE OLIGO-CENE PALEO-CENE EOCENE P A L E OZOIC PLIOCENE EARL Y CRET A CEOUS JURASSIC TRIASSIC PERMIAN MESOZOIC CENOZOIC EARL Y EARL Y MIDDLE LA TE LA TE LA TE M L E MIO-CENE OLIGO-CENE PALEO-CENE EOCENE PA L EO ZOI C PLIOCENE EARL Y CRET A CEOUS JURASSIC TRIASSIC PERMIAN M ESOZO IC C EN OZ OIC EARL Y EARL Y MIDDLE LA TE LA TE LA TE M L E 100 200 300 100 200 300

6 a 6 b

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collided with the Birdshead in the Mid-Tertiary, when the older sediments were folded. More detailed work is needed on the Mesozoic rocks, which are well exposed on the south coast of Misool and the islands immediately to the south, to clarify their role in the regional evolution.

3.6. Australian Margin summary

The wide variations in Australian Margin stratigraphies are not surprising in view of the vast area covered. The type area for the association is taken to be the shelf south of Timor (the ‘Timor Gap’). Western Irian Jaya, including the Birdshead, and the Sula Spur, are included in this asso-ciation but their basements of Late Paleozoic granites and associated extrusive rocks have more in common with central Papua New Guinea than the Northwest Shelf. Misool island is different again and lacks the terrestrial Triassic ‘red beds’ deposited elsewhere in the region prior to, and at the beginning, of the break-up of this part of Gondwanaland.

The later Mesozoic in all areas records a steady increase in water depth, from marginal marine in the Jurassic to open water bathyal in the later Cretaceous. During the Tertiary, parts of the margin were fragmented and/or involved in collisions, and Tertiary stratigraphies therefore differ considerably.

4. Banda Association

The Banda Association, which is found on the islands surrounding the Banda Sea, progresses from Upper Triassic to Lower Jurassic shallow water clastics and carbonates which are richly fossiliferous and sometimes bituminous, via unconformity to an Upper Jurassic to Paleogene condensed sequence of deep water carbonates and cherts from which clastic components are almost completely absent. The Middle to Late Jurassic hiatus is an important diagnostic feature. It is widely recognised on the Australian J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761–779

770

La

wanopo

Fault

Undifferentiated Mesozoic Cretaceous limestone Jurassic limestone Jurassic/Triassic clastics

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Milsom

Journal

Asian

Earth

Sciences

(2000)

761

–

779

771

POMPANGEO SCH IST

Low to high grade metamorphics

Reef Limestone REEF LIMESTONE

EAST SULAWESI

MELU H U FOR MATION

TOKALA FOR MATION CELEBES MOLASSE

CELEBES LIMESTONE

BURU

MATANO FOR MATION

WAH LUA COMPLEX RANA COMPLEX

DALAN FOR MATION

KU MA FOR MATION

MEFA FM.

Interbedded sandstones, shales,

siltstones, some conglomerates

Basaltic lavas and tuffs Cherty calcilutites

interbedded with conglomerates in the

lower part and marl and shale in the

upper parts

Low grade (greenschist facies) metamorphosed

clastics

Marls, calcilutites, conglomerates Conglomerate, sst, lst Coral and

foram. lst LEKO FOR MATION

FTAU WAEKEN FOR MATION WAKATIN

GH EGAN FOR MATION

H OTONG

Dolomites, shales, some bituminous, limestones

Medium-grade (greenschist to amphibolite) metamorphosed clastics

Coral reef

Sandstones Coarse to fine grained

terrigenous clastics

Reefal and nummulitic limestone

White to pink porcellaneous pelagic

limestones

Bituminous limestones, and shales

Bathyal fine-grained limestones and argillaceous limestone

MIO-CENE

OLIGO-CENE

PALEO-CENE EOCENE

PLIOCENE

EARL

CRET

CEOUS

JURASSIC

TRIASSIC

PERMIAN

MESOZOIC

CENOZOIC

EARL

MIDDLE

M L E

MIO-CENE

OLIGO-CENE

PALEO-CENE EOCENE

OIC

PLIOCENE

EARL

CRET

CEOUS

JURASSIC

TRIASSIC

PERMIAN

ESO

ENO

EARL

MIDDLE

M L

100

200

300 100

200

300

8 b 8 a

Fig. 8. (a) Stratigraphic column for East Sulawesi, after Rusmana et al. (1993). (b) Stratigraphic column for Buru, after Tjokrosapoetro et al. (1993). Numbers in circles refer to locations shown in Fig. 1. Vertical scale in m.y.

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Northwest Shelf, where it was termed ‘Wombat-type’ by Gradstein (1992) following ODP drilling on the Wombat Plateau. There are many similarities between the Mesozoic sediments of the Wombat Plateau and the Banda Associa-tion. However, whereas the Mesozoic of the plateau rests on a thick unmetamorphosed Paleozoic section, metamorphic rocks, upon which the Triassic sediments are said to rest unconformably, are common in the Banda Association, as are Cretaceous–Paleocene ophiolites. Stratigraphies in the various Banda fragments diverge significantly only after the Oligocene, when many of the blocks were deformed by thrusting.

4.1. Buton

Buton is one of the most important islands of the Banda Association because of its present close proximity to Sula-wesi (Fig. 1). Drawing in part on work by Fortuin et al. (1990) and De Smet and Hermanto (1991), Davidson (1991) deduced separation from Australia in the Late Trias-sic or Early JurasTrias-sic and a transition from pre-rift to syn-rift sedimentation in the Middle to Late Triassic. The Triassic rocks (Winto Formation) rest on pelitic phyllites and slates (Lakansai Formation) which are exposed over an area of only about 40 km2 in the northeast of the island (Smith and Silver, 1991). Both the Winto and the overlying Lower Jurassic Ogena Formation consist dominantly of limestone, but the Ogena appears to have been depos-ited in deeper water. Clastic sediments, principally shales, are common in the Winto of southern Buton. Both formations contain abundant organic material that is generally considered to be the source of the island’s asphalt deposits.

The poorly exposed later Mesozoic on Buton begins with deep marine siliceous and calcareous mudstones of the Upper Jurassic Rumu Formation and continues with the pelagic limestones with nodules and stringers of red chert of the Tobelo Formation. The Tobelo was originally classi-fied as entirely Upper Cretaceous but has now been shown to extend from the end of Rumu deposition up into the late Eocene or early Oligocene (Smith and Silver, 1991). Both the Rumu and the Tobelo were evidently laid down very slowly and their lithologies are consistent with deposition during the drift of an isolated continental fragment.

According to Davidson (1991), a hiatus at the top of the Tobelo Formation can be attributed to collision with SE Sulawesi in the Early and Middle Miocene (N11). Ophio-lites in southern Buton were probably emplaced at about this time, and compression led to uplift and the establishment of an unconformity representing a hiatus of approximately 3 m.y. The basal sediments of the coarse clastic Tondo Formation, immediately above the unconformity, are mainly carbonate detritus but ultramafic and mafic frag-ments become dominant later, indicating uplift of the ophio-lites above sea level.

Tondo Formation deposition was ended by subsidence of Buton to bathyal depths at approximately 5 Ma and the deposition of chalks and marls. Subsequent uplift was accompanied by the development of reefal carbonates. Minor compressional effects can be observed in Upper Pliocene strata, and oblique compression and associated strike-slip faulting may continue to the present day. Quaternary uplift in southern Buton, where spectacular flights of coral terraces rise to almost 500 m above sea level, has been estimated at 2.5 km, but the northern part of the island is now subsiding (Davidson, 1991). The history of post-Middle Miocene molasse deposition and ophiolite emplacement on Buton is virtually identi-cal with that of eastern Sulawesi, and the 5 Ma subsi-dence seems much more likely to have been caused by extensional collapse of the entire Sulawesi orogen, of which Buton formed a part (Milsom et al., 1999), than by the choking of a subduction zone, as suggested by Davidson (1991).

4.2. Buru

The Mesozoic succession on Buru (Fig. 8b) is virtually identical to that on Buton, even though the two islands are separated by the oceanic North Banda Basin (Fig. 1). Bitu-minous Triassic source rocks which are abundantly present in float in rivers in the northwest are directly comparable with the Triassic of Buton, as is most of the Cretaceous section. One distinctive feature of Buru, however, is the presence of some Jurassic igneous material, which may be compared with the rift phase volcanics of the Wallaby Plateau (Gradstein, 1992). Buru also differs from Buton in the abundance of metamorphic rocks, which cover almost the whole of the western two-thirds of the island. These metamorphics were divided by Tjokrosapoetro et al. (1993) into the low grade Rana and higher grade Wahlua Complexes. A recent reconnaissance along new roads into the centre of the island confirmed this distinction (D. Roques, personal communication, 2000).

As on Buton, the depositional environment on Buru became shallower in the Tertiary, but there is no direct evidence for Middle Miocene orogeny. Tjokrosapoetro et al. (1993) described the island as characterised by the absence of thrust faults, imbricated structures, melange or ophiolites. The latest phase of the island’s history has been dominated by very rapid uplift and deposition of thick, coarse alluvial fans (Fortuin et al., 1988). However, steep gravity gradients recently mapped in the southeast of the island are difficult to explain except by the presence of concealed, thrust emplaced, ophiolites.

4.3. Seram

Buru and western Seram were described by Hamilton (1979) as forming a single microcontinent, and certainly the two islands are stratigraphically very similar. For example, parallels have been drawn between the Rana J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761–779

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Metamorphics of Buru and the Tehoru Metamorphics of Seram (Linthout et al., 1989). The sediments of Seram were divided into allochthonous and para-autochthonous units by Audley-Charles et al. (1979) but this was rendered unconvincing by disagreement amongst the co-authors as to whether the key Nief Beds, which span the time interval from the Jurassic to the Oligocene, were to be assigned to the allochthon or the para-autochthon. A different view based in part on recent exploration drilling has been offered by Kemp and Mogg (1992). In this scheme, the oldest unme-tamorphosed sediments of Seram belong to the Middle to Upper Triassic, clastic-dominated, near-shore Kanikeh Formation, which grades into Lower and Middle Jurassic deep and shallow water limestones (Manusela and Saman– Saman Formations, respectively) both upwards and later-ally. If these relationships have been correctly interpreted, then all the sediments of Seram can be fitted into a single stratigraphic sequence (Fig. 6b).

The Kanikeh and Saman–Saman are contemporaneous with, and also strikingly similar to, respectively, the Winto and Ogena Formations of Buton. Only the Manusela Formation appears to lack a direct Buton equivalent. In view of the small area of Mesozoic outcrop on Buton, there may be little significance in this absence, or in the apparently different durations of the Jurassic hiatus. The similarities between Buton and Seram continue above the mid-Jurassic unconformity, since the Late Jurassic Kola Shale of Seram resembles the Rumu Formation of Buton and both are over-lain by deep water condensed sequences dominated by fora-miniferal limestones and marls which extend from the Early Cretaceous into the Paleogene (Nief Beds on Seram and Tobelo Formation on Buton). In both the Nief and Tobelo there is evidence for shallower water conditions in the Paleogene and eventual termination of carbonate sedimen-tation in the Miocene.

4.4. Western Kai and the Banda Ridges

Silicic schists, gneisses and migmatites on the western-most islands of the Kai group have been correlated by Charl-ton et al. (1991b) with the Kobipoto Complex of Seram and were considered by Honthaas et al. (1997) as uplifted parts of a former Banda forearc.

Dredging on the high standing ridges (Banda Ridges) in the central Banda Sea has recovered igneous and meta-morphic rocks (Silver et al., 1985), Triassic sediments (Villeneuve et al., 1994) and Miocene reefs (Corne´e et al., 1998). The metamorphics were originally correlated with those of the Birdshead (Silver et al., 1985) but the descrip-tions of the Triassic rocks are much more reminiscent of Buton, Buru and Seram.

4.5. Eastern Sulawesi

The geology of eastern Sulawesi is often described in terms of a simple division into belts of schist and ophiolite separated by the Lawanopo Fault (Fig. 1), but there are

schists to the north of this fault and ophiolites to the south. Patterns of metamorphism are complex. Parkinson (1998) considered that some of the metamorphic rocks formed a high temperature metamorphic sole to the ophio-litic thrusts but it is not clear how widely this interpretation can be applied. Both blueschists and greenschists are present. Metamorphic facies vary and have been used to define a number of distinct formations, but it is possible that these grade into each other. The prevalence of thrusting, and the reconnaissance nature of the mapping in many areas, leave this question open.

Gravity data indicate that the ophiolites overlie the schists on a thrust surface with variable but very shallow dip (Silver et al., 1978). Relatively small south-block up movement along the Lawanopo Fault could have created the present outcrop pattern by exposing the southern parts of the ophio-lite belt to more intensive erosion. Ultramafics south of the Lawanopo Fault, which might represent deep keels to a previously extensive thrust sheet, are associated with occa-sionally strong but very local positive gravity anomalies. If schists, rather than ophiolites, predominate at depth, then their age and origin are important in any regional synth-esis. The most widely held view is that they represent the basement of a microcontinent which collided with western Sulawesi in the mid-Tertiary (Hamilton, 1979). If this is the case, then Australasia seems the most likely ultimate source, although Parkinson (1998) inter-preted the main metamorphic formation, the Pompangeo Schist Complex, as the easternmost extension of the Mesozoic Sundaland Margin, metamorphosed in the Early Cretaceous under intermediate high-pressure conditions.

Mesozoic sediments, metamorphosed slightly or not at all, are often ignored in regional descriptions but are very widely distributed in eastern Sulawesi (Fig. 7). The main periods represented are the Triassic–Lower Jurassic (terres-trial to marginal marine Meluhu Formation and deep water Tokala Formation) and Cretaceous (deep water carbonate-chert Matano Formation). Kundig (1956) estimated the total Mesozoic section as little more than one kilometre thick on the East Arm, but it may be thicker in the Southeast Arm, where the outcrops are more extensive. He also commented on similarities between the Triassic sedi-ments along the southeastern margin of the East Arm, where they form isolated klippen and comprise bitumi-nous limestones and shales, and those of Buru. Surono (1998), in describing the Meluhu Formation, noted that palaeomagnetic determinations placed the site of deposi-tion close to the then latitude of the North Australian Margin.

The Cretaceous deep water sediments are generally spatially associated with the ophiolite and might be supposed to constitute its uppermost part. However, Parkinson (1998) cited age relationships and the reported existence of depositional contacts between the Matano Formation and the schists as arguments against J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761–779 773

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J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761 – 779 774 MIO-CENE OLIGO-CENE PALEO-CENE EOCENE PA L E O Z O IC PLIOCENE EARL Y CRET A CEOUS JURASSIC TRIASSIC PERMIAN MESOZOIC CENOZOIC EARL Y EARL Y MIDDLE LA TE LA TE LA TE M L E

h i a t u s ? h i a t u s ? erosional hiatus

Claystone and shale with interbedded limestones, calcilutites and siltstones

Interbedded calcilutites and thin shales with radiolaria, foraminifera and chert nodules

Massive to thickly bedded calcarenites, calcirudites and recrystallised limestones

ATAH OC FOR MATION

OFU FOR MATION VIQU EQU E

GR OU P

BOBO-NAR O

TIMOR (PARA)- AUTOCHTHON

TIMOR ALLOCHTHON

MAU BISSE FOR MATION AITU TU LIMESTONE BABU LU FOR MATION NIOF FOR MATION WAI LU LI

FOR MATION

OE BAAT FOR MATION

NAKFU NU FOR MATION MENU FOR MATION

MIO-CENE OLIGO-CENE PALEO-CENE EOCENE PLIOCENE Limestone and radiolarian chert with volcanics and tuffaceous clastics Volcanics and tuffaceous clastics

Agglomerate and tuff

Basalts and tuffs Shallow marine bioclastic limestone EARL Y CRET A CEOUS ME SO ZOI C C EN OZO IC LA TE LA TE

METAN FOR MATION

OCU SSI VOLCANICS

CABLAC LIMESTONE

H AU LASI FOR MATION

NONI FOR MATION

MU TIS/LOLOTOI COMPLEX

Radiolaria-rich limestones and

shales

Silts, shales and sandstones

Red crinoidal limestones and basaltic pillow lavas

Shales with minor sandstones and tuffs. Weakly metamorphosed at base.

Slates, phyllites, meta-quartzites, schists, rare marble.

Shales with thin fine grained sandstones (turbidites) Varied deep and shallow

water deposits _Scaly

clay

Interbedded calcilutites and thin shales with radiolaria, foraminifera and chert nodules

Massive glauconitic sandstone

Medium-grade metamorphics and ophiolite

AILEU FOR MATION

K O L B ANO S E Q U E NC E KE KN E N O S E Q U E N C E 100 100 200 300 9 a 9 b

Fig. 9. (a) Stratigraphic column for the Timor parautochthon and autochthon, after Sawyer et al. (1993) and Reed et al. (1996). (b) Stratigraphic column for the Timor allochthon, after Sawyer et al. (1993) and Earle (1983). Numbers in triangles refer to locations shown in Fig. 1. Vertical scale in m.y.

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this assumption. The stratigraphic column for East Sula-wesi shown in Fig. 8a is compared with the column for Buru, where the proportions of outcrop of metamorphic rocks and sediments are similar, although ophiolites are absent.

4.6. Banda Association summary

The Banda Association is characterised by a variety of metamorphic rocks, some of which may represent continen-tal basement and others which may be the metamorphic soles to ophiolite sheets, and ophiolitic rocks which are only occasionally strongly metamorphosed. The sedimen-tary record begins in the Triassic with deposition under fluvial or marginal marine conditions. Water depths increased, and carbonate deposition became more wide-spread, in the Early Jurassic. Sediments of this generally conformable sequence are frequently bituminous. They are found in outcrop on Buton (where they source asphalt deposits; Davidson, 1991) and Buru, and source oil on Seram (Peters et al., 1999).

A characteristic feature of the Banda Association is the presence of a major unconformity encompassing at least a major part of the Late Jurassic and sometimes much of the Middle Jurassic and Early Cretaceous. The sediments immediately above this unconformity are generally shales but quickly give way to condensed sequences of carbonates with cherts, deposited in environments remote from sources of clastic sedimentation. This type of sedimentation contin-ued into the Paleogene, when a second major unconformity developed, interpreted here as a consequence of collision between a microcontinent and the margin of Sundaland. The subsequent history of the association can be interpreted in terms of post-orogenic collapse and dispersal, with early molasse deposition and, in some cases, later collision with the advancing Australian Margin around the Banda Arc.

5. Timor

Most discussions of the Outer Banda Arc begin with Timor, which is logical, since it is the largest and most intensively studied of the islands, but unfortunate since it is probably also the most geologically complex (Charlton et al., 1991a). Moreover, at no time has it been equally easy to visit both the eastern and the western parts of the island, which have been described rather differently even in the most recent publications (Sawyer et al., 1993; Reed et al., 1996). It is not clear whether the differences stem merely from different approaches to mapping and interpretation or reflect real variations in geology.

It is common ground amongst all recent authors that most of the Mesozoic sediments exposed on Timor are of Austra-lian origin (see discussion in Charlton et al., 1991a). Most authors also accept the presence on Timor of a forearc, formerly separated from Australia by an oceanic basin and referred to by Carter et al. (1976) and Barber (1981) as the

allochthon and by Harris et al. (1998) as the ‘Banda Terrane’. Seismic lines across the arc near Timor have provided striking images of underthrusting by the thickly sedimented Australian continental margin (Hughes et al., 1996; Schlu¨ter and Fritsch, 1985) and suggest that coherent slices of the sedimentary cover have in some places been stripped from the downgoing slab and incorporated in the overlying collision complexes. In the same terminology, these continental shelf thrust slices constitute the parautoch-thon. Both allochthon and parautochthon are now overlain by post-orogenic sediments of the autochthon, deposited after collision.

5.1. The allochthon

Charlton et al. (1991a) listed the allochthonous elements of Timor, of which the most important were the Mutis/Lolo-toi Complex, the Palelo Group (Noni, Haulasi and Metan Formations), the Cablac Limstone and the Ocussi Volca-nics. Many authors have drawn attention to parallels between some of these formations and rocks on Sumba and Sulawesi. Earle (1983) and Haile et al. (1979) drew attention to localities in Timor where Noni Formation radi-olarian cherts of Late Jurassic or Early Cretaceous age rest directly on the metamorphics, and to similar occurrences of this globally unusual pattern in SW Sulawesi. In both cases the environment of deposition was interpreted as a forearc basin. Elsewhere, the oldest unmetamorphosed sediments were considered by Earle (1983) to be the tuffs and agglom-erates of the Metan Formation, but these have now been assigned to the Upper Eocene and Oligocene (Sawyer et al., 1993.). This discrepancy is some indication of the uncer-tainty that still surrounds the Palelo Series, an unceruncer-tainty compounded by the strong similarities noted by Sawyer et al. (1993) between some outcrops normally mapped as Palelo Group and others mapped as part of the Kolbano Series. A distinction can, however, be made on the basis of the presence of volcanic elements throughout the Palelo, as in similar and coeval rocks on both Sumba and SW Sulawesi.

The Palelo Group is succeeded unconformably by the Cablac Limestone and the Ocussi Volcanics. Harris (1992) considered the latter to be an upthrust part of the Late Miocene or Pliocene oceanic floor of the Savu Basin.

5.2. The parautochthon

Australian shelf rocks exposed on Timor range in age from Permian to Paleogene and have been divided into two main groups, termed the Kekneno and Kolbano series (Fig. 9a), separated by a hiatus occupying most of the Late Jurassic (Sawyer et al., 1993). Three distinct Permian Formations have been recognised, these being the Lower to Upper Permian Maubisse basalts and limestones, the Atahoc shales, which interfinger with the Maubisse lime-stones, and the Upper Permian (or possibly partly Triassic, Reed et al., 1996) Cribas sands, silts, shales and bioclastic J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761–779 775

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J. Milsom / Journal of Asian Earth Sciences 18 (2000) 761 – 779 MIO-CENE OLIGO-CENE PALEO-CENE EOCENE PA L E O Z O IC PLIOCENE EARL Y CRET A CEOUS JURASSIC TRIASSIC PERMIAN MESOZOIC CENOZOIC EARL Y EARL Y MIDDLE LA TE LA TE LA TE M L E

h i a t u s ?

erosional hiatus

Claystone and shale with

interbedded limestones,

calcilutites and siltstones

Interbedded calcilutites and thin shales with radiolaria, foraminifera and chert nodules

Massive to thickly bedded calcarenites, calcirudites and recrystallised limestones

ATAH OC FOR MATION

OFU FOR MATION

VIQU EQU E

GR OU P

BOBO-NAR O

TIMOR (PARA)- AUTOCHTHON

TIMOR ALLOCHTHON

MAU BISSE

FOR MATION

AITU TU LIMESTONE BABU LU FOR MATION

NIOF

FOR MATION

WAI LU LI

FOR MATION

OE BAAT FOR MATION

NAKFU NU FOR MATION MENU FOR MATION

MIO-CENE OLIGO-CENE PALEO-CENE EOCENE PLIOCENE

Limestone and

radiolarian chert

with

volcanics and

tuffaceous clastics

Volcanics and

tuffaceous clastics

Agglomerate and tuff

Basalts and tuffs

Shallow marine

bioclastic limestone

EARL Y CRET A CEOUS ME SO ZOI C C EN OZO IC LA TE LA TE

METAN FOR MATION

OCU SSI VOLCANICS

CABLAC LIMESTONE

H AU LASI

FOR MATION

NONI FOR MATION

MU TIS/LOLOTOI

COMPLEX

Radiolaria-rich

limestones and

shales

Silts, shales and sandstones

Red crinoidal limestones and basaltic pillow lavas

Shales with minor

sandstones and tuffs. Weakly

metamorphosed at base.

Slates, phyllites, meta-quartzites, schists, rare marble.

Shales with thin fine grained sandstones (turbidites) Varied deep and shallow water deposits _Scaly

clay

Interbedded calcilutites and thin shales with radiolaria, foraminifera and chert nodules

Massive glauconitic sandstone

Medium-grade metamorphics

and ophiolite

AILEU FOR MATION

K

O

L

B

ANO

S

E

Q

U

E

NC

E

KE

KN

E

N

O

S

E

Q

U

E

N

C

E

100

200

300

9 a 9 b

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this assumption. The stratigraphic column for East

Sula-wesi shown in Fig. 8a is compared with the column for

Buru, where the proportions of outcrop of metamorphic

rocks and sediments are similar, although ophiolites are

absent.

4.6. Banda Association summary

The Banda Association is characterised by a variety of

metamorphic rocks, some of which may represent

continen-tal basement and others which may be the metamorphic

soles to ophiolite sheets, and ophiolitic rocks which are

only occasionally strongly metamorphosed. The

sedimen-tary record begins in the Triassic with deposition under

fluvial or marginal marine conditions. Water depths

increased, and carbonate deposition became more

wide-spread, in the Early Jurassic. Sediments of this generally

conformable sequence are frequently bituminous. They are

found in outcrop on Buton (where they source asphalt

deposits; Davidson, 1991) and Buru, and source oil on

Seram (Peters et al., 1999).

A characteristic feature of the Banda Association is the

presence of a major unconformity encompassing at least a

major part of the Late Jurassic and sometimes much of the

Middle Jurassic and Early Cretaceous. The sediments

immediately above this unconformity are generally shales

but quickly give way to condensed sequences of carbonates

with cherts, deposited in environments remote from sources

of clastic sedimentation. This type of sedimentation

contin-ued into the Paleogene, when a second major unconformity

developed, interpreted here as a consequence of collision

between a microcontinent and the margin of Sundaland.

The subsequent history of the association can be interpreted

in terms of post-orogenic collapse and dispersal, with early

molasse deposition and, in some cases, later collision with

the advancing Australian Margin around the Banda Arc.

5. Timor

Most discussions of the Outer Banda Arc begin with

Timor, which is logical, since it is the largest and most

intensively studied of the islands, but unfortunate since it

is probably also the most geologically complex (Charlton et

al., 1991a). Moreover, at no time has it been equally easy to

visit both the eastern and the western parts of the island,

which have been described rather differently even in the

most recent publications (Sawyer et al., 1993; Reed et al.,

1996). It is not clear whether the differences stem merely

from different approaches to mapping and interpretation or

reflect real variations in geology.

It is common ground amongst all recent authors that most

of the Mesozoic sediments exposed on Timor are of

Austra-lian origin (see discussion in Charlton et al., 1991a). Most

authors also accept the presence on Timor of a forearc,

formerly separated from Australia by an oceanic basin and

referred to by Carter et al. (1976) and Barber (1981) as the

allochthon and by Harris et al. (1998) as the ‘Banda

Terrane’. Seismic lines across the arc near Timor have

provided striking images of underthrusting by the thickly

sedimented Australian continental margin (Hughes et al.,

1996; Schlu¨ter and Fritsch, 1985) and suggest that coherent

slices of the sedimentary cover have in some places been

stripped from the downgoing slab and incorporated in the

overlying collision complexes. In the same terminology,

these continental shelf thrust slices constitute the

parautoch-thon. Both allochthon and parautochthon are now overlain

by post-orogenic sediments of the autochthon, deposited

after collision.

5.1. The allochthon

Charlton et al. (1991a) listed the allochthonous elements

of Timor, of which the most important were the

Mutis/Lolo-toi Complex, the Palelo Group (Noni, Haulasi and Metan

Formations), the Cablac Limstone and the Ocussi

Volca-nics. Many authors have drawn attention to parallels

between some of these formations and rocks on Sumba

and Sulawesi. Earle (1983) and Haile et al. (1979) drew

attention to localities in Timor where Noni Formation

radi-olarian cherts of Late Jurassic or Early Cretaceous age rest

directly on the metamorphics, and to similar occurrences of

this globally unusual pattern in SW Sulawesi. In both cases

the environment of deposition was interpreted as a forearc

basin. Elsewhere, the oldest unmetamorphosed sediments

were considered by Earle (1983) to be the tuffs and

agglom-erates of the Metan Formation, but these have now been

assigned to the Upper Eocene and Oligocene (Sawyer et

al., 1993.). This discrepancy is some indication of the

uncer-tainty that still surrounds the Palelo Series, an unceruncer-tainty

compounded by the strong similarities noted by Sawyer et

al. (1993) between some outcrops normally mapped as

Palelo Group and others mapped as part of the Kolbano

Series. A distinction can, however, be made on the basis

of the presence of volcanic elements throughout the Palelo,

as in similar and coeval rocks on both Sumba and SW

Sulawesi.

The Palelo Group is succeeded unconformably by the

Cablac Limestone and the Ocussi Volcanics. Harris (1992)

considered the latter to be an upthrust part of the Late

Miocene or Pliocene oceanic floor of the Savu Basin.

5.2. The parautochthon

Australian shelf rocks exposed on Timor range in age

from Permian to Paleogene and have been divided into

two main groups, termed the Kekneno and Kolbano series

(Fig. 9a), separated by a hiatus occupying most of the Late

Jurassic (Sawyer et al., 1993). Three distinct Permian

Formations have been recognised, these being the Lower

to Upper Permian Maubisse basalts and limestones, the

Atahoc shales, which interfinger with the Maubisse

lime-stones, and the Upper Permian (or possibly partly Triassic,

Reed et al., 1996) Cribas sands, silts, shales and bioclastic

(3)

limestones. The Aileu metamorphics of the north coast are

now recognised as metamorphosed equivalents of some or

all of these formations.

The Triassic of Timor is composed of the Niof Formation,

deposited as gravity flows in a range of water depths, and

later limestones and carbonate muds of the Aitutu

Forma-tion. In western but not in eastern Timor, a massive but

probably local sandstone wedge has been given formation

status (Babulu Formation). The top of the Kekneno Series is

represented by the Late Triassic–Jurassic Wai Luli

Forma-tion. In contrast to the other members of the Series, which

are generally confined to the northern mountains, the Wai

Luli is found only in the south.

The Kolbano Series is exposed principally in thrust sheets

in the Kolbano Mountains of southern West Timor. It is not

found in the north and exposures in the east are very limited.

The base of the series is represented by sandstones and

conglomerates of the Lower Cretaceous Oe Baat Formation,

but later sediments (the Nakfunu, Menu and Ofu

Forma-tions; Fig. 9a) were deposited in clastic-starved marine

settings which were initially deep but became gradually

shallower in the Tertiary. The younger rocks of Timor are

assigned to either the allochthon or the autochthon.

5.3. Timor controversies

Four major aspects of Timor geology continue to be

controversial. One of these, concerning the relationships

between the various Permian formations, interpreted by

Audley-Charles (1968) as having been deposited in widely

separated areas but by many more recent authors as

inter-fingering (cf. Barber, 1981), has only minor implications for

later orogenic development. More significant is the question

of when orogeny actually occurred. Reed et al. (1996)

iden-tified folds in East Timor which pre-dated deposition of the

Early–Middle Miocene Cablac Limestone, but Sawyer et al.

(1993) considered the earliest orogenic phase in West Timor

to be Late Miocene. The timing of orogeny in Timor would

have been dictated by the exact shape of the Australian

Margin and would therefore have been diachronous (Harris,

1991) but the time interval suggested above seems too long

to be explained by this factor alone.

Another problem concerns the Kolbano Series, which

Sawyer et al. (1993), following earlier authors, interpreted

as accreted to Timor only in the latest Miocene or Pliocene.

However, paleomagnetic studies by Wensink et al. (1987)

placed the site of deposition of the Lower Cretaceous

Nakfunu Formation at only 20

8 S, and thus more than

1000 km north of the Australian Margin (Smith et al.,

1994). If this paleomagnetic datum is correct, the Nakfunu

must have been deposited on a rifted fragment that drifted

north ahead of the main continent. The fossil evidence is

ambiguous and Clowes (1997) described a radiolarian fauna

of mixed Tethyan and higher latitude affinities. Given strong

1 3 0 Eo

1 3 4 Eo

2 So

6 So

1 0 So

Banda Sea

expands

eastward

Timor allochthon moves

south-east from western Sulawesi

Halmahera block moves west

to collide with Sangihe Arc

Buru and Seram move

east from the collision

orogen and rotate

Sumba moves south

from western Sulawesi

Sula Spur collides with the

East Arm of Sulawesi

W Sulawesi

(remnant

collision

orogen)

1 2 6 Eo

1 2 2 Eo

Au s t ra lia n S h e lf As s o c ia t io n

S u n d a la n d Ma rg in As s o c ia t io n Ba n d a As s o c ia t io n

4 0 0 km 2 0 0

Fig. 10. Dispersion and amalgamation in eastern Indonesia. The convergence of all three assemblages in Timor is unproven, and not essential to the basic post orogenic extension hypothesis, but a possibility meriting further investigation.

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geological resemblances and the evidence for a pre-Middle

Miocene orogenic event in Timor, it seems unwise to

completely discount the possibility that the Kolbano

belongs with the Banda Association as defined in this paper.

Uncertainty also surrounds the metamorphic rocks. On

the basis of metamorphic grades indicating burial to depths

of more than 20 km, the Lolotoi Complex of East Timor was

originally described as continental basement (e.g.

Audley-Charles, 1968), although the importance of basic igneous

rocks was recognised by all early workers. In West Timor

the supposedly equivalent Mutis Complex has been more

intensively studied and comprehensively described, and is

now interpreted as a metamorphic sole overlain by a thrust

mass of variably metamorphosed ophiolitic rocks similar to

the Lamasi Complex of west Sulawesi (Sopaheluwakan et

al., 1989). It is not proven that continental metamorphic

basement outcrops in either West or East Timor.

6. Discussion

Although most aspects of the geological history of eastern

Indonesia are still controversial, there is consensus on a few

important points. It is generally agreed that eastern Sulawesi

and Buton were sutured to western Sulawesi in the Late

Oligocene or Early Miocene and that their later history

has been dominated by extensional and transcurrent

fault-ing. This suturing was regarded as two separate events by

Smith and Silver (1991) but Milsom et al. (1999) have

argued in favour of a single collision. Neogene compression

in Sulawesi has been confined to the north, where the

Celebes Sea is now being subducted beneath the North

Arm and where the Sula Spur collided with the East Arm

in the Pliocene. Further south, Bergman et al. (1996) and

Polve´ et al. (1997) have independently concluded, on the

basis of detailed geochemical studies, that Neogene

volcan-ism was a consequence of orogenic collapse and extension

rather than of subduction.

It is also generally agreed that Sumba and the related

Timor allochthon are of SE Asian origin and were closely

linked to western Sulawesi throughout the Mesozoic and

Paleogene (Soeria-Atmadja et al., 1998; Wensink, 1997).

It follows that the as yet undated oceanic crust of the Flores

Sea must be Neogene, which provides circumstantal support

for the still controversial Late Neogene dating of the North

and South Banda Basins (Rehault et al., 1994). An almost

inescapable corollary is that the Outer Banda Arc islands of

Buru and Seram, as well as the continental fragments in the

Banda Ridges, were closer to Sulawesi prior to the Late

Miocene than they are today and not further away, as in

many reconstructions (e.g. Silver et al., 1985). The virtually

identical Mesozoic stratigraphies of Buton, Buru and Seram

strongly suggest that they formed part of a single block and,

somewhat more controversially, recent work (e.g. Surono,

1998) suggests that this block included most of East

Sulawesi.

In contrast, detailed stratigraphic comparisons argue

against any correlation between these now dispersed

frag-ments and the Sula Spur. Although they share an

Australa-sian origin, there are significant differences in their times,

places and modes of separation. The Sula Spur is closely

related to New Guinea, from which it has been detached

principally by transcurrent faulting. It remained part of

that margin and a site of shallow water sedimentation

until the Cretaceous, and collided with Sulawesi only in

the Pliocene. The rocks of the Banda Association, on the

other hand, were rifted from Gondwanaland in the Jurassic,

drifted north ahead of Australia during the Cretaceous and

collided with the SE Asian Margin at the end of the

Oligo-cene. Their Mesozoic sediments resemble those of the most

distal elements of the present Australian Margin, such as the

Wombat Plateau, but were deposited on metamorphic

base-ment, instead of on older sediments. Direct correlation of

the metamorphics with any known Australian province may

prove impossible, both because of distances involved (the

nearest exposures of metamorphic rocks on the Australian

continent are several hundred kilometres from any plausible

sites of rifting) and because of the later metamorphism

asso-ciated with Tertiary collision, rifting and uplift.

The stratigraphic relationships outlined in this paper

suggest a Late Neogene history for eastern Indonesia

which is summarised in Fig. 10 and which is directly

comparable to hypotheses now being advanced to explain

the development of the western Tethys extensional basins

shown in Fig. 2. A number of propositions concerning the

process of suturing are suggested by these relationships,

which should be testable by detailed and focussed

programmes of investigation. These are that:

1. The Cretaceous sediments of East Sulawesi are not parts

of the ophiolite sequence but are direct equivalents of the

Cretaceous sediments of Buru, Buton and Seram;

2. The metamorphic rocks on which the Triassic sediments

of Buru, Buton and Seram were deposited are equivalents

of the Pompangeo Schist of eastern Sulawesi;

3. The Wai Luli and Kolbano Series rocks of Timor,

together with similar sediments on Buton, Buru, Seram

and East Sulawesi, were deposited on a microcontinent

which rifted away from the Australian continent in the

Late Jurassic and collided with West Sulawesi in the

mid-Tertiary, and that similar and distinctive faunal

assemblages will be found in all cases;

4. The main collision between the Timor ‘allochthon’

(Sunda forearc) and the ‘parautochthon’ took place

before, and not after, the Middle Miocene;

5. The palaeomagnetic inclinations in the Lower

Cretac-eous rocks of Buton, Buru, Seram and East Sulawesi

will be similar to those already obtained for the Nakfunu

Formation of Timor; and

6. Triassic palaeomagnetic inclinations of Buton, Buru and

Seram will be similar to those already obtained for the

Meluhu Formation of East Sulawesi.

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In the light of the results of these tests it should be

possi-ble to formulate a more detailed, more reliapossi-ble (and perhaps

completely different) model.

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