Abstract: The Kutai basin, East Kalimantan, is one of several large extensional basins formed in the. Middle to Late Eocene. Formed above an orogenic complex ...
Journal of the Geological Society The influence of basement reactivation on the extensional and inversional history of the Kutai Basin, East Kalimantan, SE Asia I. R. CLOKE, S. J. MOSS and J. CRAIG Journal of the Geological Society 1997; v. 154; p. 157-161 doi:10.1144/gsjgs.154.1.0157
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© 1997 Geological Society of London
Journal of the Geological Society, London, Vol. 154, 1997, pp. 157–161, 4 figs. Printed in Great Britain
The influence of basement reactivation on the extensional and inversional history of the Kutai Basin, East Kalimantan, SE Asia I. R. CLOKE 1 , S. J. MOSS 1,2 & J. CRAIG 3 Department of Geology, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK 2 Present address: School of Applied Geology, Curtin University of Technology, Perth, 6001, WA, Australia 3 New Ventures Group, LASMO International, LASMO PLC, 100 Liverpool Street, London, UK 1
Abstract: The Kutai basin, East Kalimantan, is one of several large extensional basins formed in the Middle to Late Eocene. Formed above an orogenic complex of Late Cretaceous/Early Tertiary age, basement fabrics oriented NW–SE and NE–SW strongly influenced basin extension and inversion. Remote sensing data and fieldwork show basement fabric on the margins and Tertiary cover are dominated by NE–SW, NW–SE and NNE–SSW-trending structures. Similarly, on a larger scale, NW–SE narrow linear gravity lows cut NNE–SSW highs on the gravity data within the basin. Reconstruction of the basin architecture shows NNE–SSW basin-bounding faults overlapping in a right stepping en-echelon manner. These overlaps (relay ramps) were breached by faults oriented NE–SW through reactivation of the basement fabric. Opposing antithetic and synthetic half-grabens were linked by highly oblique NW–SE transfer faults. Inversion utilized extensional faults as reverse faults; however, NW–SE-oriented structures were reactivated as zones of lateral offset along the fold-thrust belt, whilst fault kinks oriented NE–SW were reactivated as oblique-slip reverse faults. The Kutai Basin provides an example of the controls that basement heterogeneities play on both extension and inversion. Keywords: Kalimantan, Indonesia, basement, reactivation, extension.
complex (Sikumbang 1990). This complex on the southern margin of the basin is laterally offset along strike-slip faults oriented NW–SE. Outcrops of gabbroic and ultrabasic rocks on the northern margin of the Kutai basin has led some authors to postulate that the Meratus complex extends north beneath the Kutai basin. Definitive evidence has yet to be published.
Basement geology Tertiary rocks of the Kutai Basin rest with angular unconformity upon a basement of Mesozoic and older strata. The complex Pre-Tertiary geology exposed on the northern, western and eastern margins of the basin has been discussed in detail by Moss et al. (1997). Basement is exposed on the northern, western and southern margins of the Kutai basin (Fig. 1). To the north and west it consists of both the continental basement of ‘Sundaland’ and both fore-arc and foreland basin fill (Pieters et al. 1987; Pieters & Supriatna 1990). The continental basement consists of metamorphic, sedimentary, and granitic to gabbroic rocks of PermoCarboniferous to late Cretaceous/early Eocene age. Some pre-Tertiary units are intruded by late Cretaceous ‘stitching’ granites (Moss et al. 1997). Deformed and tectonized basement basalts and ultrabasics recognised throughout Kalimantan are often suggested to be ophiolites (Van de Weerd & Armin 1992), but are not part of a full ophiolitic sequence and are often too typically highly deformed for conclusions to be made (Moss et al. 1997). Brittle–ductile shear zones are common on the northern margin with dominant orientations of NW–SE and NE–SW. Many shear zones show several phases of movement evidenced by overprinting of quartz veins, and reactivation and breaching of shear bands. The fold axes of chevron-folded cherts are oriented NE–SW. NW–SE-aligned lineaments have previously been identified throughout Borneo and used to provide evidence for a through-going ‘Trans-Kalimantan shear zone’ connected to anyone of a number of major strike-slip faults associated with the extrusion of Asian crustal blocks (Wood, 1985; Hutchison 1989; Daly et al. 1991; Tate, 1992). The southern margin of the basin consists of the NNE– SSW-oriented Meratus Mountains ophiolite and volcanic arc
Gravity data Three positive gravity highs, oriented NNE–SSW, are apparent in the regional Bouguer gravity pattern for part of East Kalimantan (Fig. 2). The most prominent of these highs is the Kutai Lakes gravity high with an amplitude of about 40 mGal (Fig. 2). The Kedang Kepala high has a broader anomaly of approximately 54 mGal and is situated to the NNE of the Kutai Lakes high, offset by about 10 km in a right lateral sense by a NW–SE-oriented gravity low called the Belayan Axis (Fig. 2). To the north, the Kedang Kepala high is offset from another positive anomaly by another linear gravity zone called the Kedang–Kepala axis (Fig. 2). Gravity data to the east indicate a continuation of the Kedang Kepala axis (Sunaryo et al. 1988). The two main lows described range in value from 0.43 mGal to 21.02 mGal. Magnetic data across the region also indicate an increase in depth to magnetic basement across the Kedang–Kepala axis (Sunaryo et al. 1988). Well data (Mawai well, Fig. 1) show syn-rift sediments, with high vitrinite values (>1.2%) atop these highs. Residual Bouguer gravity data can be used to show structures within the Tertiary carapace and, although not shown on Fig. 2 for brevity, confirm the presence of the Kutai Lakes high as well as narrow linear zones of both NW–SE and NNE–SSW orientations within the sedimentary cover. 157
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Fig. 1. Geological map of the Kutai Basin and surrounding margins, Eastern Kalimantan, showing variation in basement geology and structure. Inset shows location (modified from Moss et al. 1997).
Basement trends identified using synthetic airborne radar (SAR) and Landsat TM scenes Lineaments identified in pre-rift basement and post-rift sedimentary sequences show differing orientations and abundances. Basement lineaments are predominantly oriented NW–SE and NE–SW with a weakly developed NNE–SSW fabric (Fig. 3). NW–SE lineaments consist of predominantly short features cutting through basement strata. Field data show that low angle joints orthogonal to the NW–SE trend and resulting from compressional stresses, account for the NE–SW trend (Wain & Berod 1989). Our field studies have identified NW–SE-oriented dominant fractures, duplex shear bands and S–C fabrics, showing multiples stages of reactivation. NE–SW lineaments crosscut the NW–SE direction, but poor exposure of basement rocks makes it impossible to establish sense and age of displacement. The NE–SW orientation has previously been identified in Sarawak by Tate (1992). Locally this trend reflects the faulted boundary between basement and Tertiary cover. Fractures and tight chevron folds within basement cherts on the northern margin of the basin parallel this structural grain. An additional trend oriented NNE–SSW is weakly developed within the basement and forms continuous lineaments cross-cutting early structures. Late-stage extensional faults parallel this trend. Late Cretaceous granitic dykes trending NW–SE cross-cut sheared basaltic rocks and are deformed by centimetre-scale extensional faults aligned
Fig. 2. Regional Bouguer gravity map of the Kutai Basin, showing presence of NW–SE aligned gravity lows offsetting NNE–SSW positive highs.
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Fig. 3. Satellite-derived SAR image of part of the northern margin of the Kutai Basin showing basement structures and inversion structures within Tertiary cover. Interpretation shown below.
NNE–SSW (Moss et al. 1997). The paucity of this NNE–SSW fabric within the basement suggests it may be the result of later tectonic activity.
Tertiary trends Basement lineament trends locally continue weakly into the Palaeogene and Neogene cover (Fig 3). Prominent within the Tertiary material is the NNE–SSW trend, previously termed a megalinear and linked to a Cretaceous palaeoplate boundary
(Wain & Berod 1989). Proprietary seismic data through a fold and thrusts within the basin show Eocene-age, NNE–SSW extensional faults reactivated with a reverse sense of motion (Fig. 3). Developed above these fault zones are asymmetric, west vergent anticlines and open synclines with axes parallel to the NNE–SSW fabric (Fig. 3). Seismic data quality is poor, but harpoon and whale-back structures typical of inversion (McClay & Buchanan 1991) are seen on many sections. NE–SW and NW–SE trends in the Tertiary cover result from basement reactivation. NW–SE trends parallel the Adang
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Fig. 4. 3D block diagram illustrating relationship of basement fracture patterns to Tertiary extensional structures. BRR, breached relay ramps—active in extension; NF, normal faults—active in extension and inversion; TF, transfer faults—active in extension and inversion; FA, fold axis oriented NE–SW; SD, semi-ductile shear bands—several movement phases; JL, joints—low angle (compressional), high angle (tensional). Diagram at top left shows the main orientations of structural features observed.
and Sangkulirang faults, two major structures within the Makassar Straits (Fig. 1). Reactivation of the NW–SE trend within the Tertiary cover is indicated by the presence of right and left-lateral offset of NNE–SSW-trending reverse faults (Fig. 3). Across NW–SE ‘damage zones’, fold axes swing from NNE–SSW to a NW–SE orientation and high angle thrusts strike NW–SE. The offset and folding of distinctive shale layers and the presence of sub-horizontal slickenfibres within these fault zones suggest transpressional movement. Within the Mahakam delta, long, linear, isoclinal anticlines are offset along NW–SE trends (VICO, pers. comm. 1995). Apatite fission-track data indicate a rapid cooling of basement on the NW margin of the Kutai Basin at c. 25 Ma, due to tectonic uplift (S. Moss, unpublished data). Other workers have noted an important Late Oligocene tectonic event (Van de Weerd & Armin 1992). Oligo-Miocene limestone-cored folds developed above reverse faults, and show right-lateral offset along the NE–SW trend (Fig. 3). Sunaryo et al. (1988) observed that the trends further to the east rapidly change direction to an east–west orientation and suggested that this is related to movement on an extension of the Palu–Koro fault of Sulawesi into the Mangkalihat Peninsula area (Fig. 1).
Reconstruction of extensional basin architecture The pre-inversion structural architecture in an area of the basin was restored by section balancing of seismic sections and established the extensional structure in plan view (Fig. 4). A complex series of half-graben that switch polarity along strike is evident, similar to many other rift systems (e.g. the East African Rift, Rosendahl et al. 1986). Basin-bounding faults strike NNE–SSW (Fig. 4), indicating that lineaments of this orientation observed within the Tertiary cover are likely to be the result of either extension or reverse reactivation during compression. Extensional faults overlap in an en-echelon rightstepping manner with overlapping fault segments separated by relay ramps. This is a common occurrence in extensional fault systems formed by oblique rifting over a heterogeneous basement (McClay & White 1995). Additional NE–SW faults
formed within these ramps as a result of progressive rotation within the ramp and reactivation of the underlying NE–SW basement trend, leading to the breaching of the ramp and the formation of a continuous fault trace trending NNE–SSE with several NE–SW kinks (Fig. 4). Opposing polarity half-graben were connected by a combination of hard and soft-linkage faults trending NW–SE (Fig. 4). Evidence for one of these zones is shown by the transition from a strongly inverted, east facing half-graben to a west facing half-graben (Fig. 4). The transition is accommodated via steep faults oriented NW–SE that downthrow either to the south or north. Bouguer gravity data across this zone indicate the coincidence of a NW–SE gravity low, called the Kedang Kepala axis (Fig. 2).
Reactivation of extensional fabric during inversion Conclusive evidence for compression and basin inversion is shown by late Miocene age growth synclines and thinning of strata onto anticlines within the Lower Kutai Basin (Chambers & Daley 1995). An earlier event on the NW margin of the basin shown by apatite fission-track data indicates that tectonic uplift occurred at 25 Ma (S. Moss unpublished data), substantially earlier than other dated events. Inversion seems to have occurred in a series of pulses throughout the Late Oligocene to Pliocene (Chambers & Daley 1995) and the locus of inversion appears to have migrated from west to east during this time (Moss et al. 1997). The presence of a thick post-rift sequence and over pressured shale obscures observation of inversion geometries in the basin centre and resulted in several explanations for the compressional structures (Ott 1987; Van de Weerd et al. 1990; Biantoro et al. 1992; Chambers & Daley 1995; Moss et al. 1997). Shortening directions interpreted from slickenfibres associated with compressional features indicate a predominantly WNW–ESE to NW–SE orientation. The cause of inversion is enigmatic; possible interpretations include the collision of continental fragments in the South China Sea against northwestern Borneo (Van de Weerd & Armin 1992) and of the Banggai–Sula block with east Sulawesi (Daly et al. 1991) as well as the reversal of movement on a pair of
REACTIVATION, KUTAI BASIN, E KALIMANTAN
overlapping strike-slip faults (Biantoro et al. 1992). These are all reviewed in Moss et al. (1997). A combination of early collision to the north and later collision in the east may provide an explanation for the series of punctuated inversion events (Moss et al. 1997). The degree of reactivation of previous extensional fault systems is dependant on the angle and direction of dip of the basin-bounding fault. It is apparent that east facing, NNE– SSW faults perpendicular or at a high angle to the compression direction are more strongly inverted than west facing faults. Both low-angle (25–40)) and high-angle reverse faults are seen in the field, but high-angle faults are comparatively rare. This is a result of reverse movement at depth reactivating where the fault has a lower angle, however at higher structural levels the presence of incompetent syn-rift fill leads to the propagation of new reverse faults that flatten out at shallower levels. Inversion is also accompanied by reactivation of the basement heterogeneities. NE–SW-oriented, breached relay ramps developed during extension reactivated with an oblique sense of slip. Faults trending NE–SW developed as high-angle transpressive features with right-lateral displacements. Reverse faults are offset along zones trending NW–SE (Figs 3 & 4) as a result of reactivation of NW–SE fractures.
Conclusions Remote-sensing data have enabled the identification of prominent fractures/lineaments oriented both NW–SE and NE–SW (Fig. 3) within basement rocks located on the margins of the Kutai basin. Another fracture pattern oriented NNE–SSW is faintly developed within basement, but is more prominent within Tertiary cover. Integration with regional Bouguer gravity data has extended these interpretations of the basement heterogeneities to the sub-surface and the basin centre. Gravity data have revealed a series of linear NNE–SSW-trending positive anomalies offset by NW–SE-trending linear zones of low to negative gravity anomalies. These represent prominent basement structures that have experienced a number of phases of tectonic activity, both extensional and inversional. Fold structures within Tertiary cover are oriented NNE– SSW with kinks oriented NE–SW (Fig. 3). Offset of fold axis occurs along fault zones oriented NW–SE. Seismic lines perpendicular to the thrust front show that inversion of a pre-existing, Mid-Eocene extensional basin generated the present-day structures. Restoration of the extensional architecture shows a complex series of half-grabens existed prior to inversion. NE–SW fault kinks are likely to be the result of a breaching of relay ramps separating main bounding faults oriented NNE–SSW (Fig. 4). Opposing polarity, half-grabens may have been joined by transfer faults oriented NW–SE. Compression oriented WNW–ESE reactivated the previous extensional basin architecture. NNE–SSW basin-bounding faults were reactivated in a pure reverse sense (Fig. 4), whilst NW–SE transfer faults were reactivated as strike-slip/transpressional features offsetting reverse faults and fold axes (Figs. 3 & 4). NE–SW-breached relay ramps were reactivated in an oblique slip manner and allowed ‘escape’ of the basin fill. Transpressional right-lateral faults oriented NE–SW formed as a result of reactivation of NE–SWtrending basement fractures (Fig. 4). Reactivation of basement structures with a variety of trends and ages explains the present-
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day orientation of structural features within the Kutai basin and has played a major role in controlling extensional and inversional basin architectures. This work forms part of NERC studentship GT4/94/370/G to I.R.C. Fieldwork was assisted by LASMO Runtu Ltd and the Richard C. Hasson AAPG grant to I.R.C. S.J.M.’s post-doctorate is funded by the SE Asia Research Group and the SE Asia Consortium of oil companies, ARCO Indonesia, LASMO Indonesia Ltd, MOBIL Oil Indonesia, EXXON Inc, Can Oxy Indonesia and Union Texas Petroleum, who are all gratefully acknowledged. Remote sensing data provided courtesy of LASMO Runtu Ltd. R. Hall, M. Cooper & S. Matthews are thanked for improving the quality of the original manuscript.
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Received 25 April 1996; revised typescript accepted 16 August 1996. Scientific editing by Rob Butler and Alex Maltman.
