Structural Controls on the Evolution of the Kutai Basin, East Kalimantan
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PERGAMON Journal of Asian Earth Sciences 17 (1999) 137±156 Structural controls on the evolution of the Kutai Basin, East Kalimantan 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 Eocene as a result of extension linked to the opening of the Makassar 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 Tertiary. Oset of middle Eocene carbonate horizons and rapid syn-tectonic thickening of Upper Oligocene sediments on seismic sections indicate Late Oligocene extension on NW±SE trending en-echelon extensional faults. Early middle Miocene (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 oset 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 dierent 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 Sulawesi 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 East Kalimantan 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 Indonesia (Rose and Hartono, 1978). In the proven in-place resources in the Kutai Basin are 15.81 Samarinda region the Neogene 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 Paleogene succession has 1367-9120/99 $ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0743-9547(98)00036-1 138 I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 137±156 Fig. 1. Geological Map of the Kutai Basin and surrounding margins, showing diering 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 diering 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 Borneo, 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 dierent names for gin of the basin is laterally oset along strike±slip similar sedimentary and igneous formations and high- faults oriented NW±SE. This results in the juxtaposi- light the diculty of correlation of formations due to tion of Aptian to Turonian age sedimentary rocks their highly faulted and dismembered nature (Moss against island arc 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 dierent ages exposed on the northern, western and Meratus Complex extends north beneath the Kutai southern margins of the Kutai Basin (Fig.