1999_dep model & facies of rift n inversion episodes kutai basin_moss & chambers

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IPA99-G-188

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATIONTwenty Seventh Annual Convention & Exhibition, October 1999

DEPOSITIONAL MODELING AND FACIES ARCHITECTURE OF RIFT AND INVERSION

EPISODES IN THE KUTAI BASIN, KALIMANTAN, INDONESIA

Steve J. Moss*

John L.C. Chambers**

ABSTRACT

The Kutai Basin, a large sedimentary basin in easternKalimantan, hosts significant oil and gas resourceswithin Miocene deltaic systems. We have integrated

disparate geological and geophysical surface andsubsurface data-sets to re-interpret Tertiary faciesdistributions in the basin and present models to explainthe progressive tectonic evolution of the basin, theresulting depositional environments and theirarrangements within the basin in relationship to major

basin tectonostratigraphic phases.

The basin was initiated in the Middle Eocene in

conjunction with rifting and incipient sea-floorspreading in the North Makassar Straits. The resultingseries of N-S/NE-SW trending, discrete, fault-

bounded depocentres preceeded a sag phase inresponse to thermal relaxation. Sedimentary fill of thediscrete, Eocene depocentres varies depending upon

position with respect to sediment source, palaeo-waterdepths and geometry of the half-grabens. This

strongly contrasts with the regionally uniformsedimentary styles that followed in the latter part ofthe Eocene and the Oligocene. Tectonic uplift,documented along basin margins and relatedsubsidence of the Lower Kutai Sub-basin, occurredduring the Late Oligocene. This subsidence isassociated with significant volumes of high level

basin development is important for the appreciation ofresource distribution in this basin and similar rift basinsof Borneo and SE Asia.

_______________________________________ ____* Consultant

** LASMO Venezuela BV, Caracas, Venezuela INTRODUCTION

This paper describes the depositional environments,the resultant facies associations and their architectureduring different phases of evolution of the Kutai

Basin, from the Middle Eocene to present. Previouslithostratigraphic schemes for the basin have failed torecognise the lateral variability and diachronism offacies that can be expected within rift basins such asthe Kutai Basin. These earlier studies have,unavoidably, failed to recognise the true arrangementof facies within the basin and the separate phases of

basin evolution with which these facies areassociated. The Cenozoic succession of the KutaiBasin has been subdivided into separate depositional

phases which are coincident with the onset and theend of major, basin-forming processes and significantvariations in basin-fill character. As such, thesedepositional phases are akin to megasequences,although they are not in all instances bounded by



associated with significant volumes of high-level

accumulations than those which were targeted duringinitial exploration phases in the basin.

GEOLOGY OF THE KUTAI BASIN

The Kutai Basin, located on the eastern side ofKalimantan (Figure 1), comprises the western, UpperKutai Sub-basin and the eastern, Lower Kutai Sub-

basin (Figure 2). The Upper Kutai Sub-basin todayrepresents an area of major, tectonic uplift andsubsequent erosion which resulted from inversion of

the Paleogene depocentres. The boundaries of thePaleogene-aged Kutai Basin are hard to define asextensional tectonism of Middle Eocene age resultedin a patchwork of connected and unconnected N-Sand NE-SW- oriented grabens and half-grabensacross the eastern part of Kalimantan.Both the Lower- and Upper- Kutai Sub-basins overliethese Paleogene depocentres. Two, northwest-

southeast-trending fault zones bound the basin, i.e. theAdang Fault to the south and the Sangkulirang Fault tothe north (Figure 2). Both fault zones and their relatedoffsets extend both onshore and offshore (Cloke etal., 1999). To the south of the Sangkulirang Fault isthe Bengalon River Fault Zone, a NW-SE trendingzone which borders the northern margin of the LowerKutai Sub-basin and which marks the rapid change,from north to south, of thin to thick Neogenesequences within the Lower Kutai Sub-basin. TheAdang Fault defines the southern sub-basin boundary.The Neogene section onshore deepens rapidly

between the Bengalon River and Adang fault zones,which represent hinge zones or down-to-the-basin,normal faults and were active during the LateOligocene to Miocene (Cloke et al., 1999).

In central and northern parts of Borneo, turbidites ofthe Rajang and Embaluh groups, of Late Cretaceous-Paleocene age, cross the island in a northeast-southwest-trending crescent which is referred to asthe Central Kalimantan Ranges (Figure 2). Theseturbidites overlie (?) older, more deformedbasic/ultrabasic igneous rocks and cherts The igneous

Central Kalimantan Ranges. The basement whichunderlies the Kutai Basin is defined as the top of the

Cretaceous-Paleocene metasediments (EmbaluhGroup), and its position has traditionally been difficultto determine with accuracy due to the similarities ofgeophysical signatures of both the Cretaceous-Paleocene metasediments and the compacted Eocenesection. The depth-to-basement map is based on a re-interpretation of gravity and magnetic data,constrained by field mapping (Figure 3). A broadlyarcuate feature, referred to as the Muyup Hinge

(Figure 3; Wain & Berod, 1989), trends roughlynortheast-southwest across the basin and appears tocontrol the western margin of the Lower Kutai Sub-

basin. This feature parallels broad, arcuate structureswithin outcrops of the Embaluh Group turbidites in the

basement to the northwest. The Muyup Hinge may berelated to the underlying geometry of the EmbaluhGroup and has acted as a zone of weakness during

deposition of the Tertiary sequence. A north-northeast-south-southwest trending gravity high, theKutai Lakes Gravity High (Figure 3), extendsnorthwards from the Meratus Mountains, in the south,to the Gunung Gongnyay area in the north (Figure 2).The Kutai Lakes Gravity High generally parallels theorientation of the fold axis of the SamarindaAnticlinorium in the basin (Figure 2) and is offsetalong northwest-southeast aligned lineaments which

parallel the trends of the Adang, Sankulirang andBengalon fault zones (Wain & Berod, 1989;Chambers & Daley, 1995; Cloke et al., 1997). We

believe the Meratus Mountains and the GunungGongnyay area are part of the same major, Mioceneinversion feature that extends through the Kutai Basinand along which deeply buried, Paleogene riftsediments which overlie basement have been uplifted.

In the case of the Meratus Mountains, inversion has been responsible for bringing basement rocks to thesurface, but at Gunung Gongnyay, surface exposurescomprise compacted Paleogene sediments. Within theLower Kutai Sub-basin, the Kutai Lakes Gravity Highhas been explained as representing uplift of a thick,well compacted Paleogene section overlying a



Straits produced a series of discrete, fault-boundeddepocentres (half-grabens). Dating of the sedimentary

fill was done using foraminifera and nannofossil biostratigraphy (Figure 5). During rifting a complexseries of opposing-polarity half-grabens developed,oriented NNE-SSW to NE-SW and offset along rift-related transform faults oriented ESE-WNW to SE-

NW (Figure 6) (Cloke et al., 1999). The morewesterly depocentres have more terrestriallydominated fill, with the eastern depocentres beingmore marine-dominated (Figure 6). Initial graben-fills

include alluvial fan deposits within western, interiorrifts but may be entirely marine in the eastern rifts.Initial and proximal graben-fill is coarse, poorly sortedmaterial derived directly from erosion of low-grademetamorphic turbidites of Late Cretaceous-Paleoceneage and underlying Early Cretaceous to Jurassicophiolitic crust. With progressive rifting and/orregional subsidence, there was a change in the

depositional environment from non-marine to shallowmarine/coastal to shelf and, eventually, bathyal marineenvironments within the eastern rifts (Figures 6 & 7).Within the eastern rifts, bathyal marine environmentswere more quickly established and these rifts areshale-dominated. In shallow marine areas removedfrom siliciclastic input, land-detached carbonate

platforms developed on highs. Sand-rich graben fillsare to be expected in depocentres with nearby eroding

basement highs. Five different facies associations arerecognised during this stage of the basin's evolution.They are non-marine, deltaic, shallow-marine,carbonate platform and marine shale and turbidite syn-rift facies associations. Facies associations seen on

both the northern and southern margins of the basinshow remarkably similar successions of sediment,although given the distance across the basin, individual

depocentres are unlikely to have been continuous.Factors controlling the facies include: sediment sourceand abundance, climate, rate of fault movement andsubsidence, and overall architecture of the grabensystem.

The non marine syn rift facies associations consist of

through inversion and hence is not exposed in mostareas. A sand-rich delta system of the deltaic syn-rift

facies crops out within inverted graben structures inthe northern part of the basin, (Sunaryo et al., 1988;Satyana & Biantoro 1995; Van de Weerd & Armin1992). Lithologies consist of continental sandstones,shales and coals as well as fluvio-deltaic and marinesandstones and shales with rare limestones.Deposition of conglomerates and interbeddedsandstones of the Beriun Formation was controlled bysyn-depositional, extensional faults, consistent with

rapid development of a syn-rift, fan delta during periods of localized subsidence along extensionalfaults (Sunaryo et al., 1988). Fluvio-deltaicsedimentary features such as thick, cross-bedded,channel sands with scoured conglomeratic bases, andthick carbonaceous shales and coals, are commontogether with immature conglomerates with abundant

basement fragments. Typical delta mouth-bar facies

and marine, bioturbated intervals and occasionalforaminiferal limestones confirm a marine influenceand not a lacustrine system. Although organicgeochemistry of oil samples indicate the presence ofcoastal plain/delta top lakes (Guritno & Chambers,1999), coals and carbonaceous mudstones weredeposited within delta plain and intertidal settings.

The shallow marine, syn-rift facies associationscomprise well-sorted, medium- to coarse- grained,

bioclastic, quartz sandstones. These sandstones exhibithummocky cross-stratification, swaley cross-stratification, large-scale trough cross-bedding,megaripples, planar laminations and wave ripples, aswell as containing trace fossils of the Skolithos andCruziana ichnofacies. Metre-scale lenses of

packstones and grainstones which are dominated by

larger foraminifera such as Nummulites andDiscocyclinids are intercalated with these sands. Suchcarbonates formed within a high energy, occasionallystorm influenced, shallow-marine shelfal setting. Localherring-bone cross-stratification and reactivationsurfaces within texturally-mature, medium-grained,quartz sandstones indicate sporadic tidal conditions



trails belonging to the Zoophycos ichnofacies. A richnannofossil fauna dates the formation as Late Eocene

(P14-P15; Moss & Finch, 1998), and the abundanceof nannoplankton and the ichnofacies indicates a deepmarine, low-energy, open marine environment,affected occasionally by turbidity currents. However,a recent revision of palaeoenvironments suggests thatthe section is not entirely bathyal to abyssal but may in

part be restricted marine and isolated intra-shelf basins (A. Wonders, pers comm. 1996). Benthonicforaminifera, when found, frequently indicate bathyal

depositional environments, suggesting a steep basinslope into slightly restricted, Eocene depocentres. Ashale-rich section has been intersected in wells atleast 80 km to the east of the present basementoutcrop and also in outcrop in the northeast of theKutai Basin. Noticeably different from the ageequivalent, sand-rich, deltaic syn-rift facies exposed inthe inverted graben at Gunung Gongnyay (Figure 2),

this facies association may be a distal marineequivalent or the product of sand-starved, extensionaldepositional settings (Figure 7).

The platform syn-rift facies contains a variety ofEocene carbonates. Near to the northern margin,carbonates formed low-relief shoals, bounded by algae

but with a high shale content, suggesting proximity to amuddy, fluvial discharge system. Isolated smaller

bodies of limestone are also found associated withdeltaic syn-rift sediments in the Gunung Gongnyayarea (Figure 2), and probably represent a minor facieswithin the delta fairway that developed in areas of lowclastic input. Some of these carbonates may havedeveloped in shallow shelf, comparatively sediment-starved settings, although some benthonic foraminiferacan tolerate up to 50% silicic lastic content and formed

as patch reefs or foraminiferal shoals. These probablydeveloped on basement highs adjacent to riftdepocentres and away from sediment sources. Largercarbonate build-ups and isolated platforms of Eoceneage are visible at outcrop, for example along theBengalon River (Figure 2) and the large limestoneoutlier of Gunung Khombeng (near Muara Wahau

emphasised that good quality micropaleontology wasessential in recognising the lateral arrangement of

lithofacies, as previous workers had shown the distalfacies to be younger than the proximal facies.

LATE EOCENE TO LATE OLIGOCENE SAG

PHASE FACIES ASSOCIATIONS AND

DEPOSITIONAL ARCHITECTURE

By the end of the Eocene, extension had ceased in the

Makassar Strait and within East Kalimantan andregional subsidence occurred throughout EastKalimantan (Moss et al., 1997). Instead of the localdepocentres observed within the syn-rift half-grabens,a more regional depocentre developed as a result ofmarine inundation and/or regional subsidence. Onisolated high areas, and on margins of the basin,carbonates continued to accumulate, but within the

basinal area marine shale accumulated. It seems likelythat by this time erosion had removed much of thetopographic relief created during Mid-Eocene rifting,as the input of coarse clastic sediment into the basin

became very limited. This sag phase of sedimentationcontinued into the Late Oligocene.

Two facies associations are recognised across the basin during the sag phase. The basinal shale, sag phase facies association is primarily a marine shaleunit, often conformable and transitional with theunderlying marine shale and turbidite, syn-rift facieslithologies, from which it is indistinguishable. Most ofthe basinal shale, sag phase facies associationscomprise uniform, monotonous shales and claystoneswith rare sandstones. Sporadic deposition fromturbiditic currents is evidenced by the presence of thin

sandstones with full and partial Bouma sequences.Deposition took place in an open marine, outer shelf to

bathyal environment. In parts of the basin, carbonatedeposition was continuous from the Late Eocene toLate Oligocene, but was restricted to basement highareas such as the Bengalon River and Kerenden, andto basin margin areas (Van de Weerd et al 1987;



antecedent topography, such as basement highs andcrests of fault blocks, still clearly influenced the

development of carbonates within the basin (Figure 9).LATE OLIGOCENE EVENTS IN THE KUTAI

BASIN

An important unconformity occurs within UpperOligocene rocks of the Kutai Basin (Figure 4),apparently related to a renewed pulse of uplift ofcentral Kalimantan and extension in the basin.Extensional faulting was orthogonal to the Eocene

extensional faults and under a different stress regime.Late Oligocene faulting follows pre-existing crustal

planes of weakness, particularly northwest-southeasttrending, transfer faults that separated Eocenegrabens (Cloke et al., 1999). The present-day, KutaiBasin depocentre was formed at this time, with theSangkulirang and Bengalon River fault zones in thenorth, and the Adang Fault Zone in the south, acting

as the principal hinge zones from the Late Oligoceneto Early Miocene. Late Oligocene cooling of the LateCretaceous-Paleocene metasedimentary basementwas induced by tectonic uplift along the northwesternmargin of the Kutai Basin (Moss et al., 1998). Apatitefission track data show cooling and, by inference,erosion began from around 25 Ma onwards. Modellingof the apatite data indicates an initial short period ofrapid cooling, most likely due to tectonic uplift,followed by slower cooling over a longer period due todenudational uplift (Figure 9). Concurrent with thisuplift widespread volcanism occurred. The upliftedCretaceous Embaluh Group provided sediment forEarly Miocene deposition in eastern parts of the basinfrom this time onwards. This Late Oligocene tectonicevent was responsible for a radical change in basinarchitecture and disrupted the stable sag phase by

introducing sharp topographic changes and a new andabundant sediment source, from both upliftedhinterland and active volcanoes.

Two facies associations are recognised across the basin following the Late Oligocene (~25 Ma)tectonism In basinal areas deposition was continuous

supplied by active volcanism that began in the LateOligocene, whilst carbonate debris was provided by

continued carbonate platform deposition. Platformcarbonates facies deposition continued into the EarlyMiocene when large carbonate platforms began todevelop upon the pre-existing Late Oligocenecarbonate platforms. In the northern part of the basina thin, shelf limestone was deposited across much ofthe area, in response to shallowing of the sea duringthe early part of the Late Oligocene tectonic event.This limestone acts as a regional marker horizon,

clearly visible on both SAR (Synthetic ApertureRadar) images and seismic profiles, but eventuallydisappears basinwards to the south as the facieschanges to bathyal shale. Overlying limestones,generally coralline, foraminiferal wackestones to

packstones, which developed in shallow water andlow clastic input areas, consist of isolated buildups and

platform carbonates up to 1000 m in thickness that

now form large, karstified areas on the basin margins.Coeval basinal sediments contain numerous beds ofcalci-turbidites and carbonate-rich debris flow(debrites) conglomerates. These units containnumerous clasts of shallow marine carbonate (coralsand blocks of lithified wackestone and packstone).Some beds also contain lithoclastic fragments such aschert and sandstone. A similar facies succession isalso present along the southern basin margin on thestable platform to the south of the Adang Fault Zone.On both the southern and northern margins of the

basin these redeposited carbonates may be the product of highstand shedding and progradation of the platforms as suggested for the southern margin of the basin (Saller et al., 1993). Alternatively, some mayhave been derived from an uplifted footwall crest.Isolated platform areas, such as Kerenden, also

existed basinward of the Adang Fault Zone (Van deWeerd & Armin, 1992).

LOWER MIOCENE FACIES ASSOCIATIONS

AND DEPOSITIONAL ARCHITECTURE

The Early Miocene was a period of overall regression



(Figure 11) with several inversion episodes resulting inuplift of Eocene syn-rift sections. Sediment continued

to be sourced from Mesozoic cherts and turbiditesuplifted during the Late Oligocene tectonic event,uplifted Paleogene sections in western parts of the

basin, and volcanic material from active volcanism.

Two facies associations are recognised across the basin in the Early Miocene, a deep marine post-second rift event, facies association and an EarlyMiocene (N5-N8 Zones) deltaic facies association.

The former contains Lower Miocene (N4-N6 Zones),interbedded sandstones and shales. These show

partial Bouma sequences, sandstone bedamalgamation, current ripple lamination, dishstructures, metre-thick, generally massive sandstone

beds, fining-upward sandstone beds and coarsening-upward, decimetre cycles or parasequences. Grooveand load casts and prod marks are common, as are

large syn-sedimentary slumps and sandstone dykes. Atransport direction to the southeast is indicated by thesole structures. Bathyal foraminifera in adjacentshales indicate that the coarser-grained clastics are

probably slope mass-flow deposits and part of anextensive, pro-delta turbidite, submarine-fan systemthat was established across this area in response tomovement on the Bengalon River Fault Zone andconsequent sudden, southward deepening of the basin.The sandstones are rich in volcanic fragments derivedfrom the active volcanism and some intervals alsoshow debris flows of coarse carbonate materialderived from the adjacent carbonate shelf areas. TheLower Miocene (N5-N8 Zones) deltaic faciesassociation comprises deltaic to flood-plainsandstones, shales, coals and coral reef limestones, all

previously ascribed to a number of formations.

Lithofacies terminology for the Neogene deltaicstratigraphy in the Kutai Basin has becomeincreasingly confused through misunderstandings ofcyclic deltaic stratigraphy and a widespreadmisunderstanding of the nature and timing of tectonicinversion events and their effects on the style and rateof sedimentation within the basin (Allen & Chambers

section. The time-transgressive patch-reef, coral-dominated carbonate build-ups developed in front of

the delta on the shelf margin edge during times ofreduced clastic sedimentation, especiallytransgressions. These carbonates do offer a possiblealternative to siliciclastic reservoir targets assuggested by the hydrocarbons encountered within theDian-2 well (Snedden & Sarg, 1998). To the west,deltaic to flood-plain sandstones, shales and coalswere laid down during the Early Miocene and to theeast, delta-front slope turbidite sandstones and shales

of the deep marine, post-second rift event faciesassociation were deposited (Figure 11).

MIDDLE MIOCENE TO RECENT (N9-

PRESENT) DELTAIC AGGRADATION,

PROGRADATION AND INVERSION FACIES

ASSOCIATIONS

Following inversion of the Kutai Lakes Gravity Highat the end of the Early Miocene, a flood of deltaicsediments prograded eastwards into the MahakamDelta depocentre where accommodation matchedsediment input (Figure 12), resulting in the aggradationof thick, coastal plain deposits. A delta to shelf andslope sequence similar to that documented in theLower Miocene succession is interpreted (Duval etal., 1992; Allen & Chambers, 1998). This intervalcontains the actively-exploited, petroleum systems inthe Kutai Basin with reserves in excess of 3 BBO andgas reserves in excess of 30 TCF (Graves &Swauger 1997; Paterson et al., (1997). Thesehydrocarbon reserves are sourced primarily from delta

plain coal/lignite and inter-tidal coaly, carbonaceous,mangrove mudstone source rocks. The equatorialsetting, ever-wet climate and rate of generation of

accommodation space promoted the formation of thickcoals, especially in the Middle Miocene. Thecarbonaceous, mangrove-mudstones containtransported and reworked organic debris of similarorigin to in situ coal/lignite (Paterson et al., 1997). Themudstones have been suggested to be volumetricallymore important for sourcing oils than the coals (Todd



include compressional anticlines formed duringMiocene inversions, stratigraphic traps and

combination stratigraphic and structural traps. It isworth noting that near identical play elements areencountered in the other productive Miocene sectionsof the Tertiary basins of Borneo. Following on fromthe initial, rapid delta progradation in the EarlyMiocene, Middle Miocene deltas tended to aggrade.In the Late Miocene, with another pulse of inversionto the west, the delta prograded eastward, past theMiddle Miocene shelf edges. Uplift of the Sanga-

Sanga anticline occurred at about this time (Patersonet al., 1997). Some of the anticlines in the LowerKutai Sub-basin have had up to 3000m of sectionremoved through uplift and erosion. Inland of theSamarinda Anticlinorium lacustrine sediments andthick peat beds were deposited (Kutai Lakes) inresponse to Late Miocene uplift causing entrapmentof the Mahakam river drainage and reducing fluvial

flood activity on the delta (Allen & Chambers, 1998).

TERTIARY VOLCANISM

Three groups of Tertiary igneous rocks occur in theKutai Basin (Mid to Upper Eocene, Nyaan Suitevolcanics, Upper Oligocene to Lower Miocene,Sintang Suite intrusives and volcanics and Pliocene,Metulang Suite volcanics), summarised in Moss et al.,(1997; 1998). Localised, Eocene, acidic volcanicsoccur both within the basin and on the margins andappear to be related to Eocene rifting. The LateOligocene to Early Miocene, Sintang Suite igneousrocks, which comprise shallow level intrusives andextrusives (diorites, microdiorites, dacite,microgranites and andesites), are widely distributedacross Borneo and volumetrically the most important

(Moss et al., 1997; 1998). It is probable that therewas more than one volcanic phase of Sintang activity.Volcaniclastic clasts from this phase first appear inthe Late Oligocene. The sedimentary record ofvolcanilithic sands from the Sanga-Sanga PSCsuggests a fairly discrete and short lived episode(Tanean et al 1996) as opposed to the long time

basalt, high-potassium trachyandesite to andesitecompositions.

DISCUSSION & CONCLUSIONS

The origin and evolution of the Kutai Basin have been poorly understood and various origins, such as a peripheral foreland basin, have been suggested. Bydetailing the geometry and architecture of facies andhighlighting the distinct phases in the evolution wehave firmly established the origin of the Kutai Basin

as an extensional basin. However, the classic rift basins of the world such as the East African rift, theBaikal and Rhine rifts are all narrow (50-100km), long(>1000km), highly segmented features. The KutaiBasin, although also a rift basin, does not fit this

pattern with a much wider (>100km) rift zone. Withinthis wide rift zone a multitude of initially discrete andunconnected individual rift basins (15 to 30km wide)

were established with Middle Eocene sea-floorspreading in the North Makassar Straits and CelebesSea. Broad rift zones, such as the Kutai Basin, aremore typical of areas where extension has affectedhot, weak lithosphere as opposed to cold, stronglithosphere. During the Middle Cretaceous to (?)EarlyEocene, Borneo was the site of collision of micro-continental fragments, island arcs, entrapment ofremnant oceanic crust, arc magmatism and graniteintrusion, forming the composite basement surroundingand underlying the Kutai Basin (Moss, 1998). All ofthese events would have contributed to the formationof a severely weakened, hot lithosphere by the MiddleEocene.

Late Oligocene events are important for the entireisland, with the onset of progradation and aggradation

of the Neogene Balingian, Baram, Sabah, Tarakanand Mahakam deltas into peri-Borneo basins andformation of many of the elements of the currently-active petroleum systems. These basins containhydrocarbon reserves estimated to be 10.2 BBO and58.1 TCF (Graves & Swauger, 1997). Regionally, anevent at 25Ma has been recorded in many parts of



from both southeast and northwest directions andaided by the Neogene, counter-clockwise rotation of

the island (Moss et al., 1997, 1998). As fragments ofextended South China Sea continental crust started toarrive at the North Sabah-Palawan subduction trench,subduction of the proto-South China Sea ceased at~15-14Ma. Large amounts of sediment were suppliedto the basins surrounding this uplift supplemented bysediment derived by erosion of older Paleogenedepocentres which were inverted as a result of the

NW-SE regional compression. Deposition was

sufficiently rapid to allow overpressures to develop in basin via disequilibrium compaction in pro-delta andmarine shales. The overpressure boundary itself wassubsequently utilized as an important décollementsurface in inversion and formation of detachment foldsand thin-skinned thrusts in the Lower Kutai Sub-basin(Chambers & Daley, 1995; Ferguson & McClay,1997; Paterson et al., 1997; Chambers et al., 1999).

As mentioned at the start, an aim of this paper was torecognise the real lateral arrangements of facies andfacies associations within the Kutai Basin. This is

particularly important for the understanding of theEocene syn-rift sequences in the Kutai Basin, as

potential reservoir horizons in the Eocene, such as theBeriun Formation, have been difficult to trace bothregionally and in the subsurface (e.g. Satyana &Biantoro, 1995), due to restricted grabenal depositionduring the Eocene. Stratigraphically down-dip, theBeriun Formation, for example, will pass into lowerenergy, shale-dominated facies pervasively recognisedas separate formations and often thought to be ofdifferent ages. Although most exploration in the basinhas targeted Middle Miocene and younger sequences,and exploration of the stratigraphically older parts of

the basin has not met with abundant success, themodels presented may serve as a guide to thedevelopment of Tertiary basins of SE Asia,

particularly the other basins of Borneo, which mayhave evolved in a similar fashion.

ACKNOWLEDGMENTS

recognises the assistance of LASMO Runtu co-workers; particularly Ian Carter, Tim Daley and Jeff

Towart as well as all who helped in the LASMOSamarinda Office from 1991-5 during which fieldworkwas undertaken.

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