Geology of Kalimantan_Wikibook

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The Geology of Indonesia/Kalimantan 1 The Geology of Indonesia/ Kalimantan The island of Kalimantan presently lies upon the southeastern margin of the greater Eurasian plate. It is bounded to the north by the South China Sea marginal oceanic basin, to the east by the Philippine Mobile Belt and the Philippine Sea Plate and to the south by the Banda and Sunda arc systems (Figure 1). It is bounded to the west by the Sunda Shelf and ultimately by Paleozoic and Mesozoic continental crust of the Malay Peninsula. The Greater Kalimantan Block is surrounded to the north, east, and south by plate boundaries and arc systems which are presently active or which have been active during the Tertiary and it is bounded to the west by an underexplored shelf region which possibly conceals a terrane boundary (Fuller & Richter, ?).

Transcript of Geology of Kalimantan_Wikibook

The Geology of Indonesia/Kalimantan 1

The Geology of Indonesia/KalimantanThe island of Kalimantan presently lies upon the southeastern margin of the greaterEurasian plate. It is bounded to the north by the South China Sea marginal oceanic basin,to the east by the Philippine Mobile Belt and the Philippine Sea Plate and to the south bythe Banda and Sunda arc systems (Figure 1). It is bounded to the west by the Sunda Shelfand ultimately by Paleozoic and Mesozoic continental crust of the Malay Peninsula. TheGreater Kalimantan Block is surrounded to the north, east, and south by plate boundariesand arc systems which are presently active or which have been active during the Tertiaryand it is bounded to the west by an underexplored shelf region which possibly conceals aterrane boundary (Fuller & Richter, ?).

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Kalimantan can be divided into several roughly E-W trending tectonic provinces (Figure5.1). The northern portion of the island is dominated by the Cretaceous and Eocene toMiocene Crocker-Rajang-Embaluh accretionary complex. This consists primarily ofturbidites which were being shed northeastward (present day coordinates) off of theSchwaner and younger volcanic arcs into a paralic to deep marine trench basin. Thesesediments were imbricated, deformed, and weakly metamorphosed during Creraceous andTertiary subduction and finally were intruded by late stage and post subduction intrusionsof the Oligo- Miocene Sintang Group. The Melawi-Ketungau basins and the Kutei basin (Figure 5.1) formed along the southernmargin of this complex during the Late Eocene and are separated from it by the Lupar-Lubok Antu and Boyan melange-ophiolitic zones. Scattered exposures of Cretaceous marinesediments adjacent to these basins likely record the Cretaceous fore-arc basin to the

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Schwaner arc. The Kutei basin developed primarily along an arm of the Makassar riftsystem while the Melawi-Ketungau basins and the Upper Kutei basins occupy more of afore-arc to intra-arc position to Tertiary volcanism. Tarakan and Sandakan basins areTertiary basins developed in the northeast part of Kalimantan. Similar to Kutai basin, thesebasins are sourced by deltaic system from the Kalimantan mainland. The Barito basinformed at the same time but appears to have formed as a back-arc or continental rift.Pieters et al (1987) have correlated an Eocene basal sandstone/conglomerate and Eocenevolcanics throughout all of these basins and it appears that a continuous system of Eocenerifts Formed along the margins of the uplifting and eroding Schwaner Batholith. Thesedeveloped into separate basins during the Oligocene and Miocene and sedimentation hascontinued throughout most of the Neogene. The Schwaner Barholith itself is a triangularexposure of Cretaceous granitic rocks which intrude Paleozoic and Mesozoic volcanics,volcaniclastics, and marine sediments. The only region of Kalimantan in which thisPaleozoic and Mesozoic section is well preserved is in Northwest Kalimantan and WesternSarawak (the Northwest Kalimantan domain of Williams et al (1988)) although itpresumably formed the continental crustal host for Schwaner plutonism. The easternmargin of the Barito Basin is formed by the Meratus ophiolite. This was emplaced duringthe Middle Cretaceous (Sikumbang, 1986), presumably during northwestward directedsubduction (present day coordinates). Arc volcanism in SE Kalimantan then jumpedoutboard to the Sulawesi arc system. The Meratus ophiolite separates the Barito basin fromAsem-asem basin in the southeastern portion of Kalimantan. Asem-asem basin is a Tertiarybasin which converted eastward gradually to Paternoster carbonate platform.For practical convenience and presentation, the tectonic features of Kalimantan are dividedinto two part: Tertiary Basins and Pre to Early Tertiary Highs.

5. 1. TERTIARY BASINS 5.1.1. BARITO BASIN The Barito Basin is situated along the southeastern margin of the Schwaner Shield in South Kalimantan (Fig. 8). The basin is defined by the Meratus Mountains to the east and separated from the Kutei Basin to the north by a flexure related to the Adang Fault. The basin has a narrow opening to the south towards the Java Sea. The Barito Basin is an asym- metric basin, forming a foredeep in the eastern part and a platform approaching the Schwaner Shield towards the west (Fig. 9 and Fig. 14). The Barito Basin commenced its development in the Late Cretaceous, following a micro-continental collision between the Paternoster and SW Borneo microcontinents (Metcalfe, 1996; Satyana, 1996). Early Tertiary extensional deformation occurred as a tectonic consequence of that oblique convergence. This produced a series of NW – SE trending rifts. These rifts became accommodation space for alluvial fan and lacustrine sediments of the Lower Tanjung Formation, derived from horst areas. In the earliest Middle Eocene, as the result of a marine transgression, the rift sediments becoming more fluviodeltaic and eventually marine, as transgression proceeded during the deposition of the Middle Tanjung Formation. The marine transgression subsequently submerged the rifts in late Eocene – earliest Oligocene time, resulting in the deposition of widespread marine shales of the Upper Tanjung Formation. After a short-lived marine regression in the Middle Oligocene the development of a sag basin caused renewed marine transgression. The Late Oligocene is characterized by the deposition of platform carbonates of the Berai Formation (Figs. 6 and

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7). Carbonate deposition continued into the Early Miocene, when it was terminated by increasing clastic input from the west. During the Miocene the sea regressed, due to the uplift of the Schwaner Core and the Meratus Mountains. Clastic input resulted in the deposition of the eastwards-prograding deltaic sediments of the Warukin Formation. In the late Miocene the Meratus Mountains re-emerged, followed by the isostatic subsidence of the basin which was situated in a foreland position in relation to the rising mountains. Sediments shed from this uplift were deposited in the subsiding basin, resulting. in the deposition of thousands of meters of the Warukin Formation. The uplift of the Meratus Mountains continued into the Pleistocene and resulted in the deposition of the molassic-deltaic sediments of the Pliocene Dahor Formation. This struc- tural and depositional regime still exists today. The structural development of the Barito Basin is a consequence of two distinctly separate, regimes (Fig. 6). First, an initial transtensional regime, during which sinistral shear resulted in the formation of a series of NW – SE trending wrench-related rifts, and second, a transpressional regime involving convergent uplift, which reactivated and inverted old tensile structures and resulted in wrenching, faulting and folding. The kinematics and type of Barito tectonic inversion have been discussed by Satyana and Silitonga (1994). Presently, the structural grain of the basin is charac- terized by the concentration of structures in the NNE part of the basin, typified by tight, parallel SSW – NNE trending folds, bounded towards the Meratus Mountains by high-angle easterly-dipping imbricate reverse faults, which involve the basement (Figs. 5 and 9). The presence of major wrench faults is indicated by drag or fault-bend folds and reverse fault traces. The unique concentration of structures in the NE part of the basin was interpreted by Satyana (1994) as the tec- tonic consequence of half-encirclement of the area by the two pre-Tertiary massifs: the northern Meratus Range and the North Meratus Massifs (Fig. 8). The western and southern parts of the Barito Basin was mildly tectonized and show almost no deformation structures. Thin-skinned tectonic manifestations, rep- resented by decollement and ramp anticlines are only vaguely identifiable in this portion of the basin (Satyana and Silitonga, 1993). Along the northern Central Warukin and East Tapian Fields (Fig. 3). All of the fields occur in faulted anticlines dipping to the east. The Tanjung and Kambitin Fields are associated with basement-involved structures. The Warukin and East Tapian Fields occurred in structures developed by thin-skinned tectonics within the Warukin Formation (Fig. 9). Hydrocarbons are reservoired in the Lower and Middle Tanjung Sands (Middle Eocene) and in the Lower and Middle Warukin sands (Middle Miocene) (Figs. 7, 14 and 15). Pre-Tertiary basement rocks and the Berai carbonates (late Oligocene – early Miocene), where they are fractured, have also proved to be good reservoirs, and may trap hydrocarbons if they are well positioned. The hydrocarbons were generated in, and migrated from, Lower and Middle Tanjung coals and carbonaceous shales, and Lower Warukin carbon- aceous shales. The main kitchen is located in the pre- sent basin depocentre. The sealing rocks are mainly provided by the intra-formational shales. Generation, migration and entrapment of hydrocarbons has taken place since the middle Early Miocene (20 Ma). The Barito Basin provides the best example of the effects of tectonic interaction on hydrocarbon habitat (Fig. 9). In this basin, tectonics controlled each com- ponent of the hydrocarbon habitat (petroleum system). Extensional tectonics in the Early Tertiary formed a rifted basin within which the lacustrine Tanjung shales and coals were deposited in graben areas. This lacus- trine environment was responsible for the deposition of the Tanjung source rocks. As subsidence continued and the rifted structures were submerged, widespread shales were deposited, which became an

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important seal for the underlying reservoir rocks. This condition was also responsible forthe deposition of the widely-dis- tributed Middle Tanjung reservoir rocks. Extensional faultsbecame conduits for the migration of hydrocar- bons generated in the deeper graben areas.The role of tectonics in hydrocarbon accumulation in the basin during Neogene andPleistocene time is indisputable. The implications of basin reversal in the development ofthe petroleum system in the Barito Basin is discussed in Satyana and Silitonga (1994).During the Late Miocene the basin was inverted, in as- sociation with the Meratus Uplift, toproduce an asymmetric basin; the Barito Basin, dipping gently in the NW, towards theBarito Platform, and steeply in the SE against the Meratus Uplift. Consequently the centralpart of the basin subsided rapidly, due to iso- stasy, causing the Tanjung source rocks to bedeeply buried, so that they attained the depth at which hydro- carbons were generated.Restored modelling for the Barito tectonics and pet- roleum generation (Satyana andSilitonga, 1994; Satyana, 1995; Satyana and Idris, 1996) has shown that inversion of thebasin resulted from compressional tectonism (Fig. 9). Graben-fill sequences were activelyinverted and asymmetric anticlines were generated along the reverse faults. Hydrocarbonsgenerated from the basin depocentre were expelled to fill these struc- tural traps.Structures such as the Tanjung Field were thus favorably positioned for the entrapment ofearly migrating hydrocarbons. Uplift of the Meratus Mountains was continuous during theLate Miocene, through the Pliocene, and peaked in the Plio – Pleistocene. Tanjung sourcerocks in the depocentre were already mature by the Late Miocene. Proto- invertedstructural traps formed in the early Miocene time were continuously inverted as basincompression developed, resulting in strongly positive features. Hydrocarbons filled thesetraps through the faults and along permeable sands. It is considered that in the earlyPliocene the Tanjung source rocks in this area had exhausted their liquid hydrocarbongenerating capability. At this stage gas was generated and migrated to fill the existingtraps. Plio – Pleistocene tectonism caused the whole Barito Basin to be strongly inverted(Fig. 9). This tectonic event caused both the formation of new traps and the destruction ofexisting traps. Entrapped hydrocarbons probably remigrated to the newly-formedstructures as old traps were tilted or breached by the Plio – Pleistocene inversion. At thisstage the Tanjung source rocks had ceased to generate oil and gas in the depo- centre,since the section was firmly within the dry gas window. The Lower Warukin Shales in thebasin depo- centre reached the depth of the oil window in the peak episode of tectonismduring Plio – Pleistocene times. Oil was generated and migrated to accumulate in struc-tural traps within the Warukin sands. The Warukin and East Tapian Fields were charged inthis period. The foregoing discussion depicts how critical tec- tonics are to the deposition ofreservoir and source rocks, the maturation of source rocks, the formation of structuraltraps and oil field distribution. However, tectonics may also destroy pre-existing traps.5.1.2. KUTEI BASINThe Kutei Basin is the largest (165,000 km ) and the deepest (12,000 – 14,000 meters) Tertiary sedimentary basin in Indonesia. The basin is bounded to the north by the Mangkalihat High; to the south the basin hinges on the Adang – Flexure (Adang-Paternoster Fault); to the west it is terminated by the Kuching High – part of the Kalimantan Central Ranges; and to the east the opens into the Strait of Makassar (Fig. 10). The Tertiary stratigraphic succession within the basin commenced with the deposition of Paleocene alluvial sediments of the Kiham Haloq Formation in the inner basin, close to the western border (Figs. 6, 7 and 14). The basin subsided during the late Paleocene – Middle Eocene to Oligocene, due to basement rifting, and became the site of deposition of the Mangkupa

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Shale in a marginal to open marine environment. Some coarser siliciclastics, the BeriunSands, are locally associated with the shale sequence, indicatingan interruption of basin subsidence by uplift. The basin subsided rapidly after thedeposition of the Beriun Sands, mostly through the mechanism of basin sagging, resultingin the deposition of marine shales of the Atan Formation and carbonates of the KedangoFormation (Satyana and Biantoro, 1996). Subsequent tectonic events uplifted parts of thebasin margin by the late Oligocene (Figs. 6 and 7). This uplift was associated with thedeposition of the Sembulu Volcanics in the eastern part of the basin. The secondstratigraphic phase was contemporaneous with basin uplift and inversion, which started inEarly Miocene time. During that time, a vast series of allu- vial and deltaic deposits weredeposited in the basin. They comprise deltaic sediments of the Pamaluan, Pulubalang,Balikpapan and Kampung Baru for- mations, prograding eastwards, which range in agefrom the Early Miocene to Pleistocene times. Deltaic deposition continues to the presentday, and extends eastwards into offshore Kutei Basin. At present, the structural style of theKutei Basin is dominated by a series of tight NNE – SSW trending folds (and subsidiaryfaults) that parallel the arcuate coastal line, and are known as the Samarinda Anticlinorium– Mahakam Foldbelt (Figs. 5, 10 and 11). These fold belts are characterized by tight,asymmetric anticlines, separ- ated by broad synclines, containing Miocene siliciclas- tics.These features dominate the eastern part of the basin and are also identifiable offshore.The defor- mation is increasingly more complex in the onshore direction. The western basinarea has been uplifted, A minimum of 1500 m to over 3500 m of sediments have beenremoved by a mechanism of inversion (Wain and Berod, 1989; Courteney and Wiman,1991). Not much is known about the structure of the western basin area and, althoughlarge structures are evident, a similarity in structural trend and style is not apparent fromthe available data (Ott, 1987). In this region, the tectonics may involve the basement(thick-skinned tectonics). Tectonic reversal, in terms of origin and its strain re- sponse, isnot as obvious as in the Barito Basin. Prograding deltaic sediments may have contributed tothe mechanism of structural inversion, by a mechanism of diapirism or growth-faulting,these mechanisms are very different from those which affected the Barito Basin. The originof folds and faults in the Kutei Basin remains unresolved and concepts as diverse as verticaldiapirism, gravitational gliding ( Rose and Hartono, 1978; Ott, 1987), inversion throughregional wrenching (Biantoro et al., 1992), micro-continental collision, detachment foldingabove overpressured sediments (Chambers and Daley, 1995), differential loading on deltaicsediments and an inverted delta growth fault system (Ferguson and McClay, 1997) havebeen invoked.5.1.3. TARAKAN BASIN The Tarakan Basin encompasses the basinal areas in NE Kalimantan (Fig. 12). Workers inthis area usually subdivide the NE Kalimantan basinal areas into four sub-basins: theTidung Subbasin, the Berau Sub- basin, the Tarakan Sub-basin, and the Muara Sub- basin.The Tarakan Basin of this paper includes all four sub-basins. The boundaries between thesub- basins are not always eA’ective borders, some are only hinges or fault zones. TheTarakan Basin is separated from the Kutei Basin by the Mangkalihat High or Arch (Fig. 12).To the west the basin is terminated by the Sekatak – Berau High of the Central Ranges, thebasin hinges on the Semporna High to the north, and opens eastwards and southeastwardsinto the Straits of Makassar.

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Deposition in the Tarakan Basin commenced in the Middle Eocene, simultaneously with the rifting of the Makassar Straits which separates Sulawesi from Kalimantan (Lentini and Darman, 1996) (Figs. 6 and 7). The basin subsided and opened to the east. The sea transgressed westwards and shallow marine shales of the Sembakung Formation were deposited, overlying the older Dannu basement rocks. The. transgression was interrupted by the latest Eocene uplift which resulted in the deposition of coarse clastics of Sujau Formation. During Oligocene times a carbonate plat- form (Seilor Formation) developed and continued into the Early Miocene as the Mangkabua Shales and the . reefal Tabalar Limestone. In the middle Miocene, the western basin margins were uplifted and caused open marine conditions to give way to widespread and rapid clastic deltaic deposition, which successively prograded eastwards with time. Periodic and cyclic regression – transgression during the middle Miocene to Pleistocene time caused sedimentary switching, leaving marine shales and limestones intercalated with coarse clastic deltaic sediments (The Naintupo shales, Meliat – Tabul – Santul – Tarakan – Sajau – Bunyu deltaics and the Domaring – Waru carbonates). The present structural grain of the basin is charac- terized by folds trending NW – SE and by the faults trending NE – SW (Figs. 5 and 13). Structural defor- mation becomes increasingly complex northwards. The regular NE – SW trending faults, which are normal to the direction of sedimentary thickening, suggests that they were developed contemporaneously with depo- sition, and may be the direct result of sediment loading of successive deltaic sediments. All structures in the lower basin formed as the result of thin-skinned tec- tonics (Fig. 14). Involvement of the basement charac- terizes the structures of the upper basin, approaching the Sekatak – Berau High. Tectonic inversion is almost absent in this basin. The tectonic history of the Tarakan Basin com- menced with extensional tectonics in the Middle Eocene, initiating the basin by block

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faulting, simi- lar to events in the neighbouring basins. In the Middle Miocene, the SuluSea, located to the north of the basin, was subducted below the accreted con- tinental crustof North Kalimantan, and this resulted in the extrusion of Neogene volcanics in theSemporna Peninsula and was responsible for the formation of NW – SE trending, SEplunging folds in the Tarakan Basin. These fold axes are now rep- resented by the islands ofSebatik, Bunyu and Tarakan. The folds become increasingly more com- plex towards thenorth as they approach the con- vergent margin. Some workers (Lentini and Darman, 1996;Biantoro et al., 1996) relate the for- mation of these folds to wrench tectonics in the basinitself. The thick progradation of a deltaic suc- cession during Middle Miocene to Pleistocenetime resulted in growth-faulting with rollover structures, aligned perpendicular to thesedimentary flow and subsiding towards the east.5.1.4. SANDAKAN BASINThe Sandakan Basin, located in the southern portion of the Sulu Sea, with Tertiary deltaiccomplex in the south of the basin. It is analogous in many ways to thehydrocarbon-producing Baram and Mahakam deltas, which like the Sandakan, are adjacentto Kalimantan (Figure 1). This affinity with Borneo distinguishes the Sandakan Basin fromall other sedimentary basins of the Philippines. The Sandakan Basin is filled mainly with Mio- Pliocene age fluvio-deltaic sedimentary rocks,up to 15 km thick (Figure 2). The stratigraphic section in the basin has been described byTamesis (1990). The basin is bounded on the northwest by the Cagayan Ridge and extendssouthwestward into central and southeastern Sabah. The inactive Sulu Trench and the SuluArchipelago form the eastern boundary of the basin. To the northeast, sediments aredeformed by toe-of-slope compressional folds. Northeast of these folds, the sedimentarysuccession thins to 2.5 km and downlaps onto the Southeast Sulu Sea oceanic crust,marking the northeastern boundary of the basin (Graves & Swauger, 1997).The tectonic history of this basin is not agreed upon. Back-arc and intra-arc classificationshave been assigned to the Southeast Sulu Sea. In either case, the sea-floor spreading mayhave been associated with southeast-directed subduction of a proposed proto- South ChinaSea oceanic crust, under a northeastern extension of the Borneo microcontinent (theCagayan Ridge), during Middle Miocene time (Hinz, et al., 1991). Further discussion of thebasin development is made by Hutchison (1992) and Rangin et al. (1990). 5.1.5. SARAWAK BASINThe continental shelf offshore East Malaysia belongs to an extensive shallow-water areathat connects Borneo with the Asian mainland (Fig. 2). Only the northern part of Borneo isseparated from continental Asia by deep water areas of the South China Sea. Along centralSarawak the shelf is extremely broad, generally exceeding 300 km from shelf edge to coast.It becomes narrow toward northern Sabah, where it locally is less than 100 km wide. Most of the shelf is underlain by a thick upper Tertiary sequence. Magnetic data, locallysupported by seismic data, suggest the greatest sedimentary thicknesses are in central andnorthern Sarawak, close to the present coast (Figs. 3, 4). In Sa- bah, a zone of maximumthicknesses appears to occur 60 km offshore. The main source of these sediments was theorogenic belt that runs along the southern border of Sarawak northward into Sabah. Thesemountains, that were mainly uplifted in the Eocene, now form the landward boundary of thethick upper Tertiary basin. In Sarawak, thick upper Tertiary sediments reach tar beyond the shelf edge, covering largedeepwater areas (Sarawak basin. Fig. 2). Farther north, in western Sabah, a deep relatively

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narrow trough (Sabah trough) with mostly undisturbed, horizontal sediments of probablyPliocene age, separates the thick upper Tertiary sequence beneath the shelf from the muchthinner Tertiary sequence which underlies the deep water farther offshore (Fig. 3, 4:sections 1, 2). A similarly deep, but shorter graben is found 250 km farther to thewest-northwest (Fig. 2). The abyssal plain of the China basin lies 350 km to the northwestof the Sabah trough, at a water depth of 4,000 m, and is underlain by oceanic basementwith only a thin veneer of sediments. In this area, crustal extension led to the formation ofoceanic basement, probably in middle Tertiary time, whereas in the south rifting neverwent beyond the initial graben formation. Thick upper Tertiary sediments also underlie partof the shelf in eastern Sabah, extending landward across Dent Peninsula. However, in thedeep waters to the northeast, oceanic basement appears to be at shallow depth beneath theSulu Sea (Fig. 2). In most areas seismic basement offshore corresponds to indurated Paleogene sediments.Based on projections from onshore western Sarawak and offshore well data from PeninsularMalaysia and Indonesia, basement is expected to consist of Mesozoic metamorphic andgranitic rocks, and possibly at least partly of upper Paleozoic rocks similar to those exposedin Vietnam, Peninsular Malaysia, and western Sarawak. Mesozoic metamorphic rocks havebeen described from surface outcrops in eastern Sabah (Leong, 1974). While shelf conditions prevailed in western Sarawak, a deep trough developed in centralSarawak during Cretaceous- Paleogene time, extending northward over parts of Kalimantanand western Sabah. Several thousand meters of deepwater shales and turbiditesaccumulated in this trough, the axis of which appears to have been located 100 to 200 kminland from today’s coast. Paleocene shallow-water limestones found in the subsurface ofsouthwest Luconia indicate the presence of carbonate shoals along the western flank of thePaleogene deepwater trough. The main orogenic belt of the Northwest Kalimantan basinwas strongly folded and uplifted during Eocene time, thus becoming an important sourcefor the younger Tertiary sediments.Mid-Tertiary rifting in the China basin is thought to have exerted extensional stresses thatled to the formation of a half graben and graben system in which mostly continentalsediments were deposited (Figs. 2-5). At the same time a deep trough developed in front ofthe Eocene fold belt in Sabah and northern Sarawak. It rapidly filled with a thick shale andturbidite sequence (West Crocker and Temburong formations; Liechti et al, 1960), butcarbonate shoals and reef buildups developed along the southwestern flank of the trough(Melinau Lime- stone; Liechti et al, 1960). In central Sarawak a shallower environmentprevailed with a mainly argillaceous facies deposited (Kelabit formation, Setap shale,Penian marl; Liechti et al, 1960; partly Miri Zone, Hale, 1973). Deep-marine, predominantlyshaly sequences also were de- posited in eastern Sabah, where they contain radiolaritesand spilites. These have been interpreted as trench melanges indicative of a lateOligocene-early Miocene northwest to southeast oriented subduction zone (Hamilton, 1976;Beddoes, 1976). Although no blueschist metamorphism has been observed, this zone withits highly contorted shales and the frequent radiolarites and ophiolithes shows moreindications of subduction than the southwest to northwest oriented trend of the main North-west Borneo geosyncline, which lacks typical trench melanges. Structurally. Sabah is themost complex area in northwestern Borneo, because of its megatectonic position betweenthe is- land arc system of the western Pacific and the Asian mainland.

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During the early Miocene the sea transgressed westward. Deeper marine deposits reachedthe present northern Sarawak shelf and a shallower marine wedge extended far intoIndonesian waters (Fig. 5). Locally carbonate shoals and buildups fringed the basin (e.g.Subis Limestone, Melinau Limestone, Liechti et al 1960). Extensive coastal plaincontinental deposits formed along the basin margin, with a particularly thick developmentin the present area off central/western Sarawak. Northwest to southeast oriented horst andgraben tectonics affected the area, but large parts of the area off western Sarawak havesubsequently become fairly stable, elevated, and extensively eroded. During the middleMiocene strong subsidence began off central Sarawak along a fault system of a generalnorth- northwest to south-southwest orientation. The middle Miocene sea spread into thedepressions that formed on either side of a relatively stable, elevated central area, whereextensive carbonate buildups began to form (Central Luconia). At the same time graduallyoutbuilding deltas came into existence in western and northern Sarawak and in northernSabah (Fig. 5). During the late Miocene, much of the present area off central and southernSabah underwent strong folding, initiated through basement uplifts and wrench faulting.Large parts of northern Sarawak, both onto and offshore, were also affected by this tectonicphase, though deformation generally has been weaker. Synsedimentary deformation tookplace in the thick sedimentary sequences that filled the deep depressions on either side ofthe Central Luconia carbonate platform. Deltaic outbuilding continued in western andcentral Sarawak and new deltas developed in southern and eastern Sabah (Fig. 5). Duringthe Pliocene, the sea rapidly expanded over the northward tilting shelf, depositingopen-marine clays and sands (Fig. 5). On the shelf slope, thrust folds developed faroffshore. Synsedimentary deformation continued in the deltaic areas, while another foldingphase, probably again triggered by trough basement uplifts and wrench faulting, affectedlarge parts of nearshore northern Sarawak and particularly northern Sabah (Figs. 3, 4). 5.1.6. MELAWI AND KETUNGAU BASINThe melange and accretionary rocks east of the Northwest Kalimantan domain areunconformably overlain by three sedimentary sequences; the Silat sequence, Melawi Basinsequence and Ketungau/Mandai Basin sequences. The earliest of these is the Silatsequence, which comprises a fluviatile sandstone up to 600m thick overlain by up to 2000mof lacustrine black shale. The sequence thins rapidly to the west and is not present to thewest of the Kapuas River (Fig. 2). It is folded into a tight, east plunging syncline, and limbsare in places overturned. The Silat sequence overlies the southern accretionary depositsand is unconformably overlain by rocks of the Melawi Basin. The area of outcrop of thesequence was referred to by Zeybnans van Emmichoven (1939) and Williams et al., (1984)as the Silat Fold Belt. The Silat sequence was folded before the deposition of the MelawiBasin sequence. The nature of the folding suggests the presence of thrust faults at depth(Williams et al., 1984). The Melawi Basin contains up to 5 km of fluviatile, lagoonal andmarginal marine sediments. Volcanic detritus is not abundant but van Es (1918) andWilliams and Heryanto (1986) recognised many horizons containing air-fall fragments andsilicified glass shards indicating distant contemporaneous volcanism. The source for thisdetritus is porbably from the volcanism which produced the Early Tertiary volcanics in theSchwaner Mountains. The Melawi Group unconformably overlies either the Cretaceousshelf sediments or the Silat sequence in the north and onlaps the granitic and metamorphicbasement to the south. Age diagnostic fossils are rare in the Melawi Group but a Turonianforaminiferal assemblage has been recovered from near the base (Williams and Heryanto,1986). It is an asymmetrical basin, with the maximum sediment accumulation closer to the

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norhtern margin. The rocks are folded into a gentle syncline, with maximum limb dips of 30. Folds are also asymmetric, the northern limb more steeply dipping than the southern limb.The Ketungau Basin sequence is separated from the Melawi Basin by accretionary rocksand the Boyan Melange. It is also an east- west trending basin and as is the Mandai Basin tothe east. The Tertiary sediments in the Mandai Basin are porbably correlates of theKetungau Basin sequence. The stratigraphy of the basin fill is shown in Fig. 6 (Column 5)and the total sediment thickness in the Ketungau Basin is estimated to be at least 5 km. Thelower formation is very similar to the Melawi Group which prompted Zeylmans vanEmmichoven (1939) to correlate the two. However fossils from the lowest exposed rocks ofthe Ketungau Basin are Eocene (Tan, 1979) and it is unlikely that a thick section existsbelow the fossil horizon (Williams and Heryanto, 1986). In addition the thick (approx. 2000m) fluviatile sandstone in the middle of the Ketungau sequence (Fig. 6) has no equivalent inthe Melawi Basin. Consequently the Ketungau Basin is considered younger than the MelawiBasin. The Ketungau Basins is faulted againts the Lubok Antu Melange to the north, and inplaces onlaps the Boyan Melange to the south. In other places the southern boundary of theBasin is faulted. Like the Melawi Basin, the Ketungau Basin is asymmetrical, units in thesouth being substantially thinner than their equivalents to the north. It is also folded into agentle east-trending symmetrical syncline with limb dips generally 250 The Mandai basinsediments onlap Turonian flysch north of the Boyan Melange.

5. 2. PRE- EARLY TERTIARY HIGHS 5.2.1. NW KALIMANTAN DOMAINThe oldest fossiliferous rocks of the North- west Kalimantan domain are Late Carboniferouslimestone and marble containing diagnostic fusulinids. These crop out in small areas ofboth Kalimantan (Zeylmans van Emmichoven, 1939) and Sarawak (Sanderson, 1966). InKalimantan the limestone and marble flank a unit comprising schist, phyllite and quartzitewith garnet grade greenschist facies assemblages (Fig. 2). Small areas of similar schist arepresent in Sarawak (Pimm, 1965). In Kalimantan the metamorphic rocks are intruded bybiotite granite which yields K-Ar ages from Permian to Late Triassic (Table 3, Group 1).Many of the granitic rocks contain a strorig foliation, and the Lhte Triassic ages areobtained from biotites from deformed rock The Permian dates come from hornblendecrystals from undeformed regions of the granites or from amphibolite en-claves. The olderages are interpreted as minimum intrusive ages and the Middle to Late Triassic ages as thedeformation age of the suite (Fig. 6, col. 1). Late Triassic shallow marine shales containing Monotis and Halobia were deposited on theNorthwest Kalimantan domain, and these prob- ably post-date the main Late Triassicdeformation event recorded in the granitic rocks. The shales are not in contact with theschist and granitic rocks, but Tan (1986) indicates that similar rocks in Sarawak containdetritus from the older calcareous rocks and granite, implying an unconformablerelationship. Basic and intermediate volcanic rocks are also present which may correlatewith the Late Triassic volcanics of Sarawak (Wilford and Kho, 1965; Kirk, 1968; Hon, 1978).Early Jurassic ammonites and bivalves have been identified from several localities west ofthe metamorphic part of the domain (Wing Easton, 1904). The fossils occur in shallowmarine shales, calcareous and nodular siltstone, and feldspathic conglomerate intercalatedwith biohermal limestone, oolitic and intra-clastic limestone. These appear to form aconform- able sequence with the Late Triassic strata. In the far west spilite appears to

The Geology of Indonesia/Kalimantan 12

overlie the Late Triassic to Early Jurassic sedimentary sequences, which are only middlydeformed. Regions of older slate are also present in this area which are affected by theMiddle to Late Triassic deformation event. In Sarawak, the sedimentary record from theLate Jurassic to the Late Cretaceous is fairly complete (Tan, 1986). Late Jurassic near-shoredetritus and shallow marine limestone form a marginal facies to the north-trending troughcontaining dominantly Cretaceous sandy turbidites and calcareous mudstone (Fig. 2). Tan(1986) argues that a hiatus exists between the Late Jurassic - Cretaceous strata and theLate Triassic rocks. This is supported by a structural divergence between the two units inKalimantan. In Kalimantan Late Jur- assic ammonites have also been recovered (Sato,written communication) from localities adjacent to the metamorphic rocks and thenorth-trending trough, which contains dominantly Cretaceous sandy and calcareous flyschdeposits. The trough is 40 km wide in Kalimantan, bounded on the west by Late Triassicsequences and on the east by the metamorphic rocks. The trough sequence is gently tostrongly folded; north-dipping thrust faults and folds with a north-dipping axial surface areevident in many road cuttings. These formed prior to the Senomanian, because the rocksare unconformably overlain by undeformed fluvial sandstone of that age (Muller, 1968; Tan,1983). In the far northwest (area 2 on Fig.l, Fig.6, co1.2) chert, gabbro and ultramafic rocksform isolated outcrops in Kalimantan which occur within a sequence of deformed turbiditesand pebbly mud- stone. These have also been mapped in Sarawak (Wolfenden, 1963) wherethey are Jurassic. The rocks of this area may belong to a Jurassic melange (Hamilton, 1979),but they do not form an east- west trending belt as suggested by hirn or trend to the southacross the Northwest Kalimantan do- main (Sengor, 1984, 1986). The relationship betweenthe Northwest Kalimantan domain and the Schwaner Mountains is obscured by the extentof Early Cretaceous batholiths. No equivalent sedimentary sequences are present in theSchwaner Mountains, and no granites equivalent to the deformed Permian granites havebeen found despite an intensive dating programme. Probably Triassic basic volcanics in thesouthwest (de Keyser and Rustandi, in press) may be equivalent to the Late Triassicvolcanics in Sarawak and low-grade schists in the Schwaner Mountains and the NorthwestKalimantan domain could be equivalent. 5.2.2. SCHWANER MOUNTAINS Batholiths of tonalite and granodiorite with minor mafic rocks and granite intrudinglow-grade regional metamorphic rocks underline most of the Schwaner Mountains region(Fig. 5.1). Basic volcanic rocks both older than and younger than the granitoids are alsopresent (de Keyser and Rustandi, in press). The granitoids form a belt 200 km. wide and atleast 500 km long. Chemical analyses of typical rocks from the Schwaner Mountains (Table1) indicates the I-type calc-alkaline nature of the suite. The most mafic rock analysed is anorite, and the most acid rocks are syeno-granite is a gap in composition from 67% Si02 to72% Si02 which suggests at least two batholiths are present. In the southwest, a thirdbatholith composed dominantly of granite with subordinate riebeckite-bearing alkalinegranite and syenite (de Keyser and Rustandi, in press; Table 2), has distinctive geochemicalcharacteristics com- pared to the extensive tonalite and syenogranite suites (Fig. 4). Age determination on biotite and hornblende from the hornblende from the granitoids hasbeen carried out by Haile et al. (1977) and several new determinations have been madeduring the recent mapping program. Haile et al. obtained ages ranging from Jurassic (157Ma) to Late Cretaceous (77 Ma). The current project obtained ages on 33 specimens andages range from 129 Ma to 87 Ma, falling into four main groups (Table 3). Early Cretaceousages were obtained from tonalite and granodiorite bodies (Group 2) and the middle to Late

The Geology of Indonesia/Kalimantan 13

Cretaceous ages (Group 3) were obtained from the granite batholith in the soutwest. Therange of ages from 100 Ma to 120 Ma in the tonalite and syenogranite suite may indicatetwomain magmatic episodes over that period, the second corresponding to the intrusion ofthe more siliceous batholith. Ages obtained on basic-intermediate volcanics in the areaindicate Early Tertiary volcanism tookplace in the Schwaner Mountains.5.2.3. MERATUS MOUNTAINSThe location of the Meratus Range (southeast Kalimantan) is shown in Fig. 5.1; thismountain range limits the Barito and Kutei Basins on their southeast side, separating themfrom the Kintap Basin which lies to the southeast. The directions of main folding are NNE –SSW (in the northern part) and NE – SW (in the southern part).Three major units are exposed in the southern part: The Peridotitic Nappe, overthrust(together with its metamorphic thrust sole) on the Alino formation (Koolhoven, 1935). Bothunits are unconformably overlain by the Manunggul Formation. All these Cretaceous unitsare then overlain by younger marine and continental deposits. The Peridotitic Nappe ismade up mostly of serpentinites. gabbros and plagiogranites. The metamorphic thrust soleof the Peridotitic Nappe is composed of crystalline schists and amphibolites, and is intrudedby several gabbroic an basaltic plugs.The Alino Formation is made up mainly: of volcanic and volcaniclastic rock series: lavaflows. dykes. volcanic breccias. greywackes and tuffs. The volcanogenic rocks areinterbedded with predominantly sedimentary layer, radiolaria-bearing tuffaceous clays andturbidite. Orbitolina and radiolaria-bearing limestones. ammonite-bearing argillaceouslimestones and finally cong1omerates containing pebbles and blocks of the former rockstoward the upper part of the sequence. The corresponding faunistic ages range from UpperAptian to Cenomanian (Priyomarsono, 1985). The Manunggul Formation is also made upmostly vo1canogenic rocks, but it was deposited after the over thrusting of the Peridotiticnappe: thus it has the geological character of a molasse deposit. Volcanic sediments (tuffsand greywackes) are interbedded with conglomerates, sandstones, tuffaceous c1ays, andUpper Turonian clays near the bottom of the formation. Senonian conglomerates and claybeds occur near the top of this this sedimentary sequence, transitional to the lower units ofEocene detrital Tanjung Formation. The Manunggul formation, as well as the underlyingPeriodititc Nappe, are intruded both by basalic and andesitic dyekes and by a number ofgabbroic, dioritic, microdioritic and granitic plugs. 5.2.4. RAJANG-EBALUH GROUP FOLD-THRUST BELTThis great flysch belt is the eastwards continuation of the Sibu Zone Belaga Formation ofSarawak (Kirk 1957), which swings north and north-north- east along the eastern margin ofthe Miri Zone continental margin, to reach its best known localities in the Crocker Rangenear Mount Kinabalu. In the south it has been mapped as the Late Cretaceous to EoceneLurah and Mentarang formations (BRGM 1982). These continue northwards into Sabah asthe West Crocker and Sapulut formations, respectively (Collenette 1965). The Rajang – Embaluh Group ranges in age from Late Cretaceous (Santonian) to Early Eocene. Embaluh Group strata contain upper Santonian/lower Campanian nannofossils (Moss & Finch 1998). Middle Eocene larger foraminifera are reported from outcrops of turbidites in east Kalimantan. Ter Bruggen (1935) described Palaeocene to Middle Eocene benthic foraminifera from the headwaters of the Embaluh River in Kalimantan. Picters et al. (1993) suggest a Late Cretaceous to mid-Eocene age for these rocks on the basis of regional correlation, and all available data confirm a Late Cretaceous (Santonian) to

The Geology of Indonesia/Kalimantan 14

Valaeocene/Early Eocene age for the bulk of the sediments. In Sarawak the Rajang Groupyoungs northward from the Upper Cretaceous Lupar Formation and Layer Member of theBelaga Formation to the Middle Eocene members of the Belaga Formation (Hutchisonl996). The Embaluh Group is unlikely to extend into the Middle Eocene, at least within thestudy area, because felsic agglomerates, flat-lying lava flows and vent-related intrusionsunconformably overlie or intrude the Embaluh Group along the course of the upperMahakam river. Both the volcanics and lava flows are part of the Nyaan Volcanics suiteradiometrically dated using the K – Ar technique at 48.6+ 0.5 Ma (Pieters et al. 1993) andcorrelative with other felsic intrusives across Borneo in the middle Eocene (Moss et a1.1997). Middle Eocene rocks of the Kutai Basin also unconformably overlie the EmbaluhGroup in the study area (Moss & Finch 1998).In West Sabah, this belt consists of Eocene to Oligocene turbidites, hemipelagics, andassociated broken formations (Crocker, Temburong, Trusmadi and other formations) thathave been deformed into a thrusted, steeply-dipping sequence. The sequence becomesyounger in a seaward direction (northwestward) but bedding tops face southeastward,indicating that the structure must be intricately imbricate (Hamilton, 1979). On seismic(Fig. 5b) the Fold-Thrust Belt can be seen to extend in the offshore area at least up to theBunbury-St. Joseph Ridge (that is, beneath the Inboard Belt), where it is sharply bounded tothe west by a major wrench zone. The uplifted, exposed part of the Fold-Thrust Beltprovided the main source of sediments for the Inboard Belt and subsequently for the BaramDelta and Outboard Belt depocentres (Hazerbroek & Tan, 1992).

Source: http:/ / en. wikibooks. org/ w/ index. php? oldid=1028233Contributors: Herman Darman

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