Geological Society of Malaysia, Bulletin 46 May 2003; pp. 335-351 The origin of the 'circular basins' of Sabah, Malaysia ALLAGU BALAGURU*, GARY NICHOLS AND ROBERT HALL SE Asia Research Group, Department of Geology Royal Holloway University of London, Egham, Surrey, TW20 OEX, UK *Now at Minerals and Geoscience Department Malaysia Sabah Locked Bag 2042, 88999 Kota Kinabalu, Sabah, Malaysia E-mail: [email protected] Abstract: Surface mapping, dating and radar image study of strata in southern Sabah (northern Borneo) have made it possible to revise the stratigraphy and reinterpret the structure and tectonic evolution of the area. Early Miocene regional unconformity may be equivalent to the Deep Regional Unconformity recognised offshore, below which the succession can be resolved into an Eocene accretionary complex age overlying an ophiolitic basement, and an upper Paleogene deep-water succession which formed in a forearc. The Paleogene deposits underwent syn-depositional deformation, including the development of extensive melanges, all of which lie below the unconformity. Localised limestones were deposited followed after a period of uplift and erosion in the Early Miocene, followed by an influx of clastic sediments deposited in delta and pro-deltaic environments in the Middle Miocene. These deltaic to shallow marine deposits form two coarsening-upward successions, mapped as the Tanjong and Kapilit Formations. The total thickness of these two formations remaining in the southern Sabah Basin amounts to 6,000 m, about half of previous estimates. The Early Miocene unconformity is interpreted to be the result of deformation and uplift following underthrusting of continental crust of the South China Sea which terminated Paleogene subduction beneath North Borneo. Renewed subsidence led to the development of a major Miocene depocentre above the older forearc accretionary complex. A new tectonic model is proposed for southern Sabah whereby a major transpressional deformation probably occurred during the Late Pliocene and the Tanjong. and KapilitFormations were deformed into broad NW-SE-trending synclines separated by narrow anticlines. The anticlines are sub-parallel to major faults and associated with high angle reverse faults, and positive flower structures. Secondary fold-faults formed oblique to the major faults. The structural style suggests that the NW-SE trending faults acted as major left-lateral transpressional zones and possibly produced large­ scale contractional duplexes. The faults may in part be reactivated basement structures. This deformation uplifted the area and is termed here the Meliau Orogeny. Renewed extension during the Quaternary caused some sequence repetition and widened the original synclines. The 'subcircular- to· elliptical-shaped basins' of the Meliau, Malibau and Tidung areas are structurally controlled synclines and interpreted as remnants of a single large basin, deformed in the NW -SE trending transpressional fault zones. INTRODUCTION to form the basement to the sediments of Sabah (Hutchison. 1989). Chert and other associated deep-marine sediments The geology of Sabah, a province of Malaysia in the overlying the ophiolite are Lower Cretaceous to Eocene in llorthern part of Borneo (Figs. 1 and 2), has been the age (Jasin et al., 1985). subject of a number of studies, some regional in nature The broken formations and melanges of Sabah show (e.g. Hutchison, 1988; Hutchison et aI., 2000; Rangin et characteristics of tectonic, sedimentary and diapiric origin al., 1990), others focused on particular areas or aspects of and were suggested to have formed in Early Miocene the geology (Clennell, 1991; Tongkul, 1995; Hutchison, (Clennell, 1991; Balaguru, 2001). In the Late Early Miocene i996; Noad, 1998). Previous geological maps of Sabah they were uplifted and shallow water sediments were (Wilford, 1967; Lim, 1985) indicate the distribution of the deposited unconformably on the pre-Neogene rocks which main rock formations. are more intensely deformed. The Neogene sediments are The main tectonic elements of Sabah includes a the mostly shallow marine to fluvio-deltaic and contain coal major fold-thrust belt trending north-northeast in the west beds and have formed unusual sub-circular to elliptical and bending to the east and southeast towards the shaped and fault bounded areas. They are about 20-30 Ian northeastern part of Sabah (Fig. 2). This arcuate belt wide and known as 'circular basins' of Sabah, such as the. consists of the deep-marine Eocene-Lower Miocene Rajang­ Meliau, MaIibau, Tidung, Bangan and Bukit Garam 'sub­ Crocker accretionary prism related to southeasterly basins' (Fig. 2). However, the detailed structural style and subduction of the proto-South China Sea (Rangin et al., architecture of the Neogene sediments has not been studied 1990; Tongkul, 1990; Hall, 1996). Much of the eastern before. Their structural relationships with the regional part of Sabah consists of an ophiolite complex, the melanges structural trend have often been simplified or not been and broken formations. The ophiolitic rocks are considered considered. The structural controls, including timing, Annual Geological Conference 2003, May 24-26, Kuching, Sarawak, Malaysia 336 ALLAGU BALAGURU , GARY NICHOLS & ROBERT HALL Figure 1. Simplified present-day tectonic configuration of SE Asia (from Hall, 1996) and Borneo in regional context. 1l6'N 11 7 118 119 Simeogari, (85), Bongaya (By), Sebabll (Sb~ G.odumao (G n) EARLY·LATE MIOCENE Lineaments with dip Tanjong (Tj), Mcligan (Me). SEDIMENTARY ROCKS K.pilit (K p ~ S.nd.k.o (Sn) # Balamba~ ~ . D t:r-' . Bangg' KuamuL (Km). Gennono (G r), Strike slip fau lt ~ MIDDLE MIOCENE ;/ , l..L...!J CHAOTIC DEPOSITS Ayer (Ay), Wario (Wr) Anticlinal axis fT4l EAKLY · ~ILUi)LE MIUCENE Kalumpang (Kg). Tungku (Tu) V C>Malawali ~ PYROCLASTIC DEPOSITS Labang (L b), Kul.pis (K. ~ Kudat (Kd) .. Thrust fault ~ EOCENE·EARLY MIOCENE West Crocker (wC r), Temburong (Te) SEDIMENTARY ROCKS .. t::.::SJ Sapulut (S p), Trusm.di (Tr), East Crocker (cCr) / No nnal raull BASEMENT ROCKS AND Chert-Spilitc (CS). ultrabasic rocks / Pala""Oow ASSOCIATED SEDIMENTS SOUTH CHINA SEA LATE M10CENE·QUATERNARY • IG EOUS ROCKS EARLY MIOCENE BASALT SULUSEA Labuan SccOs. CELEBES SEA SARAWAK Figure 2. Geological and structural map of Sabah (adapted from Lim 1985 and Tongkul, 1993). THE ORIGIN OF THE 'CIRCULAR BASINS' OF SABAH, MALAYSIA 337 basement controls and fault orientation, on basin formation eugeosynclioal basement of the Rajang Group of Cretaceous ~ the south-central Sabah basin are poorly known and to Oligocene age. described. The mechanism of how these structures In more recent syntheses of the stratigraphy of Sabah developed in response to tectonic activity of this region is summarised in Clennell (1996) and Hutchison et al. (2000) not fully explained. a late Eocene unconformity has been recognised which Synthetic-aperture radar (SAR) imagery for the area excludes the West Crocker and Kulapis Formations from has provided remotely-sensed data which are useful despite the Rajang Group. A Middle Miocene unconformity was the almost permanent cloud cover of the area and enabled interpreted to separate the Tanjong Formation (and the interpretation of the structural lineaments and morphology related Sandakan Formation to the north) from the older, of the study area. Biostratigraphic dating using nannofossils deeper marine deposits and melanges. These schemes has proved to be more successful than other organisms, and were based on studies of more accessible parts of Sabah to has made it possible to constrain the ages of the stratigraphic the north and west of the study area. units and timing of events. Access to the area (Fig. 3) is . Collenette (1965) proposed that the Neogene sub-basins provided by logging roads. originally comprised one major basin (Fig. 4). Lee and Tham(1989) and Clennell (1991) proposed differential TECTONIC SETTING loading and subsidence into muddy melange and diapirism as possible mechanism for theirorigiIi. Tongkol (1993) The island of Borneo formed by the Mesozoic accretion suggested the NE-SW trending regional strike-slip faults of microcontinental fragments, ophiolite terranes and island controlled the formation of isolated individual sub­ arc crust onto a Palaeozoic continental core (Hamilton, basins. ~979; Hutchison, 1989; Metcalfe, 1996) (Fig. 1). At the beginning of the Cenozoic Borneo formed a promontory of REVISED STRATIGRAPHY OF Sundaland (Hall, 1996,2002), partly separated from Asia SOUTHERN SABAH by the proto-South China Sea. The oceanic part of the proto-South China Sea was subducted during the Paleogene The revised stratigraphy of southern Sabah is shown in: and a large accretionary complex formed along the Figure 4 and Table 1 and differences can be seen by northwestern margin of Borneo. In the early Miocene comparing Figures 5 and 6. The important boundary uplift of the accretionary complex occurred as a result of between the LabanglKuamut Formations ·and the overlying underthrusting of thinned continental crust in northwest Tanjong (and locally the Gomantong Limestone) (Hamilton, 1979; Taylor and Hayes; <1983; Tan and Lamy, :Formations is significant because it is 1nterpreted to be a 1991; Hazebroek and Tan, 1993). Upliftmayhaveresulted regional unconformity which separates a lower succession also from shortening due to the counter-clockwise rotation of deformed and indurated deep water sediments and of Borneo between 20 and 10 Ma (Fuller et ai., 199,1; Hall, melanges from gently folded,