AIIIII/I1I G(oll~lJical Co4mllc( '94 JIIII( 11-12,1994, KI/ala Teren.l!.lJalll/,MaIaY.lUI

The Balingian Shear Zone, West Balingian and West Baram Lines, , and their importance in the early Cenozoic evolution of NWBorneo

ROBERT B. TATE

Department of Geology, University of Malaya 50603 Kuala Lumpur,

Abstract: A major zone ofintense deformation trending WNW has been discovered in the neighbourhood of Sg. Balingian between and . The zone probably continues offshore where a gravity lineament follows the same direction. The West Balingian and West Baram Lines mark the boundaries of the offshore hydrocarbon provinces and together with the newly discovered shear zone, appear to form a fundamental tectonic framework for this part of NW Borneo. The distribution of heat flow, igneous rocks, Oligocene deltas and Oligocene-Miocene carbonates across N Sarawak and N Kalimantan appear to be related to the framework which extends across central Borneo. The nature of the lineaments is discussed in terms of the regional tectonic evolution of northern Sarawak as well as providing clues to indicate what lies beneath this part of Borneo.

INTRODUCTION quartzites and shales are also sometimes graphitic (Fig. 3). Quartz veining in the form of stringers, ptygmatic veins and tension gashes is common, The Balingian Shear Zone especially in the more shaly rocks (Figs. 4 and 8). Intense shearing of the rocks at the Sibu-Bintulu A dm-wide low angle thrust zone with related road bridge site on the Sg. Balingian was first cleavage is visible 3 km WSW of the Balingian recognised in 1976 and the shearing then attributed bridge and elsewhere the rocks are tightly folded to a NE-trending tectonic feature related to the and cleaved. Mulu Shear Zone (McManus and Tate, 1976). New A measured section on N side ofthe road 200 m roadside exposures in the middle-upper Eocene S of the bridge is as follows:- Belaga Formation in the neighbourhood of Sg. 100.00 m sheared, cleaved dark grey siltstone Balingian between Sibu and Bintulu now reveal a 1.50 channel sand in silt major zone of deformation which seems to trend 20.00 graphitic shales approximately WNW (Fig. 1). The zone appears to 1.00 channel sand continue offshore where it is aligned with a positive 0.30 sand gravity anomaly trending WNW indicating a major 0.10 silt discontinuity at depth (Fig. 2). It is proposed to call 1.00 plane-bedded sand this shear zone the Balingian Shear Zone and the 0.30 graphitic silt main Sibu-Bintulu road crossing the Sg. Balingian 1.00 thin, multi-channel sands designated as the type locality. 2.00 disrupted sand beds with minor The rocks affected by the shear zone belong to shear zones middle-upper Eocene Belaga Formation of the 3.00 graphitic silt with quartz stringers Rajang Group. They comprise mainly deepwater 3.00 boudinaged channel sand turbidites with intercalations of purple and green 7.00 cleaved silt shaly beds, possibly of volcanic origin, the latter 3.00 thin, dm -scale channel sands exposed 3 km WSW of the bridge across the Sg. 2.00 very thin sandstones with quartz Balingian. The rocks show low grade regional stringers metamorphism; sandstones are transformed to 20.00 highly cleaved siltstone quartzites and shales become phyllitic. The bedded The effects of the major shear zone appear to quartzites show both current and graded bedding die out in a northerly direction, about 2 km N of the and shearing has disrupted the competent beds Sg. Pelagau and towards the southwest, E of Sg. into rhomboidal blocks (Fig. 5). Graphite layers Bulah. The shear zone is therefore about 8 km and coatings are frequently associated with the wide in the Balingian area.

Geol. Soc. Malaytlia, Bulletifl38, December 1995; pp. 141-151 142 ROBERT B. TATE The few structural measurements obtained from certain of its lateral continuity and trend until the Balingian exposures indicate a general N-S more evidence is available. The sections exposed at compressional direction (Fig. 10) but much more Sg. Balingian suggest a WNW trend coincident data are needed from a wider area before either the with a gravity lineament offshore which is structural interpretation or lateral extent and interpreted as the half graben, hading to direction of the shear zone, including whether there the SSW (Dr. P. Swinburn, pers. comm. 9/94). The is any horizontal component, can be established Balingian shear zone does not coincide with the satisfactorily. The structures are exceedingly West Balingian Line of Swinburn (1993) as the complex. There are at least two and probably more latter is aligned NNW and would appear to be a periods of deformation indicated by the presence of normal fault (see Figs. 2 and 9). Significantly, the at least two cleavages and an earlier cleavage is outliers of Nyalau Formation at Bukit Batu Bora, itself folded (Fig. 7). One generation of cleavage Sand E of the Usan Apau plateau also appear to be appears to be parallel to the axial plane of folds half graben hading to the N and NW. (Fig. 6). The timing of any deformation cannot be verified except that it is post-upper Eocene. It The West Balingian Line should be noted that the term "shear zone" in this The West Balingian Line, forms the western paper is used in the sense of a zone of deformation boundary of the Balingian sedimentary basins - shear whether pure shear or simple shear is not offshore (Fig. 9) and seismic data indicate there is inferred as no proof is yet available. a major fault trending WNW with about 4 km The trend of the Balingian shear zone is downthrow to the east (Swinburn, 1994, oral predicted to be WNW but as only one intersection presentation). The fault forms the eastern edge of has been observed it is perhaps premature to be the Penian high (Doust, 1981). The West Balingian

N LEGEND

S. CHINA SEA Q!i] Liang F. } (Pliocene) 4 [j!] BaIingian F [jii;INyalau F·}(u. Eoc-Mioc) o 5 10 To Binlulu ~ q!JTatauF. . , . - c:3 Arip volcanics (u. Eoc) V 1138. 3N [][J Belaga F. (m.-u. Eocene) ImBB Mersing basalt (Eocene) _ Bt. Piring granophyre (Eocene-Oligocene)

B1 BI approximate zone of intense shearing/deformation

Figure 1. Location map showing the position of the Balingian Shear zone and the regional geology (Based on Liechti et al., 1960; Wolfenden, 1960; Haile and Ho, pers. comm.). GeoL. Soc. MaLay.lia, Bulletin 58 THE BALINGIAN SHEAR ZONE, WEST BALINGIAN AND WEST BARAM liNNES, SARAWAK 143 Line can be identified on the published gravity map which identifies transform features. The West (Fig. 2) as a somewhat insignificant anomaly Baram Line is clearly a major tectonic and deep­ trending NNW, although the original gravity data seated feature as it separates areas which have show a pronounced anomaly. The onshore different geothermal parameters. There is a marked manifestation of the West Balingian Line has yet change in heat flow across the West Baram Line to be found and it perhaps occurs within the 5 km­ (Rutherford and Qureshi, 1981) and the higher wide deformation zone described here as the geothermal gradients in Luconia Province are Balingian shear zone. The West Balingian Line responsible for the predominantly gas-prone differs in azimuth from that of the Balingian shear hydrocarbons found there whereas the Baram Delta zone and they are not the same tectonic lineament. Province has lower gradients and predominantly oil-prone. Hutchison (1994) surmises that the The West Baram Line geothermally cooler gradients are as a result of cold The West Baram Line is an important tectonic oceanic crust beneath the Baram delta province boundary trending N45°W which separates the rather than the insulating effect of a +12 km thick extensive carbonates of Luconia Province from the sedimentary pile overlying hotter crust. large deltaic sedimentary basin of the Baram Delta Thus, the principal tectonic lineaments in Province (Fig. 9). It has never been defined formally. northern Sarawak form definitive boundaries to James (1984) refers to it as a major transform fault the Cenozoic sedimentary basins of northern parallel to a similar transform fault through the Sarawak but they show no consistent azimuth (Fig. Balabac Strait although no evidence is mentioned 9).

112 E 113 E GRAVITY- AREA

4 4 N N

3 3 N

Btg.Rajang o 20 40 2 2 N KM N 112 E 113 E Figure 2. Gravity - Tatau area (from Hutchison, C.S. (Ed.), CCOPIIOC, 1991). December 1995 144 ROBERT B. TATE

Figure 3. Belaga Formation, 150 m SW of bridge, Sg. Balingian. Dm-thick quartzites showing graded bedding interbedded with black phyllitic shales with graphite.

Figure 4. Ptygmatic quartz invading phyllites.

Figure 5. Competent quartzites dislocated, boudinaged and rotated by shearing.

Geo L. Soc. M ataYJia, BuLLetin 38 THE BALING IAN SHEAR ZONE , WEST BALING IAN AND WEST BARAM liNNES, SARAWAK 145

Figure 6. Sedimentary banding outlining a fold with incipient axial plane cleavage (parallel to the pen).

Figure 7. Folded cleavage. See Figure 10.

Figure 8. Sub-parallel quartz-filled joints perpendicular to foliation. Upper left: tight fold outlined by banding. See Figure 10.

December 1995 146 ROBERT B, TATE

• West Baram Lille ···.N 45° W ••••• Baram Delta ". Province luconia Carbonate ", Province "'''~~" 5' o Scale 50 sea. '. N km C h \1'1a. ", south • Tinjar fault West Balingian Line-., •••• -.N SSoW N35°W \. Ballnglan '. ". \\Province 4' N ". \, Bintulu Mukah ••••• •• h a If· g r abe n •••••• ...;. ___ Anau-Nyalau fault. . -~ ••••••• /~ (Tatau horst) N 54 E (\ ,-/'''')Mukah Balingian shear zone ...... N 75· W Mersing Line 3' N

II ' E 112' E 113' E 114'E Figure 9. The principal tectonic lineaments and offshore sedimentary provinces in northern Sarawak, POST-UPPER EOCENE SEDIMENTATION IN NORTHERN SARAWAK The stratigraphy of the Cenozoic sequences of northern Sarawak was originally defined by Liechti • a lSI (1960) but the monotonous lithostratigraphy and absence of marker horizons proved unworkable in o detail. Geiger (1963) introduced the concept of sedimentary cycles and Muller (1964) provided a biostratigraphic zonation based on pollen, Ho (1978) combined both microforaminifera and pollen to produce a comprehensive and effective biostratigraphic control, defining eight regressive sedimentary cycle units separated by thin transgressive intervals in the Oligocene to Holocene succession throughout the Sarawak shelf areas, Doust (1981) describes in detail the stratigraphic o IZI history of offshore central Sarawak and the nature of the eight sedimentary cycles in terms of coastal o plain (COL), coastal fluvio-marine (COF), lower coastal plain (LCP), holomarine inner neritic (HIN), holomarine middle neritic (HMN) Fluviomarine, inner neritic (FIN), fluviomarine, middle neritic Roadside exposures See Figure 7 (FMN) and holomarine, outer neritic (HON), o Bedding • Axial plane Prior to Cycle 1, middle Eocene. reefoid o Cleavage e Axial trace limestones began to accumulate in northern \l Secondary cleavage See Agure8 Sarawak across topographic highs in the Temala X Crinkling m Primary cleavage =bedding and Mulu areas which now form the Melinau ~ Foliation IZI Planar quartz veins lSI Secondary cleavage Limestone Formation at Melinau and Batu Gading as well as the smaller masses at Bukit Tujoh-Bukit Figure 10. Stereo plots (Schmidt net, lower hemisphere) Siman in the middle Baram. Subsequently, more of structural measurements taken at the road exposures or less continuous limestone deposition took place described in the text, at Batu Gading (middle Eocene-Lower Miocene) Geol. Soc, MaLaYJia, BuLletin 58 THE BALINGIAN SHEAR ZONE, WEST BALINGIAN AND WEST BARAM LINNES, SARAWAK 147 and Melinau (middle Eocene-Middle Miocene). The Furthermore, it is suggested that some, if not all, absence of Paleocene limestones in northern the areas shown as "Belait Formation" on the Sarawak and the scarcity of Paleocene limestones Geological Map of Sarawak (Tan, 1982), the Dulit generally in Borneo - Paleocene limestones are Range, Bt. Sekalap, Bt. Selikan and the large areas known only in scattered localities in the northern of arenaceous sediments NW of the Tinjar knee, part of the Meratus Mountains (Supriatna et al., belong to the same sequence of Oligocene deltas as 1980), is probably because much of Sundaland was those in the Nyalau Formation NE of Bintulu. In an extensive land surface at that time (Hutchison, Kalimantan, the extensive upper Oligocene-Lower 1992). Miocene Bayangkara Limestone is up to 300 m In Cycles I and II, (upper Eocene-Lower total thickness (Lefevre et al., 1982) and was Miocene), coastal plain sequences were deposited deposited to the SE of the extension of the West on the flanks of the Penian High in what is now the Baram Line in a position identical to the shelf offshore Balingian basin as well as the Nyalau limestone at Luconia (Fig. 12). Formation onshore. The "Penian high" is an Thus, there is corroborative evidence to indicate expression often used in oral presentations but it the West Baram Line continues across Borneo in has not been formally defined in the literature. a SE direction. The Penian high occupied a position offshore of the present day coastline between Mukah and Balingian TECTONIC IMPLICATIONS OF THE and is envisaged to be the source of deltaic sediments VARIOUS "LINES" in the offshore Balingian basin as well as the onshore Nyalau Formation which contains some Offshore NW Sarawak, the West Balingian Line unusually coarse-grained sandstones. In a series and West Baram Line form boundaries between of palinspastic maps, Doust (1981) and Agnostinelli sedimentary provinces and they constrain the et al. (1990), show conclusively that the Oligocene Balingian Province sedimentary basins in the NE coastline trended approximately perpendicular to and SW respectively (Swinburn, 1993). The Lines the present coast and that the Penian High formed are clearly of importance in separating the oil­ the source of prograding deltas trending NE. prone Baram DeltalBalingian Province, the gas­ In the Lower Miocene, Cycle II (Fig. 11, "A"), prone Central Luconia Provinces and the barren shelf carbonates began to accumulate offshore on areas of offshore SW Sarawak. The Balingian shear the SW side of the West Baram Line extending zone forms one side of the small sediment-filled southwards to Subis. In the present offshore area, Mukah half-graben which developed in the early Oligocene and Lower Miocene deposition to the NE Cenozoic but the zone may have had also a history of the West Baram Line was probably devoid of of repeated movement. In the upper Eocene, it clastics except perhaps thin turbidites which may formed a zone of compression and may have been occur beneath the Baram delta. In Cycle III (Fig. the focus of igneous activity, indicating a deep­ 11, "B") deltaic deposits spread further northwards, seated tectonic fracture in a ductile part of the ending deposition of shelf carbonates at Subis; by crust. In the Cenozoic, at shallower levels, it was the time of Cycle V (Fig. 11, "C"), the reeflimestones reactivated in the brittle zone and deformed by of Luconia Province had developed to their fullest extensional rifting forming a half-graben. extent and deltaic deposition had commenced in In the present offshore area, Oligocene the Baram delta which in its early development deposition to the NE of the West Baram Line was was controlled by the West Baram Line. Thus, probably devoid of clastics except perhaps thin offshore of northern Sarawak, the West Baram Line turbidites which may occur beneath the Baram acted as a steep shelf-edge from the Oligocene and delta. Onshore, the predominantly silty Setap Shale probably since the Eocene breakup of Sundaland. Formation accumulated in the Baram valley basin Onshore, the West Baram Line probably to the NE whereas the Oligocene deltas of the continues through the middle Baram across Borneo Nyalau Formation are terminated on the SW side into Kalimantan. The predominantly silty Setap of the West Baram Line. In Kalimantan, the Shale Formation accumulated in the Baram valley extensive upper Oligocene-Lower Miocene basin to the NE whereas the Oligocene deltas of the Bayangkara Limestone is gently dipping or sub­ Nyalau Formation prograde only as far as the SW horizontal suggesting that there has not been much side of the West Baram Line which seems to have movement since the time it was deposited on formed a shelf edge at that time. Paleocurrent presumably a relatively stable platform. Likewise, directions measured in the Nyalau Formation south the Central Luconia carbonates reflect prolonged of Bintulu show a NE-SW alignment (Mason, 1987) calcareous deposition on a stable platform. and the source would appear to be in the SW with Thus the West Baram Line had a controlling deltas prograding northeastwards (Fig. 12). influence on the distribution of shelf carbonates December 1995 11 t II [ II~[ CYCLE II CYCLE V 20Ma 9Ma Lower Miocene N~------5N early Upper Mioce=ne~~~~~~~~~~~ South C Sou t h

N

N ~ 2,5 5,0 3N Q 2,5 5.0 KM KM

114[

II~[ CYCLE '" Palaeofacies maps 16.SMa LEGEND lale Lower Miocene· Central Luconia. Balingian N~------~ & Baram Delta

ILCpl Lower Coaslal Plain Area of South China Sea Carbonate ~ Buildups IcoLi Coaslal ~/cOFI Coastal F1uviomarine ,.."" "" Fault N 4N ~ Holomarine Inner Neritic tmm~ rtuviomarine Inner !'Ieritic IBATI Bathyal IHMNI Holomarine Middle Neritic Hiatus II IFMNI fluviomarine Middle Neritic 11 N IHONI Holomarine Outer Neritic

Figure 11. Paleofacies maps, Central Luconia, Balingian and Baram Delta Provinces (adapted from Doust, 1981, AgnostinelIi et al., 1990 and ,Johnson et al., 1989). THE BALINGIAN SHEAR ZONE , WEST BALINGIAN AND WEST BARAM liNNES, SARAWAK 149 and Oligocene deltas extending for a distance of Kinabalu-type granodiorite and thought to be also some 800 km from Luconia to Mangalihat. Middle Miocene in age. At Long Lai in Kalimantan, Both the West Baram Line and Balingian shear tin-bearing adamellites occur in small intrusions zone seem to be offundamental tectonic importance at the intersection of Nand NW fractures not far in the evolution of the early Cenozoic history ofNW from the position of the SE extension of the West Sarawak and the presence of igneous rocks along Baram Line. The adamellites fit an isochron age of their length indicates that both structures extend 26 Ma (Bambang Setiawan and Le Bel, 1988). Tin to the deeper parts of the lithospheric crust. The has also been reported from the Lower Cretaceous Arip rhyolitic lavas, Piring granodiorite and Bukit Menyukung granite in West Kalimantan (Williams Mersing pillow basalts (Fig. 1), (Wolfenden, 1960, and Heryanto, 1986). The presence oftin is strongly Kirk, 1968) are perhaps related to the Balingian advocative of continental lithosphere at depth shear zone. The Bukit Mersing basalts lie roughly beneath this part of Borneo. along the strike from Sg. Balingian and may have There is additional evidence of continental erupted along weakness caused by the shear zone. material beneath west and central Borneo. The The considerable decomposition in the Pi ring lithophile elements antimony and arsenic occur in granodiorite may be as a result of movement on the sporadic mineralised zones extending in a belt from Balingian shear zone nearby during the later stages NW Kalimantan and west Sarawak to northern of intrusion or soon thereafter. Sarawak. The main economic deposits have been Igneous activity has occurred also along the worked in the past in West Sarawak but antimony trend of the West Baram Line across Borneo at and arsenic are known from a number of localities least since the late Oligocene (Fig. 12). Middle distributed across the Belaga Formation and as far Miocene basalts occur intruded along the Tinjar north as the middle Baram, at Long and near fault, a sub-parallel fracture (R.M. Banda, pers. Bukit Siman (Fig. 12). The latter occurrences are comm., 5/91) and the twin stocks of Bukit Kalulong reported by Haile (1962) to be in Setap Shale and Seludong located nearby are composed of Formation and are probably Miocene in age, having

5' EAST MALAYSIA N \ Sa bah i ~-"'\..~ '-., 0 -, .r---·, r :"':,-.~.,,/.,._ ...r\., ..• .sg <.. (r./·.1· .\_~. ~"g V' 4' i SebukuBay N

SbiAI <:=:=:7 ?northern limit Iiof antimony mineralisation N Ti';i:'r Dult zone ) ,,~ i\ ~ ". ~Bt Kalulong + Se lu: do~g stocks (?Mlddle Miocene) \Q) ,. 6"(3'" I 3' 0'9 ? Middle Miocene basalts (' .J upper Oligocene along f.iJult zone I . ~. . Lower Miocene ~\ N • Tubau ~ ./, granites • PaleoK"ne -II ' , limestones • " \ , Scale S arawak '-". ...., , • .1 Oligocene· o 70 ':1 Bayang1 <. granites EAST MA L A Y S I A (' (upper Oligoce ne &. / , • Tanjungredeb km ._. -Lowe r Miocene) '>, " I // t,', ' 2' \ 59·iO'Y"" " ...... ' '. ' N ~t~:e~~!~! (/ .I.I"' ~granites / 114' E 115' E 116' E 117' E l1B '~ Figure 12. The West Baram Line and its relationship to some ofthe elements ofthe geology of parts of Sarawak, E. Malaysia, Brunei and Kalimantan, Indonesia (Geology based on Tan, 1982, James, 1984 and Lefevre et al., 1982). December 1995 150 ROBERT B. TATE been mobilised by the intrusive events at Bukit whether there is a relationship. If such a Kalulong. The antimony and arsenic mineralization relationship does exist, the Balingian shear zone in the middle Baram is the most northerly probably extends much further to the east. Offshore, occurrence and coincides with the southeastwards the Balingian shear zone was reactivated in the extension of the West Baram Line. Cenozoic during a period of extension when it To summarize, the West Baram Line displays provided a weakness plane which acted as a normal the following important geological features: fault during the formation of the Mukah ha lf~ • it forms a boundary between hot and cold parts graben. In a recent paper by Jiang et al. (1994) 0 of the crust in the offshore areas. the results of geophysical investigations in the Sout • igneous activity in the form of granodioritic China Sea, seismic profiling in the northern part of plugs and Sn granites are located close to the the Zengmu (Balingian) basin has revealed a SE extension of the Line. prominent fault trending WNW near the Sarawak • basalt dykes are intruded along the sub-parallel coast and extending northwards, changing direction Tinjar fault. to NNW, for a distance of some 600 km across th • the SE extension of the West Baram Line is South China Sea. The fault is one of a series of coincident with the northern limit of Sb/As major structures showing similar trends between mineralization. the Lupar fault and the West Baram Line. The • it forms the margin of extensive carbonate faults are possibly related to the anticlockwise province offshore Sarawak as well as the NE rotation of Borneo and they may be also reactivated limit of a large limestone in Kalimantan. transforms as already indicated above. Thus, the West Baram Line has been an active Rifting of the Paleocene land surface and the tectonic lineament which extends to deep crustal breakup of Sundaland contributed largely to the levels. The Line has also had a significant initiation of most ofthe Cenozoic basins throughout geomorphological influence on deposition since the SE Asia (Hutchison, 1992). The West Baram Line early Cenozoic. and the Balingian shear zone appear to be old, deep-seated tectonic lineaments reactivated at the CONCLUSIONS beginning of the Cenozoic as part of the breakup of continental Sundaland. The nature of the main tectonic lineaments in northern Sarawak is not fully understood. Deep ACKNOWLEDGEMENTS seismic profiling has yet to be undertaken in the region and the interpretation of the tectonic The author gratefully acknowledges the helpful lineaments is largely conjectural. The West Baram discussions with Dr. KR. Chakraborty on the and West Balingian Lines both have normal fault structural aspects of this paper and thanks Prof. component characteristics but otherwise their C.S. Hutchison for bringing to the author's attentio origins are different. The West Balingian Line the importance of the West Baram Line and fo r seems to be a normal fault with a very large throw, critically reading the manuscript. The author is appears not to continue onshore and is restricted to grateful to the University of Malaya for providing the offshore oil basins; it is probably relatively research funds to carry out fieldwork in Sarawak short-lived, having formed in the early Oligocene with movement ceasing during the Middle Miocene. Conversely, the West Baram Line seems to be a REFERENCES more profound tectonic feature separating two ACNOSTlNEL LI, E., MOHAMAD RAISVDDIN BIN A KMA D TAJUDmN markedly different parts of the lithosphere and A NTO IEL LI , E. AN D M OHA 'IAD BI M OH D A RtS, 1990 extending across Borneo where it is associated with Miocene-Pliocene paleogeographic evolution of a tract high-level intrusions and associated mineralization. of Sarawak offshore between Bintulu and Miri. Geol. Soc. It may be a reactivated transform fault related to Malaysia Bull. 27, 117- 135. seafloor spreading before the advent ofthe present BAM BANG SET IAWAN AND LEBEL, L.M., 1988. Discovery of a new Indonesian island arc in the Eocene (Hayes and tin province, Long Lai area, East Kalimantan, Indonesia. Taylor, 1978) when the SE Sunda landmass began In: C.S. Hutchison (Ed.), Exploration and evaluation teclmiques for tin-tungsten granites in SE Asia and th to rift apart. Western Pacific region. Proceedings of JGep project 220 The Balingian shear zone exhibits complicated conference, Technical Bulletin 6, 61-82, SEATRAD Centre, deformation with compressive components and may Ipoh, Malaysia. be linked to nearby igneous activity, indicating its DousT, H ., 1981. Geology and explora tion history of offshore deeper features. It is tempting to link the Balingian central Sarawak. In: Michael T. Halbouty (Ed.), Energy zone with the Mersing Line but until the zone can Resources of the Pacific Region. A mer. Assoc. Petrol . be traced eastwards, it is premature to ascertain Geo!. , Studies in GeologJj, No 12. Ceo !. Soc. Ma LaYdia, BllLLetin 58 THE BALING IAN SHEAR ZONE, WEST BALING IAN AND WEST BARAM liNNES, SARAWAK 151

GEIGER, M.E., 1964. Proposed revision of the NW Borneo du Bureau de Recherches geologique et minieres 82 RDM stratigraphic terminology - The Neogene. Internal 00780, Orleans, France and Directorat Sumber Daya Report, Brunei Shell Petroleum Company, Seria, Brunei. Mineral, Bandung, Indonesia (unpublished). HAILE, N.5., 1962. The geology and mineral resources of the LIECHTI, P., ROE, F.W., AND HAILE, N.S., 1960. The geology of Suai-Baram area, northSarawak. Brit. Borneo Geol. Survey Sarawak, Brunei and the western part of North Borneo. Mem. 13 xii, 176, 19 figures, 22 tables, 32 plates, coloured Brit. Borneo Geol. Surv. Bull. 3. geological map, 1:250 000. MASON, E., 1987. Sedimentology and stratigraphy of the Nyalau HAYES, D.E. AND TAYLOR, B., 1978. Tectonics. A geophysical atlas Formation in South Bintulu, central Sarawak. B.Sc. Thesis, of the East and Southeast Asian seas (6 sheets). Geological University of Malaya (unpublished). Society of America. McMANus,J. ANDTATE,RB., 1976. Volcanic control of structures HO,I

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Manuscript received 11 November 1994

December 1995