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, ?).

Kalimantan can be divided into several roughly E-W trending tectonic provinces (Figure 5.1). The northern portion of the island is dominated by the Cretaceous and Eocene to Miocene Crocker-Rajang-Embaluh accretionary complex. This consists primarily of turbidites which were being shed northeastward (present day coordinates) off of the Schwaner and younger volcanic arcs into a paralic to deep marine trench basin. These sediments were imbricated, deformed, and weakly metamorphosed during Creraceous and Tertiary subduction and finally were intruded by late stage and post subduction intrusions of the Oligo- Miocene Sintang Group.

The Melawi-Ketungau basins and the Kutei basin (Figure 5.1) formed along the southern margin 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 marine sediments adjacent to these basins likely record the Cretaceous fore-arc basin to the Schwaner arc. The Kutei basin developed primarily along an arm of the Makassar rift system while the Melawi-Ketungau basins and the Upper Kutei basins occupy more of a fore-arc to intra-arc position to Tertiary volcanism. Tarakan and Sandakan basins are Tertiary basins developed in the northeast part of Kalimantan. Similar to Kutai basin, these basins are sourced by deltaic system from the Kalimantan mainland. The Barito basin formed 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 Eocene volcanics throughout all of these basins and it appears that a continuous system of Eocene rifts Formed along the margins of the uplifting and eroding Schwaner Batholith. These developed into separate basins during the Oligocene and Miocene and sedimentation has continued throughout most of the Neogene. The Schwaner Barholith itself is a triangular exposure of Cretaceous granitic rocks which intrude Paleozoic and Mesozoic volcanics, volcaniclastics, and marine sediments. The only region of Kalimantan in which this Paleozoic and Mesozoic section is well preserved is in Northwest Kalimantan and Western Sarawak (the Northwest Kalimantan domain of Williams et al (1988)) although it presumably formed the continental crustal host for Schwaner plutonism. The eastern margin of the Barito Basin is formed by the Meratus ophiolite. This was emplaced during the Middle Cretaceous (Sikumbang, 1986), presumably during northwestward directed subduction (present day coordinates). Arc volcanism in SE Kalimantan then jumped outboard to the Sulawesi arc system. The Meratus ophiolite separates the Barito basin from Asem-asem basin in the southeastern portion of Kalimantan. Asem-asem basin is a Tertiary basin which converted eastward gradually to Paternoster carbonate platform.

For practical convenience and presentation, the tectonic features of Kalimantan are divided into 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 asymmetric basin, forming a foredeep in the eastern part and a platform approaching the Schwaner Block 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 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- acerous 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 important seal for the underlying reservoir rocks. This condition was also responsible for the deposition of the widely-dis- tributed Middle Tanjung reservoir rocks. Extensional faults became 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 and Pleistocene time is indisputable. The implications of basin reversal in the development of the 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, to produce an asymmetric basin; the Barito Basin, dipping gently in the NW, towards the Barito Platform, and steeply in the SE against the Meratus Uplift. Consequently the central part of the basin subsided rapidly, due to iso- stasy, causing the Tanjung source rocks to be deeply buried, so that they attained the depth at which hydro- carbons were generated. Restored modelling for the Barito tectonics and pet- roleum generation (Satyana and Silitonga, 1994; Satyana, 1995; Satyana and Idris, 1996) has shown that inversion of the basin resulted from compressional tectonism (Fig. 9). Graben-fill sequences were actively inverted and asymmetric anticlines were generated along the reverse faults. Hydrocarbons generated 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 of early migrating hydrocarbons. Uplift of the Meratus Mountains was continuous during the Late Miocene, through the Pliocene, and peaked in the Plio – Pleistocene. Tanjung source rocks in the depocentre were already mature by the Late Miocene. Proto- inverted structural traps formed in the early Miocene time were continuously inverted as basin compression developed, resulting in strongly positive features. Hydrocarbons filled these traps through the faults and along permeable sands. It is considered that in the early Pliocene the Tanjung source rocks in this area had exhausted their liquid hydrocarbon generating capability. At this stage gas was generated and migrated to fill the existing traps. 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 of existing traps. Entrapped hydrocarbons probably remigrated to the newly-formed structures as old traps were tilted or breached by the Plio – Pleistocene inversion. At this stage 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 the basin depo- centre reached the depth of the oil window in the peak episode of tectonism during 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 in this period. The foregoing discussion depicts how critical tec- tonics are to the deposition of reservoir and source rocks, the maturation of source rocks, the formation of structural traps and oil field distribution. However, tectonics may also destroy pre-existing traps.

5.1.2. KUTEI BASIN

The 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 Shale in a marginal to open marine environment. Some coarser siliciclastics, the Beriun Sands, are locally associated with the shale sequence, indicating

an interruption of basin subsidence by uplift. The basin subsided rapidly after the deposition of the Beriun Sands, mostly through the mechanism of basin sagging, resulting in the deposition of marine shales of the Atan Formation and carbonates of the Kedango Formation (Satyana and Biantoro, 1996). Subsequent tectonic events uplifted parts of the basin margin by the late Oligocene (Figs. 6 and 7). This uplift was associated with the deposition of the Sembulu Volcanics in the eastern part of the basin. The second stratigraphic phase was contemporaneous with basin uplift and inversion, which started in Early Miocene time. During that time, a vast series of allu- vial and deltaic deposits were deposited in the basin. They comprise deltaic sediments of the Pamaluan, Pulubalang, Balikpapan and Kampung Baru for- mations, prograding eastwards, which range in age from the Early Miocene to Pleistocene times. Deltaic deposition continues to the present day, and extends eastwards into offshore Kutei Basin. At present, the structural style of the Kutei Basin is dominated by a series of tight NNE – SSW trending folds (and subsidiary faults) 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 basin area has been uplifted, A minimum of 1500 m to over 3500 m of sediments have been removed 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, although large structures are evident, a similarity in structural trend and style is not apparent from the 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, is not as obvious as in the Barito Basin. Prograding deltaic sediments may have contributed to the 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 origin of folds and faults in the Kutei Basin remains unresolved and concepts as diverse as vertical diapirism, gravitational gliding ( Rose and Hartono, 1978; Ott, 1987), inversion through regional wrenching (Biantoro et al., 1992), micro-continental collision, detachment folding above overpressured sediments (Chambers and Daley, 1995), differential loading on deltaic sediments and an inverted delta growth fault system (Ferguson and McClay, 1997) have been invoked.

5.1.3. TARAKAN BASIN

The Tarakan Basin encompasses the basinal areas in NE Kalimantan (Fig. 12). Workers in this area usually subdivide the NE Kalimantan basinal areas into four sub-basins: the Tidung 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 the sub- basins are not always eA’ective borders, some are only hinges or fault zones. The Tarakan 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, the basin hinges on the Semporna High to the north, and opens eastwards and southeastwards into the Straits of Makassar.

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 faulting, simi- lar to events in the neighbouring basins. In the Middle Miocene, the Sulu Sea, located to the north of the basin, was subducted below the accreted con- tinental crust of North Kalimantan, and this resulted in the extrusion of Neogene volcanics in the Semporna Peninsula and was responsible for the formation of NW – SE trending, SE plunging folds in the Tarakan Basin. These fold axes are now rep- resented by the islands of Sebatik, Bunyu and Tarakan. The folds become increasingly more com- plex towards the north 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 basin itself. The thick progradation of a deltaic suc- cession during Middle Miocene to Pleistocene time resulted in growth-faulting with rollover structures, aligned perpendicular to the sedimentary flow and subsiding towards the east.

5.1.4. SANDAKAN BASIN

The Sandakan Basin, located in the southern portion of the Sulu Sea, with Tertiary deltaic complex in the south of the basin. It is analogous in many ways to the hydrocarbon-producing Baram and Mahakam deltas, which like the Sandakan, are adjacent to Kalimantan (Figure 1). This affinity with Borneo distinguishes the Sandakan Basin from all 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 by Tamesis (1990). The basin is bounded on the northwest by the Cagayan Ridge and extends southwestward into central and southeastern Sabah. The inactive Sulu Trench and the Sulu Archipelago form the eastern boundary of the basin. To the northeast, sediments are deformed by toe-of-slope compressional folds. Northeast of these folds, the sedimentary succession 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 classifications have been assigned to the Southeast Sulu Sea. In either case, the sea-floor spreading may have been associated with southeast-directed subduction of a proposed proto- South China Sea oceanic crust, under a northeastern extension of the Borneo microcontinent (the Cagayan Ridge), during Middle Miocene time (Hinz, et al., 1991). Further discussion of the basin development is made by Hutchison (1992) and Rangin et al. (1990).

5.1.5. SARAWAK BASIN

The continental shelf offshore East Malaysia belongs to an extensive shallow-water area that connects Borneo with the Asian mainland (Fig. 2). Only the northern part of Borneo is separated from continental Asia by deep water areas of the South China Sea. Along central Sarawak 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, locally supported by seismic data, suggest the greatest sedimentary thicknesses are in central and northern Sarawak, close to the present coast (Figs. 3, 4). In Sa- bah, a zone of maximum thicknesses appears to occur 60 km offshore. The main source of these sediments was the orogenic belt that runs along the southern border of Sarawak northward into Sabah. These mountains, that were mainly uplifted in the Eocene, now form the landward boundary of the thick upper Tertiary basin.

In Sarawak, thick upper Tertiary sediments reach tar beyond the shelf edge, covering large deepwater areas (Sarawak basin. Fig. 2). Farther north, in western Sabah, a deep relatively narrow trough (Sabah trough) with mostly undisturbed, horizontal sediments of probably Pliocene age, separates the thick upper Tertiary sequence beneath the shelf from the much thinner 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 the west-northwest (Fig. 2). The abyssal plain of the China basin lies 350 km to the northwest of the Sabah trough, at a water depth of 4,000 m, and is underlain by oceanic basement with only a thin veneer of sediments. In this area, crustal extension led to the formation of oceanic basement, probably in middle Tertiary time, whereas in the south rifting never went beyond the initial graben formation. Thick upper Tertiary sediments also underlie part of the shelf in eastern Sabah, extending landward across Dent Peninsula. However, in the deep waters to the northeast, oceanic basement appears to be at shallow depth beneath the Sulu 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 Peninsular Malaysia and Indonesia, basement is expected to consist of Mesozoic metamorphic and granitic rocks, and possibly at least partly of upper Paleozoic rocks similar to those exposed in Vietnam, Peninsular Malaysia, and western Sarawak. Mesozoic metamorphic rocks have been described from surface outcrops in eastern Sabah (Leong, 1974).

While shelf conditions prevailed in western Sarawak, a deep trough developed in central Sarawak during Cretaceous- Paleogene time, extending northward over parts of Kalimantan and western Sabah. Several thousand meters of deepwater shales and turbidites accumulated in this trough, the axis of which appears to have been located 100 to 200 km inland from today’s coast. Paleocene shallow-water limestones found in the subsurface of southwest Luconia indicate the presence of carbonate shoals along the western flank of the Paleogene deepwater trough. The main orogenic belt of the Northwest Kalimantan basin was strongly folded and uplifted during Eocene time, thus becoming an important source for the younger Tertiary sediments.

Mid-Tertiary rifting in the China basin is thought to have exerted extensional stresses that led to the formation of a half graben and graben system in which mostly continental sediments were deposited (Figs. 2-5). At the same time a deep trough developed in front of the Eocene fold belt in Sabah and northern Sarawak. It rapidly filled with a thick shale and turbidite sequence (West Crocker and Temburong formations; Liechti et al, 1960), but carbonate shoals and reef buildups developed along the southwestern flank of the trough (Melinau Lime- stone; Liechti et al, 1960). In central Sarawak a shallower environment prevailed with a mainly argillaceous facies deposited (Kelabit formation, Setap shale, Penian marl; Liechti et al, 1960; partly Miri Zone, Hale, 1973). Deep-marine, predominantly shaly sequences also were de- posited in eastern Sabah, where they contain radiolarites and spilites. These have been interpreted as trench melanges indicative of a late Oligocene-early Miocene northwest to southeast oriented subduction zone (Hamilton, 1976; Beddoes, 1976). Although no blueschist metamorphism has been observed, this zone with its highly contorted shales and the frequent radiolarites and ophiolithes shows more indications of subduction than the southwest to northwest oriented trend of the main North- west Borneo geosyncline, which lacks typical trench melanges. Structurally. Sabah is the most complex area in northwestern Borneo, because of its megatectonic position between the is- land arc system of the western Pacific and the Asian mainland.

During the early Miocene the sea transgressed westward. Deeper marine deposits reached the present northern Sarawak shelf and a shallower marine wedge extended far into Indonesian waters (Fig. 5). Locally carbonate shoals and buildups fringed the basin (e.g. Subis Limestone, Melinau Limestone, Liechti et al 1960). Extensive coastal plain continental deposits formed along the basin margin, with a particularly thick development in the present area off central/western Sarawak. Northwest to southeast oriented horst and graben tectonics affected the area, but large parts of the area off western Sarawak have subsequently become fairly stable, elevated, and extensively eroded. During the middle Miocene strong subsidence began off central Sarawak along a fault system of a general north- northwest to south-southwest orientation. The middle Miocene sea spread into the depressions that formed on either side of a relatively stable, elevated central area, where extensive carbonate buildups began to form (Central Luconia). At the same time gradually outbuilding deltas came into existence in western and northern Sarawak and in northern Sabah (Fig. 5). During the late Miocene, much of the present area off central and southern Sabah underwent strong folding, initiated through basement uplifts and wrench faulting. Large parts of northern Sarawak, both onto and offshore, were also affected by this tectonic phase, though deformation generally has been weaker. Synsedimentary deformation took place in the thick sedimentary sequences that filled the deep depressions on either side of the Central Luconia carbonate platform. Deltaic outbuilding continued in western and central Sarawak and new deltas developed in southern and eastern Sabah (Fig. 5). During the Pliocene, the sea rapidly expanded over the northward tilting shelf, depositing open-marine clays and sands (Fig. 5). On the shelf slope, thrust folds developed far offshore. Synsedimentary deformation continued in the deltaic areas, while another folding phase, probably again triggered by trough basement uplifts and wrench faulting, affected large parts of nearshore northern Sarawak and particularly northern Sabah (Figs. 3, 4).

5.1.6. MELAWI AND KETUNGAU BASIN

The melange and accretionary rocks east of the Northwest Kalimantan domain are unconformably overlain by three sedimentary sequences; the Silat sequence, Melawi Basin sequence and Ketungau/Mandai Basin sequences. The earliest of these is the Silat sequence, which comprises a fluviatile sandstone up to 600m thick overlain by up to 2000m of lacustrine black shale. The sequence thins rapidly to the west and is not present to the west of the Kapuas River (Fig. 2). It is folded into a tight, east plunging syncline, and limbs are in places overturned. The Silat sequence overlies the southern accretionary deposits and is unconformably overlain by rocks of the Melawi Basin. The area of outcrop of the sequence 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 Melawi Basin 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 and marginal marine sediments. Volcanic detritus is not abundant but van Es (1918) and Williams and Heryanto (1986) recognised many horizons containing air-fall fragments and silicified glass shards indicating distant contemporaneous volcanism. The source for this detritus is probably from the volcanism which produced the Early Tertiary volcanics in the Schwaner Mountains. The Melawi Group unconformably overlies either the Cretaceous shelf sediments or the Silat sequence in the north and onlaps the granitic and metamorphic basement to the south. Age diagnostic fossils are rare in the Melawi Group but a Turonian foraminiferal 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 northern 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 rocks and the Boyan Melange. It is also an east- west trending basin and as is the Mandai Basin to the east. The Tertiary sediments in the Mandai Basin are porbably correlates of the Ketungau 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. The lower formation is very similar to the Melawi Group which prompted Zeylmans van Emmichoven (1939) to correlate the two. However fossils from the lowest exposed rocks of the Ketungau Basin are Eocene (Tan, 1979) and it is unlikely that a thick section exists below the fossil horizon (Williams and Heryanto, 1986). In addition the thick (approx. 2000 m) fluviatile sandstone in the middle of the Ketungau sequence (Fig. 6) has no equivalent in the Melawi Basin. Consequently the Ketungau Basin is considered younger than the Melawi Basin. The Ketungau Basins is faulted against the Lubok Antu Melange to the north, and in places onlaps the Boyan Melange to the south. In other places the southern boundary of the Basin is faulted. Like the Melawi Basin, the Ketungau Basin is asymmetrical, units in the south being substantially thinner than their equivalents to the north. It is also folded into a gentle east-trending symmetrical syncline with limb dips generally 250 The Mandai basin sediments onlap Turonian flysch north of the Boyan Melange.

5.2. PRE-EARLY TERTIARY HIGHS

5.2.1. NW KALIMANTAN DOMAIN

The oldest fossiliferous rocks of the North- west Kalimantan domain are Late Carboniferous limestone and marble containing diagnostic fusulinids. These crop out in small areas of both Kalimantan (Zeylmans van Emmichoven, 1939) and Sarawak (Sanderson, 1966). In Kalimantan the limestone and marble flank a unit comprising schist, phyllite and quartzite with garnet grade greenschist facies assemblages (Fig. 2). Small areas of similar schist are present in Sarawak (Pimm, 1965). In Kalimantan the metamorphic rocks are intruded by biotite 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 are obtained from biotites from deformed rock The Permian dates come from hornblende crystals from undeformed regions of the granites or from amphibolite en-claves. The older ages are interpreted as minimum intrusive ages and the Middle to Late Triassic ages as the deformation age of the suite (Fig. 6, col. 1).

Late Triassic shallow marine shales containing Monotis and Halobia were deposited on the Northwest Kalimantan domain, and these prob- ably post-date the main Late Triassic deformation event recorded in the granitic rocks. The shales are not in contact with the schist and granitic rocks, but Tan (1986) indicates that similar rocks in Sarawak contain detritus from the older calcareous rocks and granite, implying an unconformable relationship. Basic and intermediate volcanic rocks are also present which may correlate with 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 of the metamorphic part of the domain (Wing Easton, 1904). The fossils occur in shallow marine shales, calcareous and nodular siltstone, and feldspathic conglomerate intercalated with biohermal limestone, oolitic and intra-clastic limestone. These appear to form a conform- able sequence with the Late Triassic strata. In the far west spilite appears to overlie the Late Triassic to Early Jurassic sedimentary sequences, which are only middly deformed. Regions of older slate are also present in this area which are affected by the Middle to Late Triassic deformation event. In Sarawak, the sedimentary record from the Late Jurassic to the Late Cretaceous is fairly complete (Tan, 1986). Late Jurassic near-shore detritus and shallow marine limestone form a marginal facies to the north-trending trough containing dominantly Cretaceous sandy turbidites and calcareous mudstone (Fig. 2). Tan (1986) argues that a hiatus exists between the Late Jurassic - Cretaceous strata and the Late Triassic rocks. This is supported by a structural divergence between the two units in Kalimantan. In Kalimantan Late Jur- assic ammonites have also been recovered (Sato, written communication) from localities adjacent to the metamorphic rocks and the north-trending trough, which contains dominantly Cretaceous sandy and calcareous flysch deposits. The trough is 40 km wide in Kalimantan, bounded on the west by Late Triassic sequences and on the east by the metamorphic rocks. The trough sequence is gently to strongly folded; north-dipping thrust faults and folds with a north-dipping axial surface are evident in many road cuttings. These formed prior to the Senomanian, because the rocks are 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 rocks form isolated outcrops in Kalimantan which occur within a sequence of deformed turbidites and pebbly mud- stone. These have also been mapped in Sarawak (Wolfenden, 1963) where they 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 south across the Northwest Kalimantan do- main (Sengor, 1984, 1986). The relationship between the Northwest Kalimantan domain and the Schwaner Mountains is obscured by the extent of Early Cretaceous batholiths. No equivalent sedimentary sequences are present in the Schwaner Mountains, and no granites equivalent to the deformed Permian granites have been found despite an intensive dating programme. Probably Triassic basic volcanics in the southwest (de Keyser and Rustandi, in press) may be equivalent to the Late Triassic volcanics in Sarawak and low-grade schists in the Schwaner Mountains and the Northwest Kalimantan domain could be equivalent.

5.2.2. SCHWANER MOUNTAINS

Batholiths of tonalite and granodiorite with minor mafic rocks and granite intruding low-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 also present (de Keyser and Rustandi, in press). The granitoids form a belt 200 km. wide and at least 500 km long. Chemical analyses of typical rocks from the Schwaner Mountains (Table 1) indicates the I-type calc-alkaline nature of the suite. The most mafic rock analysed is a norite, and the most acid rocks are syeno-granite is a gap in composition from 67% Si02 to 72% Si02 which suggests at least two batholiths are present. In the southwest, a third batholith composed dominantly of granite with subordinate riebeckite-bearing alkaline granite and syenite (de Keyser and Rustandi, in press; Table 2), has distinctive geochemical characteristics com- pared to the extensive tonalite and syenogranite suites (Fig. 4).

Age determination on biotite and hornblende from the hornblende from the granitoids has been carried out by Haile et al. (1977) and several new determinations have been made during the recent mapping program. Haile et al. obtained ages ranging from Jurassic (157 Ma) to Late Cretaceous (77 Ma). The current project obtained ages on 33 specimens and ages range from 129 Ma to 87 Ma, falling into four main groups (Table 3). Early Cretaceous ages were obtained from tonalite and granodiorite bodies (Group 2) and the middle to Late Cretaceous ages (Group 3) were obtained from the granite batholith in the southwest. The range of ages from 100 Ma to 120 Ma in the tonalite and syenogranite suite may indicate twomain magmatic episodes over that period, the second corresponding to the intrusion of the more siliceous batholith. Ages obtained on basic-intermediate volcanics in the area indicate Early Tertiary volcanism tookplace in the Schwaner Mountains.

5.2.3. MERATUS MOUNTAINS

The Meratus Mountains or Meratus Range is a Cretaceous subduction complex, located in southeast Kalimantan (Fig. 5.1). This mountain separate Barito Basin in the west from the Asem-asem Basin which lies to the east. 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). Both units are unconformably overlain by the Manunggul Formation. All these Cretaceous units are then overlain by younger marine and continental deposits. The Peridotitic Nappe is made up mostly of serpentinites. gabbros and plagiogranites. The metamorphic thrust sole of the Peridotitic Nappe is composed of crystalline schists and amphibolites, and is intruded by several gabbroic an basaltic plugs.

The Alino Formation is made up mainly: of volcanic and volcaniclastic rock series: lava flows. dykes. volcanic breccias. greywackes and tuffs. The volcanogenic rocks are interbedded with predominantly sedimentary layer, radiolaria-bearing tuffaceous clays and turbidite. Orbitolina and radiolaria-bearing limestones. ammonite-bearing argillaceous limestones and finally cong1omerates containing pebbles and blocks of the former rocks toward the upper part of the sequence. The corresponding faunistic ages range from Upper Aptian to Cenomanian (Priyomarsono, 1985). The Manunggul Formation is also made up mostly vo1canogenic rocks, but it was deposited after the over thrusting of the Peridotitic nappe: thus it has the geological character of a molasse deposit. Volcanic sediments (tuffs and greywackes) are interbedded with conglomerates, sandstones, tuffaceous c1ays, and Upper Turonian clays near the bottom of the formation. Senonian conglomerates and clay beds occur near the top of this this sedimentary sequence, transitional to the lower units of Eocene detrital Tanjung Formation. The Manunggul formation, as well as the underlying Periodititc Nappe, are intruded both by basaltic and andesitic dyekes and by a number of gabbroic, dioritic, microdioritic and granitic plugs.

5.2.4. RAJANG-EBALUH GROUP FOLD-THRUST BELT

This great flysch belt is the eastwards continuation of the Sibu Zone Belaga Formation of Sarawak (Kirk 1957), which swings north and north-north- east along the eastern margin of the Miri Zone continental margin, to reach its best known localities in the Crocker Range near Mount Kinabalu. In the south it has been mapped as the Late Cretaceous to Eocene Lurah and Mentarang formations (BRGM 1982). These continue northwards into Sabah as the 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 Valaeocene/Early Eocene age for the bulk of the sediments. In Sarawak the Rajang Group youngs northward from the Upper Cretaceous Lupar Formation and Layer Member of the Belaga Formation to the Middle Eocene members of the Belaga Formation (Hutchison 1996). The Embaluh Group is unlikely to extend into the Middle Eocene, at least within the study area, because felsic agglomerates, flat-lying lava flows and vent-related intrusions unconformably overlie or intrude the Embaluh Group along the course of the upper Mahakam river. Both the volcanics and lava flows are part of the Nyaan Volcanics suite radiometrically dated using the K – Ar technique at 48.6+ 0.5 Ma (Pieters et al. 1993) and correlative 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 Embaluh Group in the study area (Moss & Finch 1998).

In West Sabah, this belt consists of Eocene to Oligocene turbidites, hemipelagics, and associated broken formations (Crocker, Temburong, Trusmadi and other formations) that have been deformed into a thrusted, steeply-dipping sequence. The sequence becomes younger 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 the Bunbury-St. Joseph Ridge (that is, beneath the Inboard Belt), where it is sharply bounded to the west by a major wrench zone. The uplifted, exposed part of the Fold-Thrust Belt provided the main source of sediments for the Inboard Belt and subsequently for the Baram Delta and Outboard Belt depocentres (Hazerbroek & Tan, 1992).

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