Tertiary facies architecture in the Kutai Basin, Kalimantan, Indonesia

PERGAMON

Journal of Asian Earth Sciences 17 (1999) 157±181

Tertiary facies architecture in the Kutai Basin, Kalimantan, Indonesia Steve J. Moss a, 1, John L.C. Chambers b a

Robertson Research, 69 Outram Street, Perth, 6005, WA, Australia b LASMO Runtu Ltd, Jakarta, Indonesia Received 12 January 1998; accepted 22 June 1998

Abstract The Kutai Basin occupies an area of extensive accommodation generated by Tertiary extension of an economic basement of mixed continental/oceanic anity. The underlying crust to the basin is proposed here to be Jurassic and Cretaceous in age and is composed of ophiolitic units overlain by a younger Cretaceous turbidite fan, sourced from Indochina. A near complete Tertiary sedimentary section from Eocene to Recent is present within the Kutai Basin; much of it is exposed at the surface as a result of the Miocene and younger tectonic processes. Integration of geological and geophysical surface and subsurface data-sets has resulted in re-interpretation of the original facies distributions, relationships and arrangement of Tertiary sediments in the Kutai Basin. Although much lithostratigraphic terminology exists for the area, existing formation names can be reconciled with a simple model explaining the progressive tectonic evolution of the basin and illustrating the resulting depositional environments and their arrangements within the basin. The basin was initiated in the Middle Eocene in conjunction with rifting and likely sea ¯oor spreading in the Makassar Straits. This produced a series of discrete fault-bounded depocentres in some parts of the basin, followed by sag phase sedimentation in response to thermal relaxation. Discrete Eocene depocentres have highly variable sedimentary ®lls depending upon position with respect to sediment source and palaeo water depths and geometries of the halfgraben. This contrasts strongly with the more regionally uniform sedimentary styles that followed in the latter part of the Eocene and the Oligocene. Tectonic uplift documented along the southern and northern basin margins and related subsidence of the Lower Kutai Basin occurred during the Late Oligocene. This subsidence is associated with signi®cant volumes of high-level andesitic±dacitic intrusive and associated volcanic rocks. Volcanism and uplift of the basin margins resulted in the supply of considerable volumes of material eastwards. During the Miocene, basin ®ll continued, with an overall regressive style of sedimentation, interrupted by periods of tectonic inversion throughout the Miocene to Pliocene. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Borneo is bounded by three marginal oceanic basins to the north and northeast (Fig. 1) and by mainland SE Asia (Indochina and Peninsular Malaysia) to the west and north west. To the north of Borneo is the South China Sea, formed by sea¯oor spreading between Borneo and South China of Oligocene to mid Miocene age (Briais et al., 1993). To the south, separating Borneo from Java, is the shallow Java Sea, underlain by Sundaland continental crust. Sundaland crust is believed to extend into the southwestern corner 1

Formerly at University of London Southeast Asia Research Group, Department of Geology, Royal Holloway, U.K.

of Borneo, the Schwaner Mountains (Hamilton, 1979; van de Weerd and Armin, 1992). To the east is the deep Makassar Straits and Sulawesi. Tertiary sedimentary basins, of which the Kutai Basin is the largest, cover a signi®cant area of Borneo (Fig. 1) and all appear to have a sedimentary ®ll derived from the inland parts of Borneo, suggesting that central Borneo was a€ected by extensive uplift and erosion during Tertiary times. In the central and northern parts of Borneo, turbidites of the Late Cretaceous±Early Tertiary Rajang and Embaluh Groups cross the island in a NE±SW trending crescent (Fig. 1). Where these turbidites have been extensively eroded an underlying ophiolitic-type basement of Jurassic-Early Cretaceous age is exposed. To the south-west, the Schwaner Block, composed of

1367-9120/99 $ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 7 4 3 - 9 5 4 7 ( 9 8 ) 0 0 0 3 5 - X

Fig. 1. Simpli®ed geological map of Borneo and its location within SE Asia. SR=Semitan Ridge; UKP=Upper Kutai Basin.

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Palaeozoic metamorphic rocks, form part of the Sundaland continental crust. Both the Schwaner Block and the Rajang/Embaluh Groups are intruded by considerable volumes of acidic to intermediate igneous material that appears to range in age from Early Cretaceous to Miocene. There is a shortage of reliable radiometric dating within Borneo as work done to date has relied almost exclusively on the K-Ar technique. 2. Geological setting of the Kutai Basin The Kutai Basin may divided into two (sub-) basins; a western Inner or Upper Kutai (sub-) Basin, and an eastern Outer or Lower Kutai (sub-) Basin. Today the Upper Kutai Basin is an area of major tectonic uplift as a result of Lower Miocene inversion of Paleogene depocentres and the e€ects of subsequent erosion. The boundaries of the Upper Kutai Basin are hard to constrain, as it is likely that extensional tectonics in Eocene times resulted in a patchwork of connected and unconnected graben and half-graben extending across the eastern part of Borneo, and possibly right across the island (Pieters et al. 1987). These discrete depocentres are recognised from the Barito Basin in the south through to the onshore Tarakan Basin in the north. The Lower Kutai Basin as we know it today was de®ned only during the Neogene, and overlies and encompasses many of the Palaeogene depocentres of the Upper Kutai Basin. The boundaries of the present day Kutai Basin, or its Neogene equivalent, do not correspond to the margins of any single Palaeogene depocentre. Many of the Palaeogene rifts were inverted and deeply eroded in the Neogene, further masking their true extent. The deep Makassar Straits Basin on the eastern side of the Kutai Basin (Figs. 1 and 2(a)) represents the o€shore deep-water continuation of the Kutai Basin, where the Neogene section thins onto what appears to be a Palaeogene block faulted section. The Makassar Straits Basin formed during the same Eocene extensional phase that gave rise to the initiation of the Kutai Basin (Moss et al., 1997). The Lower Kutai Basin is bounded by two northwest-southeast trending fault zones; the Adang Fault to the south and the Sangkulirang Fault to the north. These fault zones and related o€sets extend both onshore and o€shore (Cloke et al., 1997). South of the Sangkulirang Fault, within the Kutai Basin, is the Bungalun lineament (Fig. 2(b)), which together with the Adang Fault de®nes the northern and southern boundaries of the Lower Kutai Basin. The Bungalun lineament has not previously been de®ned in the literature but is a NW±SE trending lineament bordering the northern Lower Kutai Basin, controlling the path of the Bungalun River, and marking a rapid north±south

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change from thin to thick Neogene sequences within the Lower Kutai sub-basin. The onshore Neogene section rapidly deepens between the Bungalun and Adang faults. These hinge zones, or down-to-the-basin normal faults, were active during the Late Oligocene to Miocene, and are probably sited upon older pre-existing fault lines (Cloke et al., 1997). Similar parallel linear features are present north of the Mangkalihat Peninsula and within the Kutai Basin. The Muller Mountains form the western margin of the Upper Kutai Basin and comprise predominantly Cretaceous turbidites of the Rajang and Embaluh Groups. Similar Cretaceous turbidites crop out along the northern basin margin with, in places, more severely deformed Jurassic±Lower Cretaceous ophiolitic rocks (Moss, 1998). A broadly arcuate feature, referred to as the `Muyup Hinge' (Fig. 4; Wain and Berod, 1989), trends roughly NE±SW across the Kutai Basin and appears to control the western margin of the Lower Kutai Basin. Interestingly this feature parallels the broad arcuate outcrops of the Embaluh Group turbidites. The Muyup Hinge may be related to the underlying geometry of Cretaceous basement metasediments and acted as a zone of weakness during Tertiary deposition. The Tertiary history of the Kutai Basin includes a Middle to Late Eocene syn-rift phase, a Late Eocene to Oligocene sag phase and a renewed phase of tectonic activity and subsidence in the Late Oligocene to Miocene. Basin inversion, beginning at least as early as the end of the Lower Miocene, resulted in reworking of earlier sediments and ongoing deposition of `syninversion' deltaic packages. Continuing erosion from the hinterland, and from the Tertiary section, in response to Miocene and Pliocene tectonic activity, resulted in eastward prograding deltaic deposition (Chambers and Daley, 1995, Moss et al., 1997). In this paper we document the depositional environments, their architecture and resultant facies of the di€erent phases of basin evolution from Eocene to present, as well as presenting a discussion on the formation of the underlying basement. The Kutai Basin displays several gross geological features repeated elsewhere in Southeast Asia, with a Cenozoic basin ®ll overlying a thick Cretaceous turbidite section. The Middle Miocene to Pliocene deltaic section makes up the only proven economic petroleum system, although hydrocarbons have been recovered from Oligocene limestones, and to the south, in the Barito Basin, Eocene syn-rift sands of the Tanjung Formation are productive oil reservoirs. Within the Kutai Basin production began at the Sanga-Sanga ®eld in 1898, and to date in excess of three billion barrels of oil and 30 TCF of gas have been recovered. A large variety of petroleum systems can be expected in this area and, because of extensive outcrop, this is an ideal

Fig. 2. (a) Geographic map of the Kutai Basin showing areas mentioned in the text.

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Fig. 2. (b) simpli®ed geological map of the Kutai Basin.

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place to study the elements of individual petroleum systems±many of them no doubt past their prime. This paper however intends to describe the geology and facies development in the area rather than the development of the petroleum systems.

Such a fan system would be capable of depositing a thick, areally extensive turbidite system. Moss (1998) describes the Embaluh and Rajang Group turbidites of Kalimantan and Sarawak in much greater detail and suggest that these turbidites were deposited within a remnant oceanic basin during the Late Cretaceous.

3. Basement

3.2. Kutai Basin basement development

3.1. Regional basement development

The northern and western margins of the Kutai Basin margins show thick low grade metamorphosed turbidites overlying serpentinites and ultra-basic igneous rocks and gabbros representing Jurassic to Lower Cretaceous oceanic crust. To the south, where the margins include the highly deformed Meratus Mountains and the continental Schwaner Block, the situation is more complex. Mesozoic ophiolitic and `island arc rocks' of the Meratus Mountains in the southeast corner of Borneo, and overlying turbiditic material of Cretaceous age, have been interpreted as forming a suture zone between continental and oceanic/intermediate crustal types (Sikumbang 1990). The true nature of the basement rocks on either side of the Meratus Mountains, and therefore on either side of this proposed suture zone, are poorly known. Given that the basement units of the northern margin of the Kutai Basin are similar to the pre-Tertiary units of the Meratus Mountains, and that Cretaceous meta-sediments were encountered below the Tanjung ®eld (northern Barito Basin west of the Meratus Mountains; Fig. 2(a)), the Meratus Mountains may also be uplifted oceanic basement with overlying Cretaceous metasediments. We suggest that the Meratus Mountains, and their northward plunging continuation, represent a major inversion axis throughout East Kalimantan, and that these mountains once formed the basement of a major Eocene graben system which was exhumed during the Early Miocene inversion event. A NNE±SSW trending gravity high, the Kutai Lakes Gravity High (Wain and Berod, 1989, Fig. 2(b)), extends north from the Meratus Mountains in the south, to the Gongnyay area in the north. The Kutai Lakes Gravity High is o€set along NW±SE lineaments which parallel the trends of the Adang, Sankulirang and Bungalun lineaments (Wain and Berod, 1989; Chambers and Daley, 1995; Cloke et al., 1997). We would therefore link the Meratus Mountains and the Gongnyay area as major inversion features, where deeply buried Paleogene rift sediments, overlying Cretaceous metasediments and older ophiolitic material, have been uplifted during the Miocene. In the case of the Meratus Mountains, inversion has been responsible for bringing basement to the surface, but at Gongnyay, surface exposures comprise compacted Palaeogene sediments. Within the Lower Kutai Basin,

The nature of the basement surrounding and underlying the Kutai Basin has attracted much discussion in the literature (see Moss, 1998 for further discussion). We propose a new, and somewhat di€erent model for crustal development in this area to those proposed by previous workers, which we believe is consistent with regional observations, both internal and external to the island of Borneo. There is clearly an interplay between continental and oceanic basement within Borneo, with the southwest of the island made up of the Palaeozoic to Mesozoic Schwaner Block of Sundaland anity, while elsewhere, and on the margins of the Kutai Basin, variably deformed, low grade metamorphosed Late Cretaceous±Early Tertiary turbidites of the Rajang and Embaluh Groups overlie older basic/ultrabasic igneous rocks and chert. These older basement rocks comprise peridotites, serpentinised peridotites, layered gabbros, gabbros, basalts, chert and siliceous metasediments of Jurassic±Early Cretaceous age (Moss, 1998). They are referred to as ophiolitic assemblages, although extensive deformation makes identi®cation of some parts of a `typical' ophiolite sequence dicult. We believe these highly deformed, dismembered ophiolitic assemblages represent Jurassic to Early Cretaceous age oceanic crust, although it is not clear if this ocean crust originated in a truly oceanic setting or in a back-arc ocean or marginal ocean basin. Ophiolitic rocks of Jurassic to Early Cretaceous age overlain by Late Cretaceous turbidites (now locally metamorphosed) have also been described in the Meratus Mountains (Sikumbang, 1990), in the basement rocks on the northeastern and western margins of the Kutai Basin (Moss, 1998), onshore Tarakan Basin and Darval Bay in Sabah (Omang and Barber, 1996) (Fig. 1). A similar relationship has also been described in Western Sulawesi (Bergman et al., 1996). We propose that the regional occurrence of thick Late Cretaceous±Early Tertiary turbidites above ophiolitic rocks, interpreted as Jurassic±Early Cretaceous oceanic crust, suggests that a very large deep marine turbidite fan system extended across Borneo during late Cretaceous times, perhaps similar to the present-day Bay of Bengal Fan in setting and dimensions (this is shown schematically in Fig. 3).

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Fig. 3. Reconstruction based on Hall (1996) showing Late Cretaceous±Early Tertiary `Borneo fan' with Oligocene to Miocene South China Sea spreading removed and West Sulawesi closer to Borneo prior to the opening of the Makassar Straits. An inferred palaeo-Mekong river sediment supply system from Vietnam to Borneo is shown. The present-day Bengal Fan is shown on the same scale for comparison.

the Kutai Lakes Gravity High has been explained as due to the uplift of a thick, well compacted Paleogene section overlying a metasedimentary and ophiolitic basement but which still lies at considerable depth (Chambers and Daley, 1995). Although the nature of the Lower Kutai Basin basement is not known, judging by the large accommodation space generated during the Tertiary, a much thinner (due to Cenozoic extension) and therefore weaker crust, equivalent to that exposed around the northern margins of the Kutai Basin and within the Meratus Mountains, is likely. The nature of the basement in the Lower Kutai Basin will probably always remain speculative, due to its depth and the limitations of drilling. In summary, the Schwaner Block represents true continental crust, but elsewhere throughout East Kalimantan (and probably as far as South Sulawesi and beyond) the crust appears to have been originally oceanic in nature, overlain by a thick turbidite basin during the Cretaceous. This Cretaceous ocean basin was subsequently deformed, thickened through crustal shortening, metamorphosed, possibly intruded by granites and eroded, prior to the beginning of Tertiary deposition in the Mid±Late Eocene. The basement, although it originated as oceanic basement, has a mixture of continental and oceanic properties. The

Cretaceous±Paleocene history of Borneo is explored more fully in Moss (1998). Basement bordering the northwest Kutai Basin has been intruded locally by granites and diorites of Late Cretaceous age (Pieters and Supriatna, 1990) but, as stated earlier, these granites have been dated using only the K-Ar technique. As similar granites are found cross-cutting stratigraphic units as young as Miocene in age, there is doubt as to the true age of many of these intrusives. Past workers have relied heavily upon remote sensing data to determine depth to basement in the Kutai Basin Area. In terms of petroleum exploration, `basement' is de®ned as the top of the Cretaceous± Paleocene meta-sediments. In practice, both gravimetric and magnetic techniques will interpret much of the Cretaceous section as part of the Tertiary sedimentary section, as the `basement' has many similar properties to the compacted sand-rich Paleogene syn-rift section. Even using seismic methods there is diculty in resolving the basement contact, as the acoustic impedence of Cretaceous metasediments and compacted sand-rich Paleogene section is remarkably similar. Fig. 4 shows a thinner Tertiary section than was proposed previously by Wain and Berod (1989). On the Wain and Berod (1989) map `depth to basement' of up to 5 km is shown at locations where we have

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Fig. 4. Isopach map for the Kutai Basin.

found Cretaceous metasediments outcropping at the surface. Our depth to basement map has been made using existing gravity and magnetic data, as used by Wain and Berod (1989) but constrained by observations of basement gravity and magnetic response and basement outcrops.

4. Eocene syn-rift facies and depositional architecture Numerous stratigraphic terms have already been de®ned to identify Eocene and younger units within the Kutai Basin (summarised in Rose and Hartono, 1978; Wain and Berod 1989; Pieters et al., 1987, 1993). The existing lithostratigraphic terminology for the basin has been strongly in¯uenced, unavoidably, by the speci®c areas in which individuals or companies have worked. The consequence is a somewhat parochial lithostratigraphic terminology with a confusing number of di€erent formation names, which have often been applied to identical or similar units. We propose a model that relates existing formations to speci®c time frames during the development of the basin. Initially, in the Eocene, rapid facies variations were a response to the development of high relief and

the formation of local depocentres within individual rift half-grabens. A more regional depocentre, developed in response to regional subsidence in the post-rift phase towards the end of the Late Eocene, resulted in a more continuous facies distribution which has continued to the present day. Initial Eocene depocentres may have been as small as 20 km in width and contained a variety of facies ranging from proximal alluvial fan to distal restricted deep marine. Our proposed lithostratigraphy is shown in Fig. 5. Fig. 6 schematically illustrates the development of a typical Middle Eocene half-graben within the Kutai Basin. Initial and proximal graben ®ll is coarse, poorly sorted material derived directly from basement. In the case of the Kutai Basin this material is likely to be sourced from low grade metamorphic turbidites of Late Cretaceous±Early Tertiary age and underlying Early Cretaceous to Jurassic ophiolitic crust. In East Kalimantan it is possible to demonstrate that there are both terrestrial and marine initial graben ®lls, generally with the more western depocentres being terrestrial-dominated, and the eastern depocentres (closer to the Makassar Strait) being more marine-dominated (i.e. below sea-level). These initial graben ®lls probably include alluvial fan deposits within western,

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Fig. 5. Lithostratigraphy of the Kutai Basin.

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Fig. 6. Schematic model for the initial syn-rift phase of sedimentation, Late Eocene reconstruction. See text for description.

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interior rifts but may be entirely marine in the eastern rifts (Fig. 6). Polarity of half-graben bounding faults strongly a€ects facies stacking patterns, and there is good evidence to suggest that during Eocene rift formation a complex series of opposing polarity half-grabens developed, o€set along rift-related transform faults (Cloke et al., 1997). With progressive rifting, marine transgression or regional subsidence, there was an overall change in the depositional environment from non-marine progressively through to shallow marine/coastal to shelf, and eventually bathyal marine environments, within the western rifts. In a relatively small graben depocentre it may be possible to view progressively almost all facies associations along one time line, within a distance of 20 km (Fig. 6). Within the eastern rifts, more directly in¯uenced by marine conditions from the onset of rifting, bathyal marine environments were more quickly established. Factors controlling facies include: sediment source and abundance, climate, rate of fault movement and subsidence, and overall architecture of the graben system. In shallow marine areas, removed from sediment input, carbonate platforms developed. Deeper marine basins, with only small amounts of coarse sediment input, developed thick shale sections. Sand-rich graben ®lls occur in depocentres linked directly to eroding basement. Given that during the Eocene a patchwork of rift basins were formed across the East Kalimantan area in response to regional extension, lithostratigraphic terminology must be used with extreme caution and it must be recognised that lithostratigraphic terms can only be used to group similar facies groups that may or may not be time or genetically equivalent. Detailed below are the standard lithostratigraphic terms for the Upper Kutai Basin, the facies described for these formations and their interpreted relationship to Eocene rift systems (Fig. 6). We have sub-divided the period of basin evolution into distinct time slices and have illustrated the range of depositional environments and resultant facies, which we have termed facies tracts. We have attempted to list all previously used lithostratigraphic names for these sediments. 4.1. Non Marine Syn-rift Facies Tract (Fluvial±Alluvial Fan Coarse Conglomerate and Sandstone Facies±Lower Kiham Haloq Sandstone Formation; Tanjung Formation, Kuaro Formation) In the northwest corner of the Kutai Basin, the basal contact of the Tertiary sequence is well exposed in several river sections. The basal sections of the Kiham Haloq Sandstone Formation type-section consist of channel bodies (Fig. 7(a)), and epsilon crossbedding within channels indicates lateral migration of accretion surfaces. Coarse conglomerate basal lags,

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and ®ning-up beds are present and suggestive of ¯uvial deposition processes (Pieters et al., 1993; Wain and Berod, 1989; Moss et al., 1997). Alluvial fan sands and conglomerates, similar to those of the Kiham Haloq Sandstone Formation, have not been observed in the northeast part of the Kutai Basin. It is possible that, since this section occupies the deepest part of graben, it has not been brought to the surface through inversion and hence is not exposed in most areas. Alternatively, fan deltaic deposits of the Beriun Formation may be the time equivalent deposits in graben systems more distal and marine with respect to eroding basement. Middle Eocene deposition along the southern margins of the Kutai Basin and within the Barito Basin also initially included non-marine sands, coals and conglomerates of the Kuaro and Lower Tanjung Formations (Fig. 5; van de Weerd et al., 1987). The presence of coarse conglomerates within the Tanjung Formation led to the interpretation of extensional faulting a€ecting this area of the basin (van de Weerd and Armin, 1992), although the Eocene structural style is poorly resolved on available seismic data. The Lower Tanjung Formation comprises a braided stream facies, passing up into a sandy ¯uvial facies (van de Weerd et al., 1987). These in turn pass laterally and vertically into the deltaic and shallow marine facies described below. 4.2. Deltaic Syn-rift Facies Tract (Sandstones, Conglomerates, Coals, Shales of the Beriun Formation, Batu Ayau Formation, Lower Tanjung Formation, Sembakung Formation, Kayanuit Formation) The Beriun Formation is a sand-rich delta system which crops out within inverted graben structures in the northern part of the basin, (Sunaryo et al., 1988; Satyana and Biantoro, 1995; van de Weerd and Armin, 1992). Lithologies comprise continental sandstones, shales and coals, as well as ¯uvio-deltaic and marine sandstones and shales, with rare limestones. Conglomerates associated with coals and shallow marine sediments of the Beriun Formation are time equivalent to the non-marine facies tract of the rift phase. Sunaryo et al. (1988) described 600±700 m of conglomerates interbedded with sandstones in the Beriun Formation. Fluvio-deltaic sedimentary features such as thick cross-bedded channel sands with scoured conglomeratic bases, and thick carbonaceous shales and coals, are common, together with immature conglomerates containing abundant basement fragments. Typical delta mouth bar facies, marine bioturbated intervals and occasional limestones con®rm that this is a marine, and not a lacustrine system. Benthonic foraminifera, when found, frequently indicate bathyal depositional environments, suggesting a steep basin

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slope into a slightly restricted sub-basin during Eocene times. Time-structure maps presented by Satyana and Biantoro (1995) clearly show sediment thicknesses within the Beriun Formation controlled by extensional faults, and demonstrate that faulting occurred during

deposition by the growth of section from the upthrown to the downthrown side of the fault block. All of these features are consistent with deltaic deposition during localised subsidence along extensional faults, such as the Gongnyay Fault, and rapid syn-rift deposition.

Fig. 7. (a) Field photo of a ¯uvial channel incised into an earlier channel within the early syn-rift facies of the Middle Eocene Kiham Haloq Sandstone Formation. Upstream from Long Bangun. Ian Cloke for scale; (b) ®eld photo of upward bundling wave ripple above planar laminated sands within Middle to Late Eocene marine syn-rift facies. Belayan River upstream from Tabang. Coin for scale; (c) ®eld photo, looking south, of folded distal turbidites of the Batu Kelau Formation part of the marine shale and turbidite syn-rift facies. Wahau river upstream from Muara Wahau. The east vergent fold is located in the hanging-wall of a thrust, formed in response to Early Miocene inversion; (d) ®eld photo of redeposited limestone conglomerate composed of siliciclastic clasts (cherts and dark metasediments) and clasts of re-deposited shallow water carbonates including corals. Part of the Late Oligocene platform carbonate facies. North of Muara Wahau.

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Fig. 7Ðcontinued

From a seismic stratigraphic study Satyana and Biantoro (1995) proposed a syn-rift fan delta facies model for the Beriun Formation. This model is supported here. In the northwestern part of the Kutai Basin, sands, shales, mudstones and coals of the Batu Ayau Formation are equivalent in age to the Kiham Haloq Sandstone and Batu Kelau Formation. The Batu Ayau Formation comprises ®ne to medium quartzose sands, thin coals, muds, shales and carbonaceous shales.

Mudstones and shales are often heavily bioturbated. Wain and Berod (1989) recognised three subdivisions of the Batu Ayau. A ¯uvial lower part and two upper marine divisions re¯ect regression and continental style deposition, followed by transgression and a return to marine deposition on an open, low energy shelf with low clastic input. These sediments are analogous to the Lower Berai Formation of the southern part of the basin (van de Weerd et al., 1987) and the Beriun Formation, but were deposited within di€erent discrete

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depocentres. The Lower Tanjung Formation, of the southwestern part of the Kutai Basin or the northern Barito Basin, similarly contains a paralic-deltaic facies (van de Weerd et al., 1987). 4.3. Shallow Marine Syn-rift Facies Tract (Mature Sandstones and Carbonates of Upper Kiham Haloq Sandstone Formation, Ritan Limestone Member of Upper Kiham Haloq Sandstone Formation, Parts of Batu Ayau Formation, Lower Tanjung Formation, Parts of the Batu Kelau Formation) Within the northwest corner of the basin, exposures of non-marine, deltaic, syn-rift facies tracts pass vertically and laterally into well-sorted, medium- to coarsegrained sandstones of the Batu Kelau Formation. These sandstones contain hummocky cross strati®cation, swaley cross strati®cation, large trough cross-bedding, megaripples, planar laminations and wave ripples (Fig. 7(b)), as well as trace fossils of the Skolithus and Cruziana ichnofacies. Metre-scale lenses of foraminiferal packstones and grainstones dominated by larger foraminifera such as Nummulites and Discocyclinids are intercalated with these sands. These carbonates, include the Ritan Limestone member of the Kiham Haloq Sandstone Formation, formed within a high energy, occasionally storm- or ¯ood-dominated shallow marine shelfal setting. The presence locally of herring-bone cross-strati®cation and reactivation surfaces within mature, medium-grained quartz sandstones indicate tidal conditions were present at times. The Lower Tanjung Formation in the southwest corner of the basin contains shallow marine clastic sediments that overlie and are probably laterally equivalent to deltaic and alluvial facies of the same formation (described above, van de Weerd et al., 1987). Thus the northern and southern margins of the basin show remarkably similar successions of sediment, although individual depocentres are unlikely to have been continuous throughout the basin. 4.4. Marine Shale and Turbidite Syn-rift Facies Tract. (Thin Turbidite Sands and Shales, Restricted Marine (?)Bathyal, Middle to Upper Eocene±Mangkupa Formation, Marah Formation, Parts of the Batu Kelau Formation, Bongan Formation, Lower Ujoh Bilang Formation, Atan Formation, Boh Formation) The Mangkupa Formation (Sunaryo et al., 1988) is a shale-rich, deep marine facies that has been intersected in wells at least 80 km east of the present basement outcrop (Fig. 2(a)), and also occurs in outcrop in the northeastern part of the Kutai Basin. It is noticeably di€erent from the sand-rich Beriun Formation (described above) that in®lls the Gongnyay Graben (Fig. 2(a)), and may be the distal marine equivalent or

the product of sand-starved extensional depositional settings. Recent revisions in palaeontological environmental interpretation suggest that the section is possibly not entirely deep marine (bathyal to abyssal) as previously supposed, but may in part restricted marine and isolated intra-shelf basins (A. Wonders, pers. comm. 1996). The Batu Kelau Formation occurs in the northwest corner of the basin, and comprises dark shales interbedded with siltstones and ®ne sandstones (Fig. 7(c)). Sandstone beds range in thickness from four to twelve centimetres, may show grading and partial Bouma sequences, and often show load casts and groove casts. Trace fossils, although common in some beds, have minimal diversity and comprise simple feeding and grazing trails belonging to the Zoophycos ichnofacies. The formation contains a rich and biostratigraphically useful nannofossil fauna along the upper reaches of the Belayan River, dating the formation as Late Eocene (P14±P15; Moss and Finch, 1998). The abundance of nannoplankton within these sediments, and the ichnofacies, suggest a deep marine, low-energy environment. Collectively the data indicate a deep water, open marine environment, a€ected occasionally by turbidity currents. Other workers have suggested an intertidal to shallow sub-tidal (Wain and Berod, 1989) or estuarine (Pieters et al., 1993) setting. We consider that a deeper marine environment, analogous to the Bongan Formation in the southern part of the basin (van de Weerd et al., 1987) is more appropriate. Proximal and distal slope and basinal facies are recognised within the Bongan Formation (Fig. 6). 4.5. Carbonate Platform Syn-rift Facies Tract (Lower Berai, Ritan Limestone Member of Keham Haloq Sandstone Formation) A variety of Eocene carbonate occurrences have been documented within the basin. Near to the northern margin, carbonates appear to have formed low relief shoals, bound by algae, but with a high shale content, suggesting proximity to a muddy, ¯uvial discharge system. Isolated smaller bodies of limestone are also found associated with deltaic sediments in the Gunung Gongnyay area (Fig. 2(a)) and probably represent a minor facies within the delta fairway that developed in areas of low clastic input. Some of these carbonates may have developed in shallow shelf sediment-starved settings and formed as patch reefs or shoals. These probably developed on basement highs adjacent to rift depocentres. Larger carbonate buildups and isolated platforms of Eocene age are visible at outcrop, for example along the Bungalun River (Fig. 2(a)) and the large limestone outlier of Gunung Khombeng (near Muara Wahau, Fig. 2) within the northeast of the basin. Isolated buildups such as

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Gunung Khombeng appear to have nucleated upon the uplifted crest of footwall blocks (Fig. 6). During the Late Eocene, on both the northern and southern margins of the basin, extensive carbonate platforms were developed. For example Upper Eocene argillaceous limestones with larger foraminifera occur below Oligocene carbonates in the Panain-Gunung Anga and Kerenden areas of the southern margin of the basin (Saller et al., 1992, 1993) as well as in the Bungalun River Area (Wilson et al., 1998). 5. Upper Eocene to Oligocene sag phase facies and depositional architecture By the end of the Eocene, extension in the Makassar Strait and associated forces within East Kalimantan appear to have ceased and regional subsidence occurred throughout East Kalimantan (Moss et al., 1997). Instead of local depocentres, as observed within the syn-rift half-grabens, a more regional depocentre developed as a result of marine inundation of much of the area (Fig. 8). On isolated high areas, and on margins of the basin, carbonates continued to accumulate, but within the basinal area a regional marine shale was deposited. It seems likely that by this time much of the topographic relief created during the MidEocene had been removed, as input of coarse clastic sediment into the basin became very limited. The sag

171

phase of sedimentation continued into the Late Oligocene, when East Kalimantan was once again disrupted by a major extensional tectonic event. 5.1. Basinal Shale Sag Phase Facies Tract (Monotonous Shales with Thin Turbiditic Sands, Ujoh Bilang Formation, Bongan Formation, Atan Formation, Wahau Formation, Marah Formation) A monotonous, marine shale-prone unit, conformable and transitional with the underlying rift-related formations, is described as the Ujoh Bilang Formation within the Upper Kutai Basin, and as the Bongan Formation in the southern part of the basin. It is not possible to distinguish this formation from the underlying marine syn-rift formations such as the Mangkupa, Marah or Batu Kelau Formations, with which it is transitional. The major part of the Ujoh Bilang Formation comprises uniform, monotonous shales and clays, with rare sandstones, deposited in an outer shelf to bathyal environment. Sporadic deposition from distal turbiditic currents is evidenced by the presence of thin sandstones with full and partial Bouma sequences. Similarly, the Wahau Formation from the Muara Wahau area is described as consisting of claystones and thin quartz sandstones by Supriatna (1990), deposited in an open marine, outer shelf setting (Pieters et al., 1993; Moss et al., 1997).

Fig. 8. Schematic model for the Early to Late Oligocene. See text for description.

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Within the Tanjung ®eld in the northern part of the Barito Basin, the Lower Oligocene Bongan Formation contains interbedded lithoclastic±bioclastic conglomerates, limestone olistoliths and carbonate and volcaniclastic turbidite beds (van de Weerd et al., 1987), suggested to represent distal slope to basinal facies. According to Wain and Berod (1989) and van de Weerd and Armin (1992), the Lower Ujoh Bilang Formation also contains volcanogenic rich sandstones of the Len Muring Sandstone member, with volcaniclastic, quartz and bioclastic grains, deposited as debris and mass ¯ows in a deep marine environment. Shale of this facies tract has been demonstrated to lie directly upon basement, indicating that this sequence was a response to transgression that covered both syn-rift sediments and surrounding basement areas. The Ujoh Bilang Formation represents a regional shale `blanket' over the syn-rift lithologies and signi®es the establishment of deep marine sag phase environments in the Lower Oligocene (Fig. 8).

5.2. Carbonate Sag Phase Facies Tract (Thick Shallow Marine Carbonates Batu Belah Limestone Member, Berai Formation, Taballar Formation, Batu Hidup Formation, Kedango Limestone) In parts of the basin, carbonate deposition was continuous from the Late Eocene to the Late Oligocene sag phase, but restricted to basement high areas such as the Bungalun River and Kerenden, and to basin margin areas (van de Weerd et al., 1987; Saller et al., 1992, 1993; Moss et al., 1997). Carbonates on the northern side of the basin are less well-known (see Wilson et al., 1998) but are analogous in age and facies to the Berai Limestone of the Upper Berai Formation on the southern basin margin (van de Weerd et al., 1987; Saller et al., 1992, 1993). The location of antecedent topography, such as basement highs and crests of fault blocks, still clearly in¯uenced the development of this facies within the basin (Fig. 8). The Batu Baleh Limestone Member of the Ujoh Bilang is a land-detached, isolated buildup within the north west part of the basin. Previous work, primarily based on benthic foraminifera, suggested that these carbonates were of Lower Oligocene age (Pieters et al., 1993; Wain and Berod, 1989). However, more recent work (Moss and Finch, 1998), based primarily upon a well-preserved nannofossil fauna, clearly shows these limestones to be of Upper Oligocene age (NP24±25). These carbonates may have developed on an intrabasin basement high associated with an intra-basin positive gravity anomaly (Moss et al., 1997).

6. Late Oligocene events in the Kutai Basin and Borneo An important unconformity occurs within the Upper Oligocene sediments of the Kutai Basin, apparently related to a renewed pulse of extension and uplift of central Kalimantan. Oligocene extensional faults developed orthogonal to Eocene extensional faults, suggesting a di€erent stress regime. It seems likely that Oligocene faulting follows pre-existing crustal planes of weakness, particularly northwest-southeast trending transform faults that separated Eocene graben systems. An attempt has been made to show this relationship in Fig. 9. The present Kutai Basin, as we presently recognise it, formed at this time, with the Sangkulirang and Bungalun faults in the north and the Adang Fault zone in the south as the principal hinge zones. These faults acted as down-to-the-basin hinge zones from the Late Oligocene to early Miocene. Moss et al. (1998) detailed the evidence for late Oligocene cooling of the late Cretaceous±Paleocene metasedimentary basement along the northwestern margin of the Kutai Basin. From apatite ®ssion-track data they suggested a tectonic cause for the cooling and likely uplift of the area. Apatite central ages span a range between 40 and 20 Ma, clustering between 20± 25 Ma. The data are interpreted as indicating that the samples began cooling, and hence probably began to be eroded, from 025 Ma onwards, and that the uplifted Cretaceous Embaluh Group provided sediment for Early Miocene deposition in eastern parts of the basin from 025 Ma onwards. This sediment source is consistent with petrographic data from Miocene reservoirs of the Sanga Sanga PSC near the eastern margin of the basin (Tanean et al., 1996). 6.1. Post 025 Ma Basinal Sag Phase Facies Tract (Upper Ujoh Bilang Formation, Marah Formation, Pamaluan Formation) In basinal areas deposition was continuous (but probably in a more distal deeper-water environment) from the underlying sag phase. The Lower Ujoh Bilang Formation is transitional with the Upper Ujoh Bilang Formation or Marah Formation. Rare turbiditic sands and limestones occur within the shale sequence (Fig. 9). In the northwest of the basin the Upper Ujoh Bilang Formation contains a minor volcanic presence in the form of the Ujoh Bilang Volcanic Series (Wain and Berod, 1989). Volcanic debris-rich sandstones of the Len Muring Sandstone Member have been described previously (in this paper) and possibly also belong to this suite. This tectonic event in the Late Oligocene was responsible for a radical change in basin architecture and hence disrupted the stable sag phase architecture by introducing sharp topographic changes and a new and abundant sedi-

173

Fig. 9. Schematic model for the Late Oligocene. See text for description.

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ment source from both uplifted hinterland and active volcanoes. 6.2. Upper Oligocene to Lower Miocene (N3±N5) Platform Carbonates Facies Tract (Batu Hidup Formation, Taballar Formation, Berai Limestone Formation) Large carbonate platforms, variably referred to as the Batu Hidup Formation or Berai Formation, began to develop during the Late Oligocene (Fig. 9) but were frequently established on pre-existing carbonate platforms. In the northern part of the basin, initially, a thin shelf limestone, sometimes referred to as the Kedango Limestone, was deposited across much of the area in response to shallowing of the depositional environment during the early part of the Late Oligocene tectonic event. The Kedango Limestone acts as a regional marker horizon, clearly visible on both SAR images and seismic data, eventually disappearing basinward to the south, as the facies changes to bathyal shale. The Batu Hidup Limestone developed above the Kedango Limestone, in areas of suitable water depth and presumably low clastic input. The Batu Hidup Limestone consists of reef and platform carbonates up to 1000 m thick that form large karsti®ed areas at outcrop. The limestones contain identi®able coral

material in a generally foraminiferal wackestone to packstone matrix. Surrounding these areas of limestone, coeval basinal sediments contain numerous beds of calci-turbidites and carbonate-rich debris ¯ow conglomerates (Fig. 7(d)). Four lithofacies recognised within these redeposited limestones are shown in Fig. 10. These units contain numerous clasts of shallow marine carbonate (such as corals and blocks of lithi®ed wackestone and packstone) as well as lithoclastic fragments (such as chert and sandstone). These turbidites and redeposited limestones may be the product of highstand shedding and progradation of the platforms as described from the Berai Limestone on the southern margin of the basin (Saller et al., 1992, 1993) and many other carbonate platforms (cf. Droxler and Schlager, 1985; Davies et al., 1989; Emery and Meyers, 1996). Alternatively, some may have been derived from an uplifted footwall crest, analogous to Eocene to Miocene redeposited limestones of SW Sulawesi (Wilson and Bosence, 1996) although a more detailed study is required. Along the southern basin margin a similar facies succession has been described with the Berai Limestone, forming on the stable platform south of the Adang Fault Zone. Isolated platform areas such as Kerenden have been described basinward of the Adang Fault Zone (van de Weerd and Armin, 1992).

Fig. 10. Representative lithological columns for four common redeposited limestones lithofacies found in Late Oligocene to Early Miocene periplatform deposits.

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7. Lower Miocene facies and depositional architecture The Early Miocene was a period of overall regression and basin ®lling with the beginning of progradation of the proto-Mahakam River and associated deltaic sediments. Sediment continued to be sourced from Mesozoic cherts and turbidites that were uplifted during the Late Oligocene tectonic event. Further sediment sources were provided by uplifted Paleogene sediments in western parts of the basin and volcanic material from the volcanoes of the Sintang extrusives that were active at this time. 7.1. Lower Miocene Deep Marine Post-second Rift Event Facies Tract (Sandstones and Shales of the Wahau Formation, Lawa Formation, Ritan Formation, Pamaluan Formation, Loa Duri Formation) Extensive outcrops of Lower Miocene (N4±N6 Blow Zone) interbedded sandstones and shales of the Wahau Formation outcrop to the southeast of Muara Wahau (the Bungalun to Khombeng road section, Hainim and Chambers, 1994). These sandstones and shales show partial Bouma sequences, sandstone bed amalgamation, current ripple lamination, dish structures, metre-thick, generally massive sandstone beds, ®ning-up sandstone beds and coarsening-upward decimetre cycles or parasequences. Sole structures in the form of groove and load casts and prod marks are common, as are large syn-sedimentary slumps and sandstone dykes. A transport direction to the southeast is suggested from the sole structures. There was a pronounced increase in the proportion of sand being deposited during N4±N6 times, most likely in response to the changing basin topography initiated during the latest Oligocene (0N3 times) tectonic event (Fig. 9). The presence of bathyal foraminifera in adjacent shales indicate that these are probably slope mass-¯ow deposits, part of an extensive pro-delta turbidite submarinefan system that was established across this area in response to movement on the Bungalun to Khombeng Fault and a sudden deepening of the basin southward. Sands are rich in volcanic fragments derived from the Sintang volcanoes, active during the Late Oligocene and Early Miocene (see below). Some intervals also show debris ¯ows of coarse carbonate material derived from the adjacent carbonate shelf areas. To the west within the Tinjau (Fig. 2(a)) and Upper Mahakam areas the same stratigraphic interval is more proximal and coarser grained with conglomerates present in the Ritan Formation. Closer to the southern basin margin a similar facies style is apparent in the depocentre formed adjacent to the Adang Fault Zone and is ®lled with a type of turbidite sediments similar in nature to those described near the northern basin

margin. This Formation.

is

referred

175

to

as

the

Pamaluan

7.2. Lower Miocene (N6±N8 Blow Zone) Deltaic Facies Tract (Shallow Marine to Terrestrial Sandstones, Claystones and Coals of the Lower Balikpapan Beds, Pamaluan Formation, Wahau Formation. Shallow Marine Carbonates of the Lower Miocene (N6±N8)±Pulau Balang Formation, Loa Kulu Formation, Bebulu Formation) Deltaic progradation in the Kutai Basin began during N5/6 times (van de Weerd and Armin, 1992) (Fig. 11) and is apparently transitional with the thick turbidite and bathyal shale sequence described in the previous section, thus representing an overall continuing regressive sequence, as sediment supply outstripped accommodation. In the Samarinda area deltaic sedimentation was not established until N8 time. Lithofacies terminology for the Neogene deltaic stratigraphy in the Kutai Basin has become increasingly confused, largely through misunderstandings of the nature of cyclic deltaic stratigraphy, and a widespread misunderstanding of the nature and timing of tectonic inversion events and their e€ects on the style and rates of sedimentation within the basin. An original lithostratigraphic framework was proposed by Leupold and van der Vlerk (1931) based on long range shallow marine foraminifera and the lithological succession seen in the Balikpapan Bay to Sanga Sanga Area. This system was slightly modi®ed by Marks et al. (1982). An alternative system was proposed by Land and Jones (1987) as a result of regional coal exploration work in the Samarinda Area. Although Land and Jones (1987) based their study on extensive coal drilling results they failed to recognise the stratigraphic inter-relationship of the three units; Loa Kulu Formation, Batuputih Limestone and Loa Duri Formation. Chambers et al. (1992) demonstrated a more consistent stratigraphic relationship for the units proposed by Land and Jones (1987) based on extensive ®eld work carried out by LASMO Runtu Limited. In this later stratigraphic nomenclature the Batuputih Limestone (equivalent to the Bebulu Formation) is interpreted as a transitional shelf deposit between the deep marine Loa Duri Formation (elsewhere referred to as the Pamaluan or Pulau Balang beds) and the deltaic Loa Kulu Formation (elsewhere referred to as the Lower Balikpapan or Pulau Balang beds). A less confusing way to view the stratigraphic succession in the Samarinda area was presented by Chambers and Daley (1995, Fig. 12). In this stratigraphic column which is speci®c for the Samarinda Area, a single deltaic progradation is shown over an extensive syn-rift and sag phase basinal shale section. The Middle Miocene section was not deposited in this area but

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Fig. 11. Schematic model for the Early Miocene. See text for description.

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177

Fig. 12. Stratigraphic scheme for the Middle to Upper Miocene of the Samarinda area (from Siemers et al., 1994).

exists as a thick wedge to the east, as discussed in the following section. Fig. 12 summarises Neogene stratigraphic concepts developed by Chambers et al. (1992) using the lithostratigraphic terminology of Land and Jones (1987) and shows the time transgressive nature of the Batuputih Limestone which is really a patch-reef type of coraldominated carbonate build-up that developed on the shelf in areas where clastic sedimentation was not active. Netherwood and Wight (1992) have described similar delta front carbonates in the Tarakan Basin. 8. Middle Miocene to Recent deltaic progradation (N9± Present) post-inversion facies tract 8.1. Sandstones, Shales, Shallow Marine Carbonates, Coals of the Balikpapan Formation, Prangat Formation, Samboja Formation, Kampung Baru Formation and Kelinjau Formation, Kutai Lakes Formation Following the inversion tectonic event at the end of the Early Miocene a ¯ood of deltaic sediments prograded eastward from the newly formed Samarinda anticlinorium and into the Makassar Strait, where crustal accommodation appears to have matched sediment input up to the present day, to form the presently active Mahakam Delta. This sediment package is variably referred to as the Prangat and Samboja Formations or Upper Balikpapan and Kampung Baru Formations (Fig. 13). Workers in the Total Mahakam PSC describe a delta to shelf and slope sequence similar to that documented in the Lower Miocene succession (Duval et al., 1992).

This sequence is the subject of an extensive literature as it forms the present actively exploited petroleum system in the Kutai Basin, with proven reserves in excess of three billion barrels of oil, and gas reserves in excess of 30 TCF. For further details readers are referred to Paterson et al. (1997) and Duval et al. (1992). Inland of the Samarinda anticlinorium a lacustrine basin developed in response to the end Early Miocene tectonic event and lacustrine sediments and thick peat beds were deposited referred to variably as the Kutai Lakes Formation and the Kelinjau Formation. 9. Volcanism in East Kalimantan Field and petrographical studies have con®rmed three groupings of Tertiary igneous rocks in the Kutai Basin. These are the Mid to Upper Eocene Nyaan volcanics, Upper Oligocene to Lower Miocene Sintang intrusives and volcanics and Pliocene Metulang volcanics. A summary of the available data and new work was presented in Moss et al. (1997, 1998). The Sintang intrusive and volcanic rocks comprise shallow level intrusives and extrusives consisting of diorites, microdiorites, dacite, microgranites and andesites which are widely distributed across Borneo (Moss et al., 1997, 1998). Rocks of this suite are the most geographically widespread and volumetrically the most important. Volcanics of the Sintang Suite have been found to range in age from 40±8 Ma, although most of the ages cluster in the Late Oligocene to Early Miocene time interval (all K-Ar dates; Pieters and Supriatna, 1990; Doutch, 1992, van Leeuwen et al.,

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Fig. 13. Schematic model for the Late Miocene. See text for description.

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1990; Moss et al., 1997, 1998). This large range may be a re¯ection of analytical problems with the K-Ar method and more precise Ar-Ar study of this suite is required. Volcaniclastic clasts in the Lower and Upper Ujoh Bilang Formation and Bongan Formation may well have been derived from an early phase of Sintang igneous activity, whereas K-Ar dating of the Sintang Volcanics along the Telen River centre, west of Muara Wahau provides a more constrained 23±18 Ma age range (Moss et al., 1998). Sintang volcanism was also responsible for widespread distribution of volcanilithic sands in the Sanga Sanga area during the Early Miocene (Tanean et al., 1996). It is also likely that there was more than one volcanic phase which produced rocks ascribed to the Sintang igneous activity across the island of Borneo. The sedimentary record from Sanga Sanga PSC suggests a fairly discrete and short lived episode (Tanean et al., 1996) as opposed to the long time range suggested from radiometric dating of rocks ascribed to the Sintang Suite. Pliocene±Pleistocene volcanics of the Metulang (or Plateau) suite are common throughout the centre of Kalimantan. The suite comprises Upper Miocene to Pliocene andesitic stratovolcanoes and Pliocene to Pleistocene basaltic lava ¯ows which form a series of high plateaux. The rocks belong to a medium to high K calc-alkaline suite with compositions ranging from basalt, high-K trachyandesite to andesite. These volcanic centres account for many of the high plateau areas of central Borneo. Pieters et al. (1993) argue that the Metulang volcanics were formed in response to uplift and extensional tectonism following isostatic compensation of overthickened crust which developed during convergent tectonics in the Late Cretaceous to Early Tertiary. They may in part be a late stage development of the more andesitic Sintang Intrusive suite. More detailed geochemical and radiometric dating is required before we fully understand this phase of volcanism which a€ected large areas of Borneo's interior. Eocene acidic volcanics appear to be intimately associated with rifting, but the cause of the younger volcanism remains controversial. Oligocene to Lower Miocene (Sintang) and the Pliocene volcanics (Metulang) are calc-alkaline in nature and have been described as subduction type products. This is dicult to accept given that the trench to magmatic arc gap would have to be greater than 500 km. An alternative hypothesis suggested by Tanean et al. (1996) is that these volcanics are related to the melting of a thickened and shortened orogenic root. This supported somewhat by the limited geochemical data presented in Moss et al. (1998).

179

10. Discussion and conclusions Parochial lithostratigraphical nomenclature has obscured the lateral variability and diachronism of facies expected within rift basins such as the Kutai Basin. For the Kutai Basin, where localised studies, through their very nature, have unavoidably failed to recognise the true arrangement of facies within the basin and the separate phases of basin evolution with which these facies are associated. In the Kutai Basin this is particularly important for the understanding of the Eocene syn-rift sequences. Typically potential reservoir horizons in the Eocene, such as the Beriun Formation, have been dicult to trace regionally and in the subsurface (e.g. Satyana and Biantoro, 1995). This is due to the restricted nature of Eocene deposition in discrete graben. Stratigraphically the Beriun Formation, for example, will pass down-dip into lower energy, shale-dominated facies, previously regarded as separate formations (such as the Mangkupa or Batu Kelau Formations) and thought to be of di€erent ages. The origin of the Kutai Basin has for sometime been poorly understood and various origins have been suggested. An origin as a peripheral foreland basin was suggested by Pieters et al. (1993), for example. It is hoped that by detailing the geometry and architecture of facies and highlighting the distinct phases in the evolution of the Kutai Basin we have ®rmly established the origin of the Kutai Basin as an extensional basin. The basin formed through the establishment of initially discrete and unconnected half-graben and graben depocentres over a wide area of East Kalimantan. This is quite di€erent from other extensional areas, such as the East Africa Rift System where extension is concentrated along one axis forming a single rift valley. A tectonic event at the end of the Early Miocene changed the tectonic style by inverting some of the Tertiary depocentres and in places brought basement to the surface. Rapid deltaic progradation into the Makassar Strait followed, but deposition was accommodated by extended crust in the Makassar Strait, so that in excess of 5000 m of deltaic section has been deposited in the Mahakam delta depocentre from Mid Miocene to the present day. Deformation events in the Upper Miocene and Pliocene a€ected the area and are responsible for renewed uplift of the hinterland, with periodic pulses of clastic material. Acknowledgements SJM recognises the ®nancial support of the SE Asia Consortium of Companies (ARCO, LASMO, Can Oxy, Exxon, Union Texas, Mobil). Dr Irwan Bahar, the former Director of GRDC, is acknowledged for

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help in arranging ®eldwork. Dharma Satria Nas and Ian Cloke are thanked for their help and friendship in the ®eld. JLCC recognises the assistance of LASMO Runtu co-workers, particularly Ian Carter, Tim Daley and Je€ Towart as well as all who helped in the LASMO Samarinda Oce from 1991-5 during which ®eld work was undertaken. E-mail contact address: [email protected]

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