Carbonate platforms in wedge-top basins: An example from the Gunung Mulu National Park, Northern Sarawak (Malaysia)

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Marine and Petroleum Geology 26 (2009) 177–207 www.elsevier.com/locate/marpetgeo

Carbonate platforms in wedge-top basins: An example from the Gunung Mulu National Park, Northern Sarawak (Malaysia) Mario Wannier 1507 Ashford Parkway, Houston, TX 77077, USA Received 9 July 2007; received in revised form 12 December 2007; accepted 29 December 2007

Abstract The Melinau carbonate platform initiated during the Mid-Eocene on a rotating slice of the Rajang accretionary prism. The differential sedimentary loading enhanced a rotation of the mobile substratum and created an elongated, asymmetrical wedge-top basin. The extensional southern margin of the basin consists of a 2100–2200-m-thick section of Eocene-to-Oligocene carbonates. These thin laterally towards the northern margin of the basin, where a carbonate factory was active on a postulated underlying thrust. Backstepping and dismemberment of the carbonate system started during the latest Oligocene and deep-marine sedimentation became prevalent over the entire region during the Early Miocene. r 2008 Elsevier Ltd. All rights reserved. Keywords: Wedge-top basin; Carbonate platform drowning; Mobile substratum; Mulu; Melinau; Sarawak

1. Introduction A large variety of basin settings were associated with plate tectonic processes that led to the formation of the complex island arc systems of SE Asia. While it may be convenient to simply lump all these under the heading of fore-arc and back-arc basins, such classification does little to understand the highly variable tectonic setting of carbonate platforms in SE Asia. For instance, the Tertiary carbonate buildups of Papua New Guinea form in a basin that is associated with the transition from a passive margin platform to an incipient fore-deep setting (Pigram et al., 1990), the carbonate platforms of the northern Bali Sea are placed in a complex back-arc setting (Emmet, 1996). Another back-arc setting is associated extensional basement-involved rift (i.e. the Early Miocene Baturaja reefs of the West Java Sea, see Bishop, 2000). On the Luconia micro-continental block, Mid–Late Miocene reefs developed preferably on horst blocks (Mohammad Yamin Ali Corresponding author. Present address: Ruychrocklaan 131, 2597 EM Den Haag, The Netherlands. Tel.: +31 70 220 2164 (private, evening), +31 70 447 4231 (professional); fax: +31 70 447 2393 (professional). E-mail addresses: [email protected], [email protected] (M. Wannier).

0264-8172/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2007.12.004

and Abolins, 1999). Other reefs systems such as the ‘‘Thousand Islands’’ of the West Java Sea (Jordan, 1998) appear to have little, if any tectonic control. Carbonates in a fore-arc setting are reported from offshore Sumatra without much detail (Situmorang et al., 1987). This paper describes a thick carbonate unit that formed as a wedge-top basin on the Rajang accretionary prism. The concept of ‘‘wedge-top basin’’ (DeCelles and Giles, 1996) was selected as it best describes this unusual minibasin, formed on a mobile substratum within a linked extensional–compressional system. The study area is located in the northeastern inland area of Sarawak, northern Borneo (Fig. 1). It includes the Gunung Mulu National Park (GMNP), and adjacent areas between the Baram River to the west and the Limbang River to the east. The focus of this work is on Mid-Eocene to Early Miocene Melinau limestones, deposited on the Rajang accretionary prism, composed of folded deepmarine turbidites and hemipelagics (the Late Cretaceous to Paleogene Rajang flysch). The Melinau limestone presents a rare example of a larger, isolated, unrimmed carbonate system, developed on an accretionary prism (Wilson, 2002), and characterized by

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Fig. 1. NW Sarawak & Brunei geological map, redrawn and reinterpreted after Liechti et al. (1960), Sandal (1996), and Morley et al. (2003).

syndepositional tectonic activity resulting in an expanded section of vertically stacked shallow marine limestone. This paper proposes a new structural, stratigraphic and paleogeographic model for the Melinau limestone, based upon fieldwork and analogs, backed up by petrographic and biostratigraphic studies covering two separate wedgetop basins and their adjacent deep-marine basin. This study narrows down the timing for the onset of carbonate sedimentation and offers new insights on the geological events leading to the drowning of the carbonate platform. It draws largely on a reinterpretation of literature data using modern structural–stratigraphic concepts. 2. Plate tectonic framework and regional setting 2.1. Plate tectonic framework The south-eastern (SE) Eurasian margin is fragmented into a large number of microplates that have experienced a complex structural evolution during the Cenozoic, mainly as a result of the separate collisions of the Indian and Australian plates. The Mid-Eocene to recent northward penetration of India into eastern Eurasia has resulted in the formation of

a series of regional strike slip faults and shear zones that have partitioned SE Asia into a number of discrete blocks (Peltzer and Tapponnier, 1988). Following the MidPaleogene initial collision of India (55–45 Ma), a phase of strike-slip extrusion south of the Red River Fault, associated with oceanic basin formation (e.g. South China Sea) characterized much of the Oligocene (35–25 Ma). At that time, trans-tensional stress fields resulted in the formation of a large number of rift/pull-apart basins throughout the SE Asian archipelago (Rangin, 1989; Rangin et al., 1990b; Madon, 1999). As from Mid-Miocene time, the northward collision of the Australian plate caused the inversion of Indonesian basins; the combined effects of this collision and of the clock-wise rotation of the Philippine Sea plate induced the anticlock-wise rotation of Borneo (Hall, 1996), and the cessation of spreading in the South China Sea. The collision between the Philippine arc and the Eurasian margin occurred during the Pliocene. 2.2. Regional setting and structural evolution of Borneo The structural evolution of Borneo is well known in detail but still lacks a synthesis. The salient events are summarized (Fig. 2) from a variety of sources (Hutchison,

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Fig. 2. Regional structural elements and extent of the Rajang and Crocker accretionary prisms. CAP: Crocker Accretionary Prism; CS: Celebes Sea; LL: Lupar Line; MPF: Mae Ping Fault; RAP: Rajang Accretionary Prism; SCS: South China Sea; SFZ: Sabah Fracture Zone; SS: Sulu Sea; WBL: West Baram Line. Redrawn after Hutchison (1996a, b), Moss (1998), and Morley (2002).

1988, 1992, 1996a; Hutchison et al., 2000; Rangin, 1989; Rangin et al., 1990a; Benard et al., 1990; Daly et al., 1991; Hall, 1996, 2001, 2002; Pubellier et al., 2003, 2005; Morley, 2002, 2007). From the Cretaceous into the Neogene, the South China Sea region has experienced episodic sea-floor spreading, punctuated by ridge jumps and orientation changes. During this time, a number of continental fragments from Indochina drifted southward and docked against the Kalimantan continental block, a Paleozoic–Mesozoic igneous and metamorphic-cored block, itself drifted from Eurasia in the Late Jurassic. From Cenomanian till Late Miocene, a southwardoriented subduction zone was active on the northern margin of the Kalimantan block, consuming the Proto South China Sea oceanic plate. The successive docking from west to east of 3 main continental fragments, the Luconia-, Baram-, and Palawan–Reed Bank blocks eventually closed the subduction zone in stages (Hazebroek and Tan, 1993; Clennell, 1996; Sandal, 1996).

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During the Mid-Eocene, the Luconia continental block moved southward probably through slab pull, docked against the Rajang accretionary prism and associated volcanic arc, and closed the subduction zone westward of the Baram Line. The Lupar Line (Hutchison, 1996b) delineates the oceanward suture of this accretionary prism, which encompasses the Rajang Group in Sarawak and the Embaluh Group in northern Kalimantan (Moss, 1998). Inland compression and uplift associated with this event resulted in the development of the Sarawak unconformity. Subsequent relaxation within a regional trans-tensional stress field resulted in the formation of a number of extensional basins (e.g. the Makassar-East Java-rifted basin). After docking of the Luconia Block, the Rajang accretionary prism tapered out eastward of the Baram Line and was relayed to the east by the younger Crocker accretionary prism. It is in this setting that the Melinau wedge-top basin started developing, during the late part of the Mid-Eocene. Following a ridge jump in the Early Oligocene, oceanic crust started forming in the South China Sea, northward of the Baram- and Palawan–Reed Bank blocks. These two microcontinents drifted southward towards the West Crocker accretionary prism and associated volcanic arc. The Early Miocene onset of the counter clock-wise rotation of Borneo (Hall, 1996) created a new transtensional stress field that resulted in a sag phase over much of the island; the rapid subsidence (200 m/10,000 yr in Saller and Vijaya, 2002) associated with this event is the main reason for the drowning of the Kerendan carbonate platform (Saller and Vijaya, 2002) and of the Melinau carbonate platform. Docking of the Baram block in the Early–Mid-Miocene, followed by the docking of the Palawan–Reed Banks in the Mid–Late Miocene jammed this subduction zone. Inland compression associated with these two events is recorded, respectively, in the Sabah/Deep Regional unconformity and in the Shallow Regional unconformity (Hutchison, 1996b). Associated uplift destabilized part of the West Crocker accretionary prism, which slid and formed the Mid-Miocene allochthon (Rangin et al., 1990a; Ingram et al., 2004), a giant slide sheet buried offshore northwest (NW) Sabah. Extending SW to NE at the back of the Baram-Palawan/ Reed Bank blocks, the Northwest Borneo Trough acted as a subduction zone during part of the Late Miocene. At the Miocene–Pliocene transition, the southward drifting of the Dangerous Grounds block jammed this short-lived subduction zone and caused folding and thrusting on the Baram block and compressional deformation further inboard including the area of Mulu. Late Miocene to recent isostatic rebound caused the uplift of old suture zones, which were in term rapidly eroded. This led to the development of major progradational, deltaic systems in offshore basins from the NW to the SE of Borneo, and to the deposition of shelfal clastic sequences locally exceeding 10 km in thickness (Hutchison et al., 2000; Hall and Nichols, 2002).

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2.3. Northern Sarawak stratigraphy In NE Sarawak, the regional Sarawak- and Sabah/Deep Regional unconformities (Hutchison, 1996a, b) delineate three successive tectonostratigraphic units (Fig. 3). The older Rajang tectonostratigraphic unit consists of a Cretaceous to Early/Mid-Eocene flysch basin that developed as an accretionary prism along the southern margin of the Proto South China Sea. This submetamorphic to low-grade metamorphic unit constitutes the basement rock in the area of interest, where it is mapped as the Kelalan Formation (mostly turbiditic siliciclastics) and the Mulu Formation (mostly hemipelagics). As from Mid-Eocene time, deep-marine conditions became prevalent over much of northern Borneo, as the West Crocker accretionary was developing. The hemipelagic Temburong shales characterize the deepwater depositional system in the area between the West Crocker and Rajang accretionary prisms. As from late Mid-Eocene

time, the shallow marine Melinau and Batu Gading wedgetop basins started forming on the higher parts of the submerged Rajang accretionary prism. The West Crocker tectonostratigraphic unit is bracketed at the base by the Sarawak unconformity, and at the top by the Sabah/Deep Regional unconformity (DRU), which marks the docking of the Baram block and the thrusting of the Sabah volcanic arc over the Crocker basin. The development of a series of Neogene successor basins characterizes much of the subsequent evolution of NW Borneo. In line with Sandal (1996), the Setap Shale Formation characterizes the deep offshore setting of the Neogene tectonostratigraphic unit. 2.4. Chrono- and biostratigraphic reference scheme The chronostratigraphical scheme (Figs. 3 and 4) is based on the geologic time-scale of Gradstein et al. (2004). As is customary in SE Asia, the biostratigraphical subdivision of

Fig. 3. Time-rock synopsis. Chronostratigraphy after Gradstein et al. (2004); tectonostratigraphy and lithostratigraphy units, reinterpreted mainly from Liechti et al. (1960) and Sandal (1996); Tectonic events from references in chapter 2.2. DRU: Deep Regional Unconformity; SCS: South China Sea; SS: Sulu Sea.

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Fig. 4. Chrono-biostratigraphic framework and correlation of carbonate formations. Chronostratigraphy after Gradstein et al. (2004); Biostratigraphy mainly after Lunt (2004); Outcrop stratigraphy reinterpreted after Saller et al. (1993), Saller and Vijaya (2002), Adams and Haak (1962), Adams (1965), Adams and Wilford (1972), Boudagher-Fadel et al. (2000).

the Eocene–Miocene carbonates is based on the distribution of larger foraminifera. This larger foraminiferal biozonation is known as the ‘‘East Indian Letter Classification’’, a remarkably resilient framework, initially established by Van der Vlerk and Umbgrove (1927), with minor modifications introduced by Adams (1970) and Lunt (2004). While microfossils are widespread in most limestone samples, their recognition is based on random thin sections, with corresponding uncertainties in their specific identification. The pervasive faunal reworking, known to occur in parts of the Melinau Limestone Formation, further complicates the age dating. The correlation between foraminiferal biozones and the time-scale is itself subject to significant uncertainty, owing to the paucity of anchor-points and the time-variable nature of the events, many of which relate to faunal migrations. The subdivision of Lunt (2004) provides higher resolution in the Late Oligocene to Early Miocene; it has proved to be of special importance for the dating of a series of events occurring in the closing stages of the Melinau carbonate basin. Saller et al. (1993) published results of strontium isotope age dating, carried out on larger foraminifera. However, the lack of species-specific data on the foraminifera limits the value of their Strontium calibration.

3. Wedge-top basin analogs constrain the interpretive framework The Melinau carbonate platform is interpreted to have been formed in a wedge-top basin; this chapter gives a review of this structural–stratigraphic setting and describes key analogs that provide an interpretive framework. Wedge-top basins (sensu DeCelles and Giles, 1996) include one category of successor basins that develop on the unstable slope of accretionary prisms during periods of rapid sedimentation, following the emplacement of the nappe system. The sedimentary fill consists typically of thick aggrading sequences of shallow marine to non-marine clastics; shallow marine carbonates such as the Melinau limestone are unusual for this setting. The structural controls on the development of wedge-top basins include inboard extensional faulting, compensated by thrusts on their outer margin, and bounded by transform faults on their sides (Flinch, 1993). The mobile substratum surrounding these minibasins is often deformed and squeezed out in a diapiric fashion (Clennell, 1996). The initiation of the basin occurs when a portion of the mobile substratum slides downdip in a rotational movement

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Fig. 5. Prerifaine Nappe thrust front, offshore Atlantic Morocco, line drawings of seismic lines redrawn after Flinch (1993).

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and creates a shallow, wedge-like accommodation space along the extensional part of the glide plane. Differential loading by fresh sediments in this unstable wedge in turns acts as an engine that maintains the rotation of the depositional system. In time, the thrusted part of the wedge-top basin is locally exhumed and eroded, with the erosional products being redeposited downdip, as well as reincorporated in part into the wedge-top basin. This reworking is indicative for the last phase in the evolution of such basins. 3.1. Wedge-top basin analogs On the offshore Atlantic margin of Morocco, the Prerifaine nappe and its superposed wedge-top basins are excellently imaged by seismic reflection profiles. Flinch (1993) illustrates the geometry of these minibasins, which developed in the space created on the accretionary prism by the rotation of underlying gravity slides. The asymmetry of these basins is best seen on seismic dip sections (Fig. 5): they are thicker updip along the extensional margin, and thin downdip towards the toe-thrusted part of the gravity slide. These profiles also demonstrate the relationship between the rotational gravity sliding on the Prerifaine nappe and the formation of thin olistostromes downdip. In Morocco, the Supra-nappe unit and its associated wedge-top basins behaves as a deep-marine prograding system, with stratigraphically young canyon systems present downdip of the prograding wedge. These undrilled, deepwater Moroccan minibasins likely consist of clastics.

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In the Makran coastal ranges of SW Iran, Neogene wedge-top basins developed on top of soft tectonic me´langes and flysch formations of Eocene–Oligocene age (Fig. 6). These asymmetrical, oval-shaped basins formed on an incompetent substratum and are characterized by subsidence folding and faulting concurrent with depositional loading. These minibasins are typically 10–20 km in width and 20–80 km in length, with a thickness reported to reach in places 10 km; they are often arranged in an enechelon fashion. The fill of these basins consists of thick neritic, clastic sequences, but also include carbonates (coral limestones in McCall et al., 1994). Mid- to Late Miocene clastic-filled, successor basins developed on Lower Miocene deep-marine shales and me´langes have been described from the Sandakan Basin in Sabah (Clennell, 1996). These ‘‘circular’’ depocenters have a width in the order of 10 km and a thickness in excess of 3 km. They have been interpreted as forming over soft mudstones, inducing the substratum to deform and rise on the edges as mud diapirs. Seismic profiles (Fig. 7) show a marked asymmetry in the basin fill. The inboard, extensional area is characterized by a steeply dipping, thicker sedimentary sequence including possible onlap geometries against a poorly imaged boundary with the adjoining substratum (normal fault). The downdip, outboard area is characterized by a comparatively thinner sedimentary sequence, steeply rising and abruptly truncated by an unconformity, which records the progressive erosion of the outward rotating basin (thrust linked to the inboard normal fault).

Fig. 6. Makran coastal ranges geological map (SW Iran) illustrating the development of Neogene wedge-top basins overlying the Paleogene accretionary prism. Adapted from: Geological Map of Iran, Sheet No. 6 South-East Iran, 1:1,00,000. National Iranian Oil Company, Tehran (1977).

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Fig. 7. Stratigraphic architecture in Sandakan wedge-top basins, Sabah, Malaysia. (A) Interpretative line-drawing of seismic line SB6-01, redrawn from Fig. 6c in Clennell (1996). See Fig. 2 for location. (B) Time-stratigraphic interpretation of (A).

Similar wedge-top basins have been described in Italy, in particular in the Epi–Liguride sequence, where they have been referred variously as piggy-back basins (Ori and Friend, 1984), satellite basins (Ricci Lucchi, 1986) and thrust-top basins (Buttler and Grasso, 1993). See Pini (1999) for a review of olistostromes in the Northern Apennines. Thrust-top platforms (Bosence, 2005) are described as ribbon-like carbonate systems of limited thickness, developed along the positive areas of emergent thrusts. They represent a different type of carbonate platforms, structurally unrelated to the wedge-top of basins discussed here.

on a limited dataset and is likely to evolve significantly when new data become available. 4.1. Lithostratigraphy In northern Sarawak, the Melinau Limestone Formation consists in a number of individual carbonate units trending NE-SW (Fig. 1), that show significant variations in vertical thickness, lateral extent, and depositional environments. The following lithostratigraphic units are recognized from the southwest to the northeast (Chai Peng et al., 2004):

 4. The Melinau carbonates Although the Melinau limestones can be recognized and mapped easily from aerial pictures, they are known only from a limited number of outcrops. This is due to their remoteness in the midst of the Borneo rain forest, and in some cases to their inaccessibility. Outcrops are generally limited in continuity and are typically affected by severe surface weathering. The geological interpretation is based



The Batu Gading limestone and Bukit Besungai limestone crops out along the Baram River, north of the settlement of Long Lama. Shallow marine limestones up to 50 m thick form a series of isolated hills covering an area of 4 km2 (3.7 km long by 1.1 km width); the northernmost outcrop of Batu Gading has largely been quarried away. The outcrops have been described by Adams (1959), Liechti et al. (1960), Haile (1962), Adams and Haak (1962), and more recently by Ngau (1989). The up to 2200 m thick, shallow marine Melinau limestone extends over an area of some 300 km2

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(37 km long by 8 km wide, striking 30–350E); this area is now largely part of the GMNP, and the site of a large karst and cave system. The Melinau limestone has been described originally by Shell geologists (Liechti et al., 1960). Subsequent work has been carried by the Geological Survey Departments of Brunei and Sarawak (Wilford, 1961; Haile, 1962), and later by Adams (1965), who published a comprehensive biostratigraphic analysis of the limestone. This well-delineated stratigraphic unit represents a larger, individual wedge-top basin, referred to as the Melinau basin. The Selidong limestone consists in a series of isolated outcrops, some 150–240 m thick, situated circa 10 km to the NE of the GMNP area. The stratigraphic succession consists of dark blue shales, overlain by thinly bedded Globigerina limestones; the upper part of this unit includes alternation of graded calcarenites and coarse debris flows, indicating a deeper-marine depositional environment (Adams and Wilford, 1972). The Keramit limestone is an isolated outcrop to the NW of Selidong; it is some 55 m thick, and consists of two deeper-marine units with carbonate beds. The lower unit consists of thinly bedded, occasionally graded calcarenites, embedded in an overall deep-marine sequence (‘‘chalky Globigerina limestones’’ in Adams and Wilford, 1972). An upper, channel cut-and-fill sequence, overlain by debris flows has an unconformable relationship with the lower unit. The Selidong and Keramit sections document the shedding of the shallow marine Melinau basin into the Temburong deep-marine basin.

The Tujoh–Siman limestone is located some 85 km SSE of the GMNP area, in a remote jungle location; very little data are available as to its composition, thickness and age (Adams, 1965). Further research is needed to incorporate this unit within the framework of the Rajang wedge-top basins. The Late Oligocene to Early Miocene Gomantong Formation in Sabah (Boudagher-Fadel et al., 2000) is another example of a shallow marine carbonate platform, developed unconformably on top of deep-marine sediments. Its structural setting is likely analogous to that of the Melinau limestone. The Melinau Limestone Formation has not been mapped on lithological criteria, but has been subdivided on the basis of larger foraminiferal biozones, identified from a large number of samples collected by the earlier workers in the area (see Adams, 1965). Four separate larger foraminiferal biozones have been recognized in the field (Figs. 4 and 8), i.e. Ta/b (Eocene), Tc/d (Early Oligocene), Lower Te (Late Oligocene) and Upper Te (latest Oligocene–earliest Miocene). Units Tc and Td can locally be distinguished from each other; the Lower Te unit corresponds to subzones Te1–3 and the Upper Te unit to subzones Te4 and Te5 (Fig. 4). Internal unconformities are revealed uniquely by missing biozones, not by geological mapping.

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In the field, the application of the above subdivision is fraught with difficulties, owing to the occasional absence of index larger foraminifera and the often widespread reworking of older faunas in younger strata. Likewise, the identification of structural dips is often unclear, because of pervasive karstification, of superficial coating by carbonate-rich solutions or because of aliasing with joints. The available geological maps have been extrapolated from a larger number of biostratigraphically calibrated data points, and therefore include considerable uncertainties. 4.2. The Melinau wedge-top basin (Bartonian to Late Chattian) The Melinau Limestone Formation reaches its maximum development in the GMNP (Figs. 8 and 9), where it has been mapped by Shell geologists and later by Wilford (1961). The Melinau Limestone Formation has a steep NWdirected regional dip, in the order of 50–801 on its SE margin, and 20–501 on its NW edge, where it is overlain by the Temburong Formation. The limestone is often faulted (Fig. 10A) and heavily jointed (Fig. 10B); both brittle and ductile deformations are observed in thin sections (Figs. 11C, D). The reverse Melinau Fault is a NE–SW oriented, 40-km-long lineament dissecting the carbonates along an axial syncline. It possibly represents an inverted normal fault (antithetic thrusting?), but its nature requires further studies. The Melinau Limestone Formation unconformably overlays the Mulu Formation to the SE, where the limestone forms a vertical wall reaching some 1000 m in height (Figs. 10C–F). The sharp, linear contact between the 2 formations observed on aerial pictures (arrow in Fig. 10D) supports the interpretation of a faulted contact along the southern margin of the basin. The extraordinarily thick sequence of stacked shallow marine carbonates is one of the largest recorded in the whole of southeast Asia: out of some 300 carbonate units listed by Wilson (2002), the Melinau Limestone Formation is one of only two formations with a total carbonate thickness in excess of 2200 m. Adams (1965) established the limestone stratigraphy along the Melinau Gorge, the type-locality of the formation; a reinterpretation of his log together with estimated vertical thicknesses is shown on Fig. 12. Adams lists the vertical thickness estimate for the various stratigraphic units, in the different locations of the area (Table 1). The data show a doubling of the carbonate thickness on the southern margin; as mentioned in Section 4.1 there is a potentially significant error bar associated with all these estimates. Taken at face value, these rapid lateral variations in thickness and high rates of subsidence are diagnostic for deposition in an extensional tectonic setting. The contact between the Mulu Formation and the Melinau Limestone Formation has been reported as concordant (Liechti et al., 1960; Wilford, 1961; Adams,

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Fig. 8. Mulu–Melinau geological map (redrawn and reinterpreted after Wilford, 1961; Adams, 1965). The rectangle indicates the coverage of the more detailed geological map shown in Fig. 9.

1965), although exposures are rare. The abrupt burial and facies change between the deep-marine, fine-grained metasediments of the Mulu Formation and the overlying shallow marine Melinau limestones indicate that the two formations belong to separate tectonostratigraphic units. The contact is known as the Sarawak unconformity, and the associated time-gap is estimated to be in the order of some 12–14 Ma (Fig. 3). The base of the Melinau Limestone Formation logged along the SE margin of the basin (Fig. 8) consists of calcareous sandstones, red-weathered to black sandy limestone (area south of Lang Cave, in Wilford, 1961) and dolomites (base of the Melinau Gorge section in Adams, 1965; see Fig. 12). These non-fossiliferous strata are overlain by shallow marine carbonates with Nummulites javanus (Mid-Eocene, Ta marker), and Pellatispira orbitoidea (Mid–Late Eocene, Tb marker, see Fig. 11B). The boundary between the Bartonian and Priabonian has not been identified at this location. Recent fieldwork along the Melinau River opposite the Mulu airport (Ta outcrop in Fig. 9) has revealed the presence of a Late Bartonian (Ta3) foraminiferal fauna, characterized by the presence of Glomalveolina ungaroi

(Fig. 11A). This species has been described from the Late Bartonian (SBZ18) ‘‘Calcari Nummulitici’’ in northeastern Italy ((Bassi and Loriga Broglio, 1999), and is so far only known from three nearby outcrops in the Colli Berici (identification, determination and references from L. Hottinger). There, this foraminifer is present in highenergy packstones and grainstones, but it originates most probably from protected environments, as do alveolinids in general. The isolated occurrence of this foraminifer in Mulu raises interesting questions as to migration mechanisms and the age correlations between the two areas. No thickness and no contacts could be established for the Bartonian in Mulu, as this occurrence comes from a 2 m thick, 5–8 m long isolated outcrop of packstone. A Bartonian time for the onset of carbonate deposition on the shelf is further confirmed by the presence of a reworked Ta foraminiferal fauna in the basin (see chapter below). The general occurrence of Priabonian (Tb) foraminifera at or near the base of the Melinau Limestone Formation, indicates a widespread shallow marine transgression during the Late Eocene. In the GMNP area, Priabonian carbonates are free from detrital minerals; their textural

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Fig. 9. SW Melinau geological map, redrawn and reinterpreted after Wilford (1961) and Adams (1965). Fieldwork areas indicated with star. Ta outcrop refers to the location of the Bartonian Glomalveolina limestone outcrop; the quarry is the site of the rafted Melinau limestone block, next to the outcropping debris flows and olistoliths.

composition is quite uniform and consists predominantly of high-energy bioclastic limestones, dominated by the presence of red algae and foraminifera. There is no evidence for reefal development at that time. These vertically aggrading Late Eocene limestones (Fig. 12) reach a maximum thickness of some 670 m at the SE margin of the Melinau basin. Where present, the Early Oligocene (Tcd) limestones are deposited in apparent continuity with the underlying Late Eocene (Tb) beds. In the Melinau Gorge, they are characterized by a lesser presence of red algae as a skeletal constituent and by an alternation of higher-energy (Fig. 11E) and lower energy microfacies. This unit reaches a maximum thickness of 610 m at the SE margin of the basin; the absence of these biozones on the NW side of the Melinau basin (Adams, 1965) may be interpreted as nondeposition, or non-identification due to insufficient sample material. There is no record of reworked Early Oligocene (Tcd) faunas in the younger sequences, ruling out an absence through erosion. The Late Oligocene (Te1–3 biozones) carbonates are reported to be in apparent continuity with the older units (‘‘Lower Te’’ in Adams, 1965); at the SE margin, continuous subsidence and thicker development of limestones (up to 550 m thickness) result in no apparent hiatus in the sedimentation. In the Melinau Gorge, this unit is characterized at first by higher-energy facies (shoals), passing into lower-energy, matrix-supported facies (lagoonal and foreshore) and the depositional trend remains aggradational (Fig. 12). Lepidocyclinid foraminifera are

especially abundant (Fig. 11F) and corals appear more frequently than in the underlying units; however, there is no indication for significant reefal development at that time (see Wilson and Rosen, 1998 on the general paucity of reefs for this time in SE Asia). The absence of Late Oligocene on the NW side of the outcropping the Melinau limestones (Adams, 1965) may again be due to non-deposition or nonidentification (see comments above). 4.3. Backstepping, dismemberment and drowning of the Melinau carbonate system (intra Late Chattian–Early Aquitanian) The Late Chattian–Early Aquitanian carbonates (Te4 and Te5 biozones) are present mainly on the northern part of the GMNP area (Adams, 1965), within the narrow syncline along the Melinau river, and along the NW margin of the basin, where they constitute a thin fringe which wedges out towards the SW (Fig. 9). This unit has not been described in detail and its thickness estimate of up to 550 m may be exaggerated. Adams (1965) mentions ‘‘Samples rich in algal and coral debris and having an almost reef-like aspect’’, providing the first indication for coral build-ups in the Melinau Limestone Formation. On the NE margin of the basin, this unit rests in apparent conformity on Chattian (Te1–3 biozones) carbonates. At the time of reefal development on this northern area (Te4 and Te5 biozones), offshore mudstones were being deposited on the former northwest margin of the Melinau basin. To the southwest of the airport landing strip (Fig. 9),

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Fig. 10. (A–F) Mulu–Melinau outcrop pictures. (A) Fault plane in Early Oligocene (biozone Tcd) limestone; the fault is interpreted as an extensional faults, later reactivated into a reverse faults. Note the paleokarst with brecciated limestone fill (arrow; magnified within insert). Field of vision approximately 6 m by 4 m. Mulu Resort. (B) Jointed Early Oligocene (biozone Tcd) limestone, from a hill in the western axis of the airport runway; this hill has now been leveled for security purpose. (C) Eastern termination of the Melinau Limestone Formation at the Gunung Benarat. (D) Contact between Mulu and Melinau Limestone formations, southern end of the Gunung Api. The very steep contact arrow is marked by a change in the vegetation; its straight orientation indicates a faulted contact. (E) Southwestern view across the Melinau Gorge; picture (C) is taken further to the south. (F) Northeastern view towards the Gunung Api.

the deeper-marine Temburong Formation include a variety of redeposited limestone elements documenting the successive backstepping, dismemberment and drowning of the carbonate system. Along the NW flank of the basin (Fig. 9), an abandoned quarry in the basal shales of the Temburong Formation exposes a large rafted block of carbonates (Figs. 13A, B), thought to be derived from a nearby shelfal setting to the NE and displaced downslope in a southwestward direction. Particularly well-imaged analog occurrences of rafted blocks, based on 3D seismic data, have been described from the Moroccan Atlantic margin by Lee et al. (2004). The rafted block is some 12–15 m thick (Fig. 14), and stands in a normal stratigraphic position, with a paleo-

karst surface facing upward (Figs. 13C, D). The upper 6–8 m consist of massive bioclastic limestones (Fig. 13E), characterized by foraminifera, corals, bivalves and sea urchins. The high-energy textural composition of this limestone (Figs. 15A–C) indicates a shallow marine, perireefal environment. The uppermost limestone unit of the rafted block is characterized by the presence of larger domal corals, preserved in their life position (Fig. 13F). This reflects a shallowing upward trend and the development of a reefal environment. At the scale of the outcrop (50+ meters), the top surface of the block is karstified (paleo-karst), with some meter-deep dissolution pockets (Figs. 13C, D). The contact with the overlying and karst-filling shales of the Temburong Formation is sharp.

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Fig. 11. (A–F) Melinau limestone thin sections. (A) Glomalveolina ungaroi from an outcrop opposite Lapangan Terbang exit on the Melinau river (see Fig. 8). This porcellaneous larger foraminifer is of Late Bartonian age (biozone Ta3). (B) Pelletoidal, bioclastic grainstone with Pellatispira, a larger hyaline foraminifer of Priabonian age (biozone Tb). Plankwalk between Melinau Paku and Lang cave. (C) Plastic, ductile deformation in Priabonian (biozone Tb) limestone sample from Lang Cave. (D) Brittle deformation in Early Oligocene (biozone Tcd) Nummulites limestones from the Pinnacles (see Fig. 8). (E) Bioclastic grainstone from Bukit Pala (Tcd biozone). (F) Eulepinida limestone, Late Oligocene (biozone Te1–3), from a hill in the western axis of the airport runway; this hill has now been leveled for security purpose.

The shales are initially a little silty and contain siltstone lenses intercalations with scattered septaria, before passing into a thick and massive sequence of shales. The septarian nodules are characterized by a nucleus of leached, shallow marine carbonates, surrounded by an indurated matrix of deepwater hemipelagics with planktonic foraminifera (Figs. 15D–F). The presence of Miogypsinoides (without Miogypsina) from the rafted block and from the overlying smaller olistoliths (septaria) indicates a Late Chattian age (Te4 biozone). The Temburong shales can be further observed in the low hills some hundred meters to the west of the quarry (olistoliths in Fig. 9). This section is slightly younger than

the level of the quarry and shows scattered decimeter to multi-meter size carbonate olistoliths, debris flows and minor calcarenitic turbidites stretching over some 70 m apparent thickness (Figs. 16 and 17A). The vertical succession of facies from the redeposited shallow marine elements (Figs. 18A–F) indicates a shift from back-barrier (reworked particles include Halimeda plates and Austrotrillina spp. in sample#8) to reefal edge (coral-boundstone in sample #13). The olistoliths are generally angular (Figs. 17B, C), and consist of foraminiferal packstones with an abundance of Lepidocyclinidae (Fig. 17E). Some of the larger blocks include subaerially weathered coral boundstones. The joint

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Fig. 12. Melinau Gorge-type stratigraphic section, logged by Wilford and redrawn after Adams (1965).

Table 1 Melinau limestone thickness variations in the Melinau Gorge and along the southern margin of the carbonate platform, compared with the central area and northern margin (after Adams, 1965) Late Eocene thickness (m) Melinau Gorge and 490–670 Southern Flank Central and Northeast Area 7210 to 300

Early Oligocene thickness (m)

Late Oligocene thickness (m)

Latest Oligocene/earliest Miocene thickness (m)

Total thickness (m)

470–610

400–550

Absent

Absent

210–490 (minimum thickness as top is 1700–2225 eroded) 7270 to 550 490–850

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Fig. 13. (A–F) Mulu quarry outcrop pictures. (A) Mulu Quarry on the left of picture. (B) Mulu Quarry overview. Temburong shales surround the rafted Melinau limestone block, exposed in the face of the abandoned quarry (center of picture). (C and D) Upper part of the rafted block showing the paleokarst surface and the abrupt contact with the overlying, deeper-marine Temburong shales. (E) The uppermost 5 m of the rafted block consist of poorly bedded, high-energy bioclastic limestones. (F) Branching and head corals are present at the top of the rafted block. Field of vision approximately 30 cm by 20 cm.

presence of Miogypsinoides and Miogypsina is an indication for an Early Aquitanian age (Te5 biozone). The increased diversity and size of miogypsinids along the outcrop indicates a progressively younging section; the uppermost olistolith (sample#13, Fig. 17 consists of a coral boundstone likely derived from a reefal body developed at the edge of a coral barrier. The age dating of the rafted block and successive olistoliths (Te4 and Te5 biozones) described above is contemporaneous with the ultimate carbonate system development on the NE margin of the Melinau basin. Upsection from the olistoliths outcrop, the absence of shelfal shedding within the deep-marine Temburong shales signals a further backstepping-subsidence phase and marks the end of carbonate deposition in the Melinau basin.

In line with Wilson (2002), the Melinau limestone is interpreted to represent an isolated, un-rimmed carbonate basin. The asymmetry of the carbonate wedge with its thickened, probably fault-bounded southern margin, the generally shallow-marine, aggradational stratigraphic architecture, the presence of an axial syncline, the eroded northern flank leading to widespread reworking of Late Eocene faunas are diagnostic criteria for deposition in a wedge-top basin. Limestone deposition in mini-basins basins on an accretionary prism correspond to a rare shallow marine setting, where a local carbonate factory can start working, and remain completely sheltered from clastic supply. The Melinau limestone was presumably formed on a submerged paleo-high, away from any emerging domain exposing the Rajang accretionary prism; that location may

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Fig. 14. Mulu quarry log. Temburong shales with rafted block of Melinau Limestone. The limestone block is in apparent conformity with the overlying shales; the upper part shows a facies trend towards higher-energy settings and culminates with an interval characterized by larger corals in life position. The top of the block is characterized by a paleo-karst surface. The overlying shales have a deep-marine character and are rich in planktonic foraminifera. Septaria are present in the shales and have a core consisting of leached shallow marine carbonates.

have been close to the apex of the submerged Rajang nappe. 4.4. Batu Gading wedge-top basin (Bartonian–intra Late Chattian) The isolated carbonate outcrops at Batu Gading are thought to represent a smaller, separate wedge-top basin, assumed to have formed in a similar setting as the Melinau limestone described above. However, the limited data points are inadequate to constrain this interpretation spatially and structurally. The limestone outcrops at Batu Gading, Bukit Besungai and Bukit Betok (Figs. 1 and 19) are situated along the river Baram, a location that has eased their exploitation for construction materials. The quarry at Batu Gading provides a unique opportunity to log the Melinau limestone along clean and fresh faces, and to observe the contact with the underlying Kelalan Formation. There is some uncertainty in the literature (Liechti et al., 1960 vs. Adams, 1965) as to the nature of the contact between the Melinau Limestone Formation and the underlying rock sequences, referred variously as a conformable contact and an angular unconformity. The best exposed basal contact of the Melinau Limestone Formation can be observed in the limestone quarry at Batu Gading; all authors who have described this outcrop

(Haile, 1962; Adams and Haak, 1962; Ngau, 1989) have concurred in recognizing an angular contact between the Kelalan Formation and the Melinau Limestone Formation. The angular contact is well exposed and can be followed over hundreds of meters at the base of a limestone cliff in the quarry (Figs. 20A, B). Here, the underlying Kelalan Formation consists of a distal turbidite sequence (Fig. 20C) that is tightly folded and truncated; the overlying Melinau limestones have a low regional dip of some 12–151 towards the north. The Kelalan Formation at Batu Gading has apparently yielded a Cretaceous microfauna (Haile, 1962; Adams and Haak, 1962), but no diagnosis species are mentioned. Elsewhere, the Kelalan Formation has yielded Late Cretaceous planktonic foraminifera (Liechti et al., 1960) and it is assumed that the formation may extend into the Early Eocene. A schematic stratigraphic cross-section between Bukit Besungai and Batu Gading, based on Adams and Haak (1962) is shown on Fig. 21. The logs published by these authors (their Fig. 4, p. 148) are indicated by the two rectangles. The Late Cretaceous (?) folded flysch sequence of the Kelalan Formation is unconformably overlain by the 70 m thick, composite sequence of shallow marine Melinau Limestone Formation, itself overlain by the deepermarine Temburong Formation. The angular unconformity

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Fig. 15. (A–F) Mulu quarry thin sections. (A) Vuggy floatstone including leached geniculate red algae, larger foraminifera, and echinoderm fragments, cemented by sparry calcite (sample #1). (B) Packstone with poorly sorted, leached bioclasts including fragments of geniculate red algae, small and large hyaline benthic foraminifera, bivalves and echinoderms debris. Sparry calcitic cement (sample #2). (C) Floatstone–grainstone. Rounded fragments of geniculate red algae with borings, hyaline and porcellaneous benthic foraminifera, smaller bioclasts (sample #3). (D) Floatstone; leached geniculate red algae, larger foraminifera (Miogypsinoides), and echinoderm fragments in pyritic matrix (sample #5). (E and F) Leached and pyritized algal–foraminiferal packstone, reworked in wackestone matrix with planktonic foraminifera (sample #6).

between the folded flysch sequence of the Kelalan Formation and the base of the Melinau Limestone Formation is observed at both locations. A unit of sandstones grading upward into sandy limestones, some 15–18 m thick, is present at the base of the Bukit Besungai section, but wedges out towards Batu Gading. The sandstone probably originates from the underlying Rajang flysch. Its larger foraminiferal fauna is dominated by Nummulites javanus, indicating a MidEocene age (Bartonian, Ta biozone; see Lunt, 2004). This expanded, clastic-rich stratigraphic unit at Bukit Besungai is consistent with deposition in an extensional setting; Hutchison (2001) mentions the presence of half-grabens in

the near-by Sibu Zone, ‘‘developed upon the uplifted Rajang Group’’. This expanded section to the south of the Batu Gading basin is analogous to the thicker section on the southern margin of the Melinau basin. It possibly indicates similar mechanisms and vergence in the processes leading to the formation of a smaller wedge-top basin. A transgressive sequence consisting of a correlative, massive limestone unit some 35–40 m thick conformably overlies a bed of thin sandy limestones in Batu Gading and Bukit Besungai. Its rich foraminiferal fauna include several species of Pellatispira, Discocyclina, and Nummulites (Fig. 20D), diagnostic of a Late Eocene age (Priabonian, Tb biozone). It is remarkable that the massive limestone

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Fig. 16. Log of Mulu olistoliths and debris flows. A 70 m section of dark blue, hard and splintery shales, with limestone olistoliths, calcareous debris flows and subordinate calciturbidites, outcropping to the west of the Mulu quarry. The reworked shallow marine elements indicate a vertical facies succession from intra-shelf at the base (reworked particles include Halimeda plates and Austrotrillina spp. in sample #8) to shelf edge at the top (coral–boundstone in sample #13).

unit appears to keep a uniform thickness between the two outcrops, despite the fact that its top is a major sequence boundary associated with a time hiatus of some 10 Ma (Fig. 4). The overlying silty marls and limestone breccias (Figs. 20E, F) have a maximum thickness of about 10 m, and rest on an unconformable contact. The breccia is a reefal limestone including massive and branching coral forms. Besides reworked Late Eocene foraminifera, this unit has yielded Eulepidina spp., and Heterostegina borneensis, indicating a Late Oligocene age (Chattian, Te1–3 biozone). It is overlain by a 5 m thick, bedded algal–foraminiferal calcarenite that contains specimens of Miogypsinoides spp., indicating (in the absence of Miogypsina) a latest Oligocene age (Chattian, Te4 biozone). These limestones grade upward into a 3 m thick unit of thinly bedded sandy limestone and calcareous sandstones, overlain by the hard, light gray calcareous shales of the Temburong Formation. The presence of planktonic foraminifera Globoquadrina binaiensis and Paragloborotalia mayeri date the lower part of the Temburong Formation as Early Miocene (Late Aquitanian to Early Burdigalian, N5?–N6 biozones). Because of the lack of control on the location of the above samples, it is unclear if the Early Aquitanian may not have been sampled, or if it represents a condensed zone at the base of the shale sequence.

Between Bukit Besungai and Batu Gading, a structural overprint is indicated by the lateral thinning of the Chattian unit and the rapid deepening of depositional environments towards the south that characterize the onset of the Temburong Formation. These observations are compatible with a rotation of the carbonate basin from the south to the north, and mimic the structural evolution of the Melinau basin. 5. The Temburong deep-marine basin The location of the Selidong and Keramit outcrops lie at a short distance (respectively, 10 and 13 km) to the NE and N of the GMNP area (Figs. 1, 22 and 23); they are separated from the Melinau basin by a transform fault, such that their original, relative position is unknown. The carbonates encountered in these remote outcrops were initially described as being biohermal, of Eocene to Miocene age (Liechti et al., 1960) and considered as members of the Melinau Limestone Formation. The subsequent fieldwork and analyses of Adams and Wilford (1972) led to a confirmation of their age, but to a completely different assessment of their depositional environment. Based on the presence of a bathyal microfauna, of pelagic and hemipelagic sediments, including turbidite deposits, and the absence of in-situ shallow

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Fig. 17. (A–F) Mulu olistoliths outcrop pictures. (A) Overview of the logged section. The base of the section is exposed in the foreground gully; sample #18 was collected at the top of the hill, to the left. (B) Olistoliths; sample #9 was collected from the block in the foreground. (C) Larger shelf edge olistolith, where sample #13 was collected. (D) Debris flow from the base of the section, between samples #8 and 9. (E) Larger specimens of Lepidocyclina from a smaller olistolith (sample #9). (F) Larger reef-building corals within uppermost olistolith (sample #13). The field of vision is about 70 cm by 50 cm.

marine limestones, Adams and Wilford (1972) clearly demonstrated the deepwater nature of these outcrops. A deep-marine, basin to base-of-slope setting is implied by the nature of the sedimentary successions at Selidong and Keramit; they form part of the Temburong formation, developed on the West Crocker accretionary prism. 5.1. The basin to base-of-slope setting at Selidong At Selidong, the deep-marine Temburong sediments are reported by Liechti et al. (1960) to be in unconformable contact with the underlying Mulu Formation. Contrary to this observation Adams and Wilford (1972) mention an apparent conformable contact. Because of the shale-onshale contact and the poor exposures due to vegetation overgrowth, the nature of the contact has been difficult to establish. As discussed above, the contact is interpreted

as an angular unconformity, separating two successive tectonostratigraphic units (Figs. 3 and 4). The base of the section consists in a 120–150-m-thick unit of hard, dark blue calcareous shales with reworked clasts from the Mulu Formation, overlain by gray pelagic Globigerina limestone (Fig. 24). A Bartonian age is likely for the blue shales, based on the presence of planktonic foraminifer Hantkenina sp.; the pelagic limestone are dated as Priabonian by the transported larger foraminifera Pellatispira and Discocyclina. A gap in the exposures hides much of the overlying succession; it has been interpreted as time hiatus separating Late Eocene from Late Oligocene strata (Adams and Wilford, 1972). The uppermost section is 30–90 m thick and consists of thinly bedded pelagic limestones with calcarenite turbidites and debris flows. The debris flows include boulder-size components eroded from the Mulu

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Fig. 18. (A–F). Mulu olistoliths thin sections. (A) Floatstone–grainstone with pieces of rhodoliths, Halimeda plate fragments, leached larger foraminifera, and bivalve fragments (sample #8). (B) Packstone with fragments of geniculate red algae, larger foraminifera, small hyaline benthic foraminifera, bivalve fragments, serpulids, and leached bioclasts (sample #9). (C) Packstone–grainstone with fragments of geniculate red algae, larger foraminifera (Miogypsinoides), small hyaline benthic foraminifera, coral debris, and bivalve fragments (sample #10). (D) Vuggy floatstone–packstone with fragments of geniculate red algae, larger foraminifera (Lepidocyclina: occasionally leached and with syntaxial overgrowth of fibrous calcite), small hyaline benthic foraminifera, and bivalve fragments (sample #11). (E) Rudstone with scleractinian corals, larger foraminifera floating in a matrix of finer-grained shell and algal debris (sample #12). (F) Boundstone with encrusting red algae and foraminifera, echinoderm debris (sample #13).

Formation and shallow marine limestones of Eocene age. Transported miogypsinids date this section as Late Oligocene (Te4 biozone ?). The mixture of pelagic and gravity deposits points to a basin to base-of-slope setting at Selidong. 5.2. The basin to base-of-slope setting at Keramit The oldest sediments overlying the Mulu Formation are basinal, pelagic shales and marls (Fig. 24). The Keramit Marls are described by Liechti et al. (1960) as one of the most fossiliferous units in NW Borneo, with some

137 species of mainly planktonic foraminifera identified. A Bartonian age is indicated, based on the presence of planktonic foraminifer Hantkenina dumblei with Catapsydrax dissimilis, and redeposited larger foraminifer Alveolina sp. (Haak, in Liechti et al., 1960). This basinal unit is overlain by thinly bedded, pale gray, chalky Globigerina limestone, interbedded with thin calcarenitic turbidites, reflecting a base-of-slope environment. Planktonic foraminifer Chiloguembelina sp. indicates an age no younger than Rupelian. This unit is separated from the Late Oligocene deep-marine sedimentary succession by an angular contact, interpreted as a slump scar.

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Fig. 19. Batu Gading geological sketch map, redrawn after Haile, in Adams and Haak (1962). The Melinau Limestone Formation is dipping towards the north at an angle of approximately 12–151. The unconformable contact with the Kelalan Formation has been reported from Bukit Besungai, and has been observed in the quarry at Batu Gading.

The uppermost logged sequence is 10 m thick and is characterized by a change in depositional patterns, i.e. channelization with lag conglomerates (above the angular contact), the onset of debris flow deposits (presence of boulder-size, Mulu Formation clasts associated with shallow water Priabonian limestones), widespread reworking of Priabonian (Tb) foraminifera, and presence of scattered quartz sand grains, presumably reworked from the Rajang flysch. At the top of the section, brown and gray hemipelagic shales start occurring as interbeds between the pelagic chalky limestones and the calcarenitic turbidites. The presence of Globigerina binaiensis, Globigerina ciperoensis angulisuturalis and Globorotalia opima nana indicates a Late Chattian age (P22 planktonic biozone) for this upper sequence, which is consistent with the presence of redeposited Miogypsinoides spp. (Te4 biozone). 5.3. Tentative correlation between Selidong and Keramit Owing to problems of access and vegetation overgrowth, the description of these two outcrops is fragmentary, and no comprehensive lithological log is available (Adams and Wilford, 1972). However, the complementary nature of the coverage in the two outcrops, shown by the boxes in Fig. 24 makes it possible to attempt a tentative stratigraphic cross section for this basin to base-of-slope setting. The stratigraphic architecture is indicative of progressively more proximal sedimentary input, reflecting slope progradation. Initially, the basin was subject to pelagic sedimentation (Bartonian), which was then followed by deposition of mixed pelagic and carbonate, shelf-derived

turbiditic material (Priabonian). During the Rupelian and Early Chattian, the base of slope setting is interpreted to have been a zone of sedimentary by-pass. Rejuvenated sedimentary input is recorded during the Late Chattian, with include gravity-deposits of eroded shelfal carbonates (Priabonian) mixed together with basement blocks (Mulu Formation clasts). These higher energy sedimentary units suggest a possible steepening of the slope, including active fault movements along the edge of the Melinau basin. By Early Miocene time, the sedimentation appears to have been entirely hemipelagic, which marks the end of carbonate deposition in the adjacent wedge-top basin. 5.4. Scattered occurrences of shelf-derived carbonates in the basin Olistoliths embedded in Temburong shales have been reported from two outcrops further in the basin (Fig. 1). In the lower Limbang valley, southeast of the settlement of Telehak (Wilford, 1961), discontinuous occurrences of finegrained limestones in the Bergapar Anticline have been interpreted as Mid–Late Eocene olistoliths by Liechti et al. (1960). Reworked Eocene and Early Aquitanian larger foraminifera are reported together from the overlying Temburong shales. These authors also mention an isolated, late Oligocene limestone olistolith along Sungai Batu Apoi (Fig. 1), overlain by deep-marine Early Miocene shales. Liechti et al. (1960) also mention scattered occurrences of mixed Late Eocene and Early Miocene larger foraminifera in the Temburong Formation along the middle Limbang River, an area stretching from Keramit to the southeast of Selidong (Fig. 1).

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Fig. 20. (A–F) Batu Gading outcrop pictures. (A) Low-dipping, sharp ledge of Melinau Limestone (massive limestone unit) resting unconformably on the folded Kelalan Formation (foreground). (B) Angular unconformity along a quarried trench. The steeply dipping flysch sequence of the Kelalan Formation is unconformably overlain by the Melinau Limestone Formation. (C) Underside of a turbidite sandstone from the Kelalan Formation showing sole marks. The block’s dimensions are about 40 cm by 30 cm. (D) Nummulites limestone deposited under upper flow regime. Sample from the base of the Melinau Limestone Formation at Batu Gading. Field of vision is approximately 20 cm by 13 cm. (E) Unconformable contact at the top of the massive limestone unit. At this location, Late Oligocene marls alternating with thin beds of limestones unconformably overlay the Late Eocene massive limestone unit. (F) Detail of the unconformable contact described above (arrow).

6. Tentative depositional models In attempting to construct a depositional model for the Melinau limestone, the thicknesses and lithological descriptions were taken from Liechti et al. (1960) and Adams (1965), and complemented by my own research. Biozonal interpretations of larger foraminifera were adjusted to the standard developed by Lunt (2004). Because of the rough geological mapping available, the distribution and thickness of the individual limestone units in the Melinau basin have a high range of uncertainties. No attempt has been made to de-compact the sediments to their original thickness. 6.1. Structural–stratigraphic evolution A conceptual 2D model (Fig. 25A–D) depicts the evolution of the Melinau wedge-top basin along a N–S cross section, based on the structural setting and strati-

graphic development, within the framework of the analogs described above. The clastic-free nature of most of the carbonates indicates a location isolated from siliciclastic supply, probably situated on the higher, shallow marine parts of the submerged accretionary prism. Through analogy with the formation of wedge-top basins on the Prerifaine nappe, the mechanism leading to the Mid- to Late Eocene development of the Melinau basin is thought to be the gravity gliding of a thrust sheet in the underlying Rajang accretionary prism (Fig. 25A). Carbonate deposition during the Late Mid-Eocene (Bartonian) is poorly but unequivocally documented; its overall thickness is unknown and its facies indicate the presence of protected environments, adjacent to high energy, shallow marine, mixed carbonate–clastic shoals. At the same time, high-energy shallow marine shoals with a mixed carbonate–clastic content characterized the initial deposition in the smaller, neighboring Batu Gading wedge-top basin to the west, while the pelagic Keramit

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Fig. 21. Bukit Besungai–Batu Gading schematic stratigraphic cross-section, based on Adams and Haak (1962). The logs published by these authors (their Fig. 4, p. 148) are shown by the two rectangles.

Fig. 22. Selidong geological sketch map, redrawn after Adams and Wilford (1972), showing the various lithostratigraphic units mapped by these authors.

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Fig. 23. Keramit geological sketch map, redrawn after Adams and Wilford (1972), showing the various lithostratigraphic units mapped by these authors.

Marls were deposited in the adjacent deep-marine basin to the east. A subsidence phase occurred in the Melinau basin during the Late Eocene (Priabonian). In the underlying Rajang thrust sheet, a linked extensional–compressional system is thought to have created an asymmetrical accommodation space; up to 670 m of shallow marine limestones have been logged on the southern edge of the basin, while only 210–300 m have been recorded on the northern margin. The abundance of red algae and larger foraminifera and the paucity of corals point to a carbonate factory consisting of high-energy shallow marine shoals (cf. Wilson et al., 2000). The model predicts that this factory was located on the northern margin of the basin, where subsidence was comparatively less. The southern margin lithofacies recorded in the Melinau Gorge (Tb in Fig. 12) show two thicker calcarenitic units, one at the base and the other at the top, separated by an interval consisting of mixed microfacies, including packstones, wackestones and mudstones. This suggests a general transgressive–regressive cycle following

the initial onset of shallow marine carbonate sedimentation in the mini-basin. The structurally less active Batu Gading wedge-top basin only preserves some 40 m of Priabonian limestones. It is unclear if the two wedge-top basins were linked by a continuous carbonate shelf during the Priabonian marine transgression. At Selidong and Keramit, the Priabonian basin to baseof-slope sediments consist of pelagic, thinly bedded Globigerina limestones and calcarenitic turbidites. The asymmetry of the Melinau wedge-top basin was enhanced during the Early Oligocene (Rupelian): while up to 610 m of aggradational shallow marine limestones were deposited along the southern margin of the basin, carbonate sedimentation was minimal on the northern margin (Table 1, Fig. 25B). At present, the Melinau Gorge Rupelian section provides the only control point on thickness and facies, with stacking patterns inferred from vertical facies variations. Compared with the Late Eocene, the lithofacies consist dominantly of lesser energy packstones–wackestones, and include slumped elements diagnostic for a slope setting (Tc–Td in Fig. 12). The overall facies architecture with grainstones–packstones at the base and packstones– wackestones at the top indicates a transgressive trend. These relatively deeper-marine depositional environments at the southern margin of the basin strengthen the model of a northerly carbonate factory. During the Late Oligocene (Chattian), continued fault activity along the southern margin of the Melinau basin led to the deposition of some 550 m shallow marine limestones, which wedged out entirely towards the northern margin (Table 1, Fig. 25C). The Melinau Gorge section (Te1–3 in Fig. 12) shows that the southern margin of the basin was dominated by the deposition of progressively lower energy carbonates, thought to represent a more distal setting. The higher-energy unit logged between 1150 and 1200 ft is interpreted as a gravity-flow proceeding from a northerly high. The increased presence of coral fragments is an indication for a differentiation of the carbonate factory into coral reefs and algal–foraminiferal shoals. Coral reefs developed during the Late Oligocene in the nearby Batu Gading wedge-top basin. In the basin, the Selidong and Keramit area was largely a by-pass or erosional zone, probably located in a base-of-slope setting. During the latest part of the Oligocene and the earliest Miocene, the Melinau wedge-top basin experienced significant changes in its depositional history. Initially, carbonate sedimentation followed earlier trends and a minimum of 490 m of limestones were deposited along the southern margin of the basin (Table 1); the sediment thickness along that margin is likely to have been higher and subsequently truncated by erosion. Uncertainties in the location of the measured sections on the central and northeast margin mentioned by Liechti et al. (1960) make it difficult to constrain the structural– stratigraphic models. Reworking of Late Eocene shelfal foraminifera is a characteristic feature of the Late

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Fig. 24. Selidong–Keramit tentative stratigraphic cross-section, interpreted after Adams and Wilford (1972). The 2 boxes indicate the logged sections; the stratigraphy outside the boxes is speculative.

Chattian, both on the shelf and in the basin. This is a diagnostic element supporting the upward rotation of the northern margin of the basin and the erosion of the oldest sediments of the wedge-top basin (Fig. 25D). At the Selidong and Keramit locations, sedimentation resumed with the deposition of pelagic limestones, hemipelagic shales, turbidites and debris flows, in a deep-marine base-of-slope setting. The Late Chattian is marked by a phase of rapid subsidence and by the backstepping of the carbonate factories in both the Melinau and Batu Gading wedge-top basins. While a small reefal system persisted at the NE margin of the Melinau basin, the wedge-top basin was now generally below the photic zone with offshore mudstones (Temburong Formation) abruptly overlying the carbonates. At the end of the Oligocene, the Melinau remnant reef was subaerially exposed, partially karstified and destabilized, as indicated by the rafted carbonate block at Mulu. The dismemberment of the smaller, rimmed carbonate system continued during the early part of the Aquitanian, with the downdip deposition of carbonate olistoliths, coralbearing debris flows and thin carbonate turbidites. By Late Aquitanian time, the Melinau carbonate platform had drowned entirely. 6.2. Paleogeographic evolution Prior to the Miocene 451 counter-clockwise rotation of Borneo (Hall, 1996), the bounding margins of the Melinau

wedge-top basin and other structural lineaments were likely trending in a WSW–ENE direction. During the Late Eocene (Fig. 26A), the Rajang accretionary prism included a series of shallow marine depocenters that developed on the back of outboard rotating thrust sheets. Two wedge-top basins, the Melinau and Batu Gading basins are preserved today in the northern Sarawak. The Melinau basin formed in the accommodation space created between a southern normal fault and a northward rotating, detached thrust sheet. This long active extensional–compressional system was bounded laterally by transform faults. It is unclear if the two shallow marine depocenters at the Melinau and Batu Gading were linked during phases of marine transgression and if additional mini-basins existed. Eastward of the transform fault, the Crocker accretionary prism was characterized by distal, pelagic sedimentation. Continued extension along the southern margin of the Melinau wedge-top basin created the main accommodation space during the Early Oligocene (Fig. 26B). There is no record of Oligocene sedimentation in the Batu Gading basin; this absence may be due to non-deposition, as the basin may have been inactive at that time. Calciturbidites at Keramit and Selidong were presumably fed from the Melinau basin. The Late Oligocene is characterized by a marine transgression, leading to clastic-free carbonate sedimentation in both the Melinau and Batu Gading wedge-top basins

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Fig. 25. (A–D) Melinau wedge-top basin schematic evolution model. Conceptual N–S cross-section, illustrating the progressive rotation of the carbonate basin, within the linked extensional–compressional structural setting. (A) Middle Eocene (Bartonian) to Late Eocene; (B) Early Oligocene; (C) Late Oligocene; (D) Latest Oligocene to Earliest Miocene.

(Fig. 26C). The upward rotation and thrusting of the northern margin of the Melinau basin led to exhumation and erosion of the Eocene carbonates section. This structural phase is characterized by the widespread shedding of reworked Late Eocene faunas. In the Temburong basin, this is linked to turbidites and debris flows, which also incorporate elements of the Rajang Group. The latest Oligocene to Early Miocene is characterized by a drowning phase. Initially, widespread hemipelagic

sedimentation covered much of the Melinau basin, save on its NE margin (Fig. 26D), where a narrow reefal area persisted. Subaerial exposure of the remnant reef eventually terminated carbonate production in the Melinau wedgetop basin. This was followed by a phase of rapid subsidence of the rotated basin margin. The youngest remains of the latest Oligocene–Early Miocene carbonate platform are seen in the northwestern Melinau basin in

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Fig. 26. (A–D) Schematic paleogeographic reconstructions. B: Batu Gading/Bukit Besungai; M: Melinau Limestone at Mulu; K: Keramit; S: Selidong. (A) Late Eocene (35 Ma) carbonate platform piggy-backing on a South-to-North-oriented thrust sheet, downdip from a submerged paleohigh. (B) Early Oligocene (30 Ma) possible emergent northern rim of the carbonate platform and shedding of calciturbidites towards the eastern, deep-marine Temburong basin on the Crocker accretionary prism. (C) Late Oligocene (24 Ma) thrusted and emergent northern rim of the carbonate platform exposes down to Late Eocene carbonates, which are reworked and transported through mass flows and turbidites into the eastern, deep-marine Temburong basin. (D) Early Miocene (22 Ma) smaller coral reef represent the last stage of the drowning of the carbonate platform, with westward transport of rafted carbonate blocks and olistoliths.

the form of rafted blocks, olistoliths and debris flows embedded within the Temburong Formation. Because of the abrupt transition between the Melinau and Temburong formations, it is assumed that high rates of subsidence rapidly affected the whole area. These rates may have been in the order of 200 m in 10,000 years, as documented by Saller and Vijaya (2002) for the Kerendan platform.

7. Post-depositional history A phase of rapid burial continued during the Early Miocene, when the hemipelagic Temburong shales were deposited. Estimates for the maximum thickness of this formation ranges from 3600 to 5600 m (Liechti et al., 1960). The depth of burial of parts of the Melinau wedge-top

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Fig. 26. (Continued)

basin has been sufficient to induce the partial recrystallization of the Eocene limestones, as indicated by the presence of sucrosic dolomites and mylonitic limestones (Adams, 1965). The record of dead oil within the Eocene succession in the Melinau Gorge (Adams, 1965) is an indication of a consumed petroleum system, a further support for a depth of burial commensurate with the cracking of hydrocarbons. The Mid-Miocene thrusting of the West Crocker Trough (Hutchison, 1996a, b) also involved the eastern margin of the Rajang accretionary prism, including the area of study. The prominent NNE–SSW reverse Melinau Fault (Wilford, 1961; Adams, 1965) was likely formed at that

time. Deformation in the study area may have continued during the Late Miocene and Early Pliocene, in part through a linkage with the north Baram compressional phase (Tingay et al., 2005) as well as through regional isostatic rebound (Hutchison et al., 2000). Denudation of this young mountain range proceeded very rapidly, leading to high rates of sediment accumulation in the surrounding marine basins (Hutchison et al., 2000; Hall and Nichols, 2002). Subaerial exposure and karstification of the Melinau limestones started in the Late Pliocene (Farrant et al., 1995). The development of a large cave system in the GMNP may have been enhanced by volcanic-derived sulfuric acid, in a similar way to the

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processes that have created the Carlsbad caverns in New Mexico (Hill, 1987). The central Sarawak Usun Apau volcanics have been extruded during the Pleistocene, and still today vents of sulfuric gases occur on the fringe of the Melinau limestones at Mulu (hot spring in Fig. 9). 8. Conclusions The Melinau wedge-top basin consists in an isolated and unrimmed carbonate system, piggy-backing on a postulated thrust sheet that formed on the Rajang accretionary prism. The development of the wedge-top basin is structurally controlled by the gravity deformation affecting the underlying Rajang flysch. The rhomboidal-shaped Melinau basin is strongly asymmetrical along its E–W axis; it is bound by a system of normal faults on its southern margin, by lateral transform faults on its eastern and western margins, and by a postulated thrust on its northern margin. Because of the paucity of geological data in this remote and rugged jungle area, the structural setting together with analog data from other wedge-top basins is key to unraveling the evolution of the Melinau basin. The available control points are located on the southern and northern rim of the Melinau basin; further outcrops are present to the west on the smaller Batu Gading wedgetop basin and to the east in the deep-marine Temburong basin. The Melinau basin was initiated during the Mid-Eocene (Bartonian) when a shallow marine, mixed clastic–carbonate system was established on the back of a developing thrust sheet. Ongoing rotation of the thrust sheet during the Late Eocene to Early Miocene created an asymmetrical accommodation space that was filled by a largely clasticfree carbonate system. The bulk of the carbonates were formed on shallow marine shoals characterized by an association of red algae and larger foraminifera; coral reefs developed during the Late Oligocene and Early Miocene. The extensional southern margin of the Melinau basin is characterized by a 42 km thick section of carbonates, while the northern margin has only about half that sediment thickness. The basin is further characterized by an axial syncline and by an unconformity that formed during the Late Oligocene on the upward rotating northern margin, where coral reefs could develop. The end of the carbonate sedimentation is characterized by the dismantling of this reefal system through the continued upward movement of the basin’s northern margin, while the rest of the area underwent rapid subsidence. Following subaerial exposure and karstification in the latest Oligocene, the remnant reefal system was drowned and dismantled during the earliest Miocene (Early Aquitanian) in the form of rafted blocks, olistoliths and debris flows. By mid-Aquitanian time, the former area of the Melinau carbonate basin was the site of deposition of the deeper-marine Temburong shales.

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Acknowledgments I am particularly grateful to Albert W. Bally for his encompassing comments, guidance in the structural interpretation and for his introduction to key Rice University thesis and analogs. I am indebted to Brian Clark GMNP manager, and his wife Sue, for their assistance in my study, including the sampling within the park, for introducing me to the Mulu quarry area and for some financial support. Professor Lukas Hottinger accompanied me on an excursion through the GMNP and has since be a partner in discussing larger foraminifera collected in Mulu and in identifying the Glomalveolina fauna. Walter Kohli has facilitated much of my earlier trips to GMNP. I would like to thank Anyi Ngau for discussions and for access to his study on Batu Gading. My colleagues at Shell, Charlie Lee, Elizabeth Nawang, Steve and Claire Hart have helped with the logistical support and provided great companionship on a visit to Batu Gading. Peter Winefield, Steve Bergman, Guy Loftus, and Robert (Bob) Scheidemann have provided valuable comments on an initial draft of the paper. Tim Diggs and Calum Macaulay have provided support with the photography of thin sections. I acknowledge the support of my wife during the completion of this study, carried out entirely outside company time and activities. The manuscript was much improved, thanks to MPG reviews by Dan Bosence and Rasoul Sorkhabi. References Adams, C.G., 1959. Foraminifera from limestone and shale in the Batu Gading area, Middle Baram, East Sarawak. Report of the Geological Survey Department British Borneo for 1958, pp. 73–85. Adams, C.G., 1965. The foraminifera and stratigraphy of the Melinau Limestone, Sarawak, and its importance in Tertiary correlation. Quarterly Journal of the Geological Society London 121 (3), 283–338. Adams, C.G., 1970. A reconsideration of the East Indian Letter Classification of the Tertiary. Bulletin of the British Museum— Natural History Geology Series 19, 87–137. Adams, C.G., Haak, R., 1962. The stratigraphical succession in the Batu Gading area, Middle Baram, North Sarawak. In: Haile, N.S. (Ed.), The Geology and Mineral Resources of the Suai–Baram Area, North Sarawak. British Borneo Geological Survey Memoir, vol. 13, pp. 141–150. Adams, C.G., Wilford, G.E., 1972. On the age and origin of the Keramit and Selidong Limestones, Sarawak, East Malaysia. Geological Papers, Geological Survey Malaysia, vol. 1, pp. 28–42. Bassi, D., Loriga Broglio, C., 1999. Alveolinids at the Middle–Upper Eocene boundary in northeastern Italy (Veneto, Colli Berici, Vicenza). Journal of Foraminiferal Research 29/3, 222–235. Benard, F., Muller, C., Letouzey, J., Rangin, C., Tahir, S., 1990. Evidence for multiphase deformation in the Rajang–Crocker Range (northern Borneo) from landsat imagery interpretation; geodynamic implications. Tectonophysics 183, 321–339. Bishop, M.G., 2000. Petroleum systems of the northwest Java Province, Java and offshore southeast Sumatra, Indonesia. USGS Open-File Report 99-50R. Bosence, D., 2005. A genetic classification of carbonate platforms based on their basinal and tectonic setting in the Cenozoic. Sedimentary Geology 175, 49–72.

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