PERGAMON Journal of Asian Earth Sciences 17 (1999) 137±156

Structural controls on the evolution of the Kutai Basin, East

I.R. Cloke a, c, S.J. Moss a, d, J. Craig b

aUniversity of London SE Asia Research Group, Department of Geology, Royal Holloway, Egham Hill, Egham, Surrey, TW20 0EX, U.K. bLASMO PLC, 101 Bishopsgate, London, U.K. cMobil North Sea Limited, Mobil Court, 3 Clements Inn, London, WC2A 2EB, E-mail:[email protected] dRobertson Research Australia Pty Ltd, 69 Outram Street, West Perth, WA 6005, Australia

Received 10 January 1998; accepted 27 June 1998

Abstract

The Kutai Basin formed in the middle as a result of extension linked to the opening of the Straits and Philippine Sea. Seismic pro®les across the northern margin of the Kutai Basin show inverted middle Eocene half-graben oriented NNE±SSW and N±S. Field observations, geophysical data and computer modelling elucidate the evolution of one such inversion fold. NW±SE and NE±SW trending fractures and vein sets in the Cretaceous basement have been reactivated during the . O€set of middle Eocene carbonate horizons and rapid syn-tectonic thickening of Upper sediments on seismic sections indicate Late Oligocene extension on NW±SE trending en-echelon extensional faults. Early middle (N7±N8) inversion was concentrated on east-facing half-graben and asymmetric inversion anticlines are found on both northern and southern margins of the basin. Slicken-®bre measurements indicate a shortening direction oriented 2908±3108.NE±SW faults were reactivated with a dominantly dextral transpressional sense of displacement. Faults oriented NW±SE were reactivated with both sinistral and dextral senses of movement, leading to the o€set of fold axes above basement faults. The presence of dominantly WNW vergent thrusts indicates likely compression from the ESE. Initial extension during the middle Eocene was accommodated on NNE±SSW, N±S and NE±SW trending faults. Renewed extension on NW±SE trending faults during the late Oligocene occurred under a di€erent kinematic regime, indicating a rotation of the extension direction by between 458 and 908. Miocene collisions with the margins of northern and eastern Sundaland triggered the punctuated inversion of the basin. Inversion was concentrated in the weak continental crust underlying both the Kutai Basin and various Tertiary basins in whereas the stronger oceanic crust, or attenuated continental crust, underlying the Makassar Straits, acted as a passive conduit for compressional stresses. # 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Kutai Basin; Oblique extension; Inversion; Reactivation

1. Introduction The Mahakam Delta and the eastern side of the Kutai Basin have been most frequently targeted for The 60,000 km2 Kutai Basin of hydrocarbon exploration. Production began at the (Figs. 1±3) is the largest and the deepest (15 km) basin Sanga-Sanga ®eld in 1898 (Fig. 1) and the currently in (Rose and Hartono, 1978). In the proven in-place resources in the Kutai Basin are 15.81 region the section alone reaches a billion of barrels of oil equivalent with potential for thickness of up to 9 km. Tertiary sedimentation in the greater than 0.74 billion of barrels of oil equivalent basin has been fairly continuous since its inception in resources in the Palaeocene (Courteney, 1995) (Fig. 2 the middle Eocene, although the axis of deposition has and 3). To date, exploration has mainly focused on the moved eastwards with time and is now focused at the Neogene sediments, but the recognition of working mouth of the Mahakam Delta. petroleum systems within the succession has

1367-9120/99 $ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0743-9547(98)00036-1 138 ..Coee l ora fAinErhSine 7(99 137±156 (1999) 17 Sciences Earth Asian of Journal / al. et Cloke I.R.

Fig. 1. Geological Map of the Kutai Basin and surrounding margins, showing di€ering basement geology and structures. Location shown inset (Modi®ed from Moss et al., 1997). I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 139

Fig. 2. Synthetic Aperture Radar (SAR) image of part of the northern margin of the Kutai Basin showing basement structures and inversion structures within Tertiary cover. Interpretation shown below; for location see Fig. 1. 140 I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156

Fig. 3. En-echelon vein sets overprinting a crenulation cleavage, demonstrating two phases of tectonism in basement: (a) Interpretation of ®eld outcrop, demonstrating orient orientation of shear couple and conjugate nature; (b) Photograph showing orientation of veinsets, pencil shown centre right points north and is 10 cm in length. Also shows location of Fig. 3a; (c) Equal area stereonet projection summarises structural obser- vations, key is shown right. I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 141 resulted in a renewed interest in the onshore section of ment of ``Sundaland'' (the Schwaner Mountains) the Kutai Basin (Fig. 2). (Moss, 1998). The continental basement consists of This paper analyses the Paleogene and Neogene metamorphic, sedimentary, and granitic to gabbroic evolution of the Kutai Basin using a combination of rocks of Permo-Carboniferous to Cretaceous age (van geophysical and ®eld data to document the structural Bemmelen, 1949; van de Weerd et al., 1987; Pieters et controls on the evolution of the basin and the conse- al., 1987). Oceanic rocks range from chert and associ- quent implications for the regional stress regime. ated basinal sediments of Cretaceous to middle Eocene Synthetic Aperture Radar data were used to interpret age (Pieters et al., 1987) to deformed basalts and tecto- the basement fabric orientations and to interpret the nized harzburgites representing a range of di€ering en- structures identi®ed in middle Eocene and younger vironments of formation (ocean ¯oor to remnant basin-®ll. Interpretations were then extended to the ocean basin). Possible ophiolites have been identi®ed sub-surface using gravity and seismic data. Isochron throughout Kalimantan (e.g. van de Weerd and maps generated from the interpretation of regional Armin, 1992), but outcrop studies show these are typi- seismic lines recorded across the northern margin of cally highly deformed and none contain a full ophioli- the Kutai Basin were used to demonstrate spatial tic sequence. changes in the location of middle Eocene and late Brittle-ductile shear zones are common on the north- Oligocene depocentres. Section reconstructions of geo- ern margin, with dominant orientations NW±SE and seismic lines were used to demonstrate the middle NE±SW. Many shear zones show several phases of Eocene basin geometries. Field data gathered from movement evidenced by overprinting of quartz veins, both basement and Tertiary rocks is used to re®ne in- and reactivation and breaching of shear bands. The terpretations made from the geophysical data. The axes of chevron folded cherts are oriented NE±SW. ®nal part of this paper integrates the basin history into NW±SE aligned lineaments have previously been the regional plate framework and discusses impli- identi®ed throughout , using remote sensing cations for the evolution of the eastern margin of data, and have been used as evidence for a through- Sundaland. going Trans-Kalimantan shear zone, connected to any one of a number of major strike±slip faults (Wood, 1985; Hutchinson, 1989; Daly et al., 1991; Tate, 1992). 2. Basement geology and structure The connection of these structures to other regional features and extension of Asian crustal blocks have Tertiary rocks of the Kutai Basin rest with angular been disputed more recently by Cloke et al. (1997) and unconformity upon a basement of Mesozoic and older Moss et al. (1997) who have shown that NW±SE linea- strata. The Basement of Kalimantan is classi®ed as ments are in fact an inherited basement fabric and not any strata older than the middle Eocene, and makes simply the result of Tertiary ``extrusion tectonics'' as up predominantly the highlands of interior envisaged by Tapponnier et al. (1982) and Daines Kalimantan with a few isolated inliers (Fig. 1). Apart (1985). from the regional geological reviews by Van Bemmelen The southern margin of the basin consists of the (1949), Cartier and Yeats (1973), Hutchinson (1989) NNE±SSW oriented Meratus Ophiolite Complex and more lately by Moss et al. (1997), the greater part (Sikumbang, 1990), which has also previously been of detailed work has been focused on small isolated classi®ed as melange (Hamilton, 1979; Hutchinson, areas (Rose and Hartono, 1978; Pieters et al., 1987; 1989). The ophiolite has been proposed to be derived Doutch, 1992; Lumadyo et al., 1993), particularly from a back-arc basin obducted onto the eastern mar- areas of oil company interest (Gruneisen et al., 1983; gin of Sundaland during the early Cretaceous Gardner et al., 1983; van de Weerd et al., 1987; Wain (Sikumbang, 1986) and subsequently uplifted in the and Kardin, 1987; Wain and Berod, 1989) and of min- late Miocene (Rose and Hartano, 1978, van de Weerd eral interest (Bergman et al., 1988; Simmons and et al., 1987; Hutchinson, 1989) or Plio-Pleistocene (van Browne, 1990; van Leeuwen et al., 1990; Felderho€ Bemmelen, 1949). This complex on the southern mar- et al., 1996). These reports give di€erent names for gin of the basin is laterally o€set along strike±slip similar sedimentary and igneous formations and high- faults oriented NW±SE. This results in the juxtaposi- light the diculty of correlation of formations due to tion of Aptian to Turonian age sedimentary rocks their highly faulted and dismembered nature (Moss against volcanics. Outcrops of gabbroic and and Chambers, 1998). ultrabasic rocks on the northern margin of the Kutai Basement consists of a number of rock types of Basin have led some authors to postulate that the di€erent ages exposed on the northern, western and Meratus Complex extends north beneath the Kutai southern margins of the Kutai Basin (Fig. 1). To the Basin (Albrecht, 1946; Sikumbang, 1990). The simi- north and west the basement consists of both forearc larity between the basement units of the northern mar- and remnant ocean basin ®ll and the continental base- gin of the Kutai basin (e.g. Sungai Selec and Belayan 142 I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156

(Fig. 1) Cloke, 1997) and the Pre-Tertiary forming of orientations NW±SE and NE±SW (Cloke et al., 1997). the Meratus Complex suggests that this is essentially Analysis of structures identi®ed on the northern mar- continuous with the northern margin and was sub- gin is used here to demonstrate multiple deformational sequently uplifted during middle Miocene inversion. episodes (Fig. 3a and b). Two orientations of cleavage cut folded and foliated phyllites. In addition, semi- brittle shear zones with accompanying quartz veins 3. Basement orientations identi®ed using synthetic and centimetre scale dextral o€set faulting overprint airborne radar (SAR) and from ®eld data both sets of cleavage. S1 axial planar cleavage is as- sociated with fold axes oriented 1708 and plunging 298 The presence of a basement fabric inherited from towards 1608±1708 (Fig. 3c). S2 cleavage, associated previous episodes of tectonism can exert considerable with the en-echelon conjugate sigmoidal quartz veins, control on subsequent deformation. One method of is oriented 0948/428 and approximately bisects the recognising this is by the use of remote sensing data. shear couple that generated the vein sets. One set of Interpretations of the fracture orientation are then conjugate semi-brittle shear zones is aligned 1218 and integrated with other data, for instance gravity and indicates a dextral sense of movement (Fig. 3a); the magnetics, to distinguish between near surface and other set is aligned 0088 and indicates a sinistral sense deep-seated structural features. of movement. The sense of movement on the shear Basement in Borneo is distinguished on remote sen- zones suggests that they are unrelated to and post-date sing data by its coarse re¯ective nature. It forms varie- the folds. Late stage overprinting of the en-echelon gated highland areas with aligned valleys and hill vein system is indicated by dextral movement on verti- crests (Fig. 2). Lineaments identi®ed in pre- base- cal zones oriented 0628 (Fig. 3). ment and post-rift sedimentary sequences show di€er- Joint analysis of data collected from the upper ing orientations and abundances. Basement lineaments Sungai Mahakam (Fig. 4) subdivided into a high dip are predominantly oriented NW±SE and NE±SW with (greater than 608) and low dip (less than 608) demon- a more weakly developed NNE±SSW fabric (Fig. 2). strate two very di€erent modal distributions. High dip NW±SE lineaments consist of predominantly short joint data (Number = 24, Fig. 4a±c) exhibit a single fractures cutting through basement strata. Our ®eld component mode oriented 1358, with minor trends studies have identi®ed dominant fractures, duplex 0208 and 0508. Low dip joint data (Number = 13, shear bands and S-C fabrics, with multiples phases of Fig. 4d±f) exhibit dual components oriented 0308 and reactivation, oriented NW±SE which agrees with ear- 0508. The orientation of high dip data is an inherited lier work by Wain and Berod (1989). fabric, whereas the 0308 and 0508 low dip implies the NE±SW lineaments crosscut the NW±SE direction, existence of a compressional or transpressional stress but a lack of identi®able o€set features makes it im- regime (Wain and Berod, 1989) of later (middle possible to establish sense and age of displacement. Miocene to present day) age. Low dip joints develop The NE±SW orientation has previously been identi®ed orthogonal to the principal compressive direction, in Sarawak by Tate (1992). Locally this trend re¯ects which is interpreted to have been oriented between the faulted boundary between basement and Tertiary WNW±ESE to NNW±SSE. The implication of the cover (Sabins, 1983; Felderho€ et al., 1996). Fractures two modal low dip orientations is discussed at a later and tight chevron folds within basement cherts on the stage. Orientations identi®ed using ®eld data agree northern margin of the basin parallel this structural broadly with the observations of Gruneisen et al. grain. An additional trend oriented NNE±SSW is (1983), Wain and Berod (1989) and borehole breakout weakly developed within the basement and forms con- studies in the Mahakam delta (Ferguson and McClay, tinuous lineaments cross-cutting early structures. It is 1997; Busono and Syarifuddin, 1999). most clearly developed within the volcanic plateaux of the Plio-Pleistocene Metulang volcanics suggesting it is a late formed trend (Pieters et al., 1987). Late 4. Basin architecture utilising gravity, seismic and Cretaceous granitic dykes encountered on the Sungai remote sensing data Tabang (Fig. 1) trending NW±SE, cross-cut highly sheared basaltic rocks and are o€set by cm-scale exten- In the Kutai Basin, seismic data have been inte- sional faults aligned NNE±SSW (Moss et al., 1997). grated with interpretations of the surface structure de- The paucity of features related to the NNE±SSW fab- rived from remote sensing, gravity data and geological ric within the basement suggests this fabric may be the data recorded along seismic lines. The Palaeogene result of later tectonic activity. basin architecture of the Kutai Basin is poorly under- Field investigations have identi®ed numerous brittle- stood, due to the poor seismic resolution of syn-rift ductile shear zones in the basement rocks on the sediments on the data, poor and remote exposure of northern margin of the Kutai Basin, with dominant Eocene and Oligocene age sediments, Neogene inver- I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 143

Fig. 4. Fracture orientations within basement, gathered from ®eldwork on the Upper Sungai Mahakam. (a) Great circles to high angle (>608) joints; (b) Kamb contour plot of high angle (>608) joints; (c) Rose plot of high angle (>608) joints; (d) Great circles to low angle (<608) joints; Kamb contour plot of low angle (<608) joints; (e) Rose plot of low angle (<608) joints. 144 I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 sion and a lack of borehole data. This poor under- zone of late Oligocene age identi®ed on seismic data in standing of the extensional basin architecture has led the sub-surface. This fault zone is orthogonal to the to a number of models for basin evolution, such as a Eocene fabric and has importance for the stress regime trans-tensional origin (Daines, 1985; Biantoro et al., during this period. Residual Bouguer Gravity data can 1992) or a ¯exural origin (van de Weerd and Armin, be used to show structures within the Tertiary cara- 1992; Bergman et al., 1996) that are not supported by pace and, although not shown on Fig. 5 for brevity, direct data. In order to understand the basin evolution con®rms the presence of the KLGH as well as narrow it is necessary to focus on the northern and southern linear zones of both NW±SE and NNE±SSW orien- margins of the basin where Palaeogene sediments tations within the sedimentary cover. Positive Gravity (Fig. 1) are exposed. anomalies observed on the northern margin of the Kutai Basin (Fig. 5) correlate with asymmetrical sur- face anticlines, established to be a result of Neogene 5. Gravity data inversion of middle Eocene half-graben. These positive gravity anomalies are a combination of uplifted high Density models were used to constrain seismic in- density syn-rift sediments and shallow basement rocks terpretations of the structure of the northern margin (Cloke, 1997). of the basin in areas of reduced seismic clarity and A regional density model created for a 2D cross-sec- also to construct gravity maps for parts of the region. tion crossing from west of the KLGH (Fig. 5) across The data o€er the opportunity to interpret the struc- the Mahakam Delta to the thrust belt of West ture of the basin in three dimensions. Bouguer gravity Sulawesi demonstrates that the KLGH can be success- data across the Kutai Basin have previously been used fully modelled as folded high density (+2.57 mGal) to show density models of structural pro®les across the Palaeogene strata (Chambers and Daley, 1995; Cloke, basin and to postulate the location of structures within 1997). This fold developed as the result of inversion of the basin (Sunaryo et al., 1988; Wain and Berod, 1989; a deeply buried Palaeogene half-graben. The narrow- Chambers and Daley, 1995; Cloke, 1997). In addition, ness of the high is accentuated by the presence of low the regional gravity signature has been used to demon- density (+2.15 mGal) syn-inversion synclines present strate that the Kutai Basin has an extensional rather to the east and west of the high. than a foreland origin (Moss et al., 1997). Gravity models of seismic sections that intersect the Makassar Straits suggest the presence of attenuated continental 6. Remote sensing data crust and possible oceanic crust (Guntoro, 1995; Cloke, 1997). Landsat (1993) TM scenes using bands 1, 4 and 7 The Bouguer gravity values of the basin are all (courtesy of LASMO PLC) have been processed to slightly to moderately positive, with the exception of image the regional structure, and high resolution SAR the prominent highs described below, which are images (courtesy of LASMO PLC) have been used to strongly positive. Three positive gravity highs, oriented interpret the Tertiary basin ®ll of this section of the NNE±SSW, are apparent in the regional Bouguer Kutai Basin (Fig. 2). The remote sensing data enable a Gravity for part of East Kalimantan (Fig. 5). The regional interpretation of the Gongynyay and Gergaji most prominent of these highs is the Kutai Lakes fold belt (Fig. 2) to be made. The importance of this is Gravity High (KLGH) with an amplitude of about 40 to reconstruct the Paleogene basin architecture and mGal (Fig. 5). The Kedang Kepala High has a understand the interaction between the various base- broader anomaly of approximately 54 mGal and is ment fabric orientations and the extension direction. situated to the north±northeast of the KLGH, o€set We use a portion of processed ERS-1 data across the by about 10 km in a right lateral sense by a NW±SE northern margin to demonstrate a number of struc- oriented gravity low identi®ed as the Belayan Axis tural features (Fig. 2). (Fig. 5). To the north, the Kedang Kapala High is o€- The eastern portion of the data shows two west ver- set from another positive anomaly by another linear gent asymmetric anticlines (Fig. 2) each of approxi- gravity zone named the Kedang±Kepala Axis mately 70 km length, which form a prominent series of (Sunaryo et al., 1988). The two main lows range in hills trending 0258 (Fig. 2). Both folds are dissimilar to value from 0.43 to 21.02 mGal. Magnetic data cover- those encountered in the Neogene section of the Lower ing the region also indicates an increase in depth to Kutai Basin (detachment foldsÐChambers and Daley, magnetic basement from north to south across the 1995, or diapiric folds) in that they are not laterally Kedang±Kepala Axis (Sunaryo et al., 1988). This grav- continuous, tight in geometry, or separated by broad ity and magnetic anomaly corresponds with the synclines. In addition the anticlines shown in Fig. 2 Bengalon lineament (Moss and Chambers, 1998) vis- are doubly plunging (closing to NNE and SSW), re- ible on SAR data and also a linked en-echelon fault semble a whaleback morphology and have linear wes- I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 145

Fig. 5. Bouguer gravity data covering the Kutai Basin showing gravity anomalies within the basin. Colour scale shown bottom. Location on Fig. 1. 146 I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 tern sides suggestive of fault control. Instead, the structures are considered to resemble inverted half-gra- ben, and initial stratigraphic interpretations predict that the axes of these folds contain middle Eocene syn- rift sediments. Subsequent ®eldwork, although showing the Gongnyay and Gergaji fold belt (Fig. 2) to be structurally complex, proved this analysis to be essen- tially correct. The fold axes and thrust fronts are sinis- trally and dextrally o€set on a number of NW±SE trending linear fractures parallel to those observed within adjacent basement (shown in Fig. 2(b)). Fold morphology changes across the fractures, from an asymmetrical west-vergent anticline to a monocline (Fig. 2). Igneous intrusions of possible late Oligocene age lie on trend with the fractures. Similar o€sets of fold axes have been observed in the Samarinda Anticlinorum (A. Ferguson, personal communication, 1996). The o€sets may be partly controlled by base- ment fabrics. An asymmetrical anticline is observed to the west of the village of Muara Wahau (Fig. 1) and this may be traced closing towards the north. Its eastern margin coincides with a lineament that penetrates basement. This fold is east-vergent and although not as promi- nent as those previously described, is cored by late Eocene sediments, indicating it may also be an inverted half-graben. In addition to the folds described above, another im- portant observation is the presence of rhyolite dykes tentatively assumed to be of middle Eocene age (S.J.M. ®eld observations) (Fig. 2) trending NNE± SSW. The dykes are parallel to the major faults form- ing the boundary to the Gongnyay and Gergaji half- Fig. 6. Interpretation and structural reconstruction of a seismic line graben. through a middle Eocene inverted half-graben. (a) Present day sec- tion with dip information, topographic and pin line shown; (b) Restoration to late Oligocene horizon; (c) Restoration to middle Eocene syn-rift deltaics; (d) Restoration to middle Eocene syn-rift. Location shown on Fig. 2. 7. Seismic data 8. Eocene structural architecture Interpretation of seismic data across the northern margin of the Kutai Basin and the locii of particular The isochron map at Eocene level demonstrates the sedimentary packages can be used to build up an presence of two depocentres developed in the hanging understanding of the middle Eocene and Oligocene wall of major extensional faults (Fig. 7). To the east is basin architecture. The present day basin structure has the Gongnyay half-graben which is dissected further been substantially modi®ed by early-middle Miocene (N7±N8) inversion (Chambers and Daley, 1995; into two depocentres by the overlap of the Gongnyay Tanean et al., 1996; Cloke et al., 1997) but the restor- and Gergaji faults. These depocentres are now inverted ation of deformed geoseismic sections reveals the mor- and correspond to the anticlines shown on Fig. 2 and phology of cross-sections through these depocentres. are discussed above. Further to the west is the oppos- Maps generated for the total Eocene and also ing polarity, west-facing Wahau half-graben. Oligocene isochrons show the basin con®guration of Interpolation and gridding of the isochron map cover- the northern margin of the Kutai basin at these ing the Wahau half-graben is not possible, due to a speci®c times whereas Fig. 6 shows the structural res- lack of seismic data, and should therefore be viewed toration of a geoseismic section demonstrating an somewhat speculatively. Interpretations made from the inverted middle Eocene half-graben (location shown isochron map agree with residualised gravity and SAR Fig. 2). data, thus increasing the degree of con®dence. The for- ..Coee l ora fAinErhSine 7(99 137±156 (1999) 17 Sciences Earth Asian of Journal / al. et Cloke I.R.

Fig. 7. Isochron map of Eocene sequence derived from seismic grid on northern margin of the Kutai Basin. Colour scale shown top right, location shown Fig. 1. 147 148 I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 mer depocentres trend NNE±SSW and the latter almost Eocene deltaic sequence is restricted to the axis of the N±S. Gongnyay and Gergaji half-graben and abrupt thinning The fault map for the Eocene was derived from takes place at the basin bounding faults to the west and changes in isochron thickness. The orientations of faults more gradually to the east on to a ¯exural shoulder. were then integrated with other observations (e.g. trend This asymmetrical style of sedimentary thickness vari- of middle Eocene rhyolite dykes) to interpret the exten- ation is a common feature of the syn-rift in®ll of exten- sion vector for the middle Eocene rifting on this margin sional half-graben in the East African rift (Rosendahl et of the Kutai Basin. High angle reverse faults identi®ed al., 1986) or West Natuna Basin (Ginger et al., 1993). on seismic sections and in remote sensing data were Further restoration to the syn-rift ®ll (rift initiation assumed to be reactivated extensional faults. This phase of Phillips et al., 1997) demonstrates a single assumption has been shown to be valid for similar tec- asymmetrical half-graben formed in the hanging-wall of tonic con®gurations identi®ed throughout the Sunda the Gongnyay fault (Fig. 8d). Abrupt thinning of the shelf (Letouzey et al., 1990). The overlap zone between syn-rift sequence takes place to the west at the basin the Gongnyay and Gergaji half-graben is deformed by a bounding fault and more gradually at the eastern ¯ex- series of NE±SW trending faults that are interpreted to ural margin. be the result of the progressive rotation of the relay The fault pattern in the western portion of the data is ramp between the two major faults (Peacock and constrained by only two seismic lines, but by combining Sanderson, 1994). The ¯exural shoulder that forms the gravity (Fig. 5) and SAR data (Fig. 2) the pattern was eastern margin of the graben is cut by a number of expanded away from this data. Seismic data show an small scale faults, oriented NNE±SSW and arranged in anticline that lies above the thickest depocentre. This a right-stepping en-echelon manner. anticline is observed at the surface and closes to the The line interpretation (Fig. 8) shows two anticlines north and south of the line (Fig. 2). The eastern bound- that are interpreted to lie above middle Eocene depo- ary of the anticline lies above the shoulder of the half- centres identi®ed on the northern margin (location graben. In plan view the half-graben is bounded by a shown Fig. 2). Both west-vergent anticlines are bounded series of left-stepping en-echelon faults that downthrow by steep reverse faults. The footwall of the fault bound- to the west. The en-echelon nature of the fault segments ing the Gongnyay half-graben forms a complex series of is interpreted from the gravity data (Fig. 5). These show extensional faults (now reverse faults). Although the ob- a series of segmented en-echelon gravity highs. The servations are limited to this inversion fold they can be half-graben bounding fault is interpreted to extend up applied to other middle Eocene depocentres identi®ed to and penetrate the basement lying to the north (Fig. 2). Previous interpretations, made without the throughout the Kutai Basin. bene®t of seismic data have interpreted this structure as Restoration of the line interpretation shown in Fig. 8b a strike-slip fault of unknown age with approximately to the late Oligocene horizon shows the pre-inversion 20 km of dextral o€set (Moriya and Nishidai, 1996). structural architecture. Estimates for the amount of eroded section and therefore the maximum depth of burial were derived from Apatite Fission track (AFT) results (peak temperature of 908 reached at 03km 9. Late Oligocene basin architecture depth prior to rapid denudation) and therefore uplift (A. Carter, personal communication, 1997) at 23 Ma The period from the late Oligocene to the end of the (Moss et al., 1998), vitrinite re¯ectance of middle Lower Miocene is an important but poorly understood Eocene coal (%R0 = 1.1) layers and the measured dip stage in the history of the basin, and indeed of the of outcrops. The restoration demonstrates signi®cant region as a whole. In this area of Kutai Basin, changes thickening of middle Eocene syn-rift sediments across in basin architecture are related to late Oligocene exten- major extensional faults and changes in Oligocene thick- sion on a set of faults orthogonal to those active during ness across smaller faults. Similar changes in Eocene the middle Eocene (Moss and Chambers, 1998; Cloke thickness across extensional faults have been observed 1997). In other sections of the basin the `late-Oligocene by Satyana and Biantoro (1995), further to the east of unconformity' is related to renewed uplift of the basin the area of study, and have also been interpreted on margin (Satyana and Biantoro, 1995; Moss et al., 1998) proprietary seismic data across the Teweh High (Fig. 1) and compression and folding (van de Weerd et al., (now recognised as an inverted half-graben). 1987; Wain and Berod, 1989; van de Weerd and Armin, Restoration to the middle Eocene deltaics (equivalent 1992). to the rift climax phase of Phillips et al., 1997) demon- Interpretation of seismic sections and the location of strates that the rift consists of two asymmetrical syn- particular sequences were used to build up an under- thetic half-graben formed in the hanging-wall of the standing of the late Oligocene basin architecture on the Gongnyay and Gergaji faults (Fig. 6c). The middle northern margin of the Kutai Basin. Changes in iso- ..Coee l ora fAinErhSine 7(99 137±156 (1999) 17 Sciences Earth Asian of Journal / al. et Cloke I.R.

Fig. 8. Isochron map of Oligocene sequence derived from seismic grid on northern margin of the Kutai Basin. Colour scale shown top right, location shown Fig. 1. 149 150 I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 chron thickness were also used to interpret the position inversion': i.e. the expulsion of the syn-rift sediments of late Oligocene faults, the majority of which have not from the axis of the half-graben by the reverse reactiva- been re-utilised as reverse faults during inversion, tion of the major `Eocene trending' extensional faults. suggesting a directional control on inversion of faults The dating of the earliest inversion event is poorly con- (Fig. 8). In addition, the orientations of faults, integrated trolled, with a number of timings proposed (N7±N8 with other observations (e.g. mineralised veins and dyke Cloke et al., 1997; 15.5±14.5 Ma Tanean et al., 1996; orientations) were used to predict the rift kinematics for 11.8 Ma van de Weerd and Armin, 1992). The tran- this period of minor extension on the northern margin of sition from a carbonate-dominated environment to a the Kutai Basin. coal-dominated deltaic environment during the early The isochron map (Fig. 8) shows the NW±SE align- middle Miocene (N7±N8) has been used by Cloke et al. ment of the Oligocene depocentres. In the northern part (1997) to constrain the timing of inversion on the north- of the map, a series of NW±SE trending en-echelon ern margin of the Kutai Basin. Further periods of extensional faults, down-throwing to the north, form renewed inversion at 7.5, 5.5, and 3 Ma have been inter- the southern margin of a late Oligocene basin. In the preted from the increase in volcanic fragments in sand- southern part of the map a similar set of parallel en- stones forming the reservoirs to a number of oil ®elds echelon faults, in this case down-throwing to the south, (Tanean et al., 1996) and freshwater ¯ushing of reser- form the northern margin to a similar aged basin. voir rocks (Duval et al., 1992). Given the diachroneity LANDSAT TM data shows a distinct surface lithologi- of inversion observed in other sedimentary basins it is cal boundary along the southern fault zone between likely that the inversion has been caused by a number karsti®ed carbonates and shales (Moriya and Nishidai, of punctuated compressive episodes rather than a single 1996) of late Oligocene±early Miocene (Moss and event (Chambers and Daley, 1995). Chambers, 1998; Wilson et al., 1999) age, suggesting Field measurements of movement indicators (2908± some degree of fault control on deposition. A thicken- 3108) in the northern section of the basin (Fig. 9) indi- ing of the Oligocene sequence is also documented across cate compression tangential to the main Eocene NW± the Bengalon lineament (Moss and Chambers, 1998). SE trending faults, but oblique to the re-activated NE± Further to the east, an increase in depth to magnetic SW and NW±SE basement faults (Cloke et al., 1997) basement southwards across the fault zone has also (Fig. 9). This agrees approximately with borehole been observed (Sunaryo et al., 1988). Seismic data show breakout studies in the onshore section of the no evidence for displacement on a single master fault, Mahakam Delta which suggest a present day stress and in the southern section of the map faulting is regime oriented NNW±SSE (Ferguson and McClay, spread over a wider zone, as a more complex system of 1997; Busono and Syarifuddin, 1999). Seismic sections interconnecting en-echelon faults. Crustal ¯exure also across this northern margin (Fig. 6) show large anticli- appears to play a role in the increasing depth to base- nes which have been heavily reverse faulted. These are ment to the south. Gravity data show a Bouguer gravity asymmetric, with heavily faulted western limbs, steeper minimum of 15 mGal (Fig. 5) aligned NW±SE associ- than eastern limbs, indicating western vergence of the ated with this fault zone, whilst magnetic data shows an folds. Analogue modelling has shown the complications increase in the depth to magnetic basement across the that develop with inversion of an existing extensional fault zone (Sunaryo et al., 1988). The Adang Fault half-graben (McClay, 1989; McClay and Buchanan, Zone bounding the Kutai Basin to the south (Fig. 1) is 1991; Eisenstadt and Withjack, 1995). interpreted to have been active from this period Syn-inversion sedimentation shows a number of onwards. Free-air gravity data (Cloke, 1997) suggest characteristic features. In this section of the basin it is that the fault zone consists of a number of en-echelon recognised by a change from Lower Miocene deep segments, rather than a linear fault zone as convention- water clastic turbidites, with carbonate deposition on ally interpreted. highs, to a terrestrial-dominated marginal marine deltaic environment (Carter, 1994) which may be identi®ed on seismic sections onlapping onto the underlying structure 10. Middle Miocene to present day (Cloke, 1997). Recognition of the early-middle Miocene (N7±N8) deltaic sequence (Fig. 6) as a syn-inversion The present day structural architecture of the basin is sequence provides evidence for the earliest event. the result of substantial modi®cation of the combined Eocene and Oligocene basin architecture by compressive forces from the Lower Miocene to the present day. The 11. Reconstruction of the basin history and the precise orientation of this stress regime is still disputed importance of structural controls and will be discussed later. Both the southern (Teweh half-graben; Fig. 1) and Reconstruction of the pre-inversion structural archi- northern margins (Fig. 2) display evidence of `basin tecture of the Kutai Basin demonstrates that middle I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 151

Fig. 9. Slicken®bre directions for Miocene, Eocene and Oligocene thrusts, identi®ed on northern margin of the Kutai Basin, plotted on an Equal area stereonet projection.

Eocene rifting led to the development of a number of identi®ed Permo-Triassic dykes trending N or NNW, discrete asymmetrical half-graben that switched polarity approximately parallel to the boundaries of the Permo- along strike (Fig. 10a). This polarity change was accom- Triassic basin (Faerseth, 1996). Rhyolite dykes ident- modated either via hard linkage, as a result of reactiva- i®ed on the northern margin of the Kutai Basin (Fig. 2) tion of the NW±SE basement fabric (Cloke et al., are parallel to the fault traces of the Gongnyay and 1997), or soft linkage. Both bounding faults were Gergaji faults (Fig. 10a). Similar dyke orientations have oriented NNE±SSW and N±S, overlapped in an en- been observed elsewhere within the basin. These intru- echelon manner (Figs. 7 and 10a) and were initially sep- sives indicate the palaeo-stress direction during the arated by relay ramps (e.g. Gongnyay and Gergaji half- middle Eocene rift event and that the extension direc- graben; Fig. 6). tion was orthogonal to the strike of the major faults, The en-echelon and o€set nature of fault segments is but slightly oblique to the NE±SW trending basement a common feature of rifting of an oblique basement fabric. If this is correct, then the extension vector for fabric and has been documented from the North Sea middle Eocene rifting trended approximately WNW± and by analogue modelling of extension of an oblique ESE. rift (McClay and White, 1996). In a moderately oblique Further extension during the period of rift climax rift (608 angle between the trend of the rift zone and the (Phillips et al., 1997) resulted in progressive rotation of extension direction), border faults are arranged in an the relay ramp separating the Gongnyay and Gergaji en-echelon array and are shorter than those formed in half-graben and the development of faults trending an orthogonal rift (McClay and White, 1996). The ana- NE±SW aided by the reactivation of the underlying logue models suggest that the centre of the fault trace is NE±SW basement fabric. Continued fault movement orthogonal to the extension direction. led to the breaching of the ramp and the formation of a Dykes contemporaneous with extension have been continuous fault trace trending NNE±SSW with several regarded as useful palaeostress indicators (Gough and NE±SW kinks (Fig. 2). Gough, 1987), and in areas where the least principal The late Eocene and early Oligocene were a period of stress (s 3) is horizontal, rising magma tends to produce thermal subsidence with minor reactivation or rejuvena- dykes along sub-vertical, tensile fractures orthogonal to tion of tectonic activity on faults formed during the s 3. Studies on the boundaries of the North Sea have middle Eocene. The rift topography was gradually Fig. 10. Block diagrams illustrating relationship of basement fracture patterns to (a) middle Eocene (b) late Oligocene and (c) middle Miocene basin architecture and illustrating relevant stress orientations. BRRÐBreached relay rampsÐactive in extension; NFÐNormal faultsÐactive in extension and inversion; TFÐTransfer faultsÐactive in extension and inversion; HLÐHard linked fault system; SLÐSoft linked fault system; DÐmiddle Eocene rhyolite dykes; VSÐVein system of late Oligocene age; EN-FÐ`en-echelon' fault system; RFÐReverse fault; IHGÐInverted half-graben; SFÐSinistral strike±slip fault; DEFÐDextral `escape' fault; SDÐSemi-ductile shear bandsÐseveral movement phases; JLÐ JointsÐlow angle (compressional), high angle (tensional); FAÐFold axis oriented NE±SW. I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 153 in®lled and covered by sedimentation. During the late is enigmatic. Possible interpretations include the col- Oligocene the middle Eocene basin architecture was lision of continental fragments in the modi®ed by a renewed period of extension on the against north-western Borneo (van de Weerd and northern margin of the Kutai basin. AFT results data Armin, 1992) and of the Banggai-Sula block with east indicate tectonic uplift from 23 Ma onwards (Moss et Sulawesi (Daly et al., 1991), as well as the reversal of al., 1998). Extension occurred on a set of faults orthog- movement on a pair of overlapping strike±slip faults onal to those active during the middle Eocene (Fig. 9b). (Biantoro et al., 1992) and these are all reviewed in In other sections of the basin the `late-Oligocene uncon- Moss et al. (1997). A combination of early collision to formity' is related to renewed uplift of the basin margin the north and later collision in the east may provide an (Satayana and Biantoro, 1995; Moss et al., 1998) and explanation for the series of punctuated inversion compression and folding (van de Weerd et al., 1987; events. Wain and Berod, 1989; van de Weerd and Armin, The degree of reactivation of previous extensional 1992). fault systems is dependent on the dip and direction of The style of the en-echelon faults and the segmented dip of the basin bounding fault. It is apparent that east- nature of the fault segments observed in the faulting of facing, NNE±SSW faults perpendicular, or at a high the late Oligocene sequence is similar to that observed angle to the compression direction, are more strongly within the Eocene sequence, and oblique extension of inverted than west facing faults (Figs. 2 and 10c) and the underlying NW±SE basement fabric (Fig. 2) may this is also observed for detached inversion structures in provide an explanation (Fig. 9b). Pervasive mineralised the Lower Kutai Basin (Ferguson and McClay, 1997). veins, between 1 and 20 m across, host the Mt Muro Both low angle (258±408) and high angle reverse faults gold deposit on the southern margin of the basin and are seen in the ®eld, but high angle faults are compara- trend NNW±SSE. Simmons and Browne (1990) tively rare. This results from reverse movement at suggested that this orientation was orthogonal to the depth, reactivating lower angle faults, but at higher least kinematic stress indicator for extension in the structural levels the presence of an incompetent syn-rift basin at this time. It is suggested here that the transition ®ll leads to the propagation of new reverse faults, that from the middle Eocene fault population to the late ¯atten out at shallower levels, and folding of the syn- Oligocene one may represent a change from slightly rift ®ll. oblique WNW±ESE trending rifting (half-graben for- Inversion is also accompanied by reactivation of the mation) to more oblique extension trending ENE± basement heterogeneities. NE±SW oriented, breached, WSW on NW±SE en-echelon faults (Cloke, 1997). This relay ramps, developed during extension, were reacti- change in kinematic regime is inferred to be the result vated with an oblique sense of slip (Figs. 2 and 10c). of an anti- clockwise rotation in the extension direction Faults trending NE±SW developed as high angle trans- by between 458 and 908 (Fig. 10b). pressive features with right-lateral displacement. Conclusive evidence for compression and basin inver- Reverse faults are o€set along zones trending NW±SE sion is shown by late Miocene age growth synclines, (Figs. 2 and 10c) as a result of reactivation of NW±SE and the thinning of strata onto anticlines within the fractures. Lower Kutai Basin (Chambers and Daley, 1995). In summary, interpretation of the remote sensing Inversion seems to have occurred in a series of pulses data and ®eld measurements has demonstrated the pre- throughout the Late Oligocene to (Chambers sence of asymmetrical anticlines. Seismic sections show and Daley, 1995), and the locus of inversion appears to these to be developed above inverted half-graben. These have migrated from west to east during this time period have a di€erent mode of origin to the folds observed in (Moss et al., 1997, 1998). The presence of a thick post- the Neogene sediments of the Lower Kutai Basin, rift sequence and overpressured shale in the Lower although they both share the same fundamental control. Kutai Basin obscures observation of inversion geome- The formation of inversion folds on the northern and tries along the Neogene basin axis and has resulted in southern margins of the Kutai Basin is directly related several proposed explanations for the compressional to basement movements and the reactivation of pre- structures (Ott, 1987; van de Weerd et al., 1992; vious extensional faults; in contrast, fold formation in Biantoro et al., 1992; Chambers and Daley, 1995; Moss the Lower Kutai Basin is detached from any basement et al., 1997; Ferguson and McClay, 1997). block movements by the presence of a thick overpres- Shortening directions resolved from slicken®bres as- sured shale sequence (Chambers and Daley, 1995; sociate with Neogene age compressional features indi- Ferguson and McClay, 1997). Inversion of a previous cate a predominantly 2908±3108 (Fig. 9) orientation, normal fault system is still postulated to have occurred which agrees with borehole breakout studies in the at depth (Ferguson and McClay, 1997). onshore Mahakam Delta (Ferguson and McClay, 1997; One explanation for the lack of compressional struc- Busono and Syarifuddin, 1999). The cause of inversion tures within the Central Makassar Straits Basin (seismic 154 I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 line CGG-104) is that the crust is oceanic in origin; dicular to the thrust front show that inversion of a pre- however, the exact composition of the crust beneath the existing Mid-Eocene extensional basin architecture gen- basin is still disputed (cf. Bergman et al., 1996). erated the present-day structures (Fig. 6). Restoration Flexural modelling of free-air gravity pro®les over sedi- of the extensional architecture shows that a complex mentary loads, such as deltas, are one method that may series of half-graben existed prior to inversion, with be used to constrain crustal type (Watts, 1978). half-graben oriented NNE±SSW and N±S (Fig. 6). Bergman et al. (1996) have suggested that the present Opposing polarity half-graben may have been joined by day con®guration of the Makassar Straits was the result transfer faults oriented NW±SE (Fig. 10a). Half-graben of loading of stretched continental crust, but the results with a synthetic polarity overlapped in an en-echelon of ¯exural modelling of free-air gravity pro®les, calcu- manner and NE±SW fault kinks probably developed as lated for values of elastic thicknesses appropriate for the result of a partitioning of strain within the overlap- attenuated continental crust (Te = 5, 10), are unable to ping fault zones and the development of a breached generate the observed free-air gravity anomaly recorded relay ramp (Fig. 7). The NNE±SSW faults are parallel across the Mahakam Delta (Cloke, 1997). In contrast, to contemporaneous rhyolite dykes, which are indicative ¯exural modelling of the sedimentary load of the of the palaeo-stress directions during this period (Figs. 2 Neogene ®ll of the Mahakam Delta suggests that sedi- and 10a). Extension of an oblique basement fabric ments 20 km landward of the present day shelf-break (NE±SW) is one mechanical method of generating the have loaded lithosphere with a high elastic thickness structural architecture observed and this would indicate (Te) corresponding to oceanic lithosphere of 47 Ma an extension direction WNW±ESE orthogonal to the (Cloke, 1997). Landward of this point, the elastic thick- rhyolite dykes. ness appears to be less (Te = 5), suggesting stretched A further phase of tectonic activity occurred during continental crust. We therefore propose that the Central the late Oligocene, resulting in extension on a number Makassar Straits is underlain by oceanic crust of middle of segmented fault systems on the northern margin of Eocene age. Stretched continental crust is considerably the Kutai Basin. These are inferred to have reactivated weaker than oceanic crust and would deform more a basement fabric trending NW±SE (Figs. 8 and 10b). readily (basin inversion in the Kutai Basin of East The en-echelon nature of the faults and the presence of Kalimantan and the Lariang Basin of West Sulawest). contemporaneous dykes oriented NNW±SSE, identi®ed Compressional stresses from the eastern margin of on the southern margin of the Kutai Basin (Simmons Sundaland would also be transmitted through the and Browne, 1990), suggest that the extension direction Central Makassar Straits as it acted as a `passive con- was again oblique to a basement fabric trending NW± duit' to the Kutai Basin (Cloke, 1997). SE. This implies that the extension direction had rotated anticlockwise by between 458 and 908 between the middle Eocene and late Oligocene, and was there- 12. Conclusions fore aligned approximately ENE±WSW. Collisions with the northern and eastern margins of Interpretation of remote sensing data has shown pro- Sundaland from the early-middle Miocene (N7±N8) led minent fractures/lineaments oriented both NW±SE and to the inversion of favourably aligned middle Eocene NE±SW (Figs. 2±4) within basement rocks located on half-graben (Fig. 10c). The rigid oceanic crust under- the margins of the Kutai Basin. These are an inherited lying the acted as a `passive conduit' basement fabric and not the result of Tertiary defor- for compressional forces from West Sulawesi. Inversion mation, although they were reactivated at various stages was focused in areas of low strength (and therefore throughout the Tertiary. Integration with regional weak) continental crust beneath the Kutai Basin and Bouguer gravity data has extended these interpretations various basins in West Sulawesi. Contractional stresses of the basement heterogeneities to the sub-surface and were oriented WNW±ESE, as shown by the movement to the basin centre. Gravity data have identi®ed a series indicators (Fig. 9) and by borehole breakout studies of linear NNE±SSW trending positive anomalies o€set (Busono and Syarifuddin, 1999). NNE±SSW basin- by NW±SE trending linear zones of low to negative bounding faults were reactivated in a pure reverse sense gravity anomalies (Fig. 5). These represent prominent (Fig. 4). NW±SE transfer faults and late Oligocene en- basement structures that have experienced a number of echelon faults were reactivated as strike-slip/transpres- phases of tectonic activity, both extension and inver- sional features, o€setting reverse faults and fold axes sion. (Fig. 2). En-echelon folds formed in the cover above Asymmetrical fold structures within Tertiary cover reactivated NW±SE trending basement faults. NE±SW are oriented NNE±SSW an N±S, with kinks oriented breached relay ramps were reactivated in an oblique-slip NE±SW (Fig. 3). O€set of fold axes occurs along fault manner and allowed ``escape'' of the basin ®ll (Fig. 10c). zones oriented NW±SE (Fig. 2). Seismic lines perpen- Transpressional right-lateral faults oriented NE±SW I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 155 formed as a result of reactivation of NE±SW trending Busono, I., Syarifuddin, N., 1999. Regional stress alignments in Kutai Basin, East Kalimantan, Indonesia: A contribution from borehole basement fractures (Fig. 2). breakout study. Journal of Asian Earth Sciences 17, 123±135. The con®guration of the Kutai Basin at the present Carter, I., 1994. The hydrocarbon prospectivity of the northern part of day is the result of a series of tectonic episodes. In com- the Runto Block, East Kalimantan. Unpublished LASMO Runtu mon with other basins in Southeast Asia the basin was Internal Report, p. 25. formed by middle Eocene rifting (Fig. 10a). Unlike Cartier, E.G., Yeats, A.K., 1973. The Lower Tertiary in KALTIM Shell other basins this was followed by the rotation of the Contract Area, East Kalimantan. Results of the late 1972±1973 Field Surveys. KALTIM Shell, Balikpapan, Unpublished Report. extension direction and the formation of a separate Chambers, J.L.C., Daley, T., 1995. A tectonic model for the onshore population of faults during a second minor extensional Kutai Basin, East Kalimantan, based on an integrated geological episode in the late Oligocene (Fig. 10b). Structures and geophysical interpretation. Indonesian Petroleum Association, formed during both phases are explained most simply Proceedings 24th Annual Convention, Jakarta 1, 111±130. by invoking oblique extension of a pre-existing base- Cloke, I.R., 1997. Structural controls on the basin evolution of the Kutai Basin and Makassar Straits. Unpublished PhD. Thesis, ment fabric. Contractional forces from the early-middle University of London. Miocene to the present day reactivated favourably Cloke, I.R., Moss, S.J., Craig, J., 1997. The in¯uence of basement reac- aligned half-graben, all of which formed during the ®rst tivation on the extensional and inversional history of the Kutai Basin, phase of extension. Late Oligocene faults remained in Eastern Kalimantan. Journal of Geological Society, London 154, net-extension or underwent transpression (Fig. 10c). 157±161. Courteney, S., 1995. The future hydrocarbon potential of Western The Kutai Basin is an excellent example of the inter- Indonesia. In: Caughey, C.A., Carter, D., Clure, J., Gresko, M.J., action between pervasive basement fabrics and exten- Lowry, P., Park, R.K., Wonders, A. (Eds.), Proceedings of the sional and inversional deformation. International Symposium on Sequence Stratigraphy in SE Asia. Indonesian Petroleum Association, Jakarta, Indonesia, pp. 157±415. Acknowledgements Daines, S.R., 1985. Structural history of the West Natuna Basin and the tectonic evolution of the Sunda Region. Indonesian Petroleum Association, Proceedings 14th Annual Convention, Jakarta 1, 39±62. This work forms part of NERC studentship GT4/94/ Daly, M.C., Cooper, M.A., Wilson, I., Smith, D.G., Hooper, B.G.D., 370/G to I.R.C. Fieldwork was assisted by LASMO 1991. Cainozoic and basin evolution in Indonesia. Runtu Ltd and the Richard C. Hasson and Energy Marine and Petroleum Geology (Special issue: South-east Asia) 8 Minerals A.A.P.G. grant to I.R.C. Remote sensing, (1), 2±21. gravity and seismic data was provided courtesy of Doutch, H.F., 1992. Aspects of the structural histories of the Tertiary sedimentary basins of East, Central and West Kalimantan and their LASMO PLC. The SE Asia Consortium of oil compa- margins. 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