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IPA99-G-114

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATIONTwenty Seventh Annual Convention & Exhibition, October 1999

TECTONIC AND STRATIGRAPHIC EVOLUTION OF THE BONE BASIN, INDONESIA:

INSIGHTS TO THE SULAWESI COLLISION COMPLEX

Sudarmono*

ABSTRACT

The Bone Basin, located between the south andsoutheast arms of Sulawesi, is interpreted to be acomposite basin, with its origin as a subductioncomplex and suture between Sundaland andGondwana-derived micro-continents. It subsequentlyevolved as a submerged intra-montane basin.

Tectonic and stratigraphic evolution of the BoneBasin is still poorly understood due to limited data. Anew model based on integration of speculativeseismic data, published surface geology maps, andwell data is presented for the tectonic and

stratigraphic evolution of the Bone Basin. The basinhad two major periods of development: (a) Paleogeneto Early Miocene, and (b) Early Miocene to Recent.

Originally, the basin probably occupied a forearcsetting as part of a westward subduction complex

during the Paleogene to Early Miocene.Subsequently, westerly plate convergence of Australian-derived micro-continents toward thesubduction complex during the Middle to LateMiocene, dramatically changed the style of deposition, structural framework, and configuration of the basin. A Middle Miocene collision of the micro-

continents with the subduction-related accretionarycomplex, followed by collision of the micro-continents with West Sulawesi, built orogenssurrounding the Bone Basin which shed largevolumes of sediment into the northern depocenter of the Bone Basin. The collision led to an eastward

rotation of Southeast Sulawesi, which resulted inrifting and submergence of the southern part of the

basin. Compression from the collision propagated amajor back-thrust system westward to the subduction

___________________________________________ * Atlantic Richfield Indonesia Inc.

zone, generating fold belts, as indicated by the west-verging Kalosi and Majene fold belts in West

Sulawesi.

The two colliding plates began to lock during thePliocene, and the continued plate convergence wasaccommodated by strike-slip movements along theWalanae, Palukoro and many other faults.Subsequently the Bone Basin submerged into anintra-montane basin setting. Clastic sediments derivedfrom surrounding mountains to the east, north andwest were deposited progradationally southwardtoward the depocenter of the basin. Strike-slipmovements are still active, and the bathymetry of the

Gulf of Bone reflects the present day tectonic activity.

INTRODUCTION

Sulawesi is one of the five largest islands of the

Indonesian archipelago. Affected by the interplay of three major tectonic plates, Sulawesi has a verycomplex geology, and available geologic data islimited. No specific work on the Bone Basin has previously been presented.

The Bone Basin is located in the Gulf of Bone, which

is bounded to the east and west by southern arms of East and West Sulawesi, to the north by CentralSulawesi, and to the south by the Java Sea (Figure 1).The Gulf of Bone covers an area of approximately30,000 square kilometers. Water depths in the Gulf of Bone range from 200 to 2,000 meters.

This study, which presents a new model for the

tectonic and stratigraphic evolution of the Bone Basin,is primarily based on seismic interpretation of speculative seismic data, integrated with publishedsurface geology maps. A synthesis of tectonic eventswhich took place in Sulawesi will be illustrated to

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support this new model. The seismic interpretation is

put in a regional context to understand the complexgeology of Sulawesi.

DATABASE

Digicon Megapratama has shot 5,615 kilometers of

speculative seismic data in the Gulf of Bone with linespacing at about 10 km in the northern part, and 40km in the southern part. Arco purchased 75% of thisspeculative seismic data from Digicon in 1992.

The BBA-IX well, located in the northern part of theGulf of Bone, is the only direct stratigraphic control

to tie the Digicon seismic data. Unfortunately, no well

velocity data is available from the BBA-1X well, soseismic stacking velocity data were utilized to tie theseismic data to the well. The other seismic tie is theODB-1X well, which is located approximately 340km southwest of the BBA-1X well. Other usefulstratigraphic information is from published surfacegeology maps, which can be used to tie seismic

interpretation indirectly, although the maps still needsome revisions in places based on the latest

geological surveys conducted in Sulawesi. A surfacegeology map of Sulawesi published by GRDCBandung in 1975 (Sukamto, 1975) is utilized in this

study. A number of exploratory wells locatedonshore West Sulawesi also provide important

information to support the seismic interpretation.

STRATIGRAPHY

The BBA-1X well encountered marine sediments of Early Miocene to Recent age, and represents the Neogene and younger stratigraphy of the northern partof the Bone Basin. Numerous shallow wells locatedonshore West Sulawesi, which penetrated LateMiocene-Pliocene reefal carbonates and younger

sediments, also provide important information.

Paleogene stratigraphy in the northern part of theBone Basin is unknown, as the deepest penetration of the BBA-1X well is Early Miocene. Onshore WestSulawesi, the oldest Tertiary sediments are of Eoceneage, and include volcanics, limestones and fluvio-

deltaic to marginal marine clastics. Extensivecarbonate development occurred during the Oligoceneto Early Miocene, followed by deposition of volcaniclastics in the Middle to Late Miocene. To thesouthwest, the ODB-IX well provides an example of acomplete Tertiary section of Eocene to Recent

sediments, and may represent typical stratigraphy in

West Sulawesi. The well reached total depth at10,433’ within Middle-Lower Eocene marine

sediments. An important marker in the well is 190 feetof fine-grained volcanic rocks of Middle Eocene agewhich are probably extrusive porphyritic basalts. Numerous wells to the northwest of the ODB-1X well penetrated similar volcanic rocks which probablyreflect extensive eruptive volcanic activities duringthe Paleogene in West Sulawesi.

No well has been drilled in the southern part of theBone Basin. Stratigraphy in the southern part of the

basin is interpreted based on analogy and weak seismic correlation. There is no direct seismic line to

tie the southern and northern parts of the basin.Stratigraphic correlation to the ODB-1X well to thesouthwest is difficult, and has to jump across two

major faults.

REGIONAL TECTONIC SETTING

The island of Sulawesi has an unusual outline, withfour arms shaped like a letter ‘K’. This unusual shapeis closely related to the tectonic history of the island.Tectonically Sulawesi is located in a critical position

of the Indonesian archipelago, and has been subjectedto a complex interplay between the northward movingIndo-Australian plate, the westward moving Pacific plate, and the south-southeastward moving Eurasian plate.

The geology of Sulawesi is complex; it can basically be divided into three major parts: West, Central and

East Sulawesi (Figure 1). West Sulawesi isinterpreted to be part of a Cretaceous subductioncomplex along the eastern margin of Sundaland,which subsequently was rifted and drifted eastward

during Early Tertiary (Kustomo, 1991). WestSulawesi is characterized by a Tertiary volcano- plutonic complex, and shows geologicalcharacteristics similar to Western Indonesia. EastSulawesi, by comparison, is geologically similar toPapua-New Guinea, and is probably composed of

micro-continents, derived from Australia, that driftednorthwestward towards West Sulawesi following the break-up events in the Northwest Shelf of Australiaduring the Mesozoic. East Sulawesi is considered toinclude several islands to the east including Banggai

and Sula. Central Sulawesi on the other hand,comprises a metamorphic complex.



Development of the Bone Basin was influenced by

two factors: the location of the Early Tertiarysubduction complex, and Neogene plate convergence

on the eastern margin of Sundaland. The evolution of the Bone Basin during the Paleogene is indirectlyinterpreted from stratigraphic records and geologicevents in surrounding areas. The Neogene was subjectto two major stress regimes, which played animportant role in the tectonic history of the BoneBasin and Sulawesi. The first stress regime wasrelated to collision of westward moving Australian-

derived micro-continents with an accretionarycomplex located east of the Bone Basin. The collision

resulted in an obduction eastward of the accretionarycomplex onto the micro-continents. The second stress

regime was in the southeast, where Buton/TukangBesi micro-continents collided against southeastSulawesi (Davidson, 1991). These two stress regimes

were probably responsible for basin inversion on theeastern margin of Sundaland including that identified

in southwestern Sulawesi.

SEISMIC INTERPRETATION

Seismic data indicates that the Bone Basin can bestructurally divided into northern and southern areas.Locations of seismic lines for a reference in this

discussion are given in Figure 2.

Northern Bone Basin

Seismic data in the northern part of the Bone Basinshows a large compressional fold in the middle of the basin and tilting on both eastern and western flanks of the basin towards the basin center. Seismic data

indicates that the northern depocenter of the BoneBasin may contain up to 20,000 ft. of sediment.Based on seismic interpretation, five sediment packages are identified in the northern part of the

basin: A, B, C, D and E (Figure 3 and 4). Line IND-09 (Figure 3) shows folding of parallel-beddedsediment packages E and D. The originalconfiguration of the Bone Basin was synclinal with anaxis oriented N-S.

Basement and Sediment Packages E, D, & C

Sediment package E appears to onlap to a surfaceinterpreted as Basement on the eastern and westernflanks of the basin, and is probably an early basin fill

sediment which was confined to the basin center.Sediment package D is basically a succession of the E

package, but does not show onlap to Basement. It was

probably was deposited more regionally westwardthan Sediment package E. Sediment package C is

conformable to the D package, and appears to onlap toa large fold in the middle of the Bone Basin. In mapview, the fold can be seen as a southward-plunginganticline, truncated to the north (Figure 4).

Onshore Central Sulawesi (Figure 2), the continuationof the offshore anticline is eroded down to Basement,which consists in that location of a metamorphic

complex. To the east, Basement is represented as anophiolite outcrop which is associated with an

accretionary subduction complex. By extrapolation of onshore geology, it is interpreted that basement in the

northern part of the Bone Basin consists of ametamorphic complex in the middle of the basin, andan accretionary complex in the eastern part of the

basin. The E, D and C packages are uplifted andtruncated to the west.

Sediment Packages B & A

Sediment package B indicates southward progradation, and is interpreted to be an erosional product of the upfilted anticline and other upliftedstructures to the west during a low stand of sea level.

Sediment package F (at the base of sediment packageB) shows a mounding feature and is interpreted to be alow stand submarine fan. Sediment package A is asouthward prograding sequence which kept pace withsea level rise up to the present day, and is identifiedover the entire Bone Basin. It unconformably overliesthe truncated D and E packages and the metamorphiccomplex, and onlaps to the accretionary complex

basement on the eastern flank of the basin.

Stratigraphic Tie to BBA-1X

Line IND-06 (Figure 4) is used to tie seismic data tothe BBA-1X well and shows a prominent angular unconformity which was interpreted originally asBasement. In fact, no basement was penetrated by thewell. Instead, thick marine sediments of Early to LateMiocene age were encountered below the

unconformity, which are interpreted to be equivalentto sediment package E. The BBA-1X well reachedtotal depth of 10,251’ in likely Early Miocenesediments, still within sediment package E. Thelowermost part of the well, approximately 1000’ thick,

consists of conglomerate, grading upwards into fine-coarse grained sandstones, which are interbedded with



claystones and thin limestones. Conformably

overlying this interval are approximately 4,200’ of Middle to Late Miocene age sediments, which consist

of calcareous claystones and occasional thinlimestones. This entire section is assigned to sediment package E. Unconformably overlying the calcareousclaystones is a very sandy interval of Pliocene toRecent sediments, which is interpreted to be sediment package A. A lower 500' conglomeratic section of Pliocene age grades upwards into 4,400’ of sandstones, intercalated with claystones. Sediment

packages D, C, and B are absent in the BBA-1X welldue to truncation (Figure 4).

Southern Bone Basin

In contrast with the northern part of the Bone Basin,the southern part of the basin shows extension from

the beginning of basin development until the presenttime. There are two major N-S east-verging

extensional faults: the Bone Bay Fault, which cutsalong the middle of the Bone Basin, and the WalanaeFault, which is located to the east of the Bone BayFault, and extends to onshore West Sulawesi (Figure5). The southern depocenter of the Bone Basin maycontain up to 10,000’ of sediment.

Sediment Packages in Southern Area

Seismic line IND-97 (Figure 5) shows that the original basin configuration and the early basin fill of thesouthern and northern parts of the Bone Basin aresimilar. Early extension on the Bone Bay Fault took place on the west flank of the basin, which resulted inwestward tilting of sediment packages D and E.

Concurrently with the early extension, sediment package C was deposited, which onlaps to the top of sediment packages E and D. It appears that whilecompression occurred in the northern part of the Bone

Basin, extension was taking place in the southern partof the basin. When early extension ceased, it wasfollowed by deposition of sediment packages B and Aconformably over the top of sediment package C.Sediment packages B and A in the southern part of the basin are probably distal facies of sediment package B

and A identified in the northern part of the basin.Finally, late extension occurred, reactivating existingfault planes, and leading to generation of antitheticfaults on the east flank of the basin. This later movement results in a blind fault, which was probably

associated with deeper crustal lateral movement. Inthe offshore area, the Walanae Fault, located to the

east of the Bone Bay Fault, was formed by a similar

process, and clearly indicates strike-slip movementduring the later extensional phase. The Walanae Fault

was originally formed by a sag-like depression alongthe Walanae Fault location. The later strike-slipmovement cut the sag-like depression to form agraben.

INTEGRATED TECTONIC AND

STRATIGRAPHIC EVOLUTION

The following discussion is an effort to integrateobservations in the Bone Basin with tectonic and

stratigraphic data from other areas of Sulawesi, notonly to understand the history of Bone Basin

development itself, but also to understand theevolution of Sulawesi as a whole.

Based on the drilling results of the BBA-1X well,many authors interpret the Bone Basin as a Neogene

basin, since the BBA-1X well encountered sedimentsno older than Early Miocene. For example, Bergmanet al. (1994) suggest that Bone Bay resulted from thecollapse of an over-thickened Miocene orogen.

Paleogene to Early Miocene

Showing similar geology to the margin of Sundaland,it is interpreted that West Sulawesi was probably partof the Cretaceous subduction complex along themargin of Sundaland. Following the rifting andeasterly drifting of the eastern margin of Sundalandduring the Early Tertiary, the Cretaceous subductioncomplex, which originally was located nearby EastKalimantan, was shifted eastward (Kustomo, 1991).

The Bone Basin probably occupied a forearc setting as part of a westward subduction complex to the east of West Sulawesi during the Paleogene, while West

Sulawesi was probably a volcano-plutonic complex.Deposition in Bone Basin during Paleogene time was probably dominated by volcanigenic flysch-typesediments deposited on top of an accretionarycomplex basement. This model is primarily based onwidespread Paleogene volcanic outcrops in Western

Sulawesi, and information from widespreadstratigraphic records of numerous wells offshoreSouth Sulawesi. This model refers to the assumptionthat the East Sulawesi ophiolite belt probablyoriginated as an accretionary complex related to

westward Early Tertiary subduction. It is possible thatsediment package E contains Paleogene sediments.



However, no wells have yet penetrated Paleogene

sediments in the Bone Basin to support this model, nor have Paleogene sediment outcrops in Central

Sulawesi, to the north of Bone Basin, been identified.

Early to Late Miocene

A cartoon illustrating the tectonic and stratigraphicevolution of the Bone Basin from Early Miocene toPresent Day is depicted in Figures 6a, 6b and 6c.

There were two major compressional regimes duringthe Neogene in Sulawesi: one from west involving theEast Sulawesi/Banggai/Sula micro-continents, whichcollided against the Early Tertiary accretionarycomplex, and a second from the southeast involving

the Buton/Tukang Besi micro-continents, whichcollided against southeast Sulawesi. The collision of the East Sulawesi/Banggai/Sula Early Tertiary

accretionary complex in Eastern Sulawesi probablytook place during Middle Miocene, and resulted in theeastward obduction of the accretionary complex ontothe micro-continents (Simanjuntak, 1986). However,the first contact of the micro-continents and the

accretionary complex probably occurred much earlier than Middle Miocene. The accretion of the complex

probably did not take place at one time, but appears tohave occurred in several stages, within Tertiary times(Davies, 1991). The Mesozoic-Paleogene sequencesof the micro-continents were imbricated beneath theophiolite and thrust eastward.

The Buton and Muna/southeast Sulawesi collisionoccurred during Early to Middle Miocene, withmaximum regional compression, thrusting, and uplifton Buton occurring in the Middle Miocene (Davidson,1991). The Buton-Muna/southeast Sulawesi collision

might have caused uplifting and southward tilting of southeast Sulawesi, since no Tertiary sedimentary

records have been identified in most of the northern part of the area. This is in contrast with the northern part of the Bone Basin, where a very thick Tertiarysedimentary sequence occurs. Instead, the surfacegeology map (Figure 2) indicates that an extensivemetamorphic complex outcrops in most of southeastSulawesi, which probably indicates extensive erosion

and denudation has taken place. However, there is stilldebate as to whether southeast Sulawesi has been anon-depositional area since Middle Miocene due tothe uplifting, or alternatively, the uplifting and erosionoccurred in northern part of southeast Sulawesi

concurrently with the deposition of the syn-orogenicPlio-Pleistocene sediments in the southern part.

The original effect of the obduction of the

accretionary complex onto the micro-continents was acollapse of the accretionary complex basement in the

center of the Bone Basin as a consequence of thrustingand over-thickening of the accretionary complexduring Early to Middle Miocene. Deposition in theBone Basin was probably in a deep marine settingduring Early to Late Miocene. The coarse clasticinterval identified at the base of the BBA-1X well is probably a flysch-type sediment. West Sulawesi wasdominated by extensive volcanic activities during

Middle to Late Miocene. The center of the BoneBasin was probably distal from volcanic eruptions and

other sediment sources during that time, so that onlyvery fine grained materials were deposited.

Deposition on the east flank of the northern BoneBasin during Early to Middle Miocene was dominated by shelf margin carbonate development, resting on top

of the uplifted accretionary complex, while the eastflank of southern Bone Basin probably saw

hemipelagic sediment or non-deposition.

The continued westward moving micro-continentsthen collided against West Sulawesi during the LateMiocene in Central Sulawesi (Figure 6b). Theanticline in the northern part of the Bone Basin isinterpreted to be a collision effect of the micro-

continents against West Sulawesi. Material in thecollision zone was squeezed upward, pushing thesedimentary package above it to form the structure.Sediment package C, which onlaps to the structure, isdefined as syn-collision sediment. Based on the BBA-1X well, the top of the anticline is probably no older than Late Miocene. Therefore, it is interpreted thatthe structure and the collision was probably initiated

during the Late Miocene. Sediment packages B and Aare basically syn-orogenic or post-collision sediments.Sediment package B was probably deposited in a highenergy and low stand stage of sea level, while

sediment package A was deposited during sea levelrise.

The collision was confined only to the northern part of the Bone Basin (Central Sulawesi). The accretionarycomplex basement in the collision zone and overlying

sediments were compressed, squeezed upward anduplifted above sea level. During this process, theaccretionary complex was probably heated, meltedand metamorphosed. The uplifted areas weresubsequently truncated by erosion to basement level

during the Early Pliocene. The Central Sulawesimetamorphic complex probably represents the



exposed collision materials. Bergman et al. (1994)

indicated that the micro-continents were partlysubducted beneath West Sulawesi causing Late

Miocene to Pliocene extrusive and intrusivemagmatism in West Sulawesi.

In the southern part of the Bone Basin, westerlymovement of the micro-continents did not reach thecollision stage. The collision in Central Sulawesi, andmore or less the collision of Buton-Muna/southeastSulawesi, led to an anticlockwise eastward rotation of

the southeast Sulawesi with a rotation center at thecollision zone in Central Sulawesi. The rotation

resulted in two major N-S, facing to the east,extensional faults: the Bone Bay Fault and the

Walanae Fault (Figure 2). Deposition in the southern part of the Bone Basin was probably in a marinesetting with a reefal carbonate development of Late

Miocene to Pliocene age on the east and west flanks of the basin.

Compression from the collision in Central Sulawesi propagated westward into a major back-thrust systemto the subduction zone, generating foldbelts now present in the west-verging Kalosi and Majenefoldbelts in West Sulawesi (Figure 2).

Pliocene to Recent

The two colliding plates, represented by WestSulawesi and the micro-continents, began to lock during the Pliocene, and continued plate convergencewas accommodated by strike-slip movements alongthe existing Walanae, Bone Bay, Palukoro and manyother faults. The last period of movement along the

Bone Bay and Walanae Faults was associated withthis accommodation of crustal movements. Thenortheastern part of the Bone Basin underwent a stressrelease and then collapsed, which led to generation of

the west facing extensional Fault C, located along thenorthwest part of southeast Sulawesi (Figure 2), andthe propogation of other extensional listric faultsalong the eastern margin of the southern part of the basin (Figure 5). Subsequently, the Bone Basinsubmerged in an intra-montane setting with mountains

to the east, west, and north shedding clastic materialsto be deposited progradationally southward toward thedepocenter of the basin. When the two colliding plateswere locked up during the Pliocene in the northern part of Bone Basin, erosion followed, while in the

southern part of the basin extension ceased and deepmarine deposition continued. This is why no angular

unconformity is identified on seismic in the southern

part of the basin. When the syn-orogenic sedimentwas deposited in the northern part of the basin, the

southern part of the basin occupied a deeper marinesetting with deposition of hemipelagic and turbiditicsedimentary facies. The movement of the over-thickened accretionary complex was still active in thenorthern part of Bone Basin during Pleistocene asindicated on seismic data by folding and uplifting of the post-collision sediments.

The Bone Bay, Walanae, Palukoro and many other faults are still active today with strike–slip motion.

This strike-slip setting has reactivated and generatednormal faults to the south resulting in continued

subsidence of the Bone Basin. The bathymetry of theGulf of Bone reflects the present day tectonic activityin the area.

CONCLUSIONS

The Bone Basin probably occupied a forearc setting asa part of an Early Tertiary subduction complex duringthe Paleogene. After colliding against the EarlyTertiary accretionary complex during the MiddleMiocene, the west-vergent Australian-derived micro-continents continued to collide against West Sulawesi

during the Late Miocene.

The Central Sulawesi metamorphic complex probablyrepresents the exposed collision zone. Westerlymovement of the micro-continents did not reach thecollision stage with West Sulawesi in the southern part of Bone Basin. Instead, the collision in CentralSulawesi together with collision of Buton –

Muna/southeast Sulawesi led to an eastward anti-clockwise rotation of southeast Sulawesi, whichgenerated NW-SE major extensional faults in theBone Basin. The two colliding plates, represented by

West Sulawesi and the micro-continents, were lockedup during Pliocene, resulting in uplift and erosion.Plate movement was then accommodated by strike-slip movements along Walanae, Bone Bay, Palukoroand many other faults during Plio-Pleistocene.

The strike-slip movements are still active at presenttime and the bathymetry of the Gulf of Bone reflectsthis neotectonic activity.

ACKNOWLEDGEMENTS

The author wishes to thank the management of Arco



and Pertamina/BPPKA for their permission to publish

this paper. Special thanks are for drafting colleaguesfor their efforts to complete the figures of this paper.

REFERENCES

Bergman, Steven C., Coffield, Dana Q., Talbot, JamesP., Garrard, Richard J., 1994. Tertiary Tectonic andMagmatic Evolution of Western Sulawesi and theMakassar Strait, Indonesia: Evidence for a Miocene

Continent-Continent Collision. University of London,Geological Research in South East Asia, 34-42.

Davidson, J.W., 1991, The Geology and prospectivity

of Buton Island, S.E. Sulawesi, Indonesia. Proceedingof the Indonesia Petroleum Association, 20th Annual

Convention, 209-224.

Davies, I.C., 1990, Geological and Exploration review

of the Tomori PSC, Eastern Indonesia. Proceeding of the Indonesian Petroleum Association, 19th Annual

Convention 1990, 41-67.

Hasan, Kustomo, 1991, The Upper Cretaceous Flyschsuccession of the Balangbaru Formation, Southewest-Sulawesi. Proceeding of the Indonesian PetroleumAssociation, 20

th Annual Convention, 183-208.

Simanjuntak, T.O., 1986, Sedimentology and

Tectonics of the Collision Complex in the East Arm of Sulawesi, Indonesia. Ph.D. thesis University of

London. 374 p.

Sukamto, Rab., 1975, Geologic Map of Indonesia,Ujung Pandang Sheet. Geol. Res. Dev. Centre,Bandung.

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