Geology of a Miocene collision complex, Buton, eastern

RANDALL B. SMITH Department of Geology, Sonoma State University, Rohnert Park, California 94928 ELI A. SILVER Earth Sciences Board, University of California, Santa Cruz, California 95064

ABSTRACT rived from uplift and erosion of the Wolio the Miocene collision resulted from oblique, Complex, placing an upper limit on the age of southwest-directed convergence between the The island of Buton in eastern Indonesia is TBP collision. We infer a middle Miocene Pacific and Australian plates. The oblique colli- part of a Neogene collision zone that encom- time of collision, with oblique convergence sion led to development of major, left-lateral passes much of the eastern margin of Su- continuing into late Miocene time. In con- strike-slip faults within the collision zone, such lawesi. Miocene collision of microcontinents trast, sparse biostratigraphic data place the as the Sorong fault system of northwestern Irian with a west-dipping zone em- age of the SP-East Arm collision in the late Jaya. Along these faults, large continental frag- placed the Tukang Besi platform (TBP) Miocene. Separate microcontinents may have ments were removed from the northern margin against Buton, and the Sula platform (SP) collided successively with different parts of of and transported westward into against the East Arm of , and thrust the Buton-Sulawesi convergent margin, or a the Banda Sea region. The largest such fragment large ophiolite sheets eastward over the im- single large microcontinent may have been is the Sula platform (Pigram and others, 1985; bricated margins of the platforms. Reconnais- fragmented during oblique collision. Hamilton, 1979), which collided with the Su- sance observations of the resulting collision lawesi arc during Miocene time. Hamilton complex in Buton (Wolio Complex) indicate INTRODUCTION (1979) and Bowin and others (1980) identified that imbricate west-dipping thrust sheets and additional continental fragments, including the overturned folds are the predominant struc- Geologists have long recognized the impor- largely submerged Tukang Besi platform adja- tural element, although later steep faults tance of collision events in the formation of cent to , and a fragment en- offset the imbricate stack and largely control mountain belts. Early plate-tectonic models em- compassing western Seram and Buru in the present map patterns. phasized the role of collisions in which the col- northern Banda arc. On the basis of dredge haul and geophysical data, Silver and others (1985) The Wolio Complex consists of the sedi- liding elements (continental margins, island interpreted a series of northeast-trending subma- mentary Turumbia sequence, peridotite, and arcs) are structurally continuous for great dis- rine ridges in the central Banda Sea as a conti- two distinct groups of metamorphic rocks. tances along the length of the orogenic belt. The nental borderland terrane similarly displaced The Turumbia sequence makes up most of recent development of the terrane concept em- from western Irian Jaya. the eastern part of the Wolio Complex and phasizes that many collision zones have a more consists mostly of deep- water limestone rang- modest extent and commonly involve smaller, These microcontinents have clearly played a ing in age from Late Triassic through late less elongate crustal elements such as oceanic major role in the Neogene development of the Eocene or Oligocene. It is interpreted as a plateaus, microcontinents, and amalgamated arc Banda Sea region. Silver and others (1983b) deep-water facies of the western TBP margin. terranes. Eastern Indonesia (Fig. 1) provides a discussed the geologic effects on the island of Ophiolitic rocks are represented in present geologically recent example of the processes in- Sulawesi resulting from the Sula platform colli- outcrop only by massive peridotite in the volved in terrane displacement and accretion sion. Many aspects of the geology of these mi- western part of the Wolio Complex, but the (Silver and Smith, 1983). The Neogene tectonic crocontinents remain poorly understood, how- compositions of clasts in conglomerates over- evolution of this region has been dominated by a ever, including their original positions along the lying the Wolio Complex indicate that a full series of collision events of various scales. A col- New Guinea margin, the times of their detach- ophiolite succession was exposed to erosion lision is now in progress between the Banda arc ment, their subsequent movement histories, and during the collision. Metabasite and meta- and the northwest Australian margin (Bowin the nature and ages of the structures bounding chert ranging in grade from greenschist to and others, 1980; Karig and others, 1987), and them. amphibolite facies are found locally in fault the Neogene orogenic belt of Papua New Guin- The island of Buton lies between the Tukang contact with peridotite and are interpreted as ea and Irian Jaya has been interpreted as the Besi platform and southeast Sulawesi. Although the remnants of a metamorphic sole devel- result of a similar island-arc/continental-margin much of Buton is blanketed by little-deformed oped at the base of the ophiolite. Pelitic collision during Miocene time (Jacques and Pliocene and Quaternary sedimentary strata, phyllite and quartzite in northeastern Buton Robinson, 1977; Hamilton, 1979). These colli- older deformed rocks that crop out in many probably represent a slice of the continental sions have occurred on the leading edge of the areas provide evidence for a Miocene collision basement of the TBP, for similar rocks have Australian continent during its rapid northward between the Tukang Besi platform and the pre- been dredged from the northeast margin of movement away from Antarctica, which began Miocene accretionary terrane of southeast Su- the TBP and also form the pre-Mesozoic in early Eocene time. lawesi. In this paper, we describe the lithologic basement of the SP. On the basis of regional structural patterns assemblages and structure of the Miocene colli- The middle to upper Miocene Tondo For- within the New Guinea mobile belt in Irian sion complex of Buton. Our studies support mation in Buton consists of clastic strata de- Jaya, Dow and Sukamto (1984) postulated that Hamilton's (1979) inference that the Tukang

Geological Society of America Bulletin, v. 103, p. 660-678, 15 figs., 1 table, May 1991.

660

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Figure 1. Generalized tectonic map of the Banda Sea region, showing the location of Buton and the inferred continental fragments derived from the northern margin of New Guinea. Modified from Hamilton (1979), incorporating data from Silver and others (1983b, 1985).

Besi platform is a continental fragment and indi- Katili (1978), and Sukamto (1978). The central plete, north-topping ophiolite is present (Silver cate that its collision with southeast Sulawesi belt appears to be an accretionary complex and others, 1983a). No radiometric ages are probably began during middle Miocene time. formed during Cretaceous and Paleogene time available for the ophiolite, and no oceanic sedi- This is substantially earlier than the late Mio- (Hamilton, 1979). In contrast, the eastern and ments have been found in depositional contact cene emplacement of the Sula platform against western belts record a younger, Neogene episode with it. As a result, the age of formation of the the East Arm of Sulawesi, and we examine al- of westward-dipping subduction. Neogene arc ophiolite is constrained only to be older than the ternative kinematic models for these diachro- magmatism in the western belt commenced dur- Miocene clastic strata that overlap it and thus nous microcontinent collisions. ing late early Miocene time, continuing into the postdate its structural emplacement. Gravity Pliocene and sporadically into the Pleistocene data over the ophiolite show it to thicken west- GEOLOGIC SETTING OF BUTON (Sukamto, 1978; Hamilton, 1979). The imbri- ward and dip beneath the central schist belt cated eastern belt is interpreted as a Neogene along a major thrust fault (Silver and others, Buton is part of a zone of late defor- accretionary complex formed by westward- 1978,1983a). mation encompassing much of eastern Sulawesi. dipping subduction and by partial underthrust- In the eastern half of the East Arm, the ophio- Sulawesi can be divided into a western Creta- ing of the Sula and Tukang Besi platforms. lite is thrust over and intersliced with Triassic ceous and Tertiary volcanic arc (Fig. 1), a The eastern belt includes the East Arm and through lower Miocene sedimentary rocks in a central belt of schist and highly deformed sedi- eastern part of the Southeast Arm of Sulawesi, northwest-dipping imbricate thrust complex. mentary rocks, and an eastern belt dominated by and we infer that it extends southward across a This sedimentary sequence is made up of deep- an ophiolite complex imbricated with sedimen- submerged shelf to Buton. On Sulawesi, much water and shelf limestones and represents the tary rocks and mélange (Fig. 2). The tectonic of this belt is underlain by a tectonized ophiolite deeper marginal facies of the Sula platform, interpretation of these belts has been discussed complex. In the Southeast Arm, ultramafic which was underthrust beneath the ophiolite by Silver and others (1983a), Hamilton (1979), rocks dominate, but in the East Arm, a com- during the Miocene collision (Hamilton, 1979;

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/5/660/3381214/i0016-7606-103-5-660.pdf by guest on 28 September 2021 Figure 2. Geologic map of the eastern Sulawesi-Buton collision zone. The Sula and Tukang Besi platforms are interpreted as continental fragments that collided with the accretionary terrane of eastern Sulawesi during the Miocene. Land geology modified from Sukamto (1975). Offshore structural features and bathymetry from Silver and others (1983a).

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boundary of the platform is the linear, north- Explanation for Figure 2 west-trending Hamilton fault (Silver and others, 1985), the age and sense of movement of which are unknown. Dredge hauls from the Hamilton WiWi 1 111 iii Quaternary alluvium fault scarp recovered middle and upper Miocene I!I!I!I',I!I! claystone and siltstone, a variety of carbonate 2 Neogene and Quaternary chalk and reef limestone lithologies, and diabase, diamictite, chert, and slate (Silver and others, 1985). Although the na- o 0 o—er ture of the basement of the Tukang Besi plat- ' o o ° 01 Neogene coarse clastic sediments: form remains uncertain, we interpret it to be a 3 °o o° ° V Celebes Molasse (Sulawesi) and Tondo Formation (Buton) £—n ° a continental fragment, based on the following in- direct lines of evidence: (1) the shallow summit 4 Imbricated Mesozoic through Miocene passive margin depth of the platform; (2) its proximity to the sequence (primarily limestone) continental Banda ridges; (3) the presence of thick continental-margin strata within the Wolio Complex of Buton; (4) the presence of pelitic 5 Mesozoic and Paleogene sedimentary rocks and melange (may include rocks equivalent to 4) metamorphic rocks in Buton (Lakansai Forma- tion, described below) and in the dredge hauls from the Hamilton fault scarp, which we believe Pre-Neogene metamorphic rocks of central Sulawesi M represent samples of the platform basement.

Ophiolite SCOPE OF PRESENT STUDY AND PREVIOUS WORK 8 Pre-Triassic schist, phyllite, and granite Field studies of the Wolio Complex were car- High-angle fault A A Thrust fault ried out in 1979 and 1980 as an adjunct to a sedimentologic study of the clastic strata of the Miocene Tondo Formation (Smith, 1983). Buton is a mountainous island covered by tropi- cal monsoon forest. Roads are few and confined Silver and others, 1983a). A narrow zone of Southeast Arm of Sulawesi (Fig. 2). On Buton, to the southern third of the island; small boats poorly known but apparently similar Mesozoic late Miocene to Recent folding, faulting, and are required for travel around the remainder of sedimentary rocks is also exposed east of the uplift have exposed parts of the underlying colli- Buton. Exposures of the Wolio Complex are ultramafic rocks in the Southeast Arm, north of sion complex in a number of areas (Fig. 3). We limited to deeply incised stream valleys and to the Matano fault (Fig. 2; van Bemmelen, 1949; herein propose the name "Wolio Complex" for sea-cliff exposures at the northern tip of the is- Hamilton, 1979). The character of this coastal the structurally complicated, lithologically land. We examined parts of all major outcrop belt, coupled with bathymétrie data presented diverse association of pre-Miocene rocks on areas of the Wolio Complex and made recon- by Silver and others (1983b; incorporated here Buton, which we describe in subsequent sec- naissance maps of several critical areas, utilizing in Fig. 2), suggests that the western margin of tions. The Neogene strata overlying the Wolio traverse data and photogeologic interpretations the Sula platform lies west of the limit shown by Complex fall into two distinct sequences: (based on black-and-white aerial photographs at Hamilton (1979) and extends to the coast of the (1) folded and faulted Miocene clastic rocks of a scale of 1:65,000). The main focus of our re- northern Southeast Arm, as shown in Figure 1. the Tondo Formation and (2) less deformed connaissance was to characterize the petrology Locally overlying the ophiolite and imbri- upper Miocene to Pliocene chalk (Sampolakosa and depositional setting of the sedimentary cated sedimentary rocks of the eastern belt are Formation) and Quaternary reef limestone. The strata in the Wolio Complex; these are described less deformed Miocene and younger clastic sed- coarse Tondo strata contain clasts of all major in more detail elsewhere (Smith, 1983; R. B. iments of the Celebes Molasse, which were de- Wolio Complex lithologies, and the formation is Smith, unpub. data). In this paper, we briefly rived largely from uplift and erosion of the clearly a synorogenic deposit analogous to the describe the lithologic units of the Wolio Com- ophiolite (Kundig, 1956; van Bemmelen, 1949). Celebes Molasse. The overlying carbonate strata plex and present new biostratigraphic data that The age of the Celebes Molasse is poorly con- accumulated after most collision-related defor- more precisely date the deformation of the com- strained, but Kundig (1956) indicated that the mation and clastic sedimentation ceased. plex. We also reinterpret the structure of the bulk of the molasse in the East Arm is late Mio- Lying southeast of Buton, the Tukang Besi or Wolio Complex and explore tectonic models for cene and Pliocene in age. Wakatohi Islands (Fig. 2) form a northwest- its origin and its relationship with the accretion- In the Buton region, the eastern accretionary trending chain of atolls and larger elevated is- ary complex of eastern Sulawesi. belt appears to be narrower, and exposed ophio- lands exposing upper Neogene and Quaternary The first major study of the geology of Buton litic rocks are much less extensive. The nature of reef limestones, with no outcrops of pre- is the work of Hetzel (1936), who built on the the eastern belt collision complex in this region Neogene rocks. The islands rise from northwest- work of Bothe (1927) and others to publish a and its relationship to the schist belt of the adja- trending submarine ridges with broad summit geologic map of the island at a scale of cent Southeast Arm are obscured by extensive regions shallower than 200 m, separated by nar- 1:200,000 and established an informal strati- Miocene and younger strata. Neogene strata row troughs deeper than 1,000 m. We refer to graphic nomenclature. Subsequent discussions cover most of Buton, all of the adjacent island of this complex of largely submerged ridges as the of Buton in the regional compilations by van Muna, and much of the southern part of the "Tukang Besi platform." The northeastern Bemmelen (1949) and Hamilton (1979) have

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TABLE 1. PETROGRAPHIC FEATURES AND INTERPRETATION OF OPHIOLITE CLASTS IN CONGLOMERATES OF THE MIOCENE TONDO FORMATION, BUTON

Lithdogy Primary texture Metamorphic fades

Clinopyroxene Plagiodase-clinopyroxene mesocumulates > gabbro with intercumulus hornblende and quartz; cumulate lamination present Moderate to extensive replacement of Greenschist fades hornblende and clinopyroxene by actinolite. Plagioclase fresh except along fractures Hornblende-clinopyroxene Medium grained with noncumulate gabbro subophitic texture; hornblende intergranular to poikflitic

Aphyric, intergranular to subophitic Clinopyroxene completely altered to Greenschist fades actinolite and minor chlorite. with zeolite fades Plagioclase replaced by albite, overprint epidote, and actinolite. Late veins and patches of analdme, thomsonite, and other zeolites

Olivine-phyric with quench textures; Olivine replaced by iddingsite and acicular hollow plagioclase calcite; plagioclase partially replaced microphenocrysts with interstitial by zeolites. Veins and amygdules of skeletal/acicular clinopyroxene calcite, chlorite, and zeolites

Completely recrystallized Randomly oriented prismatic actinolite Greenschist fades intergrown with albite, chlorite, sphene, and epidote

been largely based on Hetzel's work. Wiryosu- jono and Hainim (1978) studied the Neogene stratigraphy of the island and formally defined QUATERNARY the Tondo and Sampolakosa Formations. Most recently, Sikumbang and Sanyoto (1984) pub- Wapulaka Formation lished a revised geologic map of Buton and re- (reef limestone) vised the nomenclature of the pre-Neogene rock co H units. Z Alluvium 3 Z THE WOLIO COMPLEX: o UPPER MIOCENE DEFINITION AND GENERAL co TO PLIOCENE FEATURES

O Sampolakosa Fm 0 (chalk and marl) We define the Wolio Complex to include the 1 H deformed sedimentary, metamorphic, and igne- C0 ous rocks of Buton that underlie the Neogene O MIDDLE TO UPPER CL MIOCENE Tondo and Sampolakosa Formations. The unit is named for the Wolio District of southwestern Tondo Formation Buton, where extensive exposures of several of j (coarse clastics) the major units of the complex occur (Fig. 4). The Wolio Complex (Fig. 5) consists of slices of Triassic through Eocene or Oligocene deep- Igjgl Wolio Complex water limestones (Turumbia sequence); serpen- tinized peridotite with associated minor amphib- olite, greenschist, and siliceous phyllite; and Thrust fault minor pelitic phyllite and slate. Contacts be- tween rock units within the complex are almost Fault invariably tectonic, and we present evidence below that the predominant structure of the N complex is a series of west-dipping thrust sheets and overturned folds. 0 25 KILOMETERS LITHOLOGIC UNITS OF THE WOLIO COMPLEX

Ophiolite

Peridotite and minor gabbro of the Kapan- Figure 3. Generalized geologic map of Buton, modified from Hetzel (1936) and Sukamto toreh Ultrabasic Complex crop out locally in the (1975) to include traverse data and photogeologic interpretations from this study. west half of Buton. The largest exposure of these

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Fig. 7- Upper Miocene through Quaternary Turumbia Bay sedimentary strata (post-collision)

Lakansai WOLIO COMPLEX i/ River

Turumbia sequence Tobelo Mtns.

CRETACEOUS AND PALEOGENE Tobelo Formation \

UPPER JURASSIC

Rumu Formation

LOWER JURASSIC

Figure 4. Geologic map of rock units Ogena Formation within the Wolio Complex, showing geographic names referred to in text. UPPER TRIASSIC Sources same as those for Figure 3. Figure numbers refer to the locations Winto Formation 5°- of detailed geologic maps. Crystalline Rocks BUTON Lakansai Formation s \ > (pelitic phyllite) Mukito Formation (amphibolite, basic and/? siliceous phyllite) Kapantoreh Ultrabasic Complex

m Mwinto /°9ena bu^' Fault " ^—* \ River Riverì Thrust fault Fig. 8 N

0 25 KILOMETERS

123° _L

rocks is in the Kapantoreh Mountains of body consist of massive to brecciated harzburg- also mapped a number of smaller exposures of southwest Buton (Fig. 4), where peridotite is ite, which is at least 50% serpentinized. Hetzel harzburgite in northwestern Buton that we have exposed in a belt 25 km long and as much as 4 (1936) reported local occurrences of gabbro as- not examined. km wide, bounded by major zones of northeast- sociated with the harzburgite, but because of Voluminous conglomerates of the Miocene trending steep faults. Outcrops on the eastern poor exposure, he was unable to determine the Tondo Formation that crop out near the perido- and western flanks of the Kapantoreh peridotite relationship between these lithologies. Hetzel tite bodies are dominated by clasts of mafic and

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ultramafic igneous rocks. Peridotite and gabbro current deformation, as shown by the absence mapping along the Mukito River (Fig. 6), clasts are most common, but some diabase and of deformational fabrics within the rocks and we distinguished two metamorphic rock units: basalt clasts are also present. We examined thin (2) metamorphic grade increased downward (1) basic, siliceous, and minor calcareous sections of a suite of 19 mafic clasts from the within the ophiolite: zeolite to greenschist facies phyllite with mineral assemblages typical of the Tondo Formation; the petrographic data are within the basalt layer, greenschist facies (with greenschist facies and (2) fine-grained, schistose summarized in Table 1. The range of mafic and local zeolite-facies overprint) within the diabase amphibolite. ultramafic clasts in the Tondo conglomerates in- layer, and greenschist to perhaps lower amphib- The phyllite unit has a maximum exposed dicates that all the elements of a complete ophio- olite facies within the upper part of the gabbro thickness of 100 m, but due to faulting, the base lite sequence were present and exposed to layer. Similar metamorphic features in many is not exposed. The unit includes basic and sili- erosion during the collision which produced the ophiolites are commonly attributed to the action ceous phyllites interlayered in intervals several Wolio Complex, with the present exposures rep- of hydrothermal circulation systems within the meters to tens of meters thick, with minor thin resenting only the lower part of the sequence. upper part of the ophiolite soon after its forma- calcareous phyllite layers. The green basic phyl- Due to the limited extent of present exposures tion at a spreading ridge (Coleman, 1977). lite is weakly to strongly foliated but lacks ob- and the overprint of later structures, it is uncer- vious compositional layering, and it contains tain whether the ophiolite remained structurally Metamorphic Rocks associated chlorite + epidote + actinolite(?) + albite + coherent during the Miocene deformation or with Ophiolite quartz, indicating metamorphism to the green- was structurally dismembered during its schist facies. The fine grain size of the basic phyl- emplacement. Metamorphic rocks of the Mukito Formation lite suggests an aphanitic basaltic protolith. The All of the ophiolitic clasts examined are occur along the western edge of the Kapantoreh siliceous phyllite is well foliated and red to gray metamorphosed to the zeolite or greenschist peridotite body in a zone about 1 km wide along and is composed of microcrystalline quartz with facies. Metamorphism of the Buton ophiolite the Mukito River (Fig. 6). These rocks are minor muscovite and chlorite. Thin composi- occurred prior to its emplacement, for the Tri- mostly metabasites and metacherts of green- tional laminae parallel the phyllitic cleavage, assic through Eocene sedimentary rocks of the schist and amphibolite facies. Unlike the meta- which contains a strong crenulation lineation. Turumbia sequence, with which the ophiolite basite clasts derived from the Buton ophiolite, On the basis of its highly siliceous composition, red color, and association with metabasites, the is in fault contact, are unmetamorphosed. The the Mukito Formation metamorphic rocks have protolith of the siliceous phyllite was probably other salient features of the metamorphism are well-developed tectonite fabrics, with strongly abyssal red chert. The thin calcareous layers are (1) metamorphism was static, involving no con- to weakly developed foliation. In detailed weakly foliated metalimestones made up of mi- crocrystalline calcite with minor quartz and muscovite. No fossils or primary sedimentary textures are preserved in the calcareous layers. The amphibolite unit has an exposed thick- ness along the traverse of about 150 m. The unit consists primarily of well-foliated, fine-grained hornblende-plagioclase schist, locally with a millimeter-scale compositional banding of alter- nating hornblende-rich and plagioclase-rich lay- ers parallel to the schistosity. Green to blue- green hornblende is the most abundant phase, Figure 5. Inferred struc- accompanied by variably altered untwinned tural and stratigraphie rela- plagioclase. Small quantities of quartz and tionships of rock units with- sphene are present in nearly all the rocks, and in the Wolio Complex and either epidote or clinozoisite is present in most overlying strata. samples. The amphibolite shows little evidence of postcrystallization deformation, and aside from the alteration of plagioclase, retrograde ef- fects are minor, limited to very rare replacement of hornblende by chorite, and to veins of albite and prehnite + clay. Near the western boundary of the eastern amphibolite outcrop, we found a zone of garnet- bearing schist within the predominant mafic amphibolite. The schist consists of plagioclase, pink garnet, quartz, biotite, and possible kyanite, with less abundant muscovite and epidote. Ret- rograde alteration is more extensive than in the amphibolite; kyanite? is almost completely re- placed by sericite, biotite extensively replaced by

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Serpentinized peridotite

B

METERS

Bend in section _ . Vertical Scale = Horizontal Scale B

Yr+Mf r^^no o o o "do a • • • o c> o o &Ì WSpri Ü ^ ;; ^ ;; ;; ;; ;;

^'«^•.V.'.'^jir o O • O • • • • too O Q/Ì OOOQDOOOOOOO Ü o o o o • • 0 0 o o r ° ° ° ° ° ° D ° ° ° o a fi^o • o q/oa o

Figure 6. Geologic map and cross section of metamorphic rocks of the Mukito Formation in the Wolio Complex, exposed along the Mukito River west of the Kapantoreh peridotite body. Inset shows the location of the map area. The metamorphic rocks are interpreted as an up-faulted fragment of the basal metamorphic "sole" of the Buton ophiolite.

chlorite, and garnet moderately altered to are discordant, suggesting that the phyllite- soles that occur at the base of many major ophi- chlorite. amphibolite contact here is also a fault. The olite sheets (Williams and Smyth, 1973; Wood- We interpret the structure of the Mukito faults are visible in aerial photographs as cock and Robertson, 1977; Spray, 1984). These Formation as a fault-bounded, south-plunging straight, north- to northwest-trending linea- soles occur at the base of the peridotite section of synform defined by the distribution of the ments, indicating that they are steeply dipping. an ophiolite and show a downward metamor- metamorphic units and the attitudes of foliations The structural relationship between the perido- phic progression from amphibolite facies (or in (Fig. 6). The amphibolite structurally overlies tite sliver and the surrounding metamorphic some cases granulite facies) at the top, to green- the phyllite and occupies the core of the struc- rocks is unknown; Figure 6 shows one possible schist facies in the lower part. All rocks within ture, but the contacts between the two units are interpretation, in which the peridotite structur- the soles are highly deformed, with well- concealed. On the east limb of the synform, the ally overlies the amphibolite at depth. The developed tectonite fabrics. The protoliths are foliations in both units are parallel, and the con- exposed thickness of the metamorphic rocks universally interpreted as mafic volcanic and tact is thus probably concordant with the folia- along the traverse is approximately 250 m. plutonic rocks, structurally interleaved in the tion but could be either a fault or a gradational The metamorphic rocks of the Mukito For- lower part with chert and other oceanic sedi- contact. The western limb of the synform is dis- mation display the metamorphic facies, tectonite mentary rocks. The prevailing interpretation of rupted by a fault-bounded sliver of peridotite, fabrics, and range of rock compositions com- these ophiolite metamorphic soles is that they and the foliations in the phyllite and amphibolite monly seen in the thin metamorphic tectonite form soon after the initial detachment of an

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ophiolite from its mantle substrate, as the ophio- metamorphism supplied by residual heat within overlies greenschist-facies metabasalt and meta- lite is thrust over adjacent oceanic crust. The the young oceanic crust of the ophiolite, and a chert, consistent with the arrangement of meta- oceanic rocks of the metamorphic sole are pro- possible contribution from frictional heating morphic facies in ophiolite metamorphic soles. gressively welded onto the bottom of the mov- (Ghent and Stout, 1981; Spray, 1984). In the The structural relationship between the Mukito ing ophiolite, with the required heat for Mukito River area, amphibolite structurally metamorphic rocks and the peridotite is ob- scured by later steep faults; traverse data permit but do not prove the interpretation that perido-

CRETACEOUS AND PALEOGENE QUATERNARY

Tobelo Formation Qal Alluvium and beach deposits Tanjung Buton \ LOWER JURASSIC Wapulaka Formation Ogena Formation (reef limestone)

UPPER TRIASSIC

Winto Formation

Synformal Anticline KILOMETERS ^37

12y Strike and dip of bedding, tops unknown

S3X Strike and dip of overturned bedding B 1000

-0

No Vertical Exaggeration

Figure 7. Geologic map and cross section of the Turuntbia Bay area, northern Buton. See Figure 4 for location of map area. The structure is dominated by northwest-dipping thrust faults, which are locally cut by later high-angle faults. Only major thrusts separating different formations are readily recognizable iin the field; additional thrusts probably are present within the Tobelo Formation and Ogena Formation. We suggest that thrust faults such as these are the predominant structures within the Wolio Complex throughout Buton.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/5/660/3381214/i0016-7606-103-5-660.pdf by guest on 28 September 2021 Figure 8. Detailed geologic map of the headwaters of the Rumu River, southeastern Buton. See Figure 4 for location of map area. Rock units of the Turumbia sequence are exposed in a complexly faulted, northeast-trending anticline. Dolomite tentatively assigned to the Ogena Formation forms the core of the anticline. Age determinations for the Tobelo Formation are based on assemblages of planktonic foraminifers identified in thin section; sample localities with corresponding sample numbers (for example, RU-14) and age determinations are indicated. Faunal lists for the samples are given in Smith (1983), Appendix 1. Age data indicate that the radiolarian-foram limestone unit within the Tobelo Formation is older (Albian) than the foraminiferal limestone (Late Cretaceous).

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tite structurally overlies the metamorphic rocks. LITHOLOGY OLIGOCENE U We believe it likely, however, that the Mukito >• L : ?•: FORAM CC LIMESTONE Formation formed at the base of the Buton

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stratigraphic position and the petrography of the least dolomitized rocks, we tentatively correlate Campanian or Maastrichtian DISCONFORMITY: (Tur-32) this unit with the Ogena Formation. • lower Eocene (Tur-29,30) Rumu Formation. The Upper Jurassic • Maastrichtian (Tur-31) Rumu Formation is known only from the cen- FAULT CONTACT: tral Rumu Mountains of southeastern Buton • upper Eocene (Tur-33) (Fig. 8). The unit consists primarily of fossilifer- • Lower Cretaceous Upper Campanian ous red calcareous mudstone grading to argil- (Tur-34,35) (Tur-25) laceous limestone, associated with brown sili- Maastrichtian ceous mudstone. The sequence of lithologies is Albian (Tur-39) (Tur-26) uncertain because of poor exposure, but traverse data suggest an alternation of calcareous and siliceous units each from several meters to as Campanian (Tur-41) much as 10 m thick. The inferred minimum thickness of the unit (assuming no tectonic repe- tition of strata) is 75 to 100 m. TURUMBIA BAY Well-preserved bivalves and belemnites are common to extremely abundant in the red cal- upper Eocene or lower careous units of the Rumu Formation. The bi- Oligocene (Tur-13,14) valves include unidentified inoceramids and a buchiid form that we identified as Malayomao- lower or middle rica malayomaorica, which is restricted to the Eocene (Tur-11) Kimmeridgian (Jeletzky, 1963). A collection of belemnites identified by B. Challinor (1982, per- sonal commun.) includes Belemnopsis aucklan- dica galoi and a single juvenile specimen of • Foraminiteral Limestone • Calcisphere-lnoceramid Limestone Hibolithes sp.; these taxa are also consistent with • Radiolarian-Foram Limestone # Micropeloid Limestone a Kimmeridgian age. The brown siliceous mud- stone intervals within the Rumu Formation are devoid of fossils, except for very poorly pre- Figure 10. Locations, microfacies, and ages of samples of the Tobelo Formation, Turumbia served radiolarians. The depositional setting of Bay area (map area same as in Fig. 7). Age assignments are based on planktonic foraminifers the formation is problematic, but we interpret identified in thin section. Fauna! lists for the numbered samples (for example, TUR-41) are the siliceous mudstone intervals as hemipelagic given in Smith (1983), Appendix 1. Microfacies are described in text. sediment that accumulated in deep water below the calcite compensation depth, and the red cal- careous intervals as redeposited sediment that mid-Cretaceous (Aptian or Albian) to late Eo- a number of widely separated localities. By originated from a much shallower submarine cene or early Oligocene. In addition, some combining biostratigraphic and structural data, bank setting. poorly dated rocks may be Late Jurassic or we have established a tentative stratigraphic se- Tobelo Formation. The most widely ex- Neocomian (Early Cretaceous) in age. Creta- quence of the microfacies (Fig. 9), which are posed of the units in the Turumbia sequence is ceous strata occur throughout Buton, but briefly described below. Most age assignments the Tobelo Formation, which crops out in Tertiary rocks of the Tobelo beds have been are based on the ranges of planktonic foraminif- northern, central, and southern Buton (Fig. 4). found only along the northern coast of the is- era identified in thin section by Zeev Reiss The Tobelo Formation consists of white to pink, land. The thickness of the formation is very (1982, personal commun.). Locations of ana- pelagic nannofossil-microfossil limestone, which poorly constrained. In view of its long age range lyzed samples are shown in Figures 8 and 10, locally includes minor interbeds of redeposited and the large outcrop areas characterized by and foraminiferal species lists are given in Smith pelagic and perhaps shelf-derived carbonate sed- moderate to steep dips, we believe that the (1983). iment. The limestone is thin to medium bedded Tobelo Limestone could be as much as several Upper Jurassic? to Neocomian (Lower Cre- or less commonly massive, and in many out- kilometers thick. taceous). Two Tobelo pelagic microfacies are crops, it contains nodules and stringers of gray Attempts to subdivide the Tobelo Formation probably pre-Aptian in age. These are (1) radio- or red chert. Planktonic foraminifers and radio- have been frustrated by the lithologic homoge- larian limestone and (2) pelagic limestone in larians are the dominant microfossils in the lime- neity of the rocks at outcrop and hand-specimen which the predominant allochems are calcite stone; the only known macrofossils are very rare scale and by their complex deformation. Petro- prisms from disaggregated inoceramid bivalve belemnites and small fragments of inoceramid graphic observations of the pelagic limestones, shells, accompanied by calcispheres. We con- bivalves. however, reveal significant variations in fossil sider the absence of planktonic foraminifers Hetzel (1936) interpreted the age of the content, which form the basis for subdividing from these facies as negative evidence for a pre- Tobelo Formation as Late Cretaceous, but new the unit into a number of pelagic microfacies. Aptian age. Although planktonic foraminifera biostratigraphic data from our study (discussed These microfacies are not found interbedded in first appeared in the Late Jurassic, they re- below) reveal a minimum age range from the single outcrops, and each has been sampled from mained a minor element in pelagic facies until

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their great evolutionary diversification in the (Pigram and others, 1982) confirms these earlier with Globotruncana contusa, G. gagnebini, G. Aptian-Albian (Masters, 1977; Tappan and age assignments. gansseri, and G. stuarti. All Late Cretaceous Loeblich, 1973). Biostratigraphic evidence, while not conclu- stages except the Coniacian are represented by Structural data from the Rumu Mountains sive, thus favors a Late Jurassic or Neocomian sampled faunas, and more closely spaced sam- are consistent with the interpretation that the age for the radiolarian limestone and calci- pling would likely confirm the presence of Co- radiolarian limestone stratigraphically underlies sphere-inoceramid prism limestone microfacies niacian strata. There is no lithologic evidence for strata of Aptian or Albian age, but outcrops of of the Tobelo Formation. If it is assumed that any significant break in sedimentation during the two microfacies are separated by a covered the unit is entirely younger than the Kimmerid- the Late Cretaceous. interval. Belemnites and fragments of buchiid gian Rumu Formation, then these Tobelo mi- Lower or middle Eocene. Limestone of this bivalves occur locally in the calcisphere-inocer- crofacies are no older than Tithonian or late interval is known from only two localities on amid prism limestone. A collection of belem- Kimmeridgian. the northern coast of Buton (Fig. 10). It is char- nites was examined by B. Challinor (1982, Aptian-Albian. Rocks of this interval are very acterized by abundant radiolarians as well as personal commun.) and identified as juvenile cherty limestone with radiolarians and plank- planktonic foraminifers, including Acaranina Belemnopsis of indeterminate species. Accord- tonic foraminifers as the principal allochems. primitiva, A. soldadoensis, and Morozovella ing to Challinor, the belemnites "have a cross- The foraminifers include Hedbergella sp., Glo- aragonensis. No rocks of Paleocene age have sectional shape similar to that of the Kimmer- bigerinelloides sp., Ticinella praeticinensis, Tici- been found in the Tobelo Formation, and we be- idgian taxon Belemnopsis moluccana BOEHM nella cf. roberti, and Biticinella algeriana. lieve that a hiatus within the unit spans the and a ventral groove similar to that of B. moluc- Upper Cretaceous. Limestones with plank- Paleocene and perhaps part of the early Eocene. cana and the Lower Cretaceous B. irianensis." tonic foraminifers of Late Cretaceous age form At one locality along the north coast of Buton, These specimens may belong to an as yet unde- the most widespread microfacies of the Tobelo this hiatus is represented by an exposed discon- scribed Early Cretaceous species, but a Late Formation. Nodules and layers of chert are formity with laminated lower Eocene pelagic Jurassic age cannot be ruled out. The abundant common, but much less abundant than in the limestone overlying massive upper Maastrich- calcispheres in the calcisphere-inoceramid prism Aptian-Albian rocks. Radiolarians are found tian limestone. We interpret the hiatus as a result limestone correspond in morphology and struc- only within and immediately adjacent to the of intense bottom water flow during Paleocene ture to Stomiosphaera moluccana and Cadosina widely spaced cherts. Planktonic foraminiferal and/or early Eocene time. fusca, forms described by Wanner (1940) and faunas are dominated by a variety of species of Upper Eocene. Foraminiferal limestone lack- Vogler (1941) from pelagic limestones in Timor, Globotruncana, accompanied by Hedbergella ing radiolarians characterizes this interval. A late Seram, Misool, and Buton. Both Wanner and sp., Globigerinelloides sp., and unidentified het- Eocene or perhaps early Oligocene age is indi- Vogler regarded the strata containing these taxa erohelicids. The rocks range in age from cated by foraminiferal faunas including Globi- as Late Jurassic or Neocomian in age. More Cenomanian, with Planomalina buxtoifi, Prae- gerina ampliapertura, GL venezuelana, and Gl. recent work on the stratigraphy of Seram globotruncana stephani, and Rotalipora cush- tripartita. (Audley-Charles and others, 1979) and Misool mani, to Maastrichtian, as indicated by rocks Interpretation of the Turumbia Sequence. Although stratigraphic relationships of the units

Figure 11. Sketches of sea-cliff exposures of folds in limestone of the Turumbia sequence, Turumbia Bay area. Each exposure is approxi- mately 3 m high; views are toward the south. A. Refolded recumbent fold in limestone of the Tobelo beds. B. Angular folds in overturned pelagic and turbidite limestones of the Winto beds. Sketches drawn by Laura Benninger from photographs.

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in the Turumbia sequence have been obscured A. Present by complex deformation, their lithologic similar- ities and ages indicate that all formed as parts of NW a single, long-lived stratigraphic succession. The earliest, Triassic part of this sequence includes clastic rocks of continental provenance, but the Jurassic through Eocene strata consist predomi- nantly of deep-water pelagic limestone. With the exception of the Upper Jurassic Rumu Forma- tion, pelagic carbonate sedimentation persisted through nearly all of this long time interval, in- dicating that deposition occurred above the CCD (carbonate compensation depth), at bath- B. late Miocene 10 Ma yal rather than abyssal depths. We infer that the Turumbia sequence accumulated on continental rather than oceanic crust We interpret the se- quence as the sedimentary cover of the deep western margin of the Tukang Besi platform, which was deformed and incorporated within the Wolio Complex during the Miocene colli- sion of the Tukang Besi platform.

Pelitic Phyllite and Slate No Vertical Exaggeration (Lakansai Formation) KILOMETERS

Pelitic and quartzitic metamorphic rocks of the Lakansai Formation crop out along the east coast of northern Buton. These rocks are Quaternary reef limestone WOLIO COMPLEX very poorly exposed, but we examined outcrops Mesozoic and along the Lakansai River near the northern limit Miocene and Pliocene Paleogene of the unit (Fig. 4). The Lakansai River outcrops Turumbia sequence are pelitic phyllite and slate with subordinate Sampolakosa Formation interbeds of quartzose and micaceous sandstone. The gray phyllite and slate are commonly highly Ophiolite weathered to a very light reddish-gray color. Miocene Tondo Formation Slaty or phyllitic cleavage is well developed, and a later crenulation cleavage is present locally. Figure 12. Schematic structural cross section across southern Buton, illustrating interpreted Thin interbeds of fine- to very fine-grained sand- structure of the Wolio Complex. A. Present section. The structure of the Wolio Complex is stone are gray-green and only weakly foliated. dominated by west-dipping thrust sheets, with ophiolite thrust over and imbricated with the The sandstones are poorly sorted wackes with a Turumbia sequence in the western part of the complex. Imbricate structure is obscured by later mixed metamorphic-plutonic provenance. The steep faults and overlap by Neogene strata. B. Section as it might have appeared in late grain assemblage is dominated by quartz (great- Miocene time, during deposition of the Tondo Formation, with effects of later steep faults er than 50%), chert and/or felsite, feldspars, removed. muscovite, and grains of slate and phyllite. The metamorphic Lakansai Formation is un- dated and is isolated from other elements of the gional considerations also favor this interpreta- platform. We therefore suggest that the meta- Wolio Complex by Neogene strata, so that its tion. The pre-Triassic basement of the Sula morphic rocks of the Lakansai Formation are stratigraphic and structural relationships to the platform consists of metapelites and quartzites also Paleozoic in age and represent a fault- rest of the complex are uncertain. Bothe (1927) similar to the Lakansai Formation (Sukamto, bounded fragment of the basement complex of and Sikumbang and Sanyoto (1984) compared 1975, 1978). The Sula platform rocks are also the Tukang Besi platform, upon which the Tu- this unit with metamorphosed Mesozoic strata undated, although Pigram and others (1985) rumbia sequence accumulated. in southeastern Sulawesi, whereas Hetzel (1936) suggested correlation with Silurian-Devonian assumed it to be pre-Mesozoic in age. Sand- turbidites in Irian Jaya that were metamor- STRUCTURE OF THE stones in the Triassic Winto beds include com- phosed prior to the middle Carboniferous. WOLIO COMPLEX mon grains of pelitic schist and phyllite that Quartzite and pelitic slate and phyllite have also could be derived from the Lakansai Formation, been dredged from the Banda ridges, and slate The Wolio Complex appears to consist of a suggesting a pre-Triassic age for the unit. Re- from the northeast margin of the Tukang Besi series of fault-bounded rock bodies, each with a

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0 tions (as mapped by Hetzel, 1936, and inter- Reef Limestone QUATERNARY preted by us from aerial photographs) have sinuous traces consistent with a moderate Sampolakosa N21 . Ma to PLIOCENE northwest dip. The older-over-younger age rela- tionship shown by several of these contacts and ^Formation N18 their discordance with nearby bedding orienta- mm* N17 tions identify them as major thrust faults. Addi- - 5 tional unmapped thrusts probably occur within LATE Tondo Figure 13. Ages of Neogene the limits of the individual formations. The Tri- N16 III Formation strata overlying the Wolio Com- assic Winto Formation forms the structurally -N15- Z plex in Buton, based on studies highest exposed thrust sheet in this area. In the of planktonic foraminifera re- northwestern part of this sheet, we have mapped 3H113P- IJJ - 10 N12 ported in Smith (1983) and Wir- an open, northeast-trending synform, but grad- N11 MIDDLE o yosujono and Hainim (1978). ing and sharp basal contacts in turbidites show N10 o Neogene zonation of planktonic that strata in both limbs of the synform are in- ? -N9—| foraminifers (N5 to N21) is from verted. We interpret this structure as the in- Tondo N8 verted limb of a large overturned anticline that Fm - 15 Blow (1969), as emended by N7 Srinivasan and Kennett (1981). has been thrust southeastward over the underly- N6 ing Cretaceous Tobelo Formation and that has EARLY Time scale is from Berggren and others (1985). undergone later refolding to form the open syn- N5 -20 form. We have not found clear evidence for WwVW such imbrication elsewhere in Buton, but we believe that it is the predominant structural style of the Wolio Complex, despite modification by rvww 30 later steep faulting. EARLY Mesoscopic folds were observed in less than Wolio Complex 10% of the outcrops of the Turumbia sequence. Most are angular, tight (Fig. 11) to isoclinal h* 35 folds. The rocks generally lack a consistently oriented cleavage, but fractures and solution seams with a wide variation in orientations are ubiquitous in the limestone units. Locally, near coherent though commonly complex internal overprinted and obscured the original structure major fault zones, spaced cleavage occurs in structure. Nowhere did we observe the pervasive of the complex and largely control the present limestones of the Tobelo Formation. This shearing and outcrop-scale sfratai disruption map pattern of rock units within the complex. cleavage is typified by smooth, anastomosing characteristic of a mélange. In mélange terrains For example, on the basis of traverse data and cleavage surfaces spaced 1 to 2 cm apart and on other humid, densely vegetated Indonesian photogeology, we interpret the present contacts closely resembles the "strong" solution cleavage islands, most outcrops consist of coherent blocks between the ophiolite and other units of the of Alvarez and others (1978). of lithologies resistant to erosion, whereas the Wolio Complex in southern Buton as steep Exposures of the ophiolite within the Wolio more easily eroded sheared matrix is rarely ex- faults that also cut the Tondo Formation. The Complex are confined to a long narrow zone in posed. The deep stream valleys in the interior of steep dips of these and other late faults within western Buton (Fig. 4), but the structural rela- Buton, however, commonly expose poorly con- the Wolio Complex are evident from the linear tionship between the ophiolite and the other solidated Neogene strata in close proximity to traces of the faults across the rugged topography. components of the Wolio Complex is obscured more resistant limestone of the Turumbia se- As a result, the original structural relationships by overlapping Neogene strata and late faulting. quence, and so we would expect to have ob- between the ophiolite and the flanking units of On the basis of the location of the ophiolite and served similar exposures of mélange matrix if it the Turumbia sequence are not readily discern- the structural style of the Turumbia sequence as were in fact present. ible from the present map pattern. observed in northern Buton, we believe that the Both the Wolio Complex and overlying Neo- The large outcrop area of the Wolio Complex ophiolite forms a west-dipping thrust sheet lying gene strata have been folded and cut by faults, at the northern tip of Buton (Fig. 7) appears to above the imbricated Turumbia sequence that but to differing degrees. The greater complexity show fewer effects of later deformation, and our forms the main, eastern part of the Wolio Com- of structure in the Wolio Complex is evidence reconnaissance observations (primarily of coast- plex (Fig. 12). This inference is consistent with that much of its deformation predates deposition al exposures) have given us some insights into the structural relationships of the ophiolite else- of the Miocene Tondo Formation. It is difficult the original structure of the Wolio Complex. where in the eastern zone of Sulawesi, discussed to separate the effects of younger and older de- Strata of the Turumbia sequence are arrayed previously. In southern Buton, rocks of the Tu- formations at either outcrop or regional scales, here in what we interpret as an imbricate stack rumbia sequence also occur west of the main however. Present exposures of the Wolio Com- of northwest-dipping thrust sheets. Bedding in Kapantoreh Mountains ophiolite body; although plex are confined to late Neogene anticlines and this region generally dips from 30° to 55° north most of this area is covered by Neogene strata, horsts; in many areas, these later structures have to northwest, and contacts between the forma- Hetzel (1936) mapped alternating narrow strips

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of peridotite and Winto beds in several river EAST ARM BUTON traverses. The Kapantori body appears to be the SULAWESI Ma lowest of several slices of ophiolite that alternate • o with slices of Turumbia sequence strata; a sim- Reef Limestone Reef Limestone QUATERNARY ilar zone of interslicing is present at the leading Celebes Pelagic Chalk PLIOCENE edge of the main ophiolite body on the East Molasse 5 Arm of Sulawesi (Kundig, 1956; Hamilton, rTTrTTilTrrirrr^^ LATE UJ 1979). Co ision Tondo Fm. Z - 10 Previous interpretations of thrust geometry in lYI^flYnYrrrnYnMTvr^^ UJ MIDDLE Buton are conflicting. Our interpretation is con- Co sionjJJJJJJilJJJ o - 15 sistent with those of Kundig (1956) and Bothe o (1927), who inferred that major thrusts in Buton EARLY - 20 dip westward. In contrast, Hetzel (1936) in- Imbricate ferred eastward dips for most thrusts in Buton. I - 25 Complex LATE AGE OF ASSEMBLY OF THE o UJ (D z WOLIO COMPLEX UoJ - 30 Wolio Complex EARLY The youngest rocks that clearly exhibit the - 35 tight folding and thrusting characteristic of the Wolio Complex are pelagic limestones of the Tobelo Formation, the youngest of which con- Figure 14. Comparison of Neogene stratigraphy and collision timing for Buton and the East tain foraminiferal faunas of late Eocene or early Arm of Sulawesi. Age data for Sulawesi are from Kundig (1956). Time scale is from Berggren Oligocene age. Throughout much of Buton, the and others (1985). Wolio Complex is overlain by less deformed Neogene sedimentary units (Fig. 13). The Mio- cene Tondo Formation is a sequence of con- trends; others offset fold axes at various angles. assembly of the Wolio Complex. The age corre- glomerate, sandstone, and mudstone at least For some of the faults, we can infer a steep dip lations reported here are based on studies of fo- 1,300 m thick that exhibits striking vertical and from their straight surface trace (determined by raminifera by H. G. Billman (1982, personal lateral facies variations (Smith, 1983). It consists multiple traverse crossings and geomorphic ex- commun.) and refer to the tropical Neogene zo- of fan-delta, turbidite fan, and slope facies de- pression in aerial photographs). For most of the nation of planktonic foraminifera by Blow posited at outer neritic to bathyal depths, and faults, however, we have insufficient field data (1969), as amended by Srinivasan and Kennett minor shallow-marine and fluvial facies. The to determine the fault attitude and sense of dis- (1981). Sample localities and faunal lists are overlying Miocene and Pliocene Sampolakosa placement. It is possible that some Wolio Com- given in Smith (1983, App. II and III). Formation consists of fine-grained pelagic fo- plex thrust faults also offset the Tondo Forma- The Tondo Formation is primarily middle to raminiferal chalk as much as 1,200 m thick tion. Folds in the Tondo Formation, however, late Miocene in age (Fig. 13). Most exposed (Hetzel, 1936; Wiryosujono and Hainim, 1978). are more open than those in the Wolio Com- sections of the Tondo are late Miocene (zone Hetzel (1936) mapped numerous major folds plex, and overturned strata are rare, so the N16 or N17). Middle Miocene rocks (N14 to that deform both the Tondo and Sampolakosa Tondo Formation has undergone significantly N15) crop out only very locally, but unpub- Formations along axes trending northeast in less horizontal shortening than has the Wolio lished biostratigraphic data from petroleum ex- southern Buton, and north in central and north- Complex. ploration wells on Buton (provided by ern Buton. Most of the folds are upright and All of the contacts we have mapped between Gulf Oil Ltd.) indicate that middle Miocene open, with limb dips less than 30°, and wave- the Tondo Formation and the Wolio Complex clastic strata (N13 to N14) are widespread in the lengths on the order of several kilometers. In are faults; we have not seen the depositional subsurface. The exposed middle Miocene sand- some areas, the Tondo Formation shows a base of the unit. The Tondo Formation appears stone and conglomerate contain abundant detri- greater degree of deformation than does the to overlap all units in the underlying Wolio tus from both the ophiolite and the Turumbia overlying Sampolakosa Formation. Moderate to Complex, however, and Tondo sandstones and sequence, and so the collisional assembly of the steep dips are common in the Tondo Formation, conglomerates are dominated by clasts of perid- Wolio Complex took place prior to the later and in several traverses, we mapped open to otite, gabbro, limestone, and chert that were part of middle Miocene time. One exploration tight folds (minimum interlimb angles 60°) with clearly derived from the Wolio Complex well in Buton, however, has penetrated lower upright to steeply inclined axial surfaces and (Smith, 1983). On the basis of the stratigraphic Miocene (N7-N9) clastic strata containing wavelengths of as much as 1 km (Smith, 1983). and structural evidence, we infer that the base of limestone-pebble conglomerate that could have In two localities in southern Buton, these fold the Tondo Formation is an angular unconform- been derived from the Turumbia sequence. This systems are truncated by an angular unconform- ity and that the formation was deposited during lower Miocene conglomerate could signal the ity at the base of the gently dipping Sampola- and subsequent to the assembly and initial de- beginning stages of collision of the Tukang Besi kosa Formation. Numerous faults cut the Tondo formation of the Wolio Complex. Biostrati- platform with Sulawesi, but the absence of Formation, typically juxtaposing different sedi- graphic data from the Tondo Formation there- ophiolite detritus from this part of the section mentary facies. Some faults strike parallel to fold fore provide an upper limit for the age of allows an alternative interpretation. The lower

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Active volcanic arc

HH Extinct volcanic arc

Pre-Neogene metamorphic belt late Miocene 10 Ma

Ophiolite

Microcontinent

Active thrust fault

late Miocene 7 Ma -.A—-A-- inactive thrust fault

Active strike-slip fault

Inactive strike-slip fault

Present 0 500 Pliocene 4 Ma Kilometers

TBP

Figure 15. Alternative reconstructions of the Neogene tectonic evolution of the Buton-eastern Sulawesi collision zone. See text for discussion. Symbols on present-day tectonic map: P, fault; K, Kolonodale fault; L, Lawanopo fault; T, Tolo thrust; H, Hamilton fault; SP, Sula platform; TBP, Tukang Besi platform; NBB, North Banda basin.

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Miocene limestone conglomerate could be unre- microcontinent that fragmented during oblique DISCUSSION lated to the collision, having been produced in- collision with Sulawesi. The Tukang Besi plat- stead by uplift associated with initial detachment form formed the western end of this microconti- In reconstruction 1, microcontinent collisions of the Tukang Besi platform from the northern nent and thus made initial contact with Sula- are confined to the Buton region and the East New Guinea margin. We prefer the latter inter- wesi. Both reconstructions assume that collision Arm of Sulawesi. The intervening part of the pretation, and a middle Miocene initiation of was underway in the Buton region by about 13 eastern Sulawesi margin (from Buton north to collision, because the bulk of the collision- Ma (middle Miocene), causing imbricate thrust- the Matano fault) would have experienced Neo- related clastic strata are middle to late Miocene ing of the deeper western margin of the Tukang gene convergence with the North Banda basin, in age. Besi platform (Turumbia sequence). For the but no collision event. The expected accreted next several million years, the Tukang Besi plat- material along this part of the convergent mar- NEOGENE TECTONIC form continued to move northward along this gin would be trench deposits and ocean-floor DEVELOPMENT OF THE EASTERN obliquely convergent boundary, leaving behind pelagic sediments (radiolarian chert, siliceous SULAWESI CONVERGENT MARGIN fragments of the Turumbia sequence in the colli- mudstone, or pelagic limestone). In reconstruc- sion complex in its "wake." We base this con- tion 2, oblique convergence and fragmentation No satisfactory model for the history of the clusion on the observation that rocks of the of the Tukang Besi-Sula platform microconti- Banda Sea basins and microcontinents has yet Turumbia sequence extend to the southwest tip nent produces a northward-migrating collision been formulated. Existing geologic data are in- of Buton, about 50 km beyond the present affecting the entire eastern Sulawesi margin. In sufficient to date securely either the basins or southwestern margin of the Tukang Besi plat- this scenario, imbricated continental-margin most of the structures bounding the microconti- form. The major, steeply dipping longitudinal strata similar to the Turumbia sequence of nents. The original relative positions of the mi- faults that cut the Wolio Complex (as well as Buton should occur along the entire margin. crocontinents on the New Guinea margin and younger strata) may have been initiated as left- A narrow coastal zone of undifferentiated their sequence of detachment and emplacement lateral strike-slip faults that partially accommo- Mesozoic sedimentary rocks extends 50 km also remain obscure. Because the northern mar- dated the northward motion of the platform. By southeast of the Matano fault before exposures gin of New Guinea has moved progressively the late Miocene (10 Ma), the Tukang Besi plat- are terminated by the southward bend of the northward relative to during form had ceased its northeastward movement coastline (Fig. 2; Sukamto, 1975). Bothe (1927) Neogene time, Silver and others (1985) pro- relative to southeast Sulawesi, and thrust defor- indicated that the southern part of this belt con- posed that later continental slivers would have mation of the Wolio Complex probably also sists of Mesozoic limestone similar to that of been emplaced north of the earlier ones. On the ceased at this time or shortly thereafter. We as- Buton, and our reconnaissance observations of basis of this assumption and other regional con- sume that subsequent relative motion between coastal exposures in this area during a gravity siderations, they suggested the following order of the North Banda basin and the Tukang Besi plat- survey (Silver and others, 1978) confirm this emplacement: (1) Banda ridges, (2) Buru and form was accommodated by left-lateral strike- interpretation. To the southeast, a shallow shelf Seram, (3) Tukang Besi platform, and (4) Sula slip along the Hamilton fault (Fig. 2), which ridge extends along the trend of the coastal platform. marks the northeastern boundary of the plat- Mesozoic belt to Manui Island, which exposes form. Although the actual sense of motion along Geologic data from Buton now place signifi- only Neogene limestone. The next exposure of this fault remains to be documented, seismic cant constraints on the timing of emplacement the Neogene convergent margin is on the island profiles across it show no evidence of major of the Tukang Besi platform. The data summa- of Wowoni, 25 km north of Buton. According thrusting or convergence (Silver and others, rized above point to a middle Miocene collision to Sukamto (1975), the pre-Neogene rocks of 1983b). of the Tukang Besi platform with the Sulawesi Wowoni consist of ultrabasic rocks thrust over subduction zone. Emplacement of the Sula plat- The reconstructions shown in Figure 15 par- Triassic sedimentary rocks similar in facies to form in its present position adjacent to the East tially incorporate previous interpretations (Sil- the Winto Formation of Buton. Wowoni there- Arm of Sulawesi (Fig. 2) appears to have oc- ver and others, 1983b) of substantial strike-slip fore appears to represent a Neogene collision curred substantially later, during late Miocene faulting in eastern and central Sulawesi in re- zone, rather than a simple subduction complex, time (Fig. 14). This conclusion is based on the sponse to the Sula platform collision. These despite the fact that it lies beyond the northern poorly constrained age of the Celebes Molasse in faults trend northwest to north and include the limit of the Tukang Besi platform. The limited the East Arm, which according to Kundig active Palu and Matano faults, the inactive available geologic data therefore support the (1956), is in part late Miocene, but primarily Lawanopo fault, and the postulated Kolonodale concept of a continuous zone of Neogene colli- Pliocene. fault (Fig. 15). Left-lateral slip on these faults sion linking Buton with the East Arm of Sula- wesi. If this interpretation is correct, then the We present two hypothetical kinematic re- accommodated substantial northwestward move- major elements of reconstruction 2 (migrating constructions for the emplacement of the Tu- ment of the Sula platform and East Arm subse- collision and fragmentation of a single large mi- kang Besi and Sula platforms that satisfy these quent to the initial collision, disrupting the crocontinent) provide the best explanation for constraints on emplacement timing (Fig. 15). ophiolite belt of eastern Sulawesi. Additional the Neogene development of the Buton-east Reconstruction 1 postulates that the Tukang consequences included major clockwise rotation Sulawesi convergent margin. Additional strati- Besi and Sula platforms were separate micro- of the North Arm of Sulawesi, with attendant graphic data are required from the eastern continents that successively collided with differ- development of the north Sulawesi trench, and a Sulawesi collision complex to more precisely ent parts of the Sulawesi convergent margin. portion of the proposed Neogene separation of constrain the age of tectonic events along this Reconstruction 2 proposes that the two plat- the South and Southeast Arms of Sulawesi to margin. forms initially formed parts of a single larger form Bone Gulf (Hamilton, 1979).

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B., 1983b, Collision, rotation, and REVISED MANUSCRIPT RECEIVED MARCH 16,1990 region: American Association of Petroleum Geologists Bulletin, v. 64, the initiation of subduction in the evolution of Sulawesi, Indonesia: MANUSCRIPT ACCEPTED JUNE 22,1990 p. 868-915. Journal of Geophysical Research, v. 88, p. 9407-9418. FINAL MANUSCRIPT RECEIVED JANUARY 25, 1991

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