Cenozoic Carbonates in Borneo: Case Studies from Northeast Kalimantan

PERGAMON Journal of Asian Earth Sciences 17 (1999) 183±201

Cenozoic carbonates in Borneo: case studies from northeast Kalimantan

M.E.J. Wilson a, *, J.L.C. Chambers b, 1, M.J. Evans c, 2, S.J. Moss a, 3, D.S. Nas d

aSE Asia Research Group University of London, Dept of Geology, Royal Holloway, Egham, Surrey TW20 0EX, UK bLASMO Runtu Ltd, Jakarta, Indonesia cMaersk Oil Indonesia Maratua AS, Jakarta, Indonesia dGeological Research and Development Centre, Bandung, Indonesia

Received 4 September 1997; received in revised form 20 July 1998; accepted 12 August 1998

Abstract

Modern and Tertiary carbonate production is, and was, extensive and diverse in the seas surrounding Borneo, and mirrors the variety of carbonate depositional systems seen in SE Asia. The availability of favourable conditions for carbonate sedimentation around Borneo was related to a combination of factors, including tectonic setting, the formation of large basinal areas, dierential subsidence providing shallow marine areas, a tropical climate and a range of local factors, such as currents or limited clastic input. A detailed sedimentological and diagenetic study was undertaken of middle Eocene to Plio±Pleistocene carbonates which developed in the north Kutai Basin and the Mangkalihat Peninsula, northeast Kalimantan. Carbonate sedimentation in this area occurred in a range of depositional environments, from mixed carbonate clastic shelves, localised and transient shoals or reefs, a variety of platform top settings to deep water redeposited carbonates. An understanding of carbonate depositional environments, spatial facies relationships, and diagenesis is essential in order to develop models for these carbonates which can be used as predictive tools in the subsurface. This study also helps to evaluate tropical carbonate development in SE Asia and the evolution of sedimentary environments in Borneo during the Cenozoic. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction developed from more commonly studied areas, such as the Bahamas, Persian Gulf or Red Sea, which are not Cenozoic carbonates are extremely varied and wide- directly applicable. Carbonates in SE Asia are mainly spread throughout SE Asia, and those found in and produced by organisms, therefore it is important to around Borneo (Fig. 1) are no exception. These car- consider the role of the various carbonate producers bonates are rarely associated with evaporites and are and how these may have varied through time. Corals mostly formed from the skeletal remains of shallow are uncommon in Paleogene carbonates, perhaps due marine organisms. Chemical precipitates, such as to a combination of evolutionary, biogeographical and ooids, are extremely rare. In order to interpret past environmental factors, and reefal frameworks have not depositional environments a model is needed which been identi®ed for this time period in SE Asian car- compares Cenozoic carbonates in SE Asia with their bonates (Wilson and Rosen, 1998). Also, Buxton and modern regional counterparts, rather than with models Pedley (1989) noted that there may have been some down slope displacement of shallow water larger benthic foraminifera by other shallow marine biota or * Corresponding author. Tel.: 017-84443592; fax: +017-84434716; foraminifera through the Cenozoic. e-mail: [email protected]. 1 SE Asia has been an extremely active tectonic area Now at LASMO, Venezuela throughout the Cenozoic and many of the carbonate successions are syntectonic or located on basement 2 Now at Anadarko Algeria Corporation, PO Box 576, Uxbridge, Middlesex, UB8 1YH, UK highs related to earlier structures. Borneo formed the eastern margin of Sundaland, the stable cratonic mar- 3 Now at Robertson Research Pty Ltd., W. Perth, Australia, 6005 gin of SE Asia, throughout much of the Tertiary

Fig. 1. Simpli®ed geological map of Borneo, showing outcrops and some subcrops of Cenozoic carbonates and modern carbonate depositional systems.

(Hall, 1996). Widespread basinal development occurred therefore occurred in a range of marine settings, where around the margins of Sundaland during the early conditions were suitable, over large parts of eastern Paleogene, and basins along the southeastern margin Borneo (Fig. 1), the Makassar Straits, the east Java rapidly became marine. Carbonate sedimentation Sea and western Sulawesi. M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201 185

Fig. 2. Maps showing the Tertiary carbonate depositional environments studied in the north Kutai Basin and on the Mangkalihat Peninsula. 186 M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201

Table 1 Summary of carbonate outcrops, their ages, constituent components and depositional environments in Borneo. The reader is referred to the principal references for more information

Formation Name Location Age Depositional setting Lithologies Biota Principal references

Terumbu Limestone Oshore NE Miocene Isolated carbonate Grainstones, Corals, coralline May and Eyles, 1985; (TL) Natuna platforms and packstones, algae and some Rudolph and Lehmann, buildups mudstones and foraminifera 1989; Dunn et al., 1996 rudstones

Luconia (LS) Luconia, Middle Carbonate buildups Mudstones, Corals, larger Epting, 1980 oshore Miocene on faulted highs. wackestones, benthic foraminifera Sarawak Four growth phases, framestones, and coralline algae some protected rudstones areas

Melinau Limestone Sarawak Late Eocene Carbonate platform, Packstones, Larger benthic Adams, 1965 (ML) (Tb) to early some marginal grainstones, some foraminifera Miocene (Te) deposits, coral patch redeposited beds. dominate reef only noted in Some dolomitization Miocene

Batu Gading (BG) Sarawak Late Eocene Platform or shoal Packstones, Larger benthic Adams and Haak, (Tb) and late deposits (Tb) wackestones and foraminifera 1962; Adams, 1965; Oligocene karsti®ed and limestone breccias dominate Abdullah and Yaw, (Te1-4)/early overlain by (Te1-4) 1993 Miocene? redeposited facies (Te) (Te1-4)

Keramit (KR) and Sarawak Late Eocene Probably slope or Marls and breccias Planktonic Adams, 1965 Selidong (SL) (Tb) some basinal deposits of foraminifera and Limestones reworked and Melinau Limestone larger benthic late (ML) foraminifera Oligocene (Te1-4)/early Miocene (Te)

Bukit Sarang Sarawak Oligocene Shallow marine Probably packstones Larger benthic Adams, 1964, 1965 Limestone (S) (Tc) shelf foraminifera

Tujoh-Siman Sarawak Palaeocene to Deep marine (Ta) Wackestones and Planktonic Adams, 1965 Limestone (TS) early Eocene and some conglomerates foraminifera and (Ta) and late conglomerates (Te) radiolaria in Ta Oligocene deposits and larger (Te1-4)/early benthic foraminifera Miocene (Te) and some corals

Subis (SB) and Sarawak Early Not given Not described Larger benthic Adams, 1965 Bekuyat (BY) Miocene foraminifera Limestones (Te5)

Balambangan/ Oshore west Late Miocene Shallow marine and Sandy limestone Foraminifera, Ali, 1992 Tigapapan Sabah to Plio± some redeposited coralline algae, Limestone (BT) Pleistocene carbonate on echinoids dominantly clastic shelf

Gomantong/ Sabah Late Shallow marine Framestones, Abundant larger Adams, 1970; Noad, Kinabatangan Oligocene to shelf, some clastic rudstones, benthic foraminifera 1996 Limestone (GL) early input packstones and and corals and Miocene (Te) mudstones coralline algae

Minor limestone in Sabah Middle Isolated and Not given Not given Heng, 1985 Sebahat Formation Miocene± transient localised (SB) Pliocene carbonates in clastic successions M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201 187

Minor limestone in Sabah Oligocene to Isolated and Not given Not given Heng, 1985 Labang/Tanjong middle transient localised Formations (LT) Miocene carbonates in clastic successions

Vanda Limestone Oshore Pliocene Delta front or shelf Framestones, Corals dominate Netherwood and Wight, (V) Tarakan edge shallow marine ¯oatstones, and larger benthic 1992 basin, NE carbonates framestones, some foraminifera and Kalimantan argillaceous some coralline algae and Halimeda

Seilor (SO) and Mangkalihat Late Eocene Extensive carbonate Framestones, Larger benthic Suessli, 1976; Buchan et Taballar (TB), Peninsula (Tb) to Mio± platform and some rudstones, foraminifera al., 1971; Achmad and Tende Hantu (TH) and Maratua Pliocene isolated buildups, packstones, dominate much of Samuel, 1984; Wilson, and Domaring ridge reworking along grainstones, platform, also corals this paper (DM) Formations margin wackestones and and coralline algae. some dolomites Corals common on margins in Oligo± Miocene

Kedango/Lebak N Kutai Late Eocene Carbonate platforms Packstones, Larger benthic This paper Limestone (KO) margin, E (Tb) to early and surrounding wackestones, foraminifera Kalimantan Miocene slope and deeper rudstones dominate and some (Te5) water facies conglomerates and corals marls

Ritan Limestone N Kutai Late Eocene Isolated Packstones, Larger benthic Moss, 1994; Moss et member (RT) and margin, E (Tb) foraminiferal shoals grainstones and foraminifera al., 1997 limestone in Batu Kalimantan wackestones dominate Kelau Formation

Bebulu/Dian East Oligocene to Delta front or shelf Framestones, Corals dominate Alam et al., this Carbonates (BB) Kalimantan late Miocene edge shallow marine ¯oatstones, and larger benthic volume; Roberts and and in carbonates framestones, some foraminifera and Sydow, 1996a and b; oshore area argillaceous some coralline algae Siemers et al., 1992 and Halimeda

Batu Belaq (Belah) Upper Kutai Oligocene Shoal or platform Packstones and Larger benthic Wain and Berod, 1989; Limestone (BQ) basin, E carbonates grainstones foraminifera Van de Weerd and Kalimantan dominate, some Armin, 1992, Moss, coralline algae 1994, Moss and Chambers, this volume

Berai (BR) and SE Mostly Extensive carbonate Packstones, Larger benthic Van de Weerd et al., Tanah Grogot (TG) Kalimantan Oligocene, platform, marginal grainstones, foraminifera 1987; van de Weerd Limestones but late deposits and framestones, dominate on much and Armin, 1992; Saller Eocene to buildups. Number rudstones, of the platform, et al., 1992; 1993 early of sequences seen at ¯oatstones, corals and coralline Miocene in platform margin wackestones, some algae also occur Barito Basin. argillaceous Active sedimentation on Paternoster Platform

The area of the Mangkalihat Peninsula and the constrain models for sedimentation and Tertiary basin north Kutai Basin formed a region of extensive shal- evolution in the area. Carbonate sedimentation low water carbonate deposition which separated the occurred in a variety of depositional environments, deeper water areas of the Kutai and Berau/Tarakan ranging from mixed carbonate±clastic shelves, localised Basins in northeast Kalimantan (Fig. 1). This study and transient foraminiferal shoals and patch reefs, describes the development and depositional environ- extensive carbonate platforms to slope and deeper ments of carbonates in northeast Borneo and helps to water basinal areas (Figs. 1 and 2, Table 1). The 188 M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201 analysis of these diverse carbonates provides analogues resulted in the formation of a widespread marine area for other SE Asian carbonates developed in similar which included large parts of eastern Borneo, as well settings. as the Makassar Straits, East Java Sea and much of Carbonates have been imaged in the subsurface by western Sulawesi (van de Weerd and Armin, 1992). seismic surveys to the north of the Mangkalihat Carbonate sedimentation was common within and Peninsula and in the northern part of the Kutai Basin around the margins of these basins where favourable (personal observation and Tomascik et al., 1997). conditions occurred in shallow marine areas with lim- These carbonate plays have been targeted for hydro- ited clastic input (Fig. 1, Table 1). Areas of carbonate carbon exploration and a number of the build-up fea- deposition are much less extensive today than during tures have been drilled. Subsurface core is limited, the Tertiary (Fig. 1), as a consequence of a compres- driving a need to develop models from outcrop data. sional regime which may have begun as early as the To achieve the objectives of the study, detailed ®eld Oligocene (Moss et al., 1998). This compression and laboratory analysis of carbonate facies, deposi- resulted in uplift, creation of more widespread land tional geometries and environments, controls on sedi- areas, the progradation of large deltas and signi®cant mentation, and diagenetic history of the carbonate clastic input into marine areas north and east of outcrops was undertaken. Data includes carbonate Borneo (Hutchison, 1989; Wilson and Moss, 1998). facies logs, outcrop samples, thin sections and biostra- Modern and Tertiary carbonates in Borneo, and tigraphic analysis. indeed throughout SE Asia, can be subdivided into four main depositional systems (Table 2). Since carbonate depositional environments are extremely com- 2. Cenozoic carbonate production in Borneo plex and aected by a wide range of factors it should be noted that the divisions shown in Table 2 are sim- Modern carbonate production and deposition is extremely widespread and varied in SE Asian waters. Table 2 When compared with outcrop and subsurface data, the Modern and Tertiary carbonate depositional systems in Borneo modern sediments are a useful analogue to Cenozoic depositional systems and carbonate rocks. Tropical (1) Localised and transient carbonates carbonate production is most proli®c in warm, shallow These occur throughout the Cenozoic as patch reefs, shoals or marine areas, where there is limited clastic and nutrient shelf margin carbonates in delta front or clastic shelf areas or on fault bounded highs (Fig. 1, Table 1). Other examples of input and the waters are well oxygenated. In the seas localised and transient carbonates would include those surrounding Borneo today these conditions are best developed around volcanic islands, such as along the Sulu developed along the east coast and bordering some of Archipelago or the North Arm of Sulawesi today, or near the islands to the west (Fig. 1). The reefal areas Kelian where upper Eocene bioclastic limestones interdigitate around the Kalimantan coast have been subdivided with volcanics (Pieters et al., 1987). (2) Mixed carbonate clastic shelves into patch reefs, atolls, fringing and barrier reefs by Areas of mixed carbonate/clastic deposition occurred around MacKinnon et al. (1996). However, this scheme does the margins of the basins, particularly during the Paleogene, or not encompass all the modern carbonate depositional when land areas became emergent due to inversion in the systems and is dicult to apply to Cenozoic carbon- Neogene. Both foraminifera and corals occur and clastic input ates in Borneo. For example, the Paternoster Platform, is common. Cenozoic examples include some of the carbonates from the north margin of the Kutai Basin (see text) and the oshore SE Kalimantan (Fig. 1), is characterised by Gomantong Limestone in Sabah (Fig. 1, Table 1) extensive areas below 30 m water depth where larger (3) Extensive carbonate platforms benthic foraminifera dominate and only localised coral These range throughout the Cenozoic, are often foraminifera reefs are developed along the northern and eastern dominated and occur in extensive shallow water areas away margins (Burollet et al., 1986). from clastic input. The Paternoster Platform is the best modern example around Borneo (Fig. 1). Carbonates in eastern Borneo Land areas in western Borneo and land connections of the Berai Limestone and those deposited along the north between Borneo and the rest of Sundaland, together margin of the Kutai Basin and the Mangkalihat Peninsula are with clastic input into the surrounding seas, prohibited Tertiary examples (Fig. 1, Table 1). The Melinau Limestone is carbonate development during much of the Cenozoic another example in Sarawak (Fig. 1, Table 1). (Lloyd, 1978). Present day reefs located oshore west (4) Slope and deeper water environments Redeposited carbonates, particularly coarse, immature units Kalimantan (Fig. 1) probably only developed during derived from shelves and platforms occur throughout the the Quaternary when the Sunda Shelf became a sea- Tertiary and were particularly well developed around shallow way (Wilson and Moss, 1998). In contrast, a deep mar- water areas with steep margins. Examples of Tertiary ine area existed in northern Borneo throughout the carbonates with well developed coarse marginal deposits Cenozoic. include the Melinau Limestone, and carbonates from the northern margin of the Kutai Basin and the Mangkalihat Regional basin initiation around the margins of Peninsula (Fig. 1, Table 1, text). Sundaland occurred during the early Paleogene and M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201 189

Table 3 Summary table showing carbonate depositional environments during the Cenozoic along the northern margin of the Kutai Basin and the Mangkalihat Peninsula

Age North margin of the Kutai Basin NE Kutai Basin and western Mangkalihat Peninsula Mangkalihat Peninsula

Middle Localised development of shallow No middle Eocene carbonate Interbedded clastic, coals and shales were deposited on Eocene marine inner shelf carbonates with exposures seen in the area. the eastern tip of the Mangkalihat Peninsula, whereas some clastic input. bathyal mudstones were deposited in the Taballar River area.

Late Eocene Localised foraminifera shoals Shallow marine moderate to low within shelfal clastic succession, energy carbonate platforms such as the Ritan Limestone. dominated by larger benthic Carbonates also developed on foraminifera in Gunung Pengabun faulted highs such as Gunung karstic outcrop areas with some Khombeng. Upper Eocene bathyal deposits to the west. shallow marine low to moderate Shallow marine mixed carbonate-clastic shelf developed energy carbonates at Gunung at the eastern tip of the Mangkalihat Peninsula. Some Gongnyay were karsti®ed, redeposition of shallow water bioclasts, lithi®ed although timing of karsti®cation carbonate and clastic material also occurred at the is unclear. eastern tip of the Peninsula, into an outer neritic or upper bathyal setting. Suggests complex basin Early Localised occurrence of shallow Carbonate production on a con®guration, with localised shallow and deeper water Oligocene marine low to moderate energy shallow marine moderate to low areas. shelf carbonates. Some energy carbonate platform redeposition along margins. dominated by larger benthic foraminifera, coralline algae and in some places coral patch reefs in Gunung Pengabun karstic outcrop area. Reworking of bioclasts and lithi®ed clasts into outer neritic to upper bathyal areas at the eastern end of the Late Localised occurrence of shallow Shallow water carbonate platform Mangkalihat Peninsula. Complex platform margin Oligocene marine low to moderate energy sedimentation with both larger between Landas and Teluk Sumbang, included high shelf carbonates within clastic benthic foraminifera and corals in energy, reef-rimmed margin and steep and possibly succession (Telen River and N of Gunung Pengabun area. In places faulted platform margin shedding massive clast- Muara Wahau). Some a steep platform margin is supported limestone breccias. Areas and periods when redeposition into deeper marine inferred. Bathyal and upper both shallow water bioclasts, including massive corals, areas. bathyal areas lay to the west and and lithi®ed clasts (both carbonate and clastic) were south of this platform area and reworked from the platform margin. Occurrence of considerable shallow water extensive shallow water, moderate to low energy material and lithi®ed carbonate carbonate platforms between Teluk Sumbang and Batu and some clastic material was Putih and in the Taballar River area. A shelfal margin shed into this area. In the with reefal development was inferred to have occurred Northern Bengalon area a region during the Oligo±Miocene in the Menumbar area. An of shallow water carbonate atoll with low energy, slightly restricted platform interior production dominated by larger facies, steep southern and eastern coral reef margins and benthic foraminifera and coralline a northwestern margin with a `step-wise' appearance is algae was bounded by a gently inferred for the interior of the Mangkalihat Peninsula. dipping ramp-type margin with some clastic input. Upper Oligocene limestones rest unconformably on deformed basement cherts and metasediments in the Kelai Area. Shallow water platform and adjacent deeper water deposits in the upper reaches of the Mahakam River. (continued on next page) 190 M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201

Early Thin carbonates units within Carbonate production on a Continued reworking of bioclasts and lithi®ed clasts into Miocene succession of pro-delta shales and shallow marine moderate to low outer neritic to upper bathyal areas on eastern end of clastics in the Telen River area. energy carbonate platform the Mangkalihat Peninsula. Continued extensive shallow dominated by larger benthic water, moderate to low energy carbonate platform foraminifera, coralline algae and sedimentation between Teluk Sumbang and Batu Putih'' in some places corals in the and in the Taballar River area. As above for the Gunung Haji karstic outcrop area. Menumbar area and the central part of the Mangkalihat Lower Miocene shelf margin, Peninsula during the late Oligocene. upper bathyal and bathyal deposits occur to the northwest of the this shallow water carbonate platform in the Northern Bengalon outcrop area. Bathyal and upper bathyal areas lay to the west and south of the Gunung Pengabun area and considerable shallow water material and lithi®ed carbonate and some clastic material was shed into this area. Localised shallow water carbonate production and some subaerial exposure are inferred for the Gunung Pengabun karstic area.

Middle No middle Miocene or younger No middle Miocene carbonates Probable continuation of extensive, moderate to low Miocene to carbonates were seen around the seen on NE margin of Kutai energy, shallow water carbonate platform sedimentation earliest northern margin of the Kutai Basin or western part of the between Teluk Sumbang and Batu Putih. A complex, Pliocene Basin. Mangkalihat Peninsula. moderate energy shelfal area with patch reef development, some tidal in¯uence and deeper water embayments or channels lay directly to the E and NE of Teluk Sumbang. To the SE of Teluk Sumbang an open marine upper bathyal setting with some in¯ux of shallow water bioclasts is inferred.

Late Not seen. Not seen. Moderate to high energy mixed carbonate-clastic shelf Pliocene± deposits containing abundant coral debris outcrop at the Quaternary eastern end of the Mangkalihat Peninsula. These deposits unconformably overlie lower Miocene redeposited facies and have been uplifted to 30±40 m above sea level. pli®ed end members, and a broad spectrum will exist were deposited on shallow water shelves bordering between these. The main depositional systems ident- land areas on the Mangkalihat Peninsula (Fig. 2b±c, i®ed include three shallow water settings. In addition, Table 3). Extensive shallow water platform deposits slope and deeper water environments may develop are exposed as massive karstic outcrops up to 600 m around any of the shallow water areas (Fig. 1, Tables 1 high on the Mangkalihat Peninsula and the northeast- and 2). ern margin of the Kutai Basin (Fig. 2, Table 3). These platform carbonates were studied in river, cave and cli sections. In north Kutai three massive karstic out- 3. Tertiary carbonates in the north Kutai Basin and the crops, which have a north±south trend, were deposited Mangkalihat Peninsula as isolated platforms surrounded by slope deposits (Fig. 4). On geological maps these carbonates have Carbonates studied on the Mangkalihat Peninsula and north margin of the Kutai Basin span the Tertiary been variously named the Lebak or Kedango and were deposited in a range of environments. Formations and assigned late Oligocene to early Isolated carbonate bodies outcrop in road and stream Miocene ages (Sukardi et al., 1995). However, the sections on the northern margin of the Kutai Basin. results of this study indicate that the platform carbon- These are laterally equivalent to clastic units of Eocene ates and adjacent slope deposits span the late Eocene and Oligocene age and were deposited as shoals or on to early Pliocene. On the nearby islands of Palawan, faulted highs (Fig. 2b±c and 3, Table 3). Mixed car- similar limestone massifs contain Paleocene foramini- bonate and clastic units of Eocene or Oligocene age fera (Robert Park, personal communication, 1998), M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201 191 suggesting that regionally the development of extensive Fig. 2 shows a number of time slice reconstructions il- platforms spanned the Cenozoic. lustrating the distribution of carbonate depositional In the Kelai river area, resedimented carbonates of environments. Many of the outcrops studied occur late Oligocene age outcrop adjacent to deformed cherts within one biostratigraphic zone. Hence, the grouping and metasediments of pre-Tertiary age (Fig. 2d). of the carbonate units into dierent time slices is in Oligo±Miocene platform carbonates were studied in part an artefact of the nature of the carbonate out- the Taballar river area on the north coast of the crops and the zonation schemes for the Tertiary. It is Mangkalihat Peninsula (Fig. 2d±f and 5, Table 3). therefore not clear whether these times slices (Table 3, Shallow water platform top and adjacent marginal Fig. 2) represent important time periods of carbonate deposits of Oligocene to Pliocene age were also ana- production and deposition, perhaps separated by local lysed from coastal, road and river sections between the or regional hiatuses. Alternatively, it may be that in village of Batu Putih and the eastern end of the some areas carbonate production was more continuous Mangkalihat Peninsula (Fig. 2d±f and 5, Table 3). A through much of the Tertiary. However, considerable shelfal margin with reef development was inferred to post-depositional faulting, or the limited or inaccess- have occurred during the Oligo±Miocene in the ible nature of outcrops do not allow dierentiation of Menumbar area on the southern part of the these two possible scenarios. Regional reconstructions Mangkalihat Peninsula (Suessli, 1976). An atoll with of the north margin of the Kutai Basin during the low energy, slightly restricted platform interior facies, Tertiary are shown in Moss and Chambers (1999). steep southern and eastern coral reef margins and a There has been considerable structural deformation northwestern margin with a `step-wise' appearance was of the carbonates and the underlying clastic succes- suggested for the interior of the Mangkalihat sions in this region, and post-depositional Miocene or Peninsula now exposed as the Tende Hantu mountains younger faulting often juxtaposes carbonate sequences (Suessli, 1976; van de Weerd and Armin, 1992). of dierent ages. Although syn-depositional Tertiary The history of carbonate deposition in the study extensional faults are suggested by the abundance of area during the Tertiary is summarised in Table 3. immature redeposited facies, success in locating faults

Fig. 3. Reconstruction showing some of the localised and transient carbonate depositional environments which developed in the upper Mahakam and Tabang/Belayan rivers during the middle to late Eocene. 192 M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201

Fig. 4. Depositional facies model of the carbonate platform in the Bengalon River area during the late Oligocene to early Miocene. M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201 193 in the ®eld was limited due to poor outcrop and over- carbonates structurally juxtaposed against upper printing by later structures. The dip of the carbonate Eocene bathyal clastics at Gunung Gongnyay (GG, successions varies from nearly horizontal to up to 50 Fig. 2c) may be another example of carbonates which degrees and dip directions vary considerably. On the developed on a faulted high. However, outcrop con- eastern end of the Mangkalihat Peninsula rocks as straints preclude the possibility of verifying this in the young as Mio±Pliocene have been aected by faulting Gunung Gongnyay area, since platform margin depos- and folding. its are not exposed and seismic data is not available. In the Gunung Khombeng section (GK, Fig. 2c), larger benthic foraminifera, miliolids and coralline 4. Carbonate depositional systems on the Mangkalihat algae are present throughout the sequence in bioclastic Peninsula and in the north Kutai Basin packstones and pack/grainstones. The occurrence of this biota, together with their robust morphologies in- 4.1. Localised and transient carbonates dicate a moderate energy setting in the shallower parts of the photic zone. Encrusting foraminifera with Localised and transient carbonates form isolated ex- curved growth forms occur abundantly in some beds. posures of limestone, often less than 30 m across and These may have been attached to seagrass in life, and less than 40 m thick, within the Kutai Basin or along again suggest deposition in the shallower parts of the its northern margin (Figs. 1 and 2c±d, Tables 1 and 3). photic zone. The carbonates vary in age from middle Eocene to ear- Carbonates in the Gunung Gongnyay section (GG, liest Miocene and lithologies within individual out- Fig. 2c) contain robust forms of larger benthic forami- crops were commonly deposited within one time nifera and fragmented coralline algae, but few imperfo- period, such as the late Eocene. Carbonate deposition rate foraminifera. These sediments are inferred to have was therefore transient or short lived on a geological been deposited in slightly deeper parts of the photic scale. The depositional systems studied vary from car- zone than those in Gunung Khombeng (GK, Fig. 2c). bonates developed on faulted highs, to foraminiferal These rocks are heavily fractured and occur in struc- shoals or small patch reefs forming on subtle basement tural contact with upper Eocene clastic sediments and highs or within clastic shelf sequences (Fig. 3). may have developed on a faulted high and/or in a delta front area away from clastic input. Subaerial ex- 4.1.1. Localised carbonates on faulted highs posure and karsti®cation of this surface is inferred During the Paleogene, the early stages of develop- from the presence of irregular cavities, partially follow- ment of the Kutai Basin are inferred to have been ing fractures, containing pendant cements and ®ne extensional (van de Weerd and Armin, 1992; Moss et sediment in®ll. Some fractures also cut the ®ne sedi- al., 1997). Within the Kutai Basin opposing polarity ment in®lls within these cavities suggesting that subaer- half-grabens developed, mostly trending NNE±SSW ial exposure occurred contemporaneously with or prior parallel to the Makassar Straits, oset by rift related to fracturing, although the exact timing of this is transform faults (Cloke et al., 1997). Facies present unknown. within these rift basins indicate that those in the east- A number of the larger carbonate platforms, such as ern part of the Kutai Basin, although locally contain- on the Mangkalihat Peninsula and in the Bengalon ing terrestrial sediments, are dominated by marine area, also developed on highs which were probably re- lithologies, whereas terrestrial deposits dominate in lated to pre-existing structures. However, because these depocentres towards the west (Moss and Chambers, carbonates are more laterally extensive and deposition 1999). spanned much of the Tertiary, these systems are Within these half-graben systems, shallow water car- described below under carbonate platforms. bonate production commonly occurred on footwall highs which were sheltered from clastic input. Deep 4.1.2. Localised foraminiferal shoals and patch reefs marine shales accumulated in adjacent hangingwall A series of isolated outcrops, with variable ages, are grabens and there may have been some resedimenta- located to the north of Muara Wahau close to the con- tion of shallow water carbonates into these hanging- tact with pre-Tertiary rocks of the basement complex wall depocentres. It is inferred from seismic lines (BC, Fig. 2c±d, Table 3). These include coral and cor- across the northern part of the Kutai Basin that car- alline algae bioclastic pack/rudstone and pack/¯oat- bonates developed on linear footwall highs. The upper stones, of probable early Oligocene age (Tc?), Eocene carbonates forming the isolated, north±south containing abundant fragmented shallow marine bio- trending karstic hill of Gunung Khombeng (GK, clasts, including recrystallized corals, coralline algae Fig. 2c) are an example of carbonates which developed and Nummulites. The limited occurrence of planktonic on a footwall high and are now exposed at the surface foraminifera combined with reworking of shallow mar- due to Neogene inversion. Fractured upper Eocene ine bioclasts, suggests a moderate energy open oceanic 194 M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201 setting. About 200 m to the north of these exposures, accommodation space and minimal subsidence. Some coral pack/¯oatstones in a marly matrix of middle of the carbonates occur within, or laterally adjacent Eocene age (Ta, probably Ta3) crop out. These lithol- to, clastic successions and probably accumulated on ogies contain ®ne quartz grains, as well as branching subtle highs within clastic shelf areas. Minor amounts corals, oysters, some larger benthic foraminifera and of ®ne to medium sand size quartz grains are present miliolids and are interpreted to have been deposited in within some of the shallow water carbonates, and the a low energy inner shelf setting. Lepidocyclina and carbonates may grade laterally into mixed carbonate± Nummulites bioclastic packstones of early Oligocene clastic shelf deposits (described below). Alternatively, age (Td), occurring as small outcrops a few metres on some parts of the shelf clastic supply may have wide, are inferred to have been deposited as a moder- bypassed areas of shallow water carbonate production. ate energy shallow water foraminifera shoal. Bioclastic packstones of late Eocene or Oligocene age (Tb±Tc), 4.2. Mixed carbonate±clastic shelves containing some fragmented recrystallized corals and coralline algae were deposited in a moderate energy, Exposures of quartzose bioclastic packstones occur shallow marine setting. Carbonate units, containing in stream sections either interbedded with, adjacent to shallow marine bioclasts and/or lithi®ed carbonate, or overlying outcrops of interbedded coals, sandstones, quartz grains and chert, sandstone, shale and metase- carbonaceous shales and claystones at the eastern tip diment clasts derived from the underlying formations of the Mangkalihat Peninsula (Fig. 2b±c, Table 2). are locally developed. These deposits indicate erosion Palynomorphs from coals and carbonaceous shales in and resedimentation from the margins of these loca- the clastic successions have been dated as middle to lised carbonates and some input of clastics, derived late Eocene. Interbedded current ripple-laminated and either from faulted footwall highs or nearby areas of trough cross-bedded sandstones, with abundant clay exposed clastic or basement rocks. drape laminae and bioturbation, suggest deposition in Other localised carbonate bodies, such as the Ritan a marginal marine setting, which included tidal ¯at Limestone (RT), or Batu Baleh/Belaq Limestone (BQ) and channel environments. occur further to the west along the northern margin of The base of this mixed carbonate±clastic sequence is the Kutai Basin (Figs. 1 and 3, Table 1). The upper not exposed. The complex structural setting of this Eocene Ritan Limestone Member of the Kiham Haloq area, together with the isolated nature of outcrops Formation consists of metre-scale packstones and along stream sections, renders it dicult to construct grainstones, containing abundant larger benthic fora- stratigraphic sections or to correlate between outcrops. minifera. These are interbedded with sandstones show- The thickness of this carbonate±clastic succession is ing hummocky and swaley cross-strati®cation, trough estimated to be a few tens of metres. Coals occur cross-bedding and planar and ripple lamination. The towards the base of the succession, whereas the mixed carbonates formed low-relief foraminiferal shoals on a carbonate±clastic lithologies occur towards the top. clastic marine shelf that was periodically in¯uenced by This section is therefore inferred to have been depos- storms or waves (Fig. 3; Moss and Chambers, 1999). ited as part of a transgressive succession passing from The Batu Belaq (or Baleh) Limestone Member of the a marginal marine to a more fully marine environ- Ujoh Bilang Formation is composed of Oligocene ment. interbedded packstones and grainstones, containing Most of the quartzose bioclastic packstones do not abundant larger benthic foraminifera. These sediments contain age diagnostic biota, but the presence of formed in a shallow marine setting, possibly as a shoal Nummulites and Pararotalia ?mecalepecenis suggests an deposit or as a small isolated platform. Nearby upper early Oligocene age (Td). Due to the close association Oligocene marls are interbedded with decimetre-scale of quartzose bioclastic packstones with the coals, beds of wackestones and packstone that contain abun- shales and sandstones, these packstones may range in dant planktonic and some larger benthic foraminifera age from late Eocene to early Oligocene. Mixed car- and represent laterally equivalent slope deposits or bonate±clastic deposits containing abundant well pre- slightly earlier pre-platform deposits. served larger benthic foraminifera and angular quartz The isolated nature of these outcrops, together with grains also occur within the Ritan Limestone Member their variable ages and facies, suggests that localised along the northern margin of the Kutai Basin and are areas of shallow water carbonate production occurred late Eocene in age (Moss and Finch, 1998). along the northern margin of the Kutai Basin from the Quartzose bioclastic packstones contain a limited, middle Eocene (Ta) through to at least the late but abundant and well preserved assemblage of com- Oligocene (Td, Fig. 2c±d). Sedimentation occurred in a mon rotaliid foraminifera, arenaceous foraminifera, a variety of shallow marine environments, such as inner few miliolids, Amphistegina, Nummulites, some shelf, foraminifera shoals or patch reefs, and the Halimeda plates, echinoid plates, sometimes abundant deposits are of limited thickness, suggesting a lack of encrusting foraminifera, and some well preserved M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201 195 planktonic foraminifera. The presence of abundant Oligocene in age, and comprise interbedded decimetre- disseminated carbonaceous plant material indicates to metre-thick packstones and pack/grainstones. These proximity to land or a marginal marine area. These contain abundant well preserved larger benthic forami- packstones are inferred to have been deposited on a nifera, such as Nummulites, Discocyclina and shallow, but open marine, mixed carbonate±clastic Pellatispira, coralline algae, imperforate foraminifera, shelf in an inner to middle neritic setting. The angular including miliolids and alveolinids, and branching cor- quartz grains were probably derived from the nearby als. Variations in texture, biotic composition and land area. growth forms between dierent beds suggest slight To the west of the carbonate platforms in the ¯uctuations in salinity and/or relative water depth, or Bengalon Area, decimetre-thick beds of mixed carbon- local variations in the physical environment on the ate±clastic lithologies are interbedded with deep mar- platform. In one lower Oligocene section, ¯oatstones ine shales or marls of early Miocene age. These and rudstones occur interbedded with the packstone. deposits ®ne upwards and commonly show ripple and These rocks contain abundant reworked branching parallel lamination towards the top of the beds, and and rounded massive corals which are inferred to have are interpreted as redeposited units. The clastic grains been derived from a nearby patch reef environment. were derived from Eocene deposits and possibly chan- Platform margin and bathyal deposits of late Eocene nelled across, or around, at least partially exposed and late Oligocene to early Miocene age have been shallow water platform deposits. Alternatively, the juxtaposed with platform deposits partly by faulting to clastic material may have been reworked form under- the south and west of Gunung Pengabun. A small out- lying units at the margins of the platforms. crop of upper Eocene shaley marls, containing abundant planktonic foraminifera and diagnostic small 4.3. Carbonate platforms and slope environments benthic foraminifera, indicates that bathyal sedimentation occurred contemporaneously with nearby shal- Extensive carbonate platforms, which spanned much low marine deposition. Upper Oligocene marls, rich in of the Cenozoic, developed on the northeast margin of planktonic foraminifera, are interbedded with graded the Kutai Basin and much of the Mangkalihat packstones and rudstones, and contain abundant frag- Peninsula (Fig. 2, Table 3). Dierences in facies and mented shallow water bioclasts. These marls are inter- diagenetic histories suggest that the platforms which preted to have been deposited along the margin and formed in the Bengalon area were isolated from a on the slope of the carbonate platform (Fig. 4). more extensive platform system which covered much Bordering other parts of the platform, beds of upper of the Mangkalihat Peninsula. It was possible to study Oligocene to lower Miocene marls with diagnostic platform margin and slope deposits associated with the western margin of the carbonate platforms in the bathyal foraminifera are interbedded with immature Bengalon area and along the northern margin of the breccias and packstones containing abundant reworked Mangkalihat Peninsula. lithi®ed carbonate clasts, fragmented shallow marine bioclasts and between 3±30% ®ne to coarse quartz 4.3.1. Carbonate platforms in the Bengalon area grains. The shallow water material and lithi®ed car- Three isolated shallow water carbonate platforms bonate clasts were resedimented from the ¯anks and developed to the north and west of the Bengalon River top of the carbonate platform. Unlithi®ed shallow (Fig. 2a). The platforms trend north±south, probably marine bioclasts are abundant in upper Oligocene rese- controlled by antecedent topography and earlier base- dimented deposits, and evidence was not seen for sub- ment structures, and they are now exposed as isolated aerial exposure in reworked shallow marine carbonate karstic outcrops, up to 600 m high. The eastern side of clasts. Both these features suggest contemporaneous the southernmost of these shallow water carbonate late Oligocene shallow water carbonate production platforms, which is late Eocene to early Miocene in and resedimentation. In comparison, localised lower age, and its adjacent platform margin deposits were Miocene redeposited units, a few hundreds of metres studied in the karstic outcrops of Gunung Pengabun apart, contain abundant lithi®ed carbonate clasts or (GP), Gunung Haji (GH) and along the Bengalon abundant fragmented shallow marine bioclasts. Some River (Fig. 2a,c and 4). To the north of Gunung Haji, of the lithi®ed carbonate clasts include karstic cavities a thinner carbonate sequence, the upper part of which with pendant cements and sediment in®lls, indicative is late Oligocene to early Miocene in age, is folded of subaerial exposure prior to resedimentation. into a series of NNE±SSW anticline±syncline pairs Therefore, major resedimentation of the platform mar- which plunge to the SSW and were studied in the gin associated with subaerial exposure and contem- upper reaches of the Bengalon River. poraneous shallow water production are inferred for The southernmost platform deposits, exposed in dierent parts of the platform during the early Gunung Pengabun, are late Eocene and early Miocene (Fig. 4). 196 M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201

The karstic exposures of Gunung Haji (GH), to the 4.3.3. Carbonate platforms of the Mangkalihat area north of Gunung Pengabun (GP, Fig. 2c and 4), Platform deposits of Oligocene and early Miocene consist of lower Miocene interbedded bioclastic pack- age were studied in the Taballar River area and to the stones, rudstones, ¯oatstones and some mud/wackes- southeast of Batu Putih, on the northern part of the tones. These include diverse, abundant and commonly Mangkalihat Peninsula (Fig. 2a, d±f, Table 3). In the well preserved bioclasts, such as corals, larger benthic Taballar River, interbedded mudstones, wackestones foraminifera, imperforate foraminifera, coralline algae and bioclastic packstones of late Oligocene and early and echinoid plates. The biota and facies indicate de- Miocene age have been juxtaposed against bathyal position on a carbonate platform in a variety of low to mudstones of middle Eocene age along a WNW±ESE moderate energy shallow marine environments which trending fault. The dominant fossils vary from abun- may have been subjected to some minor ¯uctuations in dant imperforate foraminifera, thin ¯at forms of het- salinity (Fig. 4). erosteginid larger benthic foraminifera, to a variety of Coarse-grained redeposited carbonates of early shallow marine bioclasts. These deposits accumulated Miocene age, interbedded with deep water marls, in a range of water depths on a low energy, protected outcrop to the west and northwest of Gunung Haji. inner platform setting which underwent some ¯uctu- The redeposited units contain abundant lithi®ed lime- ations in salinity (Fig. 5). Carbonates juxtaposed stone clasts, and are inferred to have been reworked against the mudstone succession have been altered to from a steep platform margin (Fig. 4). Other upper dolomites and are heavily fractured along a 1±2 km Oligocene to lower Miocene slope deposits contain zone adjacent to the fault. only a variety of fragmented shallow marine bio- On the basis of similarities in the mudstone and clasts and well preserved small benthic and plank- wackestone facies to those observed in the Taballar tonic foraminifera. Therefore the nature of the River, a low energy inner platform setting is platform margin and resedimented material varied suggested for the area between Teluk Sumbang and laterally. Upper Oligocene interbedded decimetre- Batu Putih during the Oligo±Miocene (Fig. 2d±f scale shaley marls and wacke/packstones, rich in well and 5). Close to Batu Putih, isolated outcrops of preserved planktonic foraminifera and glauconite are coral baestones, rudstones and framestones of exposed in the upper reaches of the Bengalon River. probable Miocene age suggest the development of These are interpreted to have been deposited on a patch reefs on the seaward side of some parts of gently sloping ramp-type margin (Fig. 4). Quartz, this platform. Sigmoidal packages of bioclastic pack- chert and fresh euhedral feldspar grains are present stones, up to a few metres thick, and rich in barna- in this succession. These were probably reworked cle debris, suggest the development of tidal channels from the north or east and bypassed the nearby on the platform during the Mio±Pliocene. platform deposits of Gunung Haji. The variety of Wackestones and marls of Miocene age, containing deposits to the south, west and north of Gunung abundant planktonic foraminifera, indicate that Pengabun and Gunung Haji indicate that the mor- deeper water embayments also occurred along the plat- phology of the platform margins varied from steep form margin. In the central and south-central part of the to low angle, and that a range of carbonate detritus Mangkalihat Peninsula, massive Oligo±Miocene out- was being shed from the platform areas. crops have been interpreted as shallow water carbonate platforms with reefal margins to the east and south (Suessli, 1976). On the northern part of the Mangkalihat Peninsula 4.3.2. Platform margin deposits in the Kelai area there was considerable lateral variation in platform In Sungai Kelai, west of Tanjung Redeb (Fig. 2d), margin and associated slope deposits during the redeposited carbonates interbedded with deep marine Tertiary. At the eastern end of the Peninsula, coarse shales crop out 100 m to the northwest of basement- redeposited carbonates, known as the Landas complex cherts. The redeposited carbonates in®ll Limestone Breccia, are interbedded with upper Eocene channels, have bed thicknesses of a few metres and to lower Miocene deep marine marls that contain are poorly sorted. They contain pebble to cobble abundant planktonic foraminifera. The redeposited size clasts of a variety of lithi®ed limestone clasts, as facies have bed thicknesses up to a few metres, are well as sub-angular to sub-rounded chert clasts. extremely immature, and contain fragmented shallow These upper Oligocene deposits are inferred to have water bioclasts, abundant angular clasts of a variety of been derived from a steep, and possibly faulted plat- carbonate lithologies, quartz grains, shale and chert form margin. Alternatively, the clasts of basement clasts. These deposits were shed from a steep and poss- material may have been reworked from areas of ibly faulted platform margin (Fig. 5). There is no evi- exposed basement on the platform top (Moss et al., dence in this area that the Landas Limestone Breccia 1997). occurs above an unconformity surface, as has been ...Wlo ta./Junlo sa at cecs1 19)183±201 (1999) 17 Sciences Earth Asian of Journal / al. et Wilson M.E.J.

Fig. 5. Reconstruction of the extensive carbonate platform on the Mangkalihat Peninsula during the early to middle Miocene. 197 198 M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201 suggested previously (van de Weerd and Armin, 1992). ditions. Slight ¯uctuations in salinity in¯uenced sedi- The common occurrence of fragmented shallow marine mentation on the inner parts of some of the platforms. bioclasts suggests that contemporaneous carbonate Clastic input was important on the mixed carbonate± production occurred on the platform during the late clastic shelves and aected the location of the localised Eocene to early Miocene. Further to the east, between and transient carbonates around the northern margin Landas and Teluk Sumbang, massive coral framestone of the Kutai Basin. and rudstones indicate the development of a reef- rimmed platform margin during the Oligocene. Thick isopachous shallow marine cements which formed 6. Diagenesis between the in situ corals suggest sea water ¯ushing along a high energy reef-rimmed margin (Fig. 5). The carbonates studied on both the Mangkalihat Grainstones with good porosities and permeabilities Peninsula and from the north Kutai Basin have occur landward of some of the reef-rimmed margins. similar diagenetic histories. Both have been in¯u- These lack abundant interparticle micrite characteristic enced by early marine phreatic or shallow burial of low energy inner platform areas, and show limited processes and later deeper burial processes. Early development of isopachous or blocky cements typical diagenetic eects include the micritization of bio- of high energy platform margin deposits. clasts and syntaxial overgrowths on echinoid plates. Marine acicular isopachous fringing cements and bladed or botryoidal cements are well developed 5. Controls on carbonate sedimentation along the high energy northern margin of the platform on the Mangkalihat Peninsula. However, these A wide range of regional and local factors, such as cements are rare in low energy parts of platforms tectonics, climate, oceanography and clastic input, may and on the northern margin of the Kutai Basin, in¯uence carbonate production and deposition, and in where micrite matrix dominates between bioclasts. some areas it may be dicult to distinguish some of Rare evidence for features associated with subaerial the controls on carbonate accumulation. In the north exposure occur locally in the shallow water carbon- Kutai Basin and on the Mangkalihat Peninsula the in- ates and occasionally in carbonate clasts reworked accessible nature of the outcrops and the structural from the platform margins during the Oligo± complexity of the area renders accurate correlation Miocene. These include in situ brecciation, alveolar between sections problematic, and therefore controls textures, dissolution cavities containing pendant on carbonate deposition are dicult to assess. cements and ®ne sediment in®ll. Dissolution of ara- An important in¯uence on Tertiary carbonate sedi- gonitic bioclasts was commonly observed in the mentation in the region was dierential subsidence, inner platform deposits, and more rarely the plat- with accommodation of over 500 m of shallow water form margin deposits of the Mangkalihat Peninsula. carbonates in some areas, whereas condensed lime- In contrast, the aragonitic components of the north- stones and/or non-deposition occurred in other areas. ern platform margin, and of carbonates from the Basement structures and antecedent topography northern Kutai Basin, show little sign of dissolution strongly in¯uenced the location and development of and had been neomorphically replaced by more carbonate successions. Faulting probably resulted in stable magnesium calcite. Selective leaching may large scale reworking of parts of the platform margins have occurred in a marine phreatic/shallow burial and the formation of the coarse, immature redeposited environment and/or be related to the development of facies. However, eustatic sea level ¯uctuations may a fresh water lens. The lack of dissolution of arago- have also in¯uenced resedimentation around the plat- nite components in the north Kutai Basin carbonates form margins and in part contributed towards avail- suggests that these platforms were not connected able accommodation space. The high productivity of with those on the Mangkalihat Peninsula. Other shallow water areas associated with a warm tropical diagenetic dierences between the two areas include climate, would have allowed carbonate production to the development of blocky cements, which are not keep pace with most relative rises in sea level. seen in the north Kutai Basin. Replacement of car- Outer platform carbonates, on the northeast margin bonate by dolomite rhombs occurred in a 1±2 km of the Mangkalihat Peninsula developed in a high wide strip along a major fault zone in the Taballar energy regime, probably related to a large fetch and River area. The limestones in this area had been jux- strong monsoonal winds in the Celebes Sea (Fig. 1). In taposed against a succession of deep marine shales, contrast, the inner part of the platform developed in a and it is probable that the magnesium source came relatively protected area under low to moderate energy from ¯uids released from the shales, and that altera- conditions. Deposits on the northern margin of the tion to dolomite occurred in a burial environment. Kutai Basin also accumulated under low energy con- Fracturing, sutured grain contacts, development of M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201 199 dissolution seams and stylolites, and neomorphic deposits. The dolomites in the Taballar River area, replacement of micrite by equant non-ferroan calcite locally have good porosities (5±20%) and moderate are other late diagenetic features. These features all permeabilities (tens of millidarcies), although more formed in a burial environment and are more perva- commonly a second phase of dolomite developed sively developed within carbonates from the north around the rhombs which destroyed porosity and per- Kutai Basin. meability.

7. Reservoir potential of the carbonates

An understanding of carbonate depositional en- 8. Conclusions vironments, spatial facies relationships, controls on deposition and diagenesis is essential in order to evaluate This combined ®eld and laboratory study of surface reservoir potential, and can be used as a predictive outcrops of Cenozoic age in the north Kutai Basin tool for similar subsurface carbonates. The low energy and the Mangkalihat Peninsula places new constraints depositional environment and abundant occurrence of on the age, facies variations, depositional environments micrite in carbonates from the north Kutai Basin and diagenesis of carbonates of northeast Kalimantan. result in low primary porosity. Early diagenetic eects Comparison of these carbonates with other Tertiary are mostly porosity occluding, and although some and modern carbonates in Borneo and around SE fracturing and chemical dissolution along dissolution Asia suggests that the variety of carbonate deposi- seams and stylolites has occurred, late stage equant tional environments studied in the north Kutai Basin sparry calcite development often occludes secondary and the Mangkalihat Peninsula mirrors the diversity of porosity. Up to 7% primary porosity is present in tropical carbonate production in the region. This study some of the redeposited carbonates in this area. has implications for carbonate studies and hydro- However, because compaction eects, fracturing and carbon exploration in SE Asia and stresses the import- amounts of cementation vary over short distances lat- ance of detailed analysis of surface carbonates to erally and vertically through the succession, areas with develop models for the subsurface. best developed primary or secondary porosity and per- Carbonate sedimentation in the north Kutai Basin meability development would be dicult to pinpoint in and the Mangkalihat Peninsula occurred in a range of the subsurface. In summary, carbonates studied from depositional settings, spanning the middle Eocene to the north Kutai Basin have little reservoir potential Plio±Pleistocene. These included mixed carbonate±clas- due to their unfavourable depositional and diagenetic tic shelves, localised and transient carbonates, exten- conditions. sive carbonate platforms and slope and deeper water On the Mangkalihat Peninsula low primary poros- environments. Carbonate deposition in Borneo, and ities (<5%) occurred in the low energy facies from the throughout SE Asia, also falls within these broad en- inner parts of the platform. However, aragonitic bio- vironmental settings. However, in detail, the range of clasts had commonly been leached from these deposits, facies, their relationships to each other, controls on resulting in porosities ranging between 5±15%. sedimentation and diagenetic histories are unique for Although moderate porosities occur within these each carbonate succession. Tropical Cenozoic carbon- deposits, permeabilities are generally low (less than 20 ates in SE Asia are produced mainly by organisms, millidarcies) because aragonitic bioclasts were rarely in although the dominant biota varies from larger benthic contact with each other. In contrast, good primary foraminifera, corals, coralline and green algae. In the porosities (up to 35%) were developed along the mod- north Kutai Basin and the Mangkalihat Peninsula, erate to high energy edges of the platform where little carbonate sedimentation was strongly in¯uenced by micrite was able to accumulate. Well developed shal- tectonic subsidence, basement structures, a tropical cli- low marine cements, associated with extensive sea mate, the nature of carbonate producing organisms water ¯ushing along the high energy margin resulted and probably syndepositional faulting. in almost total occlusion of primary porosity. These Depositional and diagenetic processes of the carbon- cements, or blocky cements, although less well devel- ates deposited in the north Kutai Basin under low to oped in more moderate energy sites behind the plat- moderate energy conditions were mostly porosity form margin and in many of the redeposited occluding. In comparison, both primary and secondary carbonates, are commonly present, and give some porosity occurs in carbonate deposits of the rigidity to the lithologies rendering them less resistant Mangkalihat Peninsula, where moderate energy plat- to mechanical compaction. The highest porosities, form top and redeposited carbonates deposited sea- between 5±35%, and moderate to good permeabilities ward of the platform margin are of potential reservoir (tens to hundreds of millidarcies) occur within these quality. 200 M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201

Acknowledgements Petroleum Association, 15th Annual Convention, October. pp. 155±169. The ®eldwork and subsequent laboratory analysis Buxton, M.W.N., Pedley, H.M., 1989. Short paper: A standardized model for Tethyan Tertiary carbonate ramps. Journal of the for this study was jointly funded by LASMO Runtu Geological Society, London 146, 746±748. Ltd, Maersk Oil Indonesia Maratua AS, and the Cloke, I.R., Moss, S.J., Craig, J., 1997. The in¯uence of basement reac- London University SE Asia Research Group. Their tivation on the extensional and inversional history of the Kutai support, technical assistance and approval to publish Basin, East Kalimantan. Journal of the Geological Society, this paper is gratefully acknowledged. Chandra London 154, 157±161. Dunn, P.A., Kozar, M.G., Budiyono, 1996. Application of Geoscience Tiranda, Jossy Inaray, and the LASMO ®eld crews are technology in a geologic study of the Natuna gas ®eld, Natuna Sea, thanked for invaluable assistance in the ®eld. M.E.J.W Oshore Indonesia. Proceedings of the Indonesian Petroleum and S.J.M hold (or held) postdoctoral research pos- Association, 25th Annual Convention, October. pp. 117±130. itions with the London University SE Asia Research Epting, M., 1980. Sedimentology of Miocene carbonate buildups, Group, which provided signi®cant logistical and tech- Central Luconia, oshore Sarawak. Geological Society Malaysia nical support. The ®nancial backing and input from Bulletin 12, 17±30. Hall, R., 1996. Reconstructing Cenozoic SE Asia. In: Hall, R., the consortium companies supporting the Research Blundell, D.J. (Eds.). Tectonic Evolution of Southeast Asia, Group, namely ARCO Indonesia, LASMO Indonesia Geological Society of London Special Publication, 106. pp. 153±184. Ltd, MOBIL Oil Indonesia, EXXON Inc, Canadian Heng, Y.E., 1985. Peta Geologi Negeri Sabah. Geological Survey of Petroleum Indonesia, Union Texas Petroleum and Malaysia. Unocal is acknowledged with thanks. Robertson Hutchison, C.S., 1989. Geological evolution of South-east Asia. Research, Ted Finch and Fred Banner provided bios- Clarendon Press, Oxford. Lloyd, A.R. 1978. Geological evolution of the South China Sea. tratigraphic analysis. Discussions with Ian Cloke and SEAPEX Proceedings. 4. pp. 95±137. Jon Noad clari®ed some of the ®ner points of Bornean May, J.A., Eyles, D.R., 1985. Well log and seismic character of stratigraphy. Robert Park and John Kaldi are thanked Tertiary Terumbu carbonate, South China Sea, Indonesia. for their careful reviews of this manuscript. American Association of Petroleum Geologists, Bulletin 69 (9), 1339±1358. MacKinnon, K., Hatta, G., Halim, H., Mangalik, A., 1996. The Ecology of Kalimantan. Periplus Editions, Singapore. References Moss, S.J., 1994. Report on ®eldwork in Kalimantan, July±September, 1994, 13. Unpublished Internal Report, SE Asia Research Group, University of London. Abdullah, N.T., Yaw, C.Y., 1993. Distribution of foraminiferal assem- Moss, S.J., Chambers, J.L.C., Cloke, I.R., Satria, D., Ali, J.R., blages in the upper Eocene Batu Gading Limestone, Sarawak. In: Milsom, J., Carter, A., Baker, S., 1997. New observations on the Thanasuthipitak, T. (Ed.). Symposium on biostratigraphy of main- sedimentary and tectonic evolution of the Tertiary Kutai Basin, land Southeast Asia: Facies and Paleontology. 31 January±5 East Kalimantan. In: Fraser, A.J., Matthews, S.J., Murphy, R.W. February, Chiang Mai, Thailand. pp. 231±242. (Eds.). Petroleum Geology of Southeast Asia, 126. Geological Achmad, Z., Samuel, L. 1984. Stratigraphy and depositional cycles in Society of London, Special Publication, 126. pp. 395±416. the N.E. Kalimantan Basin. Proceedings Indonesian Petroleum Moss, S.J., Carter, A., Hurford, A., Baker, S., 1998. Late Oligocene Association, 13th Annual Convention, October. pp. 109±120. uplift and volcanism on the northern margins of the Kutai Basin, Adams, C.G., Haak, R. 1962. The stratigraphic succession in the Batu East Kalimantan: implications for tectonics and sedimentation. Gading area, Middle Baram, North Sarawak. In: Haile, N. (Ed.). Journal of the Geological Society London 155, 177±192. Memoir Geological Survey Department British Territories, Borneo, 13. pp. 141±150. Moss, S.J., Finch, E.M., 1998. New Nannofossil and Foraminiferal de- Adams, C.G., 1964. On the age and foraminiferal fauna of the Bukit terminations from East and West Kalimantan and their impli- Sarang Limestone, Sarawak, Malaysia. Malaysian Geological cations. Journal of Asian Earth Sciences 15 (6), 489±506. Survey, Borneo Region, Annual Report for 1963, pp. 156±162. Moss, S.J., Chambers, J.L.C., 1999. Tertiary facies architecture in the Adams, C.G., 1965. The foraminifera and stratigraphy of the Melinau Kutai Basin, Kalimantan, Indonesia. Journal of Asian Earth Limestone, Sarawak, and its importance in Tertiary correlation. Sciences 17, 157±181. Quarterly Journal of the Geological Society, London 121, 283±338. Netherwood, R., Wight, A. 1992. Structurally-controlled, linear reefs Adams, C.G., 1970. A reconsideration of the East Indian Letter in a Pliocene Delta-front setting, Tarakan Basin, Northeast Classi®cation of the Tertiary. British Museum of (Natural Kalimantan. In: Siemers, C.T., Longman, M.W., Park, R.K., History) Geology 19 (3), 87±137. Kaldi, J.G. (Eds.). Carbonate rocks and reservoirs of Indonesia. Alam, H., Paterson, D.W., Corbin, S.G., Syarifuddin, N., Busono, I., Indonesian Petroleum Association Core Workshop Notes. 1. 1999. Reservoir potential of carbonate rocks in Kutai Basin region, Chap. 3. East Kalimantan, Indonesia. Journal of Asian Earth Sciences 17, Noad, J., 1996. The sedimentology of the Sandakan Formation. 203±214. Unpublished Internal Report, University of London SE Asia Ali, M.Y., 1992. Carbonate cement stratigraphy and timing of hydro- Research Group 165. carbon migration: an example from Tigapapan Unit oshore Sabah Pieters, P.E., Trail, D.S., Supriatna, S. 1987. Correlation of Early Geological Society of Malaysia. Bulletin 32, 185±211. Tertiary rocks across Kalimantan. Proceedings Indonesian Buchan, S.H., Campbell, R.C., Schuleman, S.F., 1971. Report on a Petroleum Association, 16th Annual Convention. pp. 291±306. reconnaissance ®eld survey of northeast Kalimantan Unpublished Roberts, H.H., Sydow, J., 1996a. The oshore Mahakam Delta: Internal Report. BP Petroleum Development Limited. Stratigraphic response of late Pleistocene to modern sea level Burollet, P.F., Boichard, R., Lambert, B., Villain, J.M. 1986. The cycle. Proceedings of the Indonesian Petroleum Association, 25th Pater Noster Carbonate Platform. Proceedings Indonesian Annual Convention, October. pp. 147±161. M.E.J. Wilson et al. / Journal of Asian Earth Sciences 17 (1999) 183±201 201

Roberts, H.H., Sydow, J., 1996b. The important role of carbonates in of the Carbonate Seminar, Jakarta, September, 1976. Indonesian the stratigraphic architecture of late Pleistocene±modern Mahakam Petroleum Association Special Publication, pp. 121±122. river delta-building cycles. Proceedings of the Indonesian Petroleum Sukardi, Djamal B., Supriatna, S., Santosa, S., 1995. Geological map Association, Abstracts, 25th Annual Convention, October. 210. of the Muaralasan Quadrangle, Kalimantan, Scale 1: 250,000. Rudolph, K.W., Lehmann, P.J., 1989. Platform evolution and Geological Research and Development Centre, Bandung. sequence stratigraphy of the Natuna platform, South China Sea. Tomascik, T., Mah, A.J., Nontji, A., Moosa, M.K., 1997. The Ecology In: Crevello, P.D., Wilson, J.L., Sarg, F., Read J.F. (Eds.). of the Indonesian Seas. Oxford University Press, UK. Controls on carbonate platform and basin development, Society of van de Weerd, A.A., Armin, R.A., 1992. Origin and evolution of the Economic Paleontologists Mineralogists Special Publication, 44. pp. 353±361. Tertiary hydrocarbon-bearing basins in Kalimantan (Borneo), Saller, A., Armin, R., Ichram, L.O., Glenn-Sullivan, C., 1992. Indonesia. American Association of Petroleum Geologists Bulletin Sequence stratigraphy of upper Eocene and Oligocene limestones, 76 (11), 1778±1803. Teweh area, Central Kalimantan. Proceedings of the Indonesian van de Weerd, A.A., Armin, R.A., Mahadi, S., Ware, P.L.B., 1987. Petroleum Association, 21st Annual Convention, October. pp. 69± Geologic setting of the Kerendan gas and condensate discovery, 92. Tertiary sedimentation and paleogeography of the northwestern Saller, A., Armin, R., Ichram, L.O., Glenn-Sullivan, C., 1993. part of the Kutei Basin, Kalimantan, Indonesia. Proceedings of Sequence stratigraphy of aggrading and backstepping carbonate the Indonesian Petroleum Association, 16th Annual Convention, shelves, Central Kalimantan, Indonesia. In: Loucks, R.G., Sarg, October. pp. 317±338. J.F. (Eds.). Carbonate sequence stratigraphy, recent developments Wain, T., Berod, B., 1989. The tectonic framework and paleogeo- and applications, American Association of Petroleum Geologists, graphic evolution of the Upper Kutei Basin. Proceedings of the Memoir, 57. pp. 267±290. Indonesian Petroleum Association, 18th Annual Convention. I. Salm, R.V., Halim, M., 1984. Marine Conservation Atlas. IUCN/ pp. 55±78. WWF Project 3108, Marine Conservation. Wilson, M.E.J., Rosen, B.R.R., 1998. Implications of the paucity of Siemers, C.T., Sutiyono, S., Wiman, S.K., 1992. Description and reservoir characterization of a Late Miocene, delta-front coral reef corals in the Paleogene of SE Asia: plate tectonics or Centre of buildup, Serang Field, Oshore East Kalimantan, Indonesia. In: Origin. In: Hall, R., Holloway, J.D. (Eds.). Biogeography and Siemers, C.T., Longman, M.W., Park, R.K., Kaldi, J.G. (Eds.) Geological Evolution of SE Asia. Backhuys Publishers, Carbonate rocks and reservoirs of Indonesia. Indonesian Amsterdam, Netherlands, pp. 165±195. Petroleum Association Core Workshop Notes. 1. Chap. 5. Wilson, M.E.J., Moss, S.J., 1998. Cenozoic palaeogeographic Suessli, P., 1976. The Tende Hantu AtollÐA lower Miocene carbonate evolution of Sulawesi and Borneo, Palaeogeography, build-up in Mangkalihat Peninsula, East Kalimantan. Proceedings Palaeoceanography and Palaeoclimatology. 145(4), 303±337.