The Balingian Shear Zone, West Balingian and West Baram Lines, Sarawak, and Their Importance in the Early Cenozoic Evolution of Nwborneo
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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, Sarawak, and their importance in the early Cenozoic evolution of NWBorneo ROBERT B. TATE Department of Geology, University of Malaya 50603 Kuala Lumpur, Malaysia Abstract: A major zone ofintense deformation trending WNW has been discovered in the neighbourhood of Sg. Balingian between Sibu and Bintulu. 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 Mukah 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-TATAU 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.