Calvert Hall 2007 West Sulawesi [PDF]

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Cenozoic evolution of the Lariang and Karama regions, North Makassar Basin, western Sulawesi, Indonesia Stephen J. Calvert1,2 and Robert Hall1



1



SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK 2 Marathon Oil Company, Beltway Office Park, Building A, 4th Floor, Jalan TB Simatupang No. 41, Jakarta 12550, Indonesia (e-mail: [email protected]) ABSTRACT: The Lariang and Karama regions of western Sulawesi, an area of



approximately 10 000 km2, were the subject of a field-based investigation with the aim of understanding the Cenozoic evolution of the North Makassar Basin. Western Sulawesi was influenced by the development of the Makassar Straits to the west and the collision of continental, ophiolitic and island-arc fragments to the east. The timing of these events has been the subject of considerable debate and it has been suggested that Neogene collisions in Sulawesi caused inversion in Borneo. A new stratigraphy for the Lariang and Karama regions of western Sulawesi, based on fieldwork, provides new and significant insights into the evolution of the Makassar Straits region. The oldest sediments are non-marine and could be as old as Paleocene; they include coals, sandstones and mudstones. Rifting had started by the Middle Eocene and continued into the Late Eocene. Syn-rift Eocene sediments were deposited in graben and half-graben in both marine and marginal marine environments. The Eocene Makassar Straits rift was highly asymmetrical; the Kalimantan margin was approximately twice the width of the Sulawesi margin. Thermal subsidence had started by the latest Eocene and by the end of the Oligocene most of western Sulawesi was an area of post-rift shelf carbonate and mudstone deposition. This shallow-marine depositional environment persisted throughout the Early Miocene and, in places, until the Middle or Late Miocene. In the Pliocene the character of sedimentation changed significantly. Uplift and erosion was followed by the deposition of coarse clastics derived from an orogenic belt to the east of the study area. The Palaeogene half-graben were inverted, there was localized detachment folding and the overlying Neogene section was folded, faulted and eroded in places. Contractional deformation in western Sulawesi dates from the Pliocene, whereas in eastern Kalimantan it dates from the Early Miocene.



KEYWORDS: Makassar Straits, Sulawesi, Lariang, Karama, rifting



INTRODUCTION The onshore portion of the Lariang and Karama basins covers an area of approximately 10 000 km2 of the coastal lowlands of western Sulawesi (Fig. 1). Based on the integration of field, biostratigraphic and remote sensing data, a new stratigraphy has been produced for the region (Fig. 2; Calvert 2000) and is a significant revision of previous work (Sukamto 1973; Hadiwijoyo et al. 1993; Ratman & Atmawinata 1993). In recent years a number of geological models have been proposed for the evolution of the Makassar Straits, which reflect insights derived from studies of eastern Kalimantan and eastern Sulawesi (Ali et al. 1996; Simandjuntak & Barber 1996; Parkinson 1998; Walpersdorf et al. 1998; Guntoro 1999; Charlton 2000; McClay et al. 2000) but lack information from western Sulawesi. Models that include the western Sulawesi region (Fig. 1) have assumed that it was affected firstly by Palaeogene extension (e.g. Hamilton 1979) which eventually led to the formation of the Makassar Straits and, secondly, by Petroleum Geoscience, Vol. 13 2007, pp. 353–368



Neogene contraction and uplift driven principally from the east (e.g. Bergman et al. 1996). The Sulawesi Neogene contraction has been widely interpreted as the explanation of inversion at the conjugate margin in East Kalimantan (Letouzey et al. 1990; Daly et al. 1991; Bransden & Matthews 1992; van de Weerd & Armin 1992; Tanean et al. 1996; Cloke et al. 1997; Longley 1997; McClay et al. 2000). Therefore, the study area is in an ideal location to observe the effects of Palaeogene extension and is adjacent to the Neogene orogenic belt of Sulawesi. This investigation of the geology of the region was carried out to help constrain the timing of Cenozoic tectonic events. Onshore oil seeps and potential offshore gas hydrates in the North Makassar Basin show that a working petroleum system exists in this frontier region which is opposite one of the largest hydrocarbon provinces in SE Asia (Calvert & Terry 2001; Jackson 2004). A post-war report by BPM (Bataafsche Petroleum Maatschappij, the principal Dutch-Shell company in Indonesia) considered the region to be the second most prospective in Sulawesi after Buton. 1354-0793/07/$15.00  2007 EAGE/Geological Society of London



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S. J. Calvert & R. Hall study area. The study area covers three 1: 250 000 quadrangles mapped by GRDC, each with its own stratigraphy (Sukamto 1973; Hadiwijoyo et al. 1993; Ratman & Atmawinata 1993). The new stratigraphy presented here (Fig. 2) is based on fieldwork carried out in 1996 and 1997 as part of a PhD at the University of London (Calvert 2000) and additional work in 2001 (Calvert & Terry 2001). Mesozoic pre-rift basement The Mesozoic basement in the Lariang and Karama regions (Fig. 3) consists of metamorphic rocks unconformably overlain by less deformed Upper Cretaceous dark shales and volcanic rocks. This basement crops out mainly in the highlands where there are peaks up to 3000 m high. Various names and ages (Table 1) have been given to the metamorphic rocks of western Sulawesi (Hadiwijoyo et al. 1993; Ratman & Atmawinata 1993). The overlying dark shales and volcanics are at least 1000 m thick and are considered laterally equivalent to basement in other parts of western Sulawesi (Table 2). These rocks have been interpreted to be the deposits of a forearc basin situated to the west of a west-dipping subduction zone (van Leeuwen 1981; Hasan 1991).



Fig. 1. Simplified geological map of Sulawesi after Hall & Wilson (2000) with box showing the location of the Lariang and Karama regions. Tectonic provinces and names of the different areas of Sulawesi referred to in the text are shown.



This paper presents an integrated summary of the geology of the Lariang and Karama regions. The new stratigraphy is described with details of the Palaeogene rift sequences. New geological mapping (Fig. 3), combined with interpretation of vintage onshore seismic data, has enabled a structural interpretation of the region. Detailed sedimentary facies analyses, combined with information from microfossil assemblages, have allowed interpretation of Cenozoic environments. The significant regional events that preceded the Eocene formation of rift basins are discussed and the basement structures throughout the Makassar Straits region are compared. The final section deals with the early Cenozoic basin style and sedimentary basin fill throughout the Makassar Straits region and comparisons are made with other rifted basins. STRATIGRAPHY No single study has focused previously on the entire Lariang and Karama region. The region was first mentioned in geological reviews of the Dutch East Indies (Hundling 1942; Beltz 1944) and summarized with other Dutch work by van Bemmelen (1949). The region was also mentioned briefly by Hamilton (1979) and Kartaadiputra et al. (1982), but it was GRDC (Geological Research and Development Centre, Indonesia, now the GSC, Geological Survey Centre) who were the first to publish geological reports on different parts of the



Volcanic rocks The terms Kambuno, Lamasi and Talaya Volcanic Units (Norvick & Pile 1976; Hadiwijoyo et al. 1993; Ratman & Atmawinata 1993) are not part of the stratigraphic scheme used here. The volcanic rocks have not been dated and, overall, little is known about them. Norvick & Pile (1976) assumed these predominantly basaltic to andesitic rocks to be, in part, laterally equivalent to their Budungbudung Formation. Probable Palaeogene basalts form part of the basement complex in the Latimojong Complex (Coffield et al. 1993) and a Cretaceous to Palaeogene Lamasi Complex was reported by Bergman et al. (1992). Volcanic rocks mapped by Hadiwijoyo et al. (1993) are unconformable on the Latimojong Formation. Volcanic rocks in the core of an anticline in the Karama River, which were mapped as the Miocene Talaya volcanic rocks by Ratman & Atmawinata (1993), are regarded here as being older than Middle Eocene. In all cases exposures previously assigned to these volcanic units and visited in this study are always closely associated with the Upper Cretaceous rocks, are always overlain by the Toraja Group and are older than Middle Eocene and, hence, mapped as such. The volcanic rock units could be either of Cretaceous age and related to accretion, or Cenozoic and related to early Palaeogene rifting. Volcanic rocks in the South Arm of Sulawesi and SE Kalimantan have been related to subduction along the southern margin of Sundaland during the Late Cretaceous (Hasan 1991; Moss et al. 1997; Parkinson et al. 1998; Soeria-Atmadja et al. 1998). In Kalimantan some volcanic rocks have been related to Early Cenozoic extensional tectonics (Hutchison 1996; Moss & Chambers 1999). However, the age and nature of these Late Cretaceous or early Cenozoic igneous rocks are poorly understood, they are not well dated, are poorly described and their chemistry could reflect older Mesozoic subduction. Palaeogene syn-rift to post-rift Toraja Group The Middle Eocene to lower Upper Oligocene Toraja Group (new name; Fig. 2) is exposed between the northern Quarles Mountains and the northeastern Molengraaff Mountains (Fig. 3). The Toraja Formation of Ratman & Atmawinata (1993) has been elevated here to group status. This change has been made



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Fig. 2. The Cenozoic stratigraphy of the Lariang and Karama regions, western Sulawesi (Calvert 2000) and summary of key tectonic events.



because useful comparisons can be made between the Palaeogene rocks in the study area, the Toraja Formation at the type area 70 km to the south (Djuri & Sudjatmiko 1974), and other formations of the same age in central and south Sulawesi. The Toraja Group rests unconformably on Mesozoic basement. The upper contact with the Neogene Lisu Formation was not seen and is believed to be conformable. In the northern Lariang region it has a faulted contact with the Lisu Formation. The Toraja Group is up to 3500 m thick and separated into two formations: the thicker marginal marine/terrestrial sedimentary rocks of the Kalumpang Formation and the more extensive marine sedimentary rocks of the Budungbudung Formation. The Budungbudung and Kalumpang formations are based upon names originally introduced by Norvick & Pile (1976). The stratigraphic ranges of samples from the Toraja Group are shown in Figure 4. The age that Hadiwijoyo et al. (1993) give for the assemblage of microfossils from an exposure of the group



is too young (based on comparison with Bolli et al. 1985; E. Finch, pers. comm. 2000). Kalumpang Formation The Kalumpang Beds of Norvick & Pile (1976) are here renamed the Kalumpang Formation. They are present north and south of the original type section and are now known to be Middle to Upper Eocene and not Lower to Middle Miocene, as originally proposed by Norvick & Pile (1976). The formation is composed of a sequence of shales, coal beds and metre-thick quartzose sandstones that outcrop in the northern Quarles Mountains (Tables 3 and 4). The type locality of the Kalumpang Formation is in the Karama River (Figs 3 and 5). The formation is approximately 3200 m thick and rests unconformably on Upper Cretaceous rocks. This formation is thought to be analogous to the ‘coalfields’ that were first



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Fig. 3. Geological map of the Lariang and Karama regions, western Sulawesi (Calvert 2000).



described in this region by Reyzer (1920). The formation passes laterally into and is conformably overlain by limestone of the Budungbudung Formation. Budungbudung Formation The type area for the Budungbudung Formation is near the Budungbudung River in the northern Karama region (Fig. 3) and the formation has an estimated minimum thickness of 1000–2000 m. Although the type section of the Budungbudung



Formation of Norvick & Pile (1976) was not visited in this study, rocks close to it and at other localities where it is mapped were studied in detail. These rocks closely resemble those described by Norvick & Pile (1976) and, therefore, the name has been retained. The Middle Eocene to Upper Oligocene Budungbudung Formation of this study is a variable sequence of variegated soft to shaly mudstones, quartzose sandstones, limestones and minor conglomerates (Tables 3 and 4). The formation is exposed in the Karama region and the northern Lariang region, where the formation is unconformable on



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Makassar Straits and West Sulawesi basins, Indonesia Table 1. Summary of the various names, ages and equivalent names used by different authors to describe the Mesozoic metamorphic rocks of western Sulawesi Area Mamuju Quadrangle



Pasangkayu Quadrangle



Palu valley and neck of Sulawesi



Name Metamorphic Rocks



South Arm



Pre-Cretaceous, possibly Triassic



Author Ratman & Atmawinata 1993



Equivalent



Hadiwijoyo et al. 1993



Glaucophane Schist of the Pompangeo Complex Lariang River Crystaline Schist Group Gneiss and schist Sopu River Gneiss Gp. Palopo Formation Gneiss and schist None given



Simandjuntak et al. 1991a



Triassic



Hadiwijoyo et al. 1993



Gumbasa Complex



Triassic–Jurassic



Hadiwijoyo et al. 1993



Toboli Complex



Early Mesozoic or even Palaeozoic Emplacement to surface in the Neogene



Sopaheluwakan et al. 1995



E. Cretaceous schist, c. 111 Ma



Parkinson 1991, 1998



Latimojong Complex



Jurassic sandstone. E. Cretaceous schist 113–132 Ma E. Cretaceous,106 Ma 114 Ma, 123 Ma, 128 Ma



Wakita et al. 1996



Pompangeo Schists



Wakita et al. 1996 Bergman et al. 1996



Similar to Bantimala Similar to Pompangeo schist ages



Eclogites Pompangeo Complex



Bantimala Complex



Barru Complex Schist in the Latimojong Mountains



Author



Wana Complex



Wana Complex



Garnet peridotite



Central Sulawesi Malili and Poso Quadrangles



Age



OTCA 1971 Sukamto 1975 OTCA 1971 Sudrajat 1981 Sukamto 1975



Simandjuntak et al. 1991a, b; Parkinson 1998 Wakita et al. 1996



Wakita et al. 1996 Parkinson 1998



Table 2. Summary of the names, ages and equivalent names used by different authors to describe the Cretaceous rocks of western Sulawesi Area



Name



Karama region



Cretaceous sedimentary rocks



Mamuju and Pasangkayu Quadrangles Kalosi Region



Latimojong Formation



South Arm



Late Cretaceous late Campanian to early Maastrichtian Late Cretaceous



Latimojong Complex referred to Mesozoic as Mesozoic basement



Mesozoic



Balangbaru Formation



Late Cretaceous n.o.t early Turonian and n.y.t late Maastrichtian (about 90–65 Ma) Early to Middle Cretaceous Late Cretaceous late Campanian to early Maastrichtian Late Cretaceous, late Campanian to early Maastrichtian



Marada Formation



This study



Age



Cretaceous rocks



Author



Equivalent



Author



Latimojong Fm. in the Malili and Poso Quadrangles Tinombo Formation Latimojong Mountains and similar to the Balangbaru and Marada Formations



Ratman & Atmawinata 1993; Hadiwijoyo et al. 1993 Hadiwijoyo et al. 1993; Parkinson et al. 1998; Coffield et al. 1993



Wakita et al. 1996



van Leeuwen 1981



Considered as part of the Bantimala Complex Balangbaru Formation



Hasan 1990



This study



Marada Formation



This study



Chamberlain & Seago (1995) Ratman & Atmawinata 1993; Hadiwijoyo et al. 1993 Coffield et al. 1993



Hasan 1992



Wakita et al. 1996



n.o.t, not older than; n.y.t, not younger than



Upper Cretaceous rocks. The upper contact with the overlying Lisu Formation (Fig. 6) was crossed in both regions but was not seen. In the Karama River the formation passes laterally into and overlies the Middle to Upper Eocene Kalumpang Formation. Interpretation of the Toraja Group The Toraja Group records subsidence in the Lariang and Karama region (Fig. 7) from the Middle Eocene through to the



early Late Oligocene (NP15–NP24). There is no firm evidence for Paleocene to Lower Eocene strata in the study area. In the Karama region Palaeogene half-graben have been interpreted from seismic lines (Calvert & Hall 2003) and from surface data. The Kalumpang Formation and lower parts of the Budungbudung Formation were deposited in NE–SW-trending halfgraben formed in a period of extension during the Middle and Late Eocene (Fig. 7a). Mapping shows that the Toraja Group terminates at regions orientated NW–SE (Fig. 3), which may have been transfer zones between individual half-graben



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Fig. 4. Stratigraphic ranges of samples from the Toraja Group based on the stratigraphic range of the nannofossil or microfossil taxa that each sample contains (Norvick & Pile 1976; Evans 1991; Chamberlain & Seago 1995). The time-scale is from Harland et al. (1990).



segments. During the Middle to Late Eocene a variety of facies was deposited and these were the infill of elongate half-graben approximately 20–40 km long and 10 km wide (Fig. 2). The presence of conglomerates near the base of the Toraja Group suggests that faulting induced strong syn-depositional relief. The Middle to Upper Eocene strata include coal, mudstone and sandstone facies of the marginal marine/terrestrial Kalumpang Formation, which is interpreted as initial basin fill. During the Late Eocene, many localized limestone shoals developed across both the Lariang and Karama regions. This is interpreted as indicating a rise in relative sea-level brought about either by continued subsidence or by eustasy. Some of the limestone shoals may have been deposited on the crests of fault blocks, as



shown by Moss & Chambers (1999) for the Kutai Basin. During the Late Eocene, rifting ceased and thermal subsidence continued during the Oligocene. The half-graben were buried beneath mudstone and limestone of the upper part of the Budungbudung Formation and sedimentation was of similar character over most of the region. There is no evidence for limestone or coarse sand deposition in the area during the Oligocene. By the mid-Oligocene, mudstone was being deposited in an outer neritic shelf environment that was only marginally deeper than that required for shelf carbonate accumulation (Norvick & Pile 1976), i.e. 200 m or less. The top part of the Budungbudung Formation coincides with a midOligocene (c. 29.5–30 Ma) eustatic lowstand (Haq et al. 1987).



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Table 3. Lithofacies of the Toraja Group Facies code Mudstone FH



FI



FL



Lithofacies



Sedimentary structures



Homogeneous mudstone (claystone siltstone, marlstone), 10–4000 cm thick Interbedded mudstone (claystone, siltstone and marlstone), 20–2100 cm thick Laminated mudstone, 12–400 cm



Tops and bases non-gradational, massive with detrital organic material in places Massive and structureless



Centimetre and millimetre laminations



Interpretation



Exposed in areas



Suspension fallout possibly rapid and reworking by organisms



III, one sample; V, two samples



Alternating suspension fallout, rapid deposition



I, one sample



Fluctuating organic input during suspension fallout or fluctuating current velocities or boundary layer clay aggregates Suspension fallout from water column of mud and volcanic ash with slight reworking by current after deposition Oxidation of ferric minerals by hamyrolysis or before deposition in an environment with low rates of organic productivity



I, two samples



FIv



Interbedded mudstone and tuff, 4–160 cm thick



Massive with occasional undulating top surfaces



Fm



Calcareous mudstone with a distinctive red or purple-pink colour, up to 70 m thick



Light grey or yellow spots 1–4 cm diameter on surface. Massive



Fb



Mudstone with a distinctive chocolate brown colour, up to 20 m thick. Contains glauconite as scattered granular clasts



Interbedded with grey and yellow mudstone in places and has grey patches on the surface



Slow deposition rates and the presence of organic matter and fluctuating oxidizing conditions



Coal 10–600 cm thick



Thin clay wisps up to 5 mm thick, overall structureless



Vegetation deposited in shallow, stagnant, fresh-to-brackish water swamps



I, three samples



Composite sandstone or fine conglomerate and mudstone beds, 20–500 cm thick



Beds are generally massive and structureless. Faint horizontal burrows



I, one sample; II, two samples; III, five samples



Sw



Interbedded sandstone and mudstone with wavy stratification up 210 cm thick. Rusty brown coloured



SS



Lenticular dm sandstone, 60–400 cm thick, usually more than 200 cm wide



Laterally discontinuous sandy mass flow, pebbles at base are ‘bedload’ or ‘traction carpet’ features reflect ‘freezing’ of flow



G



Graded beds include grading from granules and pebbles, 10–180 cm thick Massive quartzose lithic arenite beds 10–800 cm thick, usually greater than 50 cm. At least 50 m wide. Shell nodules beneath one bed at KR42



Sandstone: faint and strong parallel lamination that, in places, pinch out and consist of detrital organic matter. Symmetrical ripples on top surface. Mudstone: wavy and lenticular laminations. Usually sharp base and top, massive and structureless concave-up base matrix-supported pebbles and granules at base Simple grading-scoured bases at a few outcrops



Sandstone deposited by rapid deposition from suspension or deceleration of sediment-laden current. Mudstone deposited by suspension fallout from a water column. Interpreted as wavy bedding. Formed in a standing body of water receiving periodic inputs of sand.



I, rusty-coloured and detrital coal less than 1 mm in three samples III, one sample



Coal C Sandstone I



SQK



Conglomerate Matrix-supported structureless CMm conglomerate, up to 10 m thick Limestone L



Limestone lithofacies, up to 15 m thick. Variable proportions of coral and shell debris and microfossils. KR9, 10: bioclastic wackestone/packstone. KR29: wackestone. DG27: mudstone with burrows. DG43, 44: shelly muddy limestone



Deposition by suspension fallout from a (waning) turbulent flow



III, one sample; V, one sample I, three samples; II, one sample; III, one sample; V, five samples I, one sample; III, four samples



I, two samples



Planar and scoured or undulating bases and planar or rippled top surfaces. Planar laminations and rip-up coal clasts near base. Rare bidirectional and unidirectional cross bedding



Erosive bases due to channel incision? Variable current direction. Standing fresh or marine water.



I, nine samples



Matrix-supported clasts, structureless



Cohesive debris flow deposit similar to the deposit of a ‘catastrophic’ high viscosity mass flow



I, one sample; II, one sample; V, one sample



None. Top and base not seen



KR9, 10: reworking and deposition on a shelf. KR29: inner shelf with low terrestrial input. DG27: suspension fallout environment with moderate energy nearby. Shelf. DG43, 44: moderate energy in deeper parts of the photic zone. Bioclastic limestone blocks have been sampled in a fault zone: samples DG20, 36, 39 and 42.



I, three samples; II, two samples; IV, two float samples; V, seven samples



The relative sea-level fall shown by Haq et al. (1987) is about 200 m, which is greater than the water depth estimated for the Lariang and Karama regions at this time. This sea-level fall



should therefore have exposed these regions and caused an input of coarse clastics, for which there is no evidence. This suggests that the sea-level fall was less than that predicted by



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Table 4. Lithofacies associations of the Toraja Group Association



Lithofacies included in this association



Comments



Interpretation



Marine facies association – Budungbudung Formation



FH Fm Fb FIv C; I G; CMm; L



Shallow-marine environment with variable amounts of sediment input



Marginal marine/terrestrial association – Kalumpang Formation



C Fb; SQK SS Sw; CMm



Found in all areas of Palaeogene exposure. CMm included here as it is interbedded with L and FH in the Northern Lariang region – Area IV. CMm is also along-strike to L in Area II. C and G only in type Area II Mainly Area I, where the association is called the Kalumpang Formation. CMm included here as it is seen in a succession in the Karama River but it is unclear if it is conformable.



Haq et al. (1987). Kominz et al. (1998) suggested that the magnitude of Cenozoic sea-level change was about half that estimated by Haq et al. (1987). Subsidence continued through the Miocene to Early Pliocene, with deposition of the Lisu Formation in a setting broadly similar to that of the Budungbudung Formation. Post-rift Lisu Formation The Lisu Formation (new name) is a sequence of interbedded mudstones, greywackes and pebbly greywackes. The type locality of the Lisu Formation is in the Budungbudung River (Fig. 3) in the Karama region. The formation is approximately 2000 m thick and has a late Early Miocene to Early Pliocene age based on nannofossils and foraminifera. The lowest part of the Lisu Formation is lithologically similar to the top of the Eocene– Oligocene Toraja Group and is dominated by mudstones. The contact between the Toraja Group and the Lisu Formation was crossed in two separate places 100 km apart. There is no evidence for an angular unconformity at either location and neither has an unconformity been identified in areas to the south (see Bergman et al. 1996). The similarity in facies of the upper Toraja Group and the lower Lisu Formation suggests that the contact is conformable (Fig. 6). The distribution and age of the lithofacies, combined with the petrographic data, is the basis for interpretation of the Lisu Formation. From the late Early Miocene to early Late Miocene mudstone was deposited on a shallow-marine shelf that extended across the whole study area, an environment that had persisted from the Late Palaeogene. Coarse volcaniclastic debris and thin tuff beds in the central Lariang region show that there was some Middle Miocene volcanic activity to the east. During the Late Miocene there was a substantial input of coarse sediment, indicating increased relief to the south and east of the study area. Sand was deposited by gravity flows onto the shelf, and some was derived from a volcanic centre to the south of the Karama region. Fission track and K–Ar dating of igneous rocks east and south of the Lariang and Karama regions (Bergman et al. 1996; Bellier et al. 1998) show that uplift and magmatism was occurring in these areas. However, there is no evidence for a great elevation or large input of sediment derived from an orogenic belt. Sections in the Lariang region are mud dominated, which may indicate some basement control on subsidence. Foreland basin deposits of the Pasangkayu Formation The Pasangkayu Formation is dominated by conglomerate and sandstone beds, with an increase in the proportion of mudstones close to the present coast. The formation is unconformable on Mesozoic and older Cenozoic rocks and is



Marginal marine/terrestrial environment that allowed the preservation of organic matter.



unconformably overlain by Quaternary alluvium and limestones. The type locality of the formation is east of a large hairpin bend in the Lariang River valley (Fig. 3), where Hundling (1942) mapped Pliocene ‘Celebes Molasse’. These sediments fringe the northern Quarles and eastern Molengraaff Mountains, but the term Celebes Molasse has been dropped here because it is poorly defined and there is considerable variation in the character and age of rocks assigned to the molasse across the island (see Calvert & Hall 2003). The Pasangkayu Formation has a latest Early Pliocene to Pleistocene age (based on foraminifera and nannofossils recorded by Hadiwijoyo et al. 1993) and a thickness of between 2000 m and 3500 m. The conglomerates are the product of deposition in alluvial fans that bordered and interfingered with alluvial plain and marine deposits (Fig. 7b). The alluvial fans reached the present-day coastline in the Karama region by the Late Pliocene (well data Karama-1S, Figs 2 and 3). Alluvial fan deposition was restricted to a valley in the Lariang region and sediment transport, dispersal and distribution reflect control by the major basement lineaments that can be traced offshore. Mapped outcrop patterns suggest that deformation in front of the present-day mountains controlled the deposition of these sediments in the Lariang region. Throughout, the sediments are fundamentally different to the older Cenozoic stratigraphy and record the erosion of a mountainous belt to the east of the study area. They record the oldest Cenozoic contractional deformational event to affect both the Lariang and Karama regions. Deformation was thick skinned, involving basement, and thrusts cut through the Lisu Formation, juxtaposing Cretaceous basement with Cenozoic strata. In places there is evidence for thin-skinned deformation, with folds trending N–S and plunging away from the uplifted basement highs (Fig. 8). As the region to the east rose rapidly, the long-lived Late Palaeogene–Neogene shelf with marine sedimentation was elevated and became emergent, and a major regional angular unconformity developed (Fig. 2). This is probably the same unconformity interpreted on the PAC 201 offshore seismic line by Bergman et al. (1996). Large quantities of coarse clastic sediments of the Plio-Pleistocene Pasangkayu Formation were carried west to be deposited in alluvial fans of foreland basins at the deformation front. Deformation continued throughout the Plio-Pleistocene. Existing folds above the Palaeogene halfgraben were tightened and elevated. STRUCTURE Mesozoic and Cenozoic structural trends Three trends in the orientation of structures have been identified through fieldwork and remote sensing analysis in the Lariang and Karama regions. All three (NE–SW, NW–SE



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Fig. 5. Schematic summary log of the type section of the Kalumpang Formation measured during fieldwork (Calvert 2000).



and N–S) are present in Cenozoic rocks but only the NE–SW and NW–SE orientations are observed in Mesozoic rocks. No predominant structural trend was identified in the PlioPleistocene Pasangkayu Formation. The NE–SW lineaments in the Lariang and Karama regions include formation contacts and have been mapped by previous workers (e.g. Sukamto 1973, 1975; Hadiwijoyo et al. 1993; Ratman & Atmawinata 1993; Chamberlain & Seago 1995). The NW–SE lineaments have been interpreted as strike-slip zones (see Sukamto 1975; Hadiwijoyo et al. 1993; Ratman & Atmawinata 1993) and ‘important vertical faults’ (Norvick & Pile 1976) and they are easier to identify in the Mesozoic and Palaeogene rocks. NW–SE lineaments that control the position



of the present-day Lariang River have been interpreted as linking to the east with the NNW–SSE Palu fault zone (Sukamto 1975; Norvick & Pile 1976; Wismann 1984; Sopaheluwakan et al. 1995). The NE–SW and NW–SE basement lineaments are also seen in the Mesozoic basement fabrics in the South Arm of Sulawesi (Hasan 1990), the Kutai Basin (Cloke et al. 1997) and the Meratus region of SE Kalimantan (Guntoro 1999). The Mesozoic rocks in western Sulawesi, particularly in the South Arm of Sulawesi, have close lithological affinities with Mesozoic rocks in eastern Kalimantan (Hamilton 1979; van Leeuwen 1981; Sikumbang 1990; Parkinson 1991; Moss & Chambers 1999). Palaeomagnetic data from both areas are also similar



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Fig. 6. (a) Geological map and (b) SAR image of the area near the conformable contact between the Lisu Formation (yellow) and the Budungbudung Formation (green), northern Karama region. The actual contact has never been seen and has always been inferred from remote sensing imagery. Images of the rocks near the contact are located on the SAR image and shown in (c), (d) and (e). (c) View SE towards cliffs topped by thick amalgamated mass flow deposits of the Lisu Formation. (d) View NE showing the top of the Budungbudung Formation dominated by mudstone up to 400 m thick containing a zone NP24 nannofossil assemblage. (e) View west towards the mudstone-dominated base of the Lisu Formation.



(Fuller et al. 1999). These data support the hypothesis that these areas were positioned closer together in the Late Cretaceous. The similarities in basement trends also suggest that these areas were affected by the same pre-Cenozoic deformational events. Mesozoic plate reconstructions (e.g. Audley Charles et al. 1988) show that western Sulawesi and parts of Kalimantan were juxtaposed and accreted to SW Borneo by subduction beneath the eastern margin of Sundaland. The accretion was nearly complete by the Late Cretaceous (Metcalfe 1996), after which these areas then effectively moved as a single block (McCabe & Cole 1987; Fuller et al. 1999). The three Cenozoic structural trends are seen in other areas surrounding the Makassar Straits. In the Kalosi region, just to the south of the study area, fold-and-thrust belts trend NE– SW, NW–SE and N–S (Coffield et al. 1993). In the South Arm the faulted northern margin of the Tonasa carbonate platform trends NW–SE (Wilson et al. 2000) and Miocene reefs trend NW–SE (Ascaria 1997). In eastern Kalimantan graben trend between N–S and NE–SW and are cut by NW–SE regional features (Kusuma & Darin 1989; Cloke 1997; Satyana et al. 1999). The NW–SE regional features can be traced offshore into the Makassar Straits and along with a NE–SW fault trend are apparent on seismic reflection and gravity data (Untung et al. 1983; Wismann 1984; Bransden & Matthews 1992; Wilson & Bosence 1996; Faroppa 1998; Cloke et al. 1999) and have Plio-Pleistocene movements (Samuel et al. 1996). Basement controls on the Cenozoic strata Because of the similarity in regional trends in both basement fabric and overlying Cenozoic strata, a basement control on Cenozoic strata has been invoked for areas adjacent to the Makassar Straits (e.g. van de Weerd et al. 1987; Letouzey et al. 1990; Moss et al. 1997; Guntoro 1999; Wilson 1999). In the



Lariang and Karama regions basement structures have also influenced the orientation of Cenozoic structures. Moving from north to south three transfer zones have been recognized (Fig. 3). Around the Budungbudung River the interpreted graben and half-graben into which the Toraja Group sediments were deposited terminate at structures orientated NW–SE. This is thought to represent a major transfer zone which was active during the initial rift stage. In the southern Lariang region there is a reactivated basement fault offsetting the basement front and influencing the course of the Karossa River. This fault may have controlled the southern limit of other Palaeogene graben and half-graben beneath the present-day foreland. The southern tip of the Doda surface structure (Fig. 8), changes orientation and is coincident with the basement offset, suggesting a basement control. Oil seeps here may indicate that the Eocene coals are present in the subsurface. This Neogene structure continues to the northwest, disappearing for a while beneath alluvium and muds of the Pasangkayu Formation, and reappearing as a structure that has affected the flow of the Lariang River and acted as a barrier to deposition of the Pasangkayu Formation. The full length of this Neogene structure is comparable to the length of Palaeogene basins interpreted in the southern Karama region and probably formed due to inversion of a Palaeogene half-graben in the subsurface. The northern tip of the structure terminates against the third major and most obvious transfer zone, which has controlled the flow of the Lariang River in the hinterland, links up with the Palu fault zone and can be traced offshore. There are two areas where the Cenozoic structural trend does not resemble the conjugate lineament set identified in the basement rocks. One is just south of the Budungbudung River (Fig. 3) where north–south-orientated folds of Palaeogene and Neogene strata plunge south beneath the Karama region. The



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other is in the northern Lariang region where, again, north– south-orientated folds of the Neogene Lisu Formation plunge south beneath the Lariang region. These folds, which form gentle hills, are directly adjacent to the transfer zones. They may be localized detachment folds (Fig. 8), where the Oligocene– Miocene mudstones are acting as a decollement. Norvick & Pile (1976) recognized detachment horizons in Lower Miocene mudstones. The folds formed during the Pliocene compression event, and the uplifted basement north of the transfer zones could have caused the folds to form and plunge south due to gravitational sliding. Oligocene mudstones also act as a detachment at the conjugate margin (McClay et al. 2000) and over pressured muds form diapiric structures. Quaternary limestones now at the surface, deformation of the Pasangkayu Formation, present-day seismic activity and GPS observations all indicate that deformation and uplift in the region is continuing at present (Walpersdorf & Vigny 1998). DISCUSSION Early Cenozoic plate readjustments in SE Asia resulted in extension and subsidence over a wide area (van de Weerd & Armin 1992). Much of eastern Borneo, western Sulawesi, the Makassar Straits and the East Java Sea was a region of Middle to Late Eocene extension, subsidence and basin formation. The oldest parts of the sequences that formed in the basins may be Paleocene (e.g. van Leeuwen 1981; van de Weerd et al. 1987; Wain & Berod 1989; Kusuma & Darin 1989; Satyana et al. 1999; Fraser & Ichram 2000) but this is very uncertain because the oldest parts of the sequences are typically terrestrial and poorly dated. By the Middle Eocene, extension was well underway and Eocene rocks are interpreted as syn-rift deposits of extensional graben and half-graben. Fig. 7. (a) Schematic depositional model for the Toraja Group. The group rests unconformably on Upper Cretaceous rocks and is separated into two formations: the thicker marginal marine/ terrestrial sedimentary rocks of the Kalumpang Formation and the more extensive marine sedimentary rocks of the Budungbudung Formation. The Palaeogene half graben were inverted during the Plio-Pleistocene. (b) Schematic depositional model for the Pasangkayu Formation. The formation was deposited in a foreland basin setting during the Plio-Pleistocene and is unconformable on older Cenozoic and Mesozoic rocks. Note the inverted Palaeogene halfgraben controlling sedimentation.



Basin fill The initial basin fill is varied and includes conglomerates derived from locally significant syn-depositional topographic relief as well as non-marine to marginal marine siliciclastic sediments and coals. Eocene extensional faulting is interpreted in certain deep parts of seismic sections across the Makassar Straits (Bergman et al. 1996) and in the Kutai Basin in eastern Borneo, directly west of the Lariang and Karama region (Cloke 1997; Moss & Finch 1997; Moss & Chambers 1999). In the



Fig. 8. Schematic cross-section C–C based on seismic line BP-90-09 near the Doda oil seeps in the southern Lariang region. The cross-section is 35 km long. Location of line is shown on Figure 3.



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Lariang and Karama regions the oldest Cenozoic sediments are at the base of the Toraja Group, where a marine transgression is recorded at the base of the Budungbudung Formation and dated as Middle Eocene. These rocks are similar in age and character to the oldest parts of the Kutai Basin (van de Weerd & Armin 1992; Moss et al. 1997; Moss & Chambers 1999). During the Middle–Late Eocene, marine sediments were deposited on both margins of the Makassar Straits rift. Further east from the straits terrestrial–marginal marine sediments were deposited (Kalumpang Formation). At both margins the initial basin fill was controlled by the development of fault-controlled topography and the geometry of graben and half-graben. Conglomerates were derived from faulted margins. Shelf sandstone and mudstone accumulated in the marine areas and isolated limestones formed on tilt-block highs away from the main sedimentary input points. Coal, quartzose sandstone and interbedded mudstone accumulated in the terrestrial–marginal marine areas. During the Late Eocene the initial fault-controlled relief was buried, and a low-lying topographic area is inferred to the east. Carbonate sedimentation had spread to all areas of western Sulawesi and extensive platforms developed in the South Arm (Wilson & Bosence 1996). During the Oligocene deeper-marine outer-shelf conditions were more extensive and mudstone was deposited, though carbonate platforms persisted in some areas of the South Arm. Sedimentological data from Palaeogene strata in the Kutai Basin of eastern Kalimantan indicate that they were deposited in deeper-water settings than strata seen on land in western Sulawesi and, by the Oligocene, parts of the Kutei were ‘bathyal’ (van de Weerd & Armin 1992; Chambers & Daley 1995; Moss et al. 1997; Moss & Chambers 1999; Feriansyah et al. 2000). Basin geometry Throughout western Sulawesi, Palaeogene successions are no greater than about 4000 m thick in the main depocentres (Evans 1991; Garrard et al. 1992). In the Lariang and Karama regions the Oligocene is about 400 m thick compared to Eocene strata which are between 1000 m and 3000 m thick. Throughout western Sulawesi, moving up section from the Eocene to the Oligocene, there is a decrease in volcanic material, a decrease in lithofacies types, an increase in carbonates during the Late Eocene, an increase in mudstone during the Oligocene and no conclusive evidence for extensive deepwater areas. These observations have two important implications. First, they show that by the Oligocene any land to the east of western Sulawesi was not highly elevated and was probably low lying and close to sea-level. Secondly, they show that sedimentation kept pace with subsidence that had decreased by the Oligocene. It would appear that following Eocene extension, fault-induced topography was buried and western Sulawesi became a stable shallow-marine shelf during the Late Eocene which deepened slightly in the Oligocene as the post-rift thermal subsidence phase started. The Palaeogene shelf in western Sulawesi may subsequently have been modified and removed by Late Neogene compression and uplift. However, it is believed that the eastern limit of the Palaeogene shelf cannot have been positioned any further east than the present-day ‘Median Line’ (Brouwer 1934; Brouwer et al. 1947), which is thought to have been a major Palaeogene structure (Parkinson 1991). This eastern limit lies no more than approximately 200 km to the east of the present coastline of the Lariang and Karama region. The distance from the former shelf to the deep central part of the Makassar Straits is much less than that from the



equivalent shelf in Kalimantan (Fig. 9). Not only do the basins in eastern Kalimantan record deep-water Palaeogene fill, they also cover a much wider area than those in western Sulawesi. The zone of early Cenozoic extension was. in fact. about twice as wide in eastern Kalimantan (c. 400 km, Cloke 1997; >100 km, Moss & Chambers 2000) than it was in western Sulawesi (Fig. 9, see also reconstructions in Wilson & Moss 1999). At the end of the Oligocene the Makassar Straits was approximately 800–900 km wide, which is considerably wider than the present-day width of 250–300 km. Palaeogene–Neogene contact The Palaeogene–Neogene section is difficult to date due to the lack of numerous key taxa (E. Finch, pers. comm. 2000). The upper part of the Toraja Group and the lower part of the Lisu Formation are lithologically similar as they are both made up of mudstones. The Early to Middle Miocene sea-level drop on the eustatic curve, which is reported from the basins of SE Asia (Longley 1997), may have exposed the type area and eroded Lower and Middle Miocene sediments, thus forming a disconformity between the Toraja Group and the Lisu Formation. This part of the stratigraphy is also relatively condensed in comparison to the younger Neogene and older Palaeogene. There could be a local detachment in the study area which could explain why it has been inferred by others that there is an unconformity at this level of the stratigraphy (e.g. Norvick & Pile 1976). However, there is no evidence for an early Neogene regional unconformity in any other parts of western Sulawesi (Grainge & Davies 1983; Coffield et al. 1993, 1997; Bergman et al. 1996; Guritno et al. 1996; Pertamina 1996; Wilson et al. 2000) and the contact has been interpreted here to be conformable (Fig. 2). Comparison with other rifted margins The timing of early Cenozoic basin development is similar throughout the Makassar Straits region on both margins of the North Makassar Basin. Extension started sometime in the early Cenozoic. Non-marine syn-rift sediments may date from the Paleocene, in which case the ‘pre-break-up’ extensional phase could have started as early as c. 60 Ma. However, the oldest dated syn-rift deposits in the study area and the Kutai Basin are Middle Eocene marine sediments, NP15 (48.1–43.9 Ma) and P11–P12 (47.2–41.4 Ma; Cloke 1997), respectively, suggesting extension started by about 50 Ma. The development of oceanic crust immediately north of the Makassar Straits basin in the Celebes Sea is estimated as starting around 44 Ma (magnetic anomaly C20; Hall 1996) but it is possible that older crust has been subducted at the Sulu trench. The oldest Cenozoic oceanic crust in the region may be younger than the oldest dated Cenozoic sediment. Thus, the pre-break-up extensional phase could have lasted for up to 15 Ma or could have been as short as 5 Ma. It is not clear if the oceanic crust propagated into the North Makassar Straits, as magnetic anomalies have not been identified using any potential field data. In the South Makassar Straits there is no evidence to suggest that rifting reached the stage of oceanic spreading (Burollet & Salle 1981; Situmorang 1982, 1984; Wismann 1984; Wilson 1999) and highly attenuated continental crust is inferred to underlie this basin (Wilson 2000). Both sides of the South Makassar Straits were shallowmarine carbonate platforms until the Early Miocene (Wilson & Moss 1999). Recent seismic studies suggest attenuated continental rather than oceanic crust in the central North Makassar Straits (Nur’Aini et al. 2005; Puspita et al. 2005). Whether the rift was abortive or successful (Ziegler & Cloetingh 2004), west of Lariang and Karama the rifting stage did last for a period of 5–10 Ma and the North Makassar Basin is situated between attenuated continental crust in the South Makassar Basin and oceanic crust in the Celebes Sea.



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Fig. 9. True scale cross-sections of the asymmetrical Makassar Straits at the present day and inferred during the late Palaeogene (based on information in Chambers & Daley 1995; Bergman et al. 1996; Cloke 1997; McClay et al. 2000; PAC 201 seismic line; and this study).



The difference in age between the oldest sediment in the Makassar Straits region and the oldest oceanic crust in the Celebes Sea is not dissimilar to that of small ocean basins. Stratigraphic data from the Woodlark Basin (Taylor et al. 1999), the Red Sea and Gulf of Aden (Hughes & Beydoun 1992) show that strain/rift localization and oceanic accretion can propagate within a zone that is already extending. Tectonic and stratigraphic data from many margins (e.g. Northwest Australia, Grand Banks, Iberia, West Africa, Brazil) document the existence of late stage regional subsidence (that may allow a transgression) accompanied by only minor brittle deformation just prior to continental break-up (Driscoll & Karner 1998). One of the unusual features of the Makassar Straits is the width of the zone of extension. Typically the width of the zone of extended continental crust is 50–150 km (Louden & Chian 1999), although it can be as large as 400–500 km (e.g. Orphan Basin, Keen et al. 1987; the northern Gulf of Thailand, Chantraprasert 2000). Removing the postulated oceanic crust of Cloke et al. (1999) and uniting the two sides of the North Makassar Straits show that the early rift basins formed within a comparatively wide zone of extended continental lithosphere of about 600 km width. If the central Makassar Straits are continental (Nur’Aini et al. 2005; Puspita et al. 2005), this would be still wider. It has been suggested that the broad zone of extension beneath the Makassar Straits (which one can now consider to include both the Kutei Basin and the Lariang and Karama regions) is a result of it being heated and weakened during Mesozoic accretionary events (Moss & Chambers 1999)



or of long-term subduction (Hall & Morley 2004). Rifting may have been aided by spreading and foundering of the crust after these events (cf. Letouzey et al. 1990). The Makassar Straits and its margins are also highly asymmetrical. The 600 km of extended continental lithosphere is subdivided into about 400 km on the west margin (eastern Kalimantan) and about 200 km on the east margin (western Sulawesi, Fig. 9). Early models of lithospheric extension (e.g. Buck et al. 1988) postulated either symmetrical rifting (pure shear) or asymmetrical rifting (simple shear), despite the fact that there are very few documented examples of a complete profile across both margins of rifts. This is because igneous extrusive rocks commonly obscure the deep margin structure (Louden & Chian 1999). However, in the southern Labrador Sea there is a complete set of profiles from both sides of a rift that are not obscured by igneous extrusive rocks. By interpreting this profile, and two less complete pairs from both sides of the North Atlantic, Louden & Chian (1999) have shown that continental break-up in these regions was asymmetric. They show that one margin has a broader zone of extended crust – which could be analogous to the Kutai Basin – than the other, which could be analogous to the Lariang and Karama regions of western Sulawesi. Conceptual models of asymmetric rifts by Ziegler et al. (1998) show that the plate margin with a narrow zone of extension, the upper-plate margin (analogous to western Sulawesi), is weaker than oceanic lithosphere and the conjugate margin with a broader zone of extension (the lower plate,



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analogous to eastern Kalimantan). They suggest that the upper plate margin (western Sulawesi) is the most likely candidate for compressional reactivation. Reactivation, which started in the latest Early Pliocene in most of Sulawesi, is also apparent from the new geological mapping described in this paper (Fig. 3). The contraction is related to the collision of the Banggai–Sula microcontinent with eastern Sulawesi. In western, central and the east arm of Sulawesi there are Plio-Pleistocene sedimentary rocks, often referred to as ‘molasse’ or ‘synorogenic sediments’, which are also related to the arrival of this fragment (e.g. Hundling 1942; Davies 1990; Coffield et al. 1993; Guritno et al. 1996; Purnomo et al. 1999; Sudarmono 2000). The Pliocene orogeny in Sulawesi resulted in development of the present-day mountains, which are up to 3 km high. Since the Pliocene, crustal movements in Sulawesi have been accommodated in a predominantly sinistral wrench-fault system that cuts through the whole island. These faults include reactivated NW–SE Palaeogene transform faults within the study area. CONCLUSIONS Two major unconformities were identified in this study of the Lariang and Karama regions of western Sulawesi: one between Upper Cretaceous basement rocks and Eocene shelf sediments, and a younger one between Lower Pliocene shelf sediments and Plio-Pleistocene syn-orogenic sediments. Non-marine sediments at the base of the lower Cenozoic sections could be as old as the Paleocene, but the oldest dated sediments are marine and record a transgression in the Middle Eocene that must post-date the initiation of rifting in the region. The Eocene sediments were deposited in graben and half-graben in both marine and marginal marine environments. The Makassar Straits Eocene rift was highly asymmetrical. The zone of extension of the western margin (Kalimantan) was approximately twice as wide as that of the eastern margin (Sulawesi). The post-rift subsidence phase had started by the Late Eocene. In the Late Eocene carbonate shoals and shelf mudstones developed on both margins of the Makassar Straits and, by the end of the Oligocene, most of western Sulawesi was an area of shelf carbonate and mudstone deposition. During the Early Miocene microcontinental fragments collided with the SE arm of Sulawesi but, in western Sulawesi, there is no evidence for a break in marine deposition in the South Arm. In the study area the lowermost Miocene has not been found but there is no evidence in western Sulawesi for either a significant regional unconformity, or input of orogenic sediment. Instead, throughout the Early Miocene and, in places, until the Middle or Late Miocene, carbonates and mudstones were deposited on a shallow-marine continental margin. Early Miocene collisions in eastern Sulawesi did not cause orogeny in western Sulawesi. It was only during the Pliocene that the character of sedimentation across the whole of western, central and eastern Sulawesi changed significantly. Uplift and erosion was followed by the deposition of coarse clastics derived from an orogenic belt to the east of the study area. To the west of the orogenic belt there was syn-orogenic sedimentation, inversion and folding above Palaeogene half-graben, detachment folding and thrusting, and the development of intra-basinal unconformities and minibasins. In the Late Pliocene the study area changed from a passive margin to a foreland basin setting and sedimentation rates doubled. The contractional deformation of western Sulawesi dates from the Pliocene, whereas that on the opposite side of the Makassar Straits in eastern Kalimantan began in the Early Miocene, indicating they have different causes. Inversion in Kalimantan was not driven by deformation to the east in Sulawesi.



The project on which this manuscript is based ran from 1996–2000 and was funded by Amerada Hess and the SE Asia Research Group at Royal Holloway supported by a consortium of oil companies. Logistical field support was provided by Mac Endharto, GRDC, Bandung, Indonesia. Unocal Indonesia are acknowledged for allowing a return to the field area in 2001. The authors thank Mike Cottam who helped greatly in the final preparation of the figures.



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Received 6 August 2006; revised typescript accepted 5 May 2007.